MWIR HgCdTe avalanche photodiodes

This paper reports results obtained on mid-wave infrared (MWIR) x=0.3 Hg1-xCdxTe avalanche photodiodes (APDs) that utilize a cylindrical "p-around-n" front side illuminated n+/n-/p geometry. This "p-around-n" geometry favors electron avalanche gain. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole to electron ionization ratio, k=alpha(h)/alpha(e), of zero. At 6 V bias and 77 K, gains are typically near 50, and gains of over 100 have been measured at higher biases. Response times have been modeled and measured on these devices. The modeling indicates that the geometry and dimensions of the diode control the diffusion limited device bandwidth. Rise times of less than 0.35 nsec (1GHz bandwidth) should be possible according to this analysis. To date 10% to 90% rise times as low as 1 nsec have been measured. The gain is approximately noiseless up to gains of over fifty which is consistent with insignificant hole ionization (k=0). The noiseless gain behavior reported here is inconsistent with the original theory of McIntyre that predicts an excess noise factor of 2 for the k=0 case. The explanation for these results will require application of the modified "history dependent" theory for excess noise later proposed by McIntyre.