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.

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This paper reports on the demonstration of QCDs based on ZnO/ZnMgO QWs grown by molecular beam epitaxy on an m-plane ZnO substrate. The TM-polarized intersubband absorption is peaked at 3 µm wavelength. The sample has been processed in the form of square mesas with sizes ranging from 10x10 µm 2 up to 100x100 µm 2 . The I-V characteristics reveal that 86% of the 260 devices are operational and that the surface leakage current is negligible at room temperature, which is not the case at 77 K. The photocurrent spectroscopy of 100x100 µm 2 QCDs reveals a photocurrent resonance at 2.8 µm wavelength, i.e. slightly blue-shifted with respect to the intersubband absorption peak. The photocurrent persists up to room temperature. The calibrated peak responsivity amounts to 0.15 mA/W under irradiation at Brewster's angle of incidence. This value allows to estimate the transfer efficiency (1.15 %) of the photoexcited electrons into the active QW of the next period.

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