Gaudencio Paz-Martínez

We investigate the frequency-dependent nonlinearities of an AlGaN/GaN high electron mobility transistor (HEMT) involved in terahertz (THz) detection by means of 2-D Monte Carlo (MC) simulations. The analysis shows that, below 1 THz, the responsivity roll-off is mainly governed by the passive microwave behavior of the device rather than by any limitation of the intrinsic detection mechanism. At higher frequencies, an inverse extraction method able to provide the intrinsic nonlinearity coefficients of the device reveals just a marginal broad enhancement around 1 THz, followed by a steep roll-off.

The influence on current–voltage and capacitance-voltage characteristics of different inhomogeneous profiles for the energy barrier ( ϕ B ) along the Schottky contact of GaN Schottky barrier diodes is studied by means of a home-made Monte Carlo simulator. An insight analysis of the electron concentration and electric field inside the device allows explaining the strong influence of inhomogeneities on the electronic transport across the metal semiconductor interface. In reverse bias, the low-barrier regions increase the number of tunneled electrons and these carriers tend to move towards the surrounded low populated zones inside the semiconductor. Therefore, two-dimensional effects appear and the current flowing through the device depends not only on the value and area of the low-barrier regions but also on its relative location. On the contrary, the potential profile is not affected much by low-barrier inhomogeneities, since they are pinched-off by the surrounding high-barrier zones. As a consequence, the effective ϕ B obtained from the C–V characteristics essentially corresponds to the (high-barrier) nominal ϕ B , while the effective ϕ B obtained from forward I–V characteristics is strongly impacted by the low-barrier regions. In the latter case, lower effective values of the barrier are obtained together with an increase in the ideality factor. These effects connect the physical performance of these devices with the effective ϕ B measured in fabricated ones.

The responsivity of sub-THz zero-bias detectors based on GaN high-electron mobility transistors (HEMTs) is measured up to 110 GHz. A predictive model based on static coefficients extracted from the dc output curves together with S-parameter measurements is able to replicate the detection results both when the radio frequency power is fed into the drain or the gate terminals. The gate-drain coupling, counteracting the contribution of the direct modulation of the drain voltage to the RF responsivity, is at the origin of the frequency roll-off of the detection, and therefore, the frequency performance is improved when shrinking the gate size of the transistors. The extraction of the small-equivalent circuit allows understanding the influence of the parasitic elements and the scaling of the gate width of the transistors on the detection performance, as well as extrapolating the results of the measurements to predict the performances at higher frequencies. In addition, the analysis of the influence of the different equivalent-circuit elements allows linking the values of the responsivity and noise equivalent power (NEP) to the physics of the transistor operation and thus determining at which frequency the detection roll-off appears as a function of the gate length and width, and confirming that both must be reduced as much as possible to optimize the transistor high-frequency operation, the latter having a more significant influence.

In this work, by means of Monte Carlo (MC) simulations, the current responsivity of AlGaN/GaN HEMTs operating as zero-bias detectors is analyzed, reaching the THz frequency range. Two approaches are used for the calculation of the responsivity, trying to get physical information on the detection mechanisms from the comparison of their respective results. First, we determine the responsivity directly from the MC values of dc drain current shift originated by an input ac drain voltage excitation. As second approach, the responsivity is estimated from a closed-form expression involving the MC calculation of both the dc I–V curves of the transistor (to determine Taylor expansion coefficients) and the ac response in terms of the Y parameters (to be converted into S parameters). Both approaches coincide at low frequency, but the closed-form expression starts to deviate from the correct result at frequencies around 1 THz. Moreover, a modest plasma resonance is visible in the 2–5-THz range.

The hysteresis cycles observed in the reverse leakage current measured at low temperatures in GaN-on-sapphire Schottky barrier diodes (SBDs) have been deeply studied and interpreted in terms of trap-assisted tunneling (TAT) compatible with the existence of a trap energy band (TEB) near the metal-semiconductor interface. The analysis of the energies for which the maximum tunneling current occurs, both direct and through this trap band, allows us to explain the behaviors found at different temperatures. Starting from empty trap states by previous illumination, transient current measurements performed under different preconditioning voltages evidence a progressive partial filling/release of the trap energy levels, confirmed as well by pulsed measurements. Captured/released electrons modify the number of trap states available for tunneling and thus the current level. As a consequence, tunneling processes in the go and return paths take place through levels with different occupations within the TEB, originating the hysteresis cycle.

A generic matrix model for the responsivity of millimeter-wave (mmW) and sub-THz square law detectors is proposed. The model is valid for any number of ports and takes the biasing conditions into account, the power distribution, the phase shifts between ports, and the frequency dependence of the detector response. The proposed approach is experimentally validated up to 67 GHz on an AlGaN/GaN high-electron-mobility transistor (HEMT). It could, however, be used for other types of detectors, including diodes and CMOS detectors. In addition, a reliable mathematical method is presented, allowing the direct determination of the maximum attainable responsivity and the associated optimum excitation. This method avoids a computing time-consuming evaluation of the above quantities by constrained optimization and thus provides an efficient way to evaluate the performances of a square law detector only based on its dc and $S$ -parameters characterization.

In this paper, we report an analysis of reverse current mechanisms observed in GaN Schottky barrier diodes leading to hysteretic behavior of the I–V curves at low temperature. By means of DC measurements from 33 to 475 K, we demonstrate the presence of two leakage mechanisms when comparing the experiments with the results obtained using a unified model to predict the ideal reverse current of the diode. Poole–Frenkel emission is the dominant mechanism for temperatures above 200 K, while trap-assisted tunneling prevails for lower temperatures, where also, hysteresis cycles are revealed by means of DC dual-sweep voltage measurements. The energy of the corresponding traps has also been determined, being around 0.2 and 0.45 eV, respectively. The hysteresis phenomenon is attributed to the bias-induced occupancy of the energy states originating the leakage-current processes, which leads to the reduction of the reverse current after a high negative voltage is applied to the diode.

The behavior of GaN-based high-electron-mobility transistors (HEMTs) as microwave zero-bias detectors is very dependent on the configuration of the bias (current or voltage), the operation temperature, and whether the radio-frequency power is fed in the drain or the gate terminal. When the signal is injected into the drain, the negative current responsivity shows a bell-shape dependence on $V_{\rm GS}$ centered slightly above the threshold voltage for all the studied frequencies (1–43 GHz) and temperature ranges (8–400 K). In the case of the voltage responsivity, depending on the temperature range (associated with the presence or absence of drain leakage current), an increase or decrease of the responsivity is observed in subthreshold conditions. For the gate-injection (GI) configuration, as expected by the capacitive gate–drain coupling, the voltage responsivity at low frequency is null, but only for $V_{\rm GS}$ above threshold. Surprisingly, in subthreshold conditions, it is very high and positive, contrary to the negative values intuitively expected for this configuration. The origin of this unexpected behavior, taking place in both gate and drain injection (DI) configurations, is that the drain terminal is self-biased at the zero-current (ZC) operating point ( $V_{\rm DS}$ being largely negative). An analytical model based on static coefficients obtained from dc measurements can explain the mechanisms behind the observed dependencies of the experiments.

The detection capability of GaN-based nano-diodes is measured up to 110 GHz in two configuration schemes: voltage and current responsivity. The ratio between both figures of merit allows one to extract the AC resistance of the diode, showing a very flat value in all the frequency spans. An optimization on the geometrical parameters is performed, finding that the narrower the channel, the higher the voltage responsivity, and the higher the number of channels in parallel, the higher the current responsivity. The expected dependence of the noise equivalent power with the width and number of channels is confirmed. The proposed configuration for best performance of these devices as RF detectors is to allocate the maximum number of channels in parallel and operate in a current responsivity mode.

Two-terminal devices based on an AlGaN/GaN nanochannel exhibit a significant hysteresis in their current–voltage curve due to their large surface-to-volume ratio. Surface effects at the sidewalls of the trenches defining the nanochannel are the key for such behavior. By applying large positive/negative voltages, it is possible to trap/de-trap electrons in/from the surface states, thus modulating the conductance of the channel. In this work, the characteristic times of the involved slow processes (of the order of tens or hundreds of s) are studied from 70 up to 200 K, and the possible memory applications due to the charge retention are explored.