Giorgia Longobardi

Impact of Donor Traps on the 2DEG and Electrical Behavior of AlGaN/GaN MISFETs

As an important step in understanding trap-related mechanisms in AlGaN/GaN transistors, the physical properties of surface states have been analyzed through the study of the transfer characteristics of a MISFET. This letter focused initially on the relationship between donor parameters (concentration and energy level) and electron density in the channel in AlGaN/GaN heterostructures. This analysis was then correlated to dc and pulsed measurements of the transfer characteristics of a MISFET, where the gate bias was found to modulate either the channel density or the donor states. Traps-free and traps-frozen TCAD simulations were performed on an equivalent device to capture the donor behavior. A donor concentration of 1.14×1013 cm-2 with an energy level located 0.2 eV below the conduction band edge gave the best fit to measurements. With the approach described here, we were able to analyze the region of the MISFET that corresponds to the drift region of a conventional HEMT.

The dynamics of surface donor traps in AlGaN/GaN MISFETs using transient measurements and TCAD modelling

This paper presents a detailed and correlated (i) Id-Vg, (ii) Cgg-Vg, and (iii) transient analysis of donor traps in a SiN/GaN/AlGaN/GaN Metal-Insulator-Semiconductor Field-Effect Transistor (MISFET) fabricated on a silicon substrate. We explain for the first time that the long-time constants are due to the close coupling between the emission/capture processes on one hand and the transient transport of electrons across the GaN/AlGaN barrier on the other. Emission and capture time constants were extracted for several bias conditions and temperatures. Moreover, we have developed a TCAD model that consistently gives a good match to DC, AC, and transient experimental results.

Modelling of an AlGaN/GaN Schottky diode and extraction of main parameters

This paper describes a method to extract the ideality factor, barrier height and series resistance of a lateral AlGaN/GaN heterostructure power Schottky diode using a simple I-V measurement in on-state and sub-threshold domains. An analytical model previously developed for Gallium Arsenide (GaAs) and Silicon vertical diodes [1] is applied to lateral AlGaN/GaN Schottky diodes and calibrated using extensive experimental results. The validity of the model at increased temperatures (up to 428K) is also investigated and the dependence of the ideality factor and barrier height with temperature are obtained and assessed against those previously reported in the literature [2].

SOI multidirectional thermoelectric flow sensor for harsh environment applications

In this paper we present a novel SOI CMOS multidirectional thermoelectric flow sensor. For the first time to the best knowledge of the authors, the sensor is shown to be able to cope with harsh environment conditions, such as 150 °C ambient temperature and humidity up to 75%.

Modelling 2DEG charges in AlGaN/GaN heterostructures

In this paper we compare different approaches to calculating the charge density in the 2DEG layer of AlGaN/GaN HEMTs. The methods used are (i) analytical theory implemented in MATLAB, (ii) finite-element analysis using semiconductor TCAD software that implements only the Poisson and continuity equations, and (iii) 1D software that solves the Poisson and Schrödinger equations self-consistently. By using the 1D Poisson-Schrödinger solver, we highlight the consequences of neglecting the Schrödinger equation. We conclude that the TCAD simulator predicts with a reasonable level of accuracy the electron density in the 2DEG layer for both a conventional HEMT structure and one featuring an extra GaN cap layer. In addition, while the sheet charge density is not significantly affected by including Schrödinger, its confinement in the channel is found to be modified.

On the physical operation and optimization of the p-GaN gate in normally-off GaN HEMT devices

In this study, an investigation is undertaken to determine the effect of gate design parameters on the on-state characteristics (threshold voltage and gate turn-on voltage) of pGaN/AlGaN/GaN high electron mobility transistors (HEMTs). Design parameters considered are pGaN doping and gate metal work function. The analysis considers the effects of variations on these parameters using a TCAD model matched with experimental results. A better understanding of the underlying physics governing the operation of these devices is achieved with a view to enable better optimization of such gate designs.

The effect of the surface fixed charge and donor traps on the C(V) and transfer characteristics of a GaN MISFET — Experiment and TCAD simulations

Fixed charge and surface traps at the passivation/semiconductor interface play a major role in both the on-state and off-state performance as well as reliability of AlGaN/GaN high-voltage transistors. This paper reports a comprehensive analysis of these fixed charges and donor traps using C(V) measurements of a Metal-Insulator-Semiconductor Field-Effect Transistor (MISFET) fabricated alongside a highvoltage HEMT. For the first time, we have correlated the C(V) measurements with the Id-Vg characteristics of the MISFET and have carefully matched them with corresponding TCAD simulations for detailed explanations of the phenomena involved. We have also carried out capacitance measurements at different frequencies and investigated the formation of an inversion layer at the passivation/semiconductor interface and its dependence on the surface charge and donor traps as well as frequency.

On the Source of Oscillatory Behaviour during Switching of Power Enhancement Mode GaN HEMTs

With Gallium Nitride (GaN) device technology for power electronics applications being ramped up for volume production, an increasing amount of research is now focused on the performance of GaN power devices in circuits. In this study, an enhancement mode GaN high electron mobility transistor (HEMT) is switched in a clamped inductive switching configuration with the aim of investigating the source of oscillatory effects observed. These arise as a result of the increased switching speed capability of GaN devices compared to their silicon counterparts. The study identifies the two major mechanisms (Miller capacitance charge and parasitic common source inductance) that can lead to ringing behaviour during turn-off and considers the effect of temperature on the latter. Furthermore, the experimental results are backed by SPICE modelling to evaluate the contribution of different circuit components to oscillations. The study concludes with good design techniques that can suppress the effects discussed.

On the vertical leakage of GaN-on-Si lateral transistors and the effect of emission and trap-to-trap-tunneling through the AIN/Si barrier

Vertical leakage in lateral GaN devices has a significant contribution to the overall off-state current at high blocking voltages and high temperatures. It could could lead to premature breakdown before avalanche or dielectric breakdown occur. This paper identifies via experimental results and TCAD simulations the main physical mechanisms responsible for the vertical leakage through the epi and transition layer structure: (i) silicon impact ionization, (ii) electron injection across the AIN nucleation layer/silicon interface, (ill) space charge limited current. In particular, the trap-to-trap model, accounting for the leakage current across the nucleation layer, was implemented showing to be dominant at lower voltages (<200V).

On the impact of substrate electron injection on dynamic Ron in GaN-on-Si HEMTs

Abstract The impact of electron injection from the substrate on the dynamic Ron of GaN-on-Si High Electron Mobility Transistors (HEMTs) has been investigated by means of back-bias transient and vertical leakage measurements and TCAD simulations. A strong correlation between electrons injected from the substrate and on-state drain current transients is demonstrated. Moreover, the contribution of the electron-type traps in the buffer layer as opposed to the usually studied hole-like traps to the dynamic Ron is discussed. In particular, the impact of electron-like traps for different levels of substrate leakage current is studied. A TCAD model has been developed and calibrated by taking into account both off-state vertical leakage and on-state drain current transient experimental results. The proposed charge dynamic has also been assessed against state-of-the-art theories. This analysis contributes to a deeper understanding of the complex scenario of different types of traps in the buffer layer of GaN-on-Si devices and highlights the impact that trap-states can have on the on-state and off-state currents.