We report on the analysis of thermally-limited operation of quantum-dot infrared photodetectors ͑QDIPs͒. A device model is developed and used to calculate the QDIP detectivity as a function of the structural parameters, temperature, and applied voltage, as well as to determine the conditions for the detectivity maximum. The QDIP detectivity is compared with that of quantum-well infrared photodetectors ͑QWIPs͒. This work clarifies why the existing QDIPs are still inferior to QWIPs and shows that a significant improvement in the QDIP performance can be accomplished by the utilization of dense QD arrays with small QDs.

This paper deals with the comparison of quantum well, quantum wire and quantum dot infrared photodetectors (QWIPs, QRIPs and QDIPs, respectively) based on physical analysis of the factors determining their operation. The operation of the devices under consideration is associated with the intersubband (intraband) electron transitions from the bound states in QWs, QRs and QDs into the continuum states owing to the absorption of infrared radiation. The redistribution of the electric potential across the device active region caused by the photoionization of QWs, QRs and QDs affects the electron injection from the emitting contact. The injection current provides the effect of current gain. Since the electron thermoemission and capture substantially determine the electric potential distribution and, therefore, the injection current, these processes are also crucial for the device performance. To compare the dark current, responsivity and detectivity of QWIPs, QRIPs and QDIPs we use simplified but rather general semi-phenomenological formulae which relate these device characteristics to the rates of the thermoemission and photoemission of electrons from and their capture to the QWs and the QR and QD arrays. These rates are expressed via the photoemission cross-section, capture probability and so on, and the structural parameters. Calculating the ratios of the QWIP, QRIP and QDIP characteristics using our semi-phenomenological model, we show that: the responsivity of QRIPs and QDIPs can be much higher than the responsivity of QWIPs, however, higher responsivity is inevitably accompanied by higher dark current; the detectivity of QRIPs and QDIPs with low-density arrays of relatively large QRs and QDs is lower than that of QWIPs; the detectivity of QRIPs and QDIPs based on dense arrays can significantly exceed the detectivity of QWIPs.

We present the results of a new analytical model for the analysis of the dark current in realistic quantum dot infrared photodetectors (QDIPs). This model includes the effect of the space charge formed by electrons captured in QDs and donors, the self-consistent electric potential in the QDIP active region, the activation character of the electron capture and its limitation by the Pauli principle, the thermionic electron emission from QDs and thermionic injection of electrons from the emitter contact into the QDIP active region, and the existence of the punctures between QDs. The developed model yields the dark current as a function of the QDIP structural parameters, applied voltage, and temperature. It explains some features of the dark current characteristics observed experimentally.

We propose a device model for quantum dot infrared photodetectors (QDIPs) with relatively large lateral spacing between QDs as occurs in QDIPs fabricated and experimentally investigated recently. The developed model accounts for the self-consistent potential distribution and features of the electron capture and transport in realistic QDIPs in dark conditions. The model is used for the calculation of the dark current as a function of the structural parameters, applied voltage and temperature. It explains a rather sharp increase in the dark current with increasing applied voltage and its strong sensitivity to the density of QDs and the doping level of the active region. The calculated dependences are in good agreement with available experimental data. The obtained characteristics of QDIPs are compared to those of QWIPs with similar parameters.

We develop a device model for a high-electron-mobility transistor (HEMT) affected by the incoming terahertz radiation. The model takes into account the electron plasma oscillations in the HEMT channel, tunnelling of electrons from the channel into the gate layer and electron transit-time effects in this layer. It is shown that the excitation of plasma oscillations accompanied by the delay in the electron propagation across the gate layer and a strong nonlinearity of the tunnelling current can result in significant features of the HEMT high-frequency linear and nonlinear characteristics. We derive a formula for the HEMT gate-to-source/drain admittance. We also calculate the variation of the dc current induced by the terahertz radiation and the HEMT detection responsivity. It is found that the detection responsivity exhibits sharp resonant peaks corresponding to the frequencies of plasma oscillations. The resonant plasma frequencies and the positions of the admittance and detection responsivity peaks depend on the gate length and the lengths of the contact regions (source-to-gate and gate-to-drain spacings) and can be tuned by the gate voltage. The coincidence of the plasma and transit-time resonances can lead to a marked sharpening of the responsivity peaks.