Roberto Pierobon

Surface-related drain current dispersion effects in AlGaN-GaN HEMTs

Drain current dispersion effects are investigated in AlGaN-GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects characterized by time constants in the order of 10/sup -5/-10/sup -4/ s cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

Richardson's constant in inhomogeneous silicon carbide Schottky contacts

The electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC) is reported in this article. In spite of the nearly ideal behavior of the contact at room temperature (n=1.05), the electrical behavior monitored in a wide temperature range exhibited a deviation from the ideality at lower temperatures, thus suggesting that an inhomogeneous barrier has actually formed. A description of the experimental results by the Tung’s model, i.e., considering an effective area of the inhomogeneous contact, provided a procedure for a correct determination of the Richardson’s constant A**. An effective area lower than the geometric area of the diode is responsible for the commonly observed discrepancy in the experimental values of A** from its theoretical value in silicon carbide. The same method was applied to Ti/4H–SiC contacts.

Degradation mechanisms of GaN-based LEDs after accelerated DC current aging

This work presents the results of an extensive DC current aging and failure analysis carried out on blue InGaN/GaN LEDs which identify failure mechanisms related to package degradation, changes in effective doping profile, and generation of deep levels. DLTS and photocurrent spectra indicate the creation of extended defects in devices aged at very high current density.

Performance evaluation of a Schottky SiC power diode in a boost PFC application

The performance of a 600 V, 4 A silicon carbide (SiC) Schottky diode (Infineon SDP04S60) is experimentally evaluated. A 300 W boost power factor corrector with average current mode control (PFC) is considered as a key application. Measurements of overall efficiency, switch and diode losses and conducted electromagnetic interference (EMI) are performed both with the SiC diode and with two ultra-fast, soft-recovery, silicon power diodes, namely the RURD460 and the recently presented STTH5R06D. The paper compares the results to quantify the impact of the recovery current reduction provided by SiC diode on these key aspects of the converter behavior.

Influence of gate-leakage current on drain current collapse of unpassivated GaN∕AlGaN∕GaN high electron mobility transistors

We report on a correlation between the gate leakage currents and the drain current collapse of GaN∕AlGaN∕GaN high electron mobility transistors. Unpassivated devices on intentionally undoped and doped (Si, 5×1018cm−3) heterostructures were investigated. We observed in the devices that the larger the gate leakage current, the smaller the drain current collapse measured at 50ns gate-voltage pulse turn on, and this correlation is independent of the doping of the structure. The correlation holds for two orders of magnitude in the gate-leakage current and up to 15% in drain current collapse. We believe that the leakage current can modulate trapped surface charge so that the time constant of the current collapse becomes much faster and dependent on the amount of leakage current itself.

Experimental/numerical investigation on current collapse in AlGaN/GaN HEMT's

Rf current collapse is investigated in AlGaN/GaN HEMTs by means of pulsed, transient, and small-signal measurements. Numerical device simulations are presented, showing that the concomitant presence, at the ungated device surface, of polarization-induced charges and hole traps can explain, without invoking any other hypothesis, all dispersion effects observed experimentally.

Hot-electron-stress degradation in unpassivated GaN/AlGaN/GaN HEMTs on SiC

The paper discusses long term on-state and off-state stress on GaN/AlGaN/GaN HEMTs on SiC substrates. Hot carrier effects and their dependence on bias conditions are evaluated with electroluminescence measurements. Both hot-electron stress conditions produce drain current gate-lag dispersion and gate current decrease. However on- and off- state stresses induce degradation in different gate-to drain surface device regions, i.e. close to the drain contact for the on-state stress and close to the gate contact in the off-state stress. Furthermore, a correlation between gate-leakage current and gate-lag dispersion is also observed.

Experimental and numerical analysis of gate- and drain-lag phenomena in AlGaAs/InGaAs PHEMTs

Gate- and drain-lag phenomena are investigated in AlGaAs-InGaAs pseudomorphic HEMTs by comparing experimental transient and pulsed characteristics with simulated ones. A consistent interpretation for experimental data is provided, relying on the assumption that acceptor-like surface traps are present at the ungated surface between gate and source/drain contacts.

Temperature Stability of Breakdown Voltage on SiC Power Schottky Diodes with Different Barrier Heights

The static and dynamic electrical characterization of power Schottky rectifiers both with Ti and Ni2Si as Schottky metals having low negative coefficient of the breakdown voltage versus temperature will be presented in this paper. The values of the barrier height are respectively 1.28eV and 1.68eV, as extracted using the Tung’s model for inhomogeneous contacts from forward currentvoltage characteristics. These values were found to be in good agreement with those obtained by means of capacitance-voltage measurements. The breakdown voltage shows an almost linear dependence from the temperature for both types of devices. The extracted coefficients are respectively -0.08V/°C and -0.11V/°C, thus guarantying stable and reliable behaviour. Very short reverse recovery time at RT and at 125°C confirms the good thermal stability of these devices.

Study of breakdown dynamics in InAlAs/InGaAs/InP HEMTs with gate length scaling down to 80 nm

In this paper we present the correlation between the impact ionization gate current with the S/sub 22/ scattering parameter measured in the 50 MHz - 6 GHz frequency range in InAlAs/InGaAs/InP HEMTs. Devices with shorter gate length presenting larger I.I. gate current have shown larger inductive component in the output admittance Y/sub 22/ at low frequencies.