Characterization of HgCdTe MWIR Back-Illuminated II-VI-07 ... - Voxtel

Characterization of HgCdTe MWIR Back-Illuminated
Electron-Initiated Avalanche Photodiodes
M. B. Reine (Email: marion.reine@baesystems.com), J. W. Marciniec, K. K. Wong, T. Parodos,
J. D. Mullarkey, P. A. Lamarre, S. P. Tobin, R. W. Minich and K. A. Gustavsen
BAE Systems
Lexington, Massachusetts 02421
M. Compton and G. M. Williams
Voxtel Inc.
Beaverton, Oregon 97005
Presented at:
U.S. Workshop on the Physics and Chemistry of II-VI Materials
Baltimore, Maryland
October 30 – November 1, 2007
To be published in: Journal of Electronic Materials 37, June 2008
Dr. Marion B. Reine
Principal Engineering Fellow
BAE Systems
2 Forbes Road
Lexington, Massachusetts 02421
781-863-3043 (-3638 FAX)
marion.reine@baesystems.com
-1-

Characterization of HgCdTe MWIR Back-Illuminated
Electron-Initiated Avalanche Photodiodes
M. B. Reine (Email: marion.reine@baesystems.com), J. W. Marciniec, K. K. Wong, T. Parodos,
J. D. Mullarkey, P. A. Lamarre, S. P. Tobin, R. W. Minich and K. A. Gustavsen
BAE Systems, Lexington, Massachusetts 02421
M. Compton and G. M. Williams
Voxtel Inc., Beaverton, Oregon 97005
ABSTRACT
This paper reports new characterization data for large-area (250×250 µm²) back-illuminated planar n-
on-p HgCdTe electron-initiated avalanche photodiodes (e-APDs). These e-APDs were fabricated in p-type
HgCdTe films grown by LPE on CdZnTe substrates, and were bump-mounted to fanout boards and
characterized in the back-illuminated mode. We previously reported [J. Electronic Materials 36, 1059 (2007)]
that these arrays exhibit avalanche gain that increases exponentially with reverse bias voltage, with gainversus-bias
curves that are quite uniform from element to element, and with a maximum gain of 648 at -11.7
V at 160 K for a cutoff wavelength of 4.06 µm. In this paper we report new data on these large-area backilluminated
e-APDs, including devices from a third LPE film with a longer cutoff wavelength, 4.29 µm at 160
K, whose data support the exponential dependence of gain on cutoff wavelength for the same bias voltage we
reported for the first two films with cutoff wavelengths of 3.54 and 4.06 µm at 160 K, in agreement with
Beck’s empirical model for gain versus voltage and cutoff wavelength in HgCdTe e-APDs. Our lowest gainnormalized
current density at 80 K and zero FOV is 0.3 µA/cm² at -10.0 V for a cutoff wavelength of 4.23 µm
at 80 K. We report data for the temperature dependence of gain over the 80-200 K temperature range. We
report, for the first time, the dependence of measured gain on junction area for widely-spaced circular diodes
with radii from 20 µm to 175 µm. We interpret the variation of measured gain with junction area in terms of
edge-enhanced electric field, and fit the gain versus junction radius data with a two-gain model with a lower
interior gain and a higher edge gain. We report data for F(M)≈1 for gain up to 150 at 196 K. We describe the
features of the abrupt breakdown phenomenon seen in most of our devices at high reverse bias voltages.
Keywords: HgCdTe, photodiode, avalanche photodiode, APD, infrared detector
-2-

INTRODUCTION
The Hg 1-x Cd x Te electron-initiated avalanche photodiode 1,2 (e-APD) has attracted considerable
attention over the past seven years, from both a technological and a device physics perspective. As a highperformance
infrared avalanche photodiode, the HgCdTe e-APD has ideal properties. It exhibits essentially
single-carrier multiplication, hence the gain is an exponential function of reverse bias voltage. The gain
mechanism is such that the excess noise multiplication factor F(M) is independent of gain, and is essentially
unity for many conditions. The bandwidth is a weak function of gain, and gain-bandwidths of 1.1 THz have
recently been reported. These remarkable properties of the HgCdTe e-APD are made possible by the unique
features of the electron energy band structure of this widely-applicable semiconductor alloy. Inspired by the
seminal papers from Beck et al. 1 and Kinch et al., 2 many other groups have since corroborated these ideal
properties in a variety of HgCdTe device configurations, with cutoff wavelengths ranging from 2.2 µm to
10.3 µm.
In previous papers, 3,4 we reported data for HgCdTe MWIR back-illuminated planar n-on-p e-APD
4×4 arrays with large unit cell area, 250×250 µm², fabricated by ion beam milling in single-layer p-type
HgCdTe films grown by LPE on CdZnTe substrates. These are the largest-area HgCdTe e-APD photodiodes
yet reported. The 4×4 arrays were bump-mounted to fanout boards and characterized in the back-illuminated
mode. The maximum gain measured is 648 at -11.7 V for a device in a 4×4 array with a cutoff wavelength of
4.06 µm at 160 K. Gain versus bias voltage curves are quite uniform from element to element. For the same
reverse bias voltage, the gains measured at 160 K for elements in 4×4 arrays with two different cutoff
wavelengths (3.54 and 4.06 µm at 160 K) increase exponentially with cutoff wavelength, in agreement with
Beck’s empirical model 1 for avalanche gain in HgCdTe e-APDs. Spot scan data at 79 K show that both the
V=0 response and the gain at V=-5.0 V are quite uniform and well-behaved over the junction area, with
evidence of edge-enhanced gain. These were the first spot scan data for avalanche gain ever reported for
HgCdTe e-APDs. Capacitance versus voltage data are consistent with an ideal abrupt junction having an n-
region donor concentration equal to the indium counterdoping concentration in the LPE film.
-3-

In this paper we report new data on MWIR back-illuminated e-APDs, both the 250×250 µm²
elements we reported previously as well as widely-spaced variable-area circular photodiodes. We report data
over an expanded temperature range, 80-200 K; our previous data were all for 160 K. We report data for
devices from a new LPE film with a longer cutoff wavelength, 4.29 µm at 160 K, which support the
exponential increase of gain with cutoff wavelength for the same bias voltage that we reported previously for
two films with shorter cutoff wavelengths of 3.54 and 4.06 µm at 160 K. We report data for gain-normalized
current density at 80 K and zero FOV, with our lowest value being 0.3 µA/cm² at -10.0 V for a cutoff
wavelength of 4.23 µm at 80 K. We report data for the temperature dependence of gain over the 80-200 K
temperature range. We report, for the first time, the dependence of measured gain on junction area for a
family of widely-spaced circular diodes with diameters ranging from 40 µm to 350 µm. We interpret the
variation of measured gain with junction area in terms of edge-enhanced electric field, which is consistent
with edge features seen in our spot scan data for gain. We obtain a good fit the data for gain versus junction
area with a two-gain model, having a lower interior gain and a higher edge gain. We report data for the noise
multiplication factor F(M), which show F(M)≈1 for gain up to 150 at 196 K, for a cutoff wavelength of 4.06
µm at 160 K. We describe the characteristics of an abrupt breakdown phenomenon that is seen in nearly all
our devices at sufficiently high reverse bias voltage.
HISTORICAL BACKGROUND
Reports of avalanche gain in reverse-biased HgCdTe photodiodes appeared in the literature as early
as 1977. Tredwell 5 reported “…avalanche multiplication in photoresponse as large as 30…” at large reverse
bias in front-illuminated planar n-on-p photodiodes with a cutoff wavelength of 2.15 µm at 295 K, formed by
Hg in-diffusion in bulk-grown p-type HgCdTe. In 1982, Shin et al. 6 reported a gain of 15 at -77 V reverse
bias for a back-illuminated planar n-on-p HgCdTe photodiode with a cutoff wavelength of 1.25 µm. formed
by boron implant in an LPE film grown on a CdTe substrate.
The physics of avalanche multiplication in HgCdTe began to become better understood with the
efforts of the French groups to develop HgCdTe APDs for fiber optic detection at 1.3-1.5 µm. 7-14 The relative
-4-

advantages of the HgCdTe energy band structure for both electron-initiated and hole-initiated APDs were
elucidated through the theoretical analyses of Leveque et al. 14
They describe two regimes in which the ratio
k=α h /α e of the hole ionization coefficient α h to the electron ionization coefficient α e is either much greater
than unity or much less than unity. For cutoff wavelengths shorter than approximately 1.9 µm (x=0.56 at 300
K), they predict that α h >>α e because of resonant enhancement of the hole ionization coefficient when the
energy bandgap is near or equal to the spin-orbit splitting energy ∆ 0 (=0.938 eV from Ref. 15). This regime,
with k=α h /α e >>1, is favorable for low-noise APDs with hole-initiated avalanche. de Lyon et al. 15 exploited
this resonant enhancement regime with back-illuminated six-layer HgCdTe hole-initiated SAM (Separate
Avalanche and Multiplication) APD 25-element arrays with 50×50 µm² unit cells, grown in situ by MBE on
CdZnTe, with an absorber cutoff wavelength of 1.6 µm and a multiplication region cutoff wavelength of 1.3
µm (matched to the spin-orbit splitting energy), which exhibited gains of 30-40 at 85-90 V reverse bias at 300
K.
For cutoff wavelengths longer than approximately 1.9 µm, Leveque et al. 14 predict that the hole
ionization coefficient is non-resonant and decreases with increasing cutoff wavelength, while the electron
ionization coefficient increases rapidly because of the decreasing electron effective mass, so that the ratio
k=α h /α e is quite small, also a condition favorable for low-noise APDs, but now with electron-initiated
avalanche.
There were several early reports of experimental verification of the predicted small values of k=α h /α e
in HgCdTe with cutoff wavelengths longer than 1.9 µm. Nguyen Duy et al. 8 reported k=α h /α e =0.1 for a 2.5
µm front-illuminated planar implanted n-on-p photodiode at 300 K, with a gain of 30 at -30 V bias.
DeWames et al. 16 reported avalanche gain with an exponential dependence on reverse bias voltage, with a
gain of 4 at -5 V, and estimated a value for k=α h /α e < 0.1 for back-illuminated implanted n-on-p HgCdTe
photodiodes grown by LPE on a CdTe/sapphire substrate with a cutoff wavelength of 4.64 µm at 120 K. By
fitting M(V) and I NOISE (V) of p-absorber lateral-collection “loophole” diodes with a cutoff wavelength of 11
-5-

µm at 77 K, Elliott et al. 17 deduced that the reverse bias I(V) curves (for reverse bias beyond -1.4 V) are
limited by avalanche (impact ionization) mainly due to one carrier (electron); the largest gain reported is 5.9
at -1.4 V. Reisinger et al. 18 measured a gain of 50 at -5 V in their front-illuminated Hg-diffused n-on-p
photodiodes with a cutoff of 11.95 µm at 77 K, and concluded that α e >100×α h from analysis of the shape of
spectral response curve as function of reverse bias voltage.
It was, however, the paper by Beck et al. 1 in 2001 that first reported the clear and compelling
advantages of the electron-initiated avalanche process, based on data on MWIR HgCdTe lateral-collection “paround-n”
photodiodes with p-type absorber regions: avalanche gain increases exponentially with reverse bias
voltage, is quite uniform from element to element, and is essentially noiseless (i.e., F(M)≈1.0). Soon
thereafter, a theory by Kinch et al. 2,19,20 of the physics of electron and hole avalanche multiplication in
HgCdTe related the large inequity between α e and α h to three key features of the HgCdTe energy band
structure: the electron effective mass is much smaller than the heavy hole effective mass, which means that
electrons have a much higher mobility, a much lower scattering rate by optical phonons, and a factor-of-two
lower ionization threshold energy; there are no subsidiary minima in the conduction band to which energetic
electrons can scatter; and the light holes are not important. The key features of this theory were substantiated
by Monte Carlo simulations by the group at the University of Texas at Austin. 21
Since the seminal papers of Beck et al. 1 and Kinch et al., 2,19,20 there have been reports of HgCdTe e-
APDs in other configurations and architectures, all of which corroborate the essential features of the electroninitiated
avalanche process initially reported by Beck et al. 1
Baker et al. 22,23 report a 1.55 µm range-gated
320×256 FPA for 3D imaging with a lateral-collection “loophole” HgCdTe e-APD array having 24×24 µm²
unit cells, cutoff wavelengths of 4.2-4.6 µm, and gains over 100 at -7 V, operating at 90 K. Back-illuminated
implanted planar n + -n - -p e-APD 1×64 arrays grown by MBE on CdZnTe are reported by Vaidyanathan et al. 24
with a gain of 1000 at -10.5 V for a 4.2 µm cutoff at 78 K, and a gain >100 at -3.5 V for a 10.3 µm cutoff at
78 K. Hall et al. 25 report gain at 78 K of 100 at -12 V for a back-illuminated MWIR n-p-P structure grown in
situ by MOVPE on GaAs with an x=0.36 gain layer.
-6-

More recent results on HgCdTe e-APDs in the front-illuminated lateral-collection “p-around-n”
architecture from the DRS group include characterization 26-28 of both SWIR (2.2 µm) and LWIR (9.7 and 10.8
µm), and a 128×128 gated IR imaging FPA 29,30 with 40×40 µm² unit cells, cutoff wavelengths of 4.2 to 5 µm,
median gains of 946 at -11 V, and a sensitivity of less than one photon for a 1 µs gate width.
A gain of 5300 at -12.5 V is reported in the paper by Perrais et al. 31,32 for a back-illuminated planar p-
i-n 30×30 µm² photodiodes with cutoff wavelength of 5.0 µm at 78 K, formed by boron implantation in a
HgCdTe film grown by MBE. This is the highest reported gain for a HgCdTe e-APD, and is consistent with
the gains reported for all other e-APDs with shorter cutoff wavelengths, given that the e-APD gain increases
exponentially with both cutoff wavelength and reverse bias voltage. Recently, this group reported 32,33 the first
measurements of pulse response and bandwidth in HgCdTe e-APDs, with a bandwidth value of ~400 MHz for
1.55 µm radiation, in a device with a 5.3 µm cutoff wavelength at 80 K, and with a gain of ~2800 at -11 V,
corresponding to a gain-bandwidth product of ~ 1.1 THz. The bandwidth exhibits only a weak decrease with
increasing gain, dropping from 500 MHz at unity gain to 400 MHz at 2800, in agreement with the theoretical
predictions for a k=0 APD. 26,27 This group also reported 34 gain of 4 at -1.3 V for a cutoff wavelength of 9.2
µm at 77 K, and a gain of 20 at -22.5 V for a cutoff wavelength of 2.4 µm at 150 K.
HgCdTe e-APD DESIGN
The device cross section for our back-illuminated planar n-on-p HgCdTe e-APD is shown in Figure 1.
It is fabricated in a single-layer p-type HgCdTe film grown on an IR-transparent CdZnTe substrate. Radiation
is incident on the CdZnTe surface, which may have a thin-film antireflection coating or interference filter.
Radiation is absorbed within the p-type region, creating electron-hole pairs that diffuse and drift toward the
depletion region. The p-side is much more heavily doped than the n-side, so nearly the entire width W of the
depletion region lies within the n-region. When the photogenerated minority carrier (electron) reaches the
edge of the depletion region, it is accelerated by the strong electric field. It gains energy rapidly and begins
the avalanche multiplication process. As reverse bias voltage is applied, the depletion region expands further
into the n-region, toward the surface, and the field increases, resulting in higher avalanche gain.
-7-

This architecture has a number of advantages. It is a planar structure. It has high fill factor and high
quantum efficiency, with spatially uniform response over its active area. It can be bump-mounted onto a
fanout board or onto a Readout Integrated Circuit (ROIC) chip for high-gain multiplexed Focal Plane Arrays.
The vertical geometry allows high bandwidth because photocarriers have only a short distance to traverse to
the depletion region, and the traversal time may be shortened by acceleration in the effective electric field set
up by the favorable compositional grading in the LPE film. 35
ARRAY FABRICATION
Both individual 4×4 array chips and Test Chips with circular variable-area diagnostic diodes and 4×4
arrays were fabricated in single-layer p-type HgCdTe films grown by horizontal-slider Liquid Phase Epitaxy
(LPE) from Te-solution onto 4×6 cm² near-lattice-matched CdZnTe substrates. (Ref. 35 reviews LPE
HgCdTe technology at BAE Systems.) The unit cell size in the 4×4 arrays is 250×250 µm². The acceptor is
the doubly-ionized Hg vacancy. The Hg vacancy concentration in the p-layer is 2×10 16 cm -3 . Indium
counterdoping at a concentration of 4.5×10 14 cm -3 was introduced during film growth to establish the donor
concentration in the n-region. SIMS profiles confirmed that the indium is uniformly distributed through the
thickness of the p-layer at the target concentration. Planar junctions were formed by ion beam milling. No
antireflection coating was used on these first devices, hence there is a 21% reflection loss at the CdZnTe
entrance surface.
EXPERIMENTAL DETAILS
Both individual 4×4 arrays and Test Chips were hybridized to fanout boards and packaged in flat
packs, which were mounted into bottom-looking variable-temperature pour-fill liquid-nitrogen dewars for
characterization in the back-illuminated mode. A sapphire window was used, with either an F/5 cold shield or
zero-FOV arrangement. Dark current and dark current plus dc photocurrent were measured versus bias
voltage with an HP4155A Parameter Analyzer. The dc photocurrent was generated by radiation from an
unfiltered 1000 K blackbody. Gain versus voltage curves were determined from successive traces of dark
-8-

current and dark current plus dc photocurrent versus bias voltage by the method described in our previous
papers (Ref. 3-4).
ARRAY CHARACTERIZATION DATA
Dependence of Gain on Cutoff Wavelength
Fig. 2 shows data for gain versus reverse bias voltage, for T=160 K, for the two films from our
previous work 3,4 along with new gain data for a third film with a longer cutoff wavelength. The gain data for
this new film show the same behavior as we reported for the first two films: an exponential increase in gain
with reverse bias, and excellent uniformity of gain from element to element. As expected, the gain at a given
bias voltage is higher for the new film with the longer cutoff wavelength.
Fig. 2 shows that there is reasonably good agreement between our M(V) data at 160 K for these three
films and the empirical model of Beck 1 (dashed curves) for avalanche gain M(V) in HgCdTe e-APDs:
M(V)
= 1+
2
[2(V−V
th ) / Vth
]
, Vth
= 6.8 ×
E
G
(1)
where the values for the energy band gap E G that appear in the equation for the threshold voltage V th were
calculated from the measured cutoff wavelengths λ CO for the three films with the usual relationship
E G =hc/λ CO . Beck’s model has only one adjustable parameter, the constant of proportionality between the
threshold voltage V th and the energy gap E G . Beck uses a value of 6.8 to fit their M(V) data at T=77 K for
their front-illuminated “p-around-n” HgCdTe e-APDs with cutoff wavelengths ranging between 2.6 µm and
10.8 µm. The plots of Beck’s model in Fig. 2 (dashed curves) are done with this same value of 6.8.
Dark Current at F/5 and FOV=0
Fig. 3 shows I(V) data for a 250×250 µm² photodiode taken at 80 K for both F/5 and FOV=0
conditions. Also shown are I(V) data for F/5 with the device illuminated by a 1000 K unfiltered blackbody,
from which gain was determined. Gain data are plotted on the right-hand axis. At -10.0 V the gain is 307,
about a factor of two higher than the gain of 150 at 160 K, shown in Fig. 2, measured for the same bias
-9-

voltage in other devices from this same film (163). This is consistent with the general trend for higher gain at
lower temperatures.
It can be seen in Fig. 3 that the 80 K dark current for F/5 is dominated by gain-multiplied
photocurrent from the room background photon flux, except at the highest bias voltages. This is more easily
seen by the plot of the gain-normalized dark current for F/5, which is basically independent of bias voltage,
except at the highest bias voltages. The I(V) data for FOV=0, however, show a more rapid rise with reverse
bias voltage than the gain, probably due to a tunneling-related leakage current mechanism, and it is this
mechanism which is responsible for the upturn in the F/5 dark current near -10 V.
The dark current measured at FOV=0 and 80 K for the device in Fig. 3 is 0.13 µA at -10.0 V, which,
for a gain of 307, corresponds to a gain-normalized dark current of 0.42 nA and a gain-normalized dark
current density of 0.67 µA/cm². The lowest values we have seen among our 250×250 µm² devices from this
film are about a factor of two below this. The data in Fig. 3 show that this FOV=0 dark current is probably a
tunneling-related leakage current, which we believe can be lowered through improvements in device design
and processing.
Dependence of Gain on Temperature
Fig. 4 shows data for gain versus reverse bias voltage taken for three 250×250 µm² elements in Test
Chip 235-G for four temperatures: 80, 120, 160 and 200 K. The trend for higher gain at lower temperatures is
clearly shown by the data.
Data for gain -9.0 V are plotted versus temperature in Fig. 4, for the devices of Fig. 3 and for devices
from another film with a shorter cutoff wavelength. Both films show a similar dependence of gain on
temperature.
The dashed curves in Fig. 4 are calculated from the Beck model 1 in Eq. 1 for fixed values of Hg 1-
xCd x Te alloy composition. These curves show the decrease in gain that would result if the only change with
temperature were the change in energy band gap. The data show a stronger decrease with increasing
-10-

temperature than the dashed curves, due to the additional effect of the reduction in ionization rate at higher
temperature.
The only other published data 28 for temperature dependence of gain in HgCdTe e-APDs are those of
the DRS group for a device with a cutoff wavelength of 2.63 µm at 300 K. Their data, also plotted in Fig. 4,
show a similar dependence on temperature.
Dependence of Gain on Diode Area
In our e-APD Test Chip design, there is a family of widely-spaced circular diagnostic photodiodes
with diameters ranging from 40 µm to 350 µm. We noticed a small but significant decrease in gain with
increasing area among the variable-area devices within a given Test Chip. This dependence of the gain versus
voltage curves on junction area is shown in Fig. 6 for four photodiodes from Test Chip 235G at 160 K. This
dependence is also shown in Fig. 7, where the gains at -7.0 V for a set of variable-area diodes in this Test
Chip at 80 K are plotted versus junction area. The gains at -7.0 V range from 50 for the largest diode to 70
for the smallest.
It is customary to use variable-area diodes 36 to determine the lateral optical collection length L OPT
from the dependence of measured photocurrent on junction area. This is illustrated in Fig. 8, which shows
data for the photocurrent, due to a 1000 K blackbody, measured at V=0 and 80 K for the variable-area diodes
in the Test Chip of Figs. 6 and 7, plotted versus junction area. With no lateral collection, the measured
photocurrent would be a linear function of junction area. With lateral collection, the smaller-area diodes have
larger photocurrent than expected. The solid curve in Fig. 8 is a best fit to the data of the equation
I PH =I PH-1D (1+L OPT /R J )², where I PH is the measured photocurrent, I PH-1D /(πR J ²) is the one-dimensional
photocurrent per unit area collected by a diode with no lateral collection, and R J is the diode radius. The best
fit to the data in Fig. 8 gives a value for L OPT of 12 µm, which is acceptably close to the 6 µm value calculated
with the model for p-type HgCdTe described in our previous papers. 3,4
We interpret the observed dependence of gain on junction area, shown in Fig. 7, as being due to each
diode having two regions with different gains, an interior region with gain M 1D and an edge region with gain
-11-

M E . The edge gain would be higher than the interior gain due to the higher electric field at the edge arising,
for example, from junction curvature or from a narrower n-region at the surface. Minority photocarriers
collected laterally over a length L OPT by diffusion from beyond the edge of the junction would experience the
edge gain, whereas those collected within the interior of the junction area would experience the interior gain.
This leads to a simple expression for the measured gain as a function of diode radius:
M
2
2 2
M1DR
J
+ M
E[(R
J
+ LOPT
) − R
J
]
= (2)
2
(R
J
+ LOPT
)
The solid curve in Fig. 7 is the best fit of this equation to the gain data. With L OPT set equal to 12 µm, the
value determined from the analysis in Fig. 8, the best fit of this equation to the data of Fig. 7 yields values for
the edge gain of 45 and the interior gain of 85.
Spatial Dependence of Avalanche Gain
Our previous papers 3,4 reported spot scan data for avalanche gain at V=-5 V in these large-area diodes
at 80 K. Those data (Fig. 9 in Ref. 34) showed that the gain is uniform within the interior of the junction area,
but that there is a hint of higher gain at the edges. This higher gain at the edges is seen more clearly in the
spot scan data in Fig. 9 of another element in that same 4×4 array. The spatially-dependent avalanche gain,
defined as the ratio of the profile at -5.0 V to the profile for V=0, is plotted in Fig. 9 on the right-hand axis.
The avalanche gain is spatially uniform over the interior of the photodiode, with an average value of 10.8,
which is in close agreement with the gain data at 80 K for this film taken in the full-illumination configuration
(see Fig. 3). There is a small but consistently observed increase in the FWHM at -5.0 V compared to the V=0
case, with the value of 3.8 µm for the data in Fig. 9, which is responsible for the small increases in the gain
profile at the edges of the junction seen in Fig. 9. This increase is most probably not due to the voltageinduced
increase in the depletion width, because the higher doping of the p-region causes the depletion region
width to increase inward, toward the n-contact. We interpret this increase as due to higher gain at the edges
of the diode, which is consistent with the dependence of measured gain on junction area in Fig. 7.
Excess Noise Factor F(M)
-12-

Noise measurements were done at Voxtel, Inc. A Test Chip from film 163, packaged in a flatpack,
was mounted inside a windowed vacuum cryochamber. Stable, switchable flood illumination is projected
through the window with an OZ Optics OZ-2000 1550 nm stabilized fiber-coupled diode laser. Bias voltage
is applied to the diode by an HP4145A semiconductor parameter analyzer (SPA). A bias-T couples the DC
component of the diode current to the SPA, and sends the AC component to an HP8447D high-speed preamp.
The preamp can feed either a spectrum analyzer or a noise figure meter. In this way, both the average gain
(calculated in the usual way from the DC component of the photocurrent) and the noise power spectral
intensity at a given frequency and bias voltage can be read simultaneously. An HP8566B spectrum analyzer
is used to choose frequency bands with low environmental noise, and an HP8970B noise figure meter makes
calibrated noise power measurements.
Noise power measurements are taken both in the dark and under illumination. By subtracting the
noise power of the dark current, the noise power of the photocurrent can be isolated. This allows one to
measure the multiplication noise and reject environmental noise. The HP8970B noise figure meter outputs
noise power spectral intensity (S P in W/Hz), and the impedance of the test circuit must be found to convert
these measurements to noise current spectral intensity (S I in A²/Hz). We use the noise measurement at unity
gain (low bias voltage) for this normalization, rather than a measurement of the diode impedance in isolation,
because this gives the most direct measurement of the relevant impedance – including effects from the
mounting of the detector, its interaction with the preamplifier and noise figure meter, and anything specific to
the frequency band used for the noise power measurement. The normalization is accomplished using
Schottky’s theorem, S I = 2 q I (A²/Hz), along with a direct measurement of the DC photocurrent I PH :
R = S P / S I = S P / (2 q I PH ) (3)
Between 20 and 30 independent measurements of the light and dark noise power at unity gain are used to find
R. Subsequently, noise current spectral intensity S I is measured at a variety of different values of M, taking
10 independent measurements of light and dark noise power for each gain point. Finally, excess noise factor
F(M) is calculated using the noise current spectral intensity theorem for avalanche multiplication:
-13-

S I = R S P = 2 q M² F(M) I PH (4)
The effect of our methodology to accurately measure the test circuit impedance under the same conditions as
the noise measurements is that the excess noise factor curve is normalized so that F(M)=1 at M=1. However,
there is always experimental variation associated with the noise power measurement, so this measurement of
the test circuit impedance is not absolutely accurate. Accordingly, one finds some scatter in the F(M) data at
higher gain, and instances in which F(M) appears to fall below 1. This is an experimental limitation which
we attempt to minimize by averaging a large number of noise measurements, and by selecting relatively
‘quiet’ RF frequency bands for our measurements, but it cannot be eliminated entirely.
Data for the excess noise factor F(M) obtained by this methodology for a large-area diode at 196 K
are shown in Fig. 10 plotted versus gain. Data were taken at a frequency of 20 MHz with a bandwidth of 1
MHz. F(M) is nearly independent of gain and is close to unity out to a gain of 150, corresponding to -9.0 V
bias.
Observations on Abrupt Breakdown
A common feature seen in the reverse-bias I(V) characteristics of the diodes from these films is an
abrupt increase in dark current at large reverse bias voltage. This abrupt breakdown occurs over a narrow
range of bias voltage, with the current rising abruptly its “normal” value to the upper limit set by the
HP4155A over a 0.1-0.2 V voltage interval. This abrupt breakdown is illustrated for several devices in Fig.
11. This abrupt breakdown limits the largest reverse bias voltage that can be applied to a given diode, and
hence limits the highest achievable gain. The importance of this abrupt breakdown was emphasized recently
by DeWames. 37
This abrupt breakdown is seen in all our diodes at sufficiently large reverse bias, unless other leakage
current mechanisms, all of which vary much less rapidly with bias voltage, cause excessive current at lower
bias voltages and prohibit voltages large enough to reach the abrupt breakdown.
-14-

The breakdown voltage increases with increasing temperature in a given diode. This can be seen in
the data in the upper plot in Fig. 11, where the breakdown voltage increases from -10.4 V at 80 K to -11.7 V
at 160 K.
The breakdown voltage increases with energy band gap for devices of comparable design and
processing. This can be seen in Fig. 11. The lower plot shows a breakdown voltage of -12.8 V for a cutoff
wavelength of 3.54 µm at 160 K, whereas the upper plot shows a breakdown voltage of -11.7 at 160 K for a
cutoff of 4.05 µm at 160 K.
The breakdown voltage may vary somewhat from element to element within an array or within a film,
but often the breakdown voltages tend to be the same within an array or within a film. For example, the lower
plot in Fig. 11 shows I(V) data for a set of variable-area diodes, all with the same breakdown voltage.
After experiencing breakdown, some diodes recover immediately, while some diodes are permanently
shorted.
Based on these observations, we believe the abrupt breakdown is not a fundamental limit to the
largest bias voltage that can be applied. We believe that this breakdown can be pushed to larger voltages
through modifications to device design and processing.
SUMMARY AND CONCLUSIONS
This paper has presented new characterization data for MWIR back-illuminated planar HgCdTe e-
APDs.
Gain versus voltage data for a new film with a longer cutoff wavelength, 4.29 µm at 160 K, support
the exponential increase of gain with cutoff wavelength for the same bias voltage that we reported previously
for two films with shorter cutoff wavelengths of 3.54 and 4.06 µm at 160 K. The gain versus voltage data at
160 K for all three films agree reasonably well with the Beck empirical model.
Dark current versus voltage data at 80 K for F/5 are dominated by the gain-multiplied photocurrent
due to background radiation, while at FOV=0 the dark current increases more rapidly than the gain,
suggesting a tunneling-related leakage current. The lowest values of gain-normalized dark current density at
-15-

80 K we measured are about 0.3 µA/cm² at -10.0 V for 250×250 µm² devices with a cutoff wavelength of
4.23 µm at 80 K. This is about one to two orders of magnitude above the lowest values reported by DRS 26 for
80 K and -10.0 V, for comparable cutoff wavelengths. We believe that lower dark currents at large reverse
bias will result from improved device designs and processing.
We reported data for gain versus temperature for 250×250 µm² devices from two films over the 80-
200 K temperature range.
This paper presented the first report of the dependence of measured gain on junction area for a family
of widely-spaced circular diodes with diameters ranging from 40 µm to 350 µm, with measured gains of 70
for the smallest diodes and 50 for the largest. We interpret the variation of measured gain with junction area
in terms of edge-enhanced electric field, which is consistent with edge features seen in our data for the spatial
behavior of gain. We obtain a good fit the data for measured gain versus junction area with a simple two-gain
model, having a lower interior gain and a higher edge gain for laterally collected minority photocarriers.
We reported data for the noise multiplication factor F(M), which show F(M)≈1 for gain up to 150 at
196 K, for a cutoff wavelength of 4.06 µm at 160 K.
We described the characteristics of an abrupt breakdown phenomenon that is seen in nearly all our
devices at sufficiently high reverse bias voltage. The breakdown voltage increases with increasing
temperature, is larger for shorter cutoff wavelength, and tends to be the same within a film. Some diodes
recover after breakdown while others are permanently shorted. We believe the abrupt breakdown in our
devices is technological, not fundamental, and that higher breakdown voltages will result from improved
device designs and processing.
ACKNOWLEDGEMENTS
Characterization of MWIR HgCdTe e-APDs was done at the BAE Systems facility in Lexington,
Massachusetts, and was funded by BAE Systems internal funds. Data were also taken Voxtel, Inc. in
Beaverton, Oregon. The devices were made and initially characterized under funding from Voxtel, Inc. under
-16-

NASA Contract CA28C from the Jet Propulsion Laboratory, Pasadena, California. We acknowledge the
collaboration and support of our colleagues at BAE Systems, Gary R. Woodward and Dr. Paul LoVecchio.
REFERENCES
1. J.D. Beck, C-F Wan, M.A. Kinch and J.E. Robinson, "MWIR HgCdTe avalanche photodiodes," Proc.
SPIE 4454, 188-197 (2001).
2. M.A. Kinch, J.D. Beck, C-F Wan, F. Ma and J. Campbell, "HgCdTe Electron Avalanche Photodiodes," J.
Electronic Mat. 33, 630-639 (2004).
3. M.B. Reine, J.W. Marciniec, K.K. Wong, T. Parodos, J.D. Mullarkey, P.A. Lamarre, S.P. Tobin, K.A.
Gustavsen and G.M. Williams, “HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiode
Arrays,” Proc. SPIE 6294, 629503 (2006).
3. M. B. Reine, J. W. Marciniec, K. K. Wong, T. Parodos, J. D. Mullarkey, P. A. Lamarre, S. P. Tobin, K. A.
Gustavsen and G. M. Williams, “HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiode
Arrays,” J. Electronic Materials 36, 1059 (2007).
5. T.J. Tredwell, "(Hg,Cd)Te Photodiodes for Detection of Two-Micrometer Infrared Radiation," Optical
Engineering 16, 237 (1977).
6. S.H. Shin, J.G. Pasko, H.D. Law and D.T. Cheung, “1.22 µm HgCdTe/CdTe Avalanche Photodiodes,”
Appl. Phys. Lett. 40, 965 (1982).
7. C. Verie, F. Raymond, J. Besson and T.N. Duy, “Bandgap Spin-Orbit Splitting Resonance Effect in Hg 1-
xCd x Te Alloys,” J. Crystal Growth 59, 342 (1982).
-17-

8. T. Nguyen Duy, A. Durand and J.L. Lyot, "Bulk Crystal Growth of Hg 1-x Cd x Te for Avalanche Photodiode
Applications," Mat. Res. Soc. Symp. Proc. 90, 81 (1987).
9. R. Alabedra, B. Orsal, G. Lecoy, G. Pichard, J. Meslage and P. Fragnon, "An Hg 0.3 Cd 0.7 Te Avalanche
Photodiode for Optical-Fiber Transmission Systems at λ = 1.3 µm," IEEE Trans. Electron Devices ED-32,
1302 (1985).
10. G.P. Lecoy, B. Orsal and R. Alabedra, "Impact Ionization Resonance and Auger Recombination in Hg 1-
xCd x Te (0.6 ≤ x ≤ 0.7)," IEEE Trans. Quantum Electronics QE-23, 1145 (1987).
11. B. Orsal, R. Alabedra, M. Valenza, G.P. Lecoy, J. Meslage and C.Y. Boisrobert, "Hg 0.4 Cd 0.6 Te 1.55-µm
Avalanche Photodiode Noise Analysis in the Vicinity of Resonant Impact Ionization Connected with Spin-
Orbit Split-Off Band," IEEE Trans. Electron Devices ED-35, 101 (1988).
12. B. Orsal, R. Alabedra, A. Maatougui and J.C. Flachet, "Hg 0.56 Cd 0.44 Te 1.6- to 2.5-µm Avalanche
Photodiode and Noise Study Far from Resonant Impact Ionization," IEEE Trans. Electron Devices ED-38,
1748 (1991).
13. Y. Liu, S. R. Forest, R. Loo, G. Tangonan and H. Yen, "Anomalous multiplication in Hg 0.56 Cd 0.44 Te
avalanche photodiodes," Appl. Phys. Lett. 61, 2878 (1992).
14. G. Leveque, M. Nasser, D. Bertho, B. Orsal and R. Alabedra, “Ionization energies in Cd x Hg 1-x Te
avalanche photodiodes,” Semicond. Sci. Technol. 8, 1317 (1993).
-18-

15. T.J. de Lyon, B. Baumgratz, G. Chapman, E. Gordon, A.T. Hunter, M. Jack, J.E. Jensen, W. Johnson, B.
Johs, K. Kosai, W. Larsen, G.L. Olson, M. Sen and B. Walker, “Epitaxial Growth of HgCdTe 1.55 µ
Avalanche Photodiodes by Molecular-Beam Epitaxy,” Proc. SPIE 3629, 256-67 (1999).
16. R.E. DeWames, J.G. Pasko, D.L. McConnell, J.S. Chen, J. Bajaj, L.O. Bubulac, E.S. Yao, G.L. Bostrup,
R. Zucca, G.M. Williams, A.M. Blume and T.P. Weismuller, “Radiatively Limited MWIR Photovoltaic
Detectors Fabricated on Sapphire Substrates,” Extended Abstracts, 1989 U.S. Workshop on the Physics and
Chemistry of II-VI Materials, San Diego, California, Oct. 3-5, 1989.
17. C.T. Elliott, N.T. Gordon, R.S. Hall and G. Crimes, "Reverse breakdown in long wavelength lateral
collection Cd x Hg 1-x Te diodes," J. Vac, Sci. Technol. A8, 1251-3 (1990).
18. A.R. Reisinger, F.J. Weaver, J.J. Voelker, S.J. Caputi and T.H. Myers, “Avalanche Multiplication in Hg-
Diffused Long-Wave Infrared HgCdTe Heterodyne Photodiodes,” 1995 (unpublished).
19. M. A. Kinch, Fundamentals of Infrared Detector Materials, Chap. 7, “HgCdTe Electron Avalanche
Photodiodes (EAPDs)” (SPIE Press, Bellingham, Washington, 2007).
20. M. Kinch, “A Theoretical Model for the HgCdTe Electron Avalanche Photodiode,” J. Electronic
Materials 37, this issue (2008).
21. F. Ma, X. Li, J.C. Campbell, J.D. Beck, C-F Wan and M.A. Kinch, “Monte Carlo simulations of
Hg 0.7 Cd 0.3 Te avalanche photodiodes and resonance phenomenon in the multiplication noise,” Appl. Phys.
Lett. 83, 785 (2003).
-19-

22. I. Baker, S. Duncan and J. Copley, “A Low Noise, Laser-Gated Imaging System for Long Range Target
Identification,” Proc. SPIE 5406, 133 (2004).
23. I. Baker, P. Thorne, J. Henderson, J. Copley, D. Humphreys and A. Millar, “Advanced multifunctional
detectors for laser-gated imaging applications,” Proc. SPIE 6206, 620608 (2006).
24. M. Vaidyanathan, A. Joshi, S. Xue, B. Hanyaloglu, M. Thomas, M. Zandian, D. Edwall, G. Williams, J.
Blackwell, W. Tennant and G. Hughes, “High Performance Ladar Focal Plane Arrays for 3D Range
Imaging,” 2004 IEEE Aerospace Conference Proceedings, pp. 1776-81 (2004).
25. R.S. Hall, N.T. Gordon, J. Giess, J.E. Hails, A. Graham, D.C. Herbert, D.J. Hall, P. Southern, J.W.
Cairns, D.J. Lees and T. Ashley, “Photomultiplication with low excess noise factor in MWIR to optical fiber
compatible wavelengths in cooled HgCdTe mesa diodes,” Proc. SPIE 5783, 412 (2005).
26. J. Beck, C. Wan, M. Kinch, J. Robinson, P. Mitra, R. Scritchfield, F. Ma and J. Campbell, “The HgCdTe
Electron Avalanche Photodiode,” Proc. SPIE 5564, 44 (2004).
27. J. Beck, C. Wan, M. Kinch, J. Robinson, P. Mitra, R. Scritchfield, F. Ma and J. Campbell, “The HgCdTe
Electron Avalanche Photodiode,” J. Electronic Mat. 35, 1166 (2006).
28. P. Mitra, J.D. Beck, M.R. Skokan, J.E. Robinson, C.A. Musca, J.M. Dell and L. Faraone, “SWIR
hyperspectral detection with integrated HgCdTe detector and tunable MEMS filter,” Proc. SPIE 6295,
62950G (2006).
-20-

29. J. Beck, M. Woodall, R. Scritchfield, M. Ohlson, L. Wood, P. Mitra and J. Robinson, “Gated IR imaging
with 128 × 128 HgCdTe electron avalanche photodiode FPA,” Proc. SPIE 6542, 654217 (2007).
30. J. Beck, M. Woodall, R. Scritchfield, M. Ohlson, L. Wood, P. Mitra and J. Robinson, “Gated IR Imaging
with 128 × 128 HgCdTe Electron Avalanche Photodiode FPA,” J. Electronic Materials 37, this issue (2008).
31. G. Perrais, O. Gravrand, J. Baylet, G. Destefanis and J. Rothman, “Gain and Dark Current Characteristics
of Planar HgCdTe Avalanche Photo Diodes,” J. Electronic Materials 36, 963 (2007).
32. J. Rothman, G. Perrais, P. Ballet, L. Mollard, S. Gout and J.-P. Chamonal, “Latest Development of
HgCdTe APDs at CEA LETI-Minatec,” J. Electronic Materials 37, this issue (2008).
33. G. Perrais, J. Rothman, J. Baylet, J. P. Chamonal and G. Destefanis, “HgCdTe Avalanche Photodiodes
Response Time Depending on Gain and Temperature,” J. Electronic Materials 37, this issue (2008).
34. J. Rothman, G. Perrais, G. Destefanis, J. Baylet, P. Castelein, and J.-P. Chamonal, “High performance
characteristics in pin MW HgCdTe e-APDs,” Proc. SPIE 6542, 654219 (2007).
35. P. LoVecchio, K. Wong, T. Parodos, S.P. Tobin, M.A. Hutchins and P.W. Norton, “Advances in liquid
phase epitaxial growth of Hg 1-x Cd x Te for SWIR through VLWIR photodiodes,” Proc. SPIE 5564, 65 (2004).
36. M. B. Reine, K. R. Maschhoff, S. P. Tobin, P. W. Norton, J. A. Mroczkowski and E. E. Krueger, "The
Impact of Characterization Techniques on HgCdTe Infrared Detector Technology," Semicond. Sci. Technol.
8, 788 (1993).
-21-

37. R. DeWames, “Properties of HgCdTe Avalanche Photodiodes,” J. Electronic Materials 37, this issue
(2008).
-22-

Unit Cell
CdTe Passivation Indium Bump N-Contact Metal
P-Contact
N + HgCdTe Layer
N- HgCdTe W(V)
P-HgCdTe
CdZnTe Substrate
IR Radiation
Fig. 1. Cross-section of the back-illuminated planar n-on-p HgCdTe e-APD.
1000
235-G, λ CO =4.29 µm
Els 32, 34, 40, 44, 85, 89
n-on-p e-APDs
Voxtel Test Chips Lots 1, 2
T=160 K
Area=250x250 µm²
100
163-A1, λ CO =4.06 µm
Els 49, 51, 54, 62
Gain
177-A2, λ CO =3.54 µm
Els 36, 38, 48, 88
10
1
Beck model :
M(V) = 1+
2
V
th
= 6.8×
E
[2(V−Vth
) / Vth
]
G
-13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Bias Voltage (V)
Fig. 2. Data for gain versus reverse bias voltage at 160 K for elements from three 4×4 e-APD arrays
fabricated in films with different cutoff wavelengths. The three dashed curves are calculated from the
phenomenological model of Beck et al. for avalanche gain in HgCdTe e-APDs.
-23-

1E-4
1E-5
F/5, with BB
163A1-B1-15, T=80 K
λ CO =4.23 µm at 80 K
250x250 µm²
1E+9
1E+8
1E-6
1E+7
Current (abs value) (A)
1E-7
1E-8
1E-9
1E-10
I DARK /M
F/5, Dark
FOV=0
1E+6
1E+5
1E+4
1E+3
Gain
1E-11
1E-12
I DARK /M
Gain
1E+2
1E+1
1E-13
1E+0
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Voltage (V)
Fig. 3. Data at 80 K for I(V) and I(V)/M(V) for both F/5 and FOV=0 conditions. Also shown are I(V) data
for F/5 with radiation from a 1000 K blackbody. Gain data are plotted on the right-hand axis. The dark
current data for FOV=0 may have been affected by our measurement apparatus between -5 V and 0 V.
1E+3
T=80 K
T=120 K
235-G
λ CO =4.54 µm at 80 K
Elements 32, 44, 85
Area = 250x250 µm²
F/5
1E+2
Gain
1E+1
T=160 K
T=200 K
1E+0
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Voltage (V)
Fig. 4. Gain versus reverse bias voltage data for three elements in Test Chip 235-G, for temperatures of 80,
120, 160 and 200 K.
-24-

1000
100
Gain at -9.0 V
10
235-G, El 32, Lco=4.53 µm at 80 K
163A1-B1, El 8, Lco=4.23 µm at 80 K
DRS, Lco=2.63 µm at 300 K
Beck model, x fixed
1
0 50 100 150 200 250 300
Temperature (K)
Fig. 5. Data for gain at -9.0 V plotted versus temperature for large-area photodiodes from two films with
cutoff wavelengths of 4.53 and 4.23 µm at 80 K, along with published data for a DRS device at 300 K with a
cutoff wavelength of 2.63 µm.
1E+3
1E+2
Test Chip 235G
λ CO =4.29 µm at 160 K
Els 4, 8, 12, 24
Circular diodes
T=160 K
Gain
1E+1
40 µm dia, El 4
60 µm dia, El 8
100 µm dia, El 12
350 µm dia, El 24
1E+0
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Voltage (V)
Fig. 6. Data for gain versus reverse bias voltage at 160 K for four widely-spaced circular diagnostic diodes
with diameters of 40, 60, 100 and 350 µm. Gain increases monotonically with decreasing diameter.
-25-

80
70
M(-7.0 V) data
Best fit, Lopt=12.0 µm
60
Gain at -7.0 V
50
40
30
20
10
Curve is best fit to :
M
M =
with
M
1D
1D
R
2
J
+ M
= 45,
E
(R
J
M
[(R
+ L
E
+ L
= 85,
L
)
2
J OPT
2
OPT
)
OPT
− R
2
J
]
= 12.0 µ m
235G
λ CO = 4.54 µm at 80 K
Circular, R J = 20-175 µm
T = 80 K, F/5
0
1E-5 1E-4 1E-3
Junction Area (cm²)
Fig. 7. Data for the gains measured at V=-7.0 V for widely-spaced circular diagnostic diodes at 80 K. Solid
curve is the best fit to the data with a two-gain model.
1E-5
Curve: least-squares fit to:
I PH = I PH1D (1+L OPT /R J )²
with L OPT = 12.0 µm
A J =200x200 µm²
Photocurrent at V=0 (A)
1E-6
1E-7
235G
λ CO = 4.54 µm at 80 K
slope=1
Circular, R J = 20-175 µm
T = 80 K, F/5
1E-8
1E-5 1E-4 1E-3
Junction Area (cm²)
Fig. 8. Data for photocurrent at V=0 for widely-spaced circular diagnostic diodes at 80 K, due to incident
radiation from a 1000 K blackbody, plotted versus junction area. The solid curve is the best fit to the data,
yielding a value for the lateral optical collection length L OPT of 12 µm. The data for the 250×250 µm² devices
are shown for comparison.
-26-

1E-7
10000
Relative Response
1E-8
1E-9
1E-10
V = -5 V
V = 0
Gain, avg = 10.8
163-A1, El 56
λ CO =4.24 µm at 80 K
T=79 K
With filter 1000
FWHM:
210.97 µm at 0 V
214.73 µm at -5 V
∆=3.76 µm
100
10
Ratio
1E-11
1
-250 -200 -150 -100 -50 0 50 100 150 200 250
Distance (µm)
Fig. 9. Spot scan profiles for a 250×250 µm² unit cell in a 4×4 e-APD array at 79 K, for V=0 and V=-5.0 V.
The ratio of the -5.0 V data to the V=0 data is defined as the gain, and is plotted on the right-hand axis. The
gain is spatially uniform in the interior of the diode, with an average value of 10.8 at -5.0 V. There is
evidence of gain enhancement seen at the edges of the junction.
5
4
3
163A1, El 80
λ CO =4.06 µm at 160 K
250x250 µm²
T = 196 K
V = -0.1 V to -9.0 V
Voxtel data
k=0
k=0.025
El 80
F(M)
2
1
F(M) normailzed to 1 at V=-0.1 V
0
0 20 40 60 80 100 120 140 160
Gain
Fig. 10. Data for the excess noise multiplication factor F(M) plotted versus gain, at T=196 K, taken at Voxtel
Inc. The two solid curves are plots of the standard McIntyre equation for F(M).
-27-

Current, absolute value (A)
1E-4
1E-5
1E-6
1E-7
1E-8
200 K
160 K
120 K
163A1-B1, El 15
λ CO =4.05 µm at 160 K
250x250 µm²
FOV=0
80 K
1E-9
-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Voltage (V)
1E-3
Current (absolute value) (A)
1E-4
1E-5
1E-6
1E-7
177A2
λ CO =3.54 µm at 160 K
T = 173 K
Voxtel Data
350 µm dia.
250 µm dia.
150 µm dia.
100 µm dia.
60 µm dia.
40 µm dia.
1E-8
1E-9
-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
Voltage(V)
Fig. 11. Data illustrating general features of the abrupt breakdown phenomena seen in most of our devices.
Upper plot: Data for dark current (FOV=0) for a diode with a cutoff wavelength of 4.05 at 160 K, for
temperatures of 80, 120, 160 and 200 K, illustrating the increase of breakdown voltage with increasing
temperature. Lower plot: Data for dark current versus reverse bias voltage, for T=173 K, for widely-spaced
circular diagnostic photodiodes with a shorter cutoff wavelength of 3.54 µm at 160 K, illustrating the general
increase in breakdown voltage with decreasing cutoff wavelength.
-28-