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

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-