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

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 gain–bandwidth 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 one-dimensional 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 × 10⁷ cm/s and v _h = 1.0 × 10⁷ 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 μm and x _j = 2 μm, respectively, for a gain of M = 100.