Study of the Transit-Time Limitations of the Impulse Response in Mid-Wave Infrared HgCdTe Avalanche Photodiodes

Abstract

The impulse response in frontside-illuminated mid-wave infrared HgCdTe electron avalanche photodiodes (APDs) has been measured with localized photoexcitation at varying positions in the depletion layer. Gain measurements have shown an exponential gain, with a maximum value of M = 5000 for the diffusion current at a reverse bias of V b = 12 V. When the light was injected in the depletion layer, the gain was reduced as the injection approached the N+ edge of the junction. The impulse response was limited by the diode series resistance-capacitance product, RC, due to the large capacitance of the diode metallization. Hence, the fall time is given by the RC constant, estimated as RC = 270 ps, and the rise time is due to the charging of the diode capacitance via the transit and multiplication of carriers in the depletion layer. The latter varies between t 10-90 = 20 ps (at intermediate gains M < 500) and t 10-90 = 70 ps (at M = 3500). The corresponding RC-limited bandwidth is BW = 600 MHz, which yields a new absolute record in gainbandwidth product of GBW = 2.1 THz. The increase in rise time at high gains indicates the existence of a limit in the transit-time-limited gain-bandwidth product, GBW = 19 THz. The impulse response was modeled using a onedimensional deterministic model, which allowed a quantitative analysis of the data in terms of the average velocity of electrons and holes. The fitting of the data yielded a saturation of the electron and hole velocity of v e = 2.3 9 10 7 cm/s and v h = 1.0 9 10 7 cm/s at electric fields E > 1.5 kV/cm. The increase in rise time at high bias is consistent with the results of Monte Carlo simulations and can be partly explained by a reduction of the electron saturation velocity due to frequent impact ionization. Finally, the model was used to predict the bandwidth in diodes with shorter RC = 5 ps, giving BW = 16 GHz and BW = 21 GHz for x j = 4 lm and x j = 2 lm, respectively, for a gain of M = 100.

Comparison of flash lidar detector options

“…10 The modeled fundamental, carrier transit time limited, BW is greater than 10 GHz. 22 Linear mode HgCdTe e-APDs as fabricated at DRS are front-side illuminated cylindrical p/n−/n+ HgCdTe homojunction photodiodes in the HDVIP ® (HDVIP ® stands for "high-density vertically integrated photodiode.") configuration, as shown in Fig.…”

Infrared Avalanche Photodiode Detectors

“…However, they reported that the planar APD structure provides fill factor~ 100 % and the collection efficiency > 90%, whereas HDVIP diode has advantage to yields low dark current, high quantum efficiency and high operability. 39,47 . They are first to measure the impulse response time for front side illuminated n-on-p MWIR HgCdTe e-APD at 77 K with λ = 1.55 μm laser pulse 47 .…”

HgCdTe APDS for space applications

“…1, such high bandwidths has not yet been achieved in HgCdTe APDs due to slow carrier collection through diffusion and/or impedance mismatch (BW=40-600 MHz). Impulse response-time measurements have however shown that bandwidths of 10 to 20 GHz should be achievable in SWIR and MWIR APDs designed for fast response [10,12].…”

“…The latter is again related to exclusive electron multiplication, which guarantees that the response time is limited by a single transit time of the depletion region width for each type of carrier [9]- [12]. The experimental and numerical studies of the carrier dynamics during the multiplication suggests that bandwidths in the range of 10-30 GHz should be achievable in HgCdTe APDs at gains higher than 100, although the highest bandwidth so far is in the range of 600 MHz to 1 GHz [10,12] . These characteristics are complemented by a high gain homogeneity which makes HgCdTe APDs suitable for imagery application.…”