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

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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-
Page 1 and 2: Characterization of HgCdTe MWIR Bac
Page 3 and 4: INTRODUCTION The Hg 1-x Cd x Te ele
Page 5 and 6: advantages of the HgCdTe energy ban
Page 7 and 8: More recent results on HgCdTe e-APD
Page 9 and 10: current and dark current plus dc ph
Page 11 and 12: temperature than the dashed curves,
Page 13: Noise measurements were done at Vox
Page 17 and 18: NASA Contract CA28C from the Jet Pr
Page 19 and 20: 15. T.J. de Lyon, B. Baumgratz, G.
Page 21 and 22: 29. J. Beck, M. Woodall, R. Scritch
Page 23 and 24: Unit Cell CdTe Passivation Indium B
Page 25 and 26: 1000 100 Gain at -9.0 V 10 235-G, E
Page 27 and 28: 1E-7 10000 Relative Response 1E-8 1

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

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