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

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
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