Mitsutoshi Akita

A study on current collapse in AlGaN/GaN HEMTs induced by bias stress

Drain current collapse in AlGaN/GaN HEMTs has been studied systematically by applying bias stress to the device. The collapse was suppressed by light illumination with energy smaller than the bandgap. The position dependence of the light illumination and the measurement of series source and drain resistances revealed that the collapse was caused by the surface states between the gate and drain electrodes, which captured electrons injected from the gate. The current collapse was eliminated by the passivation of the device surface with Si/sub 3/N/sub 4/ film.

High-frequency measurements of AlGaN/GaN HEMTs at high temperatures

High-frequency measurements of the 1.3-/spl mu/m-long gate AlGaN-GaN HEMTs have been performed at temperatures ranging from 23 to 187/spl deg/C. The cutoff frequency f/sub T/ decreased with increasing temperature. It was 13.7 and 8.7 GHz at 23 and 187/spl deg/C, respectively. The effective electron velocities /spl upsi//sub eff/ in the channel evaluated from the total delay time versus I/sub D/-inverse relation were 1.2 and 0.8/spl times/10/sup 7/ cm/s at 23 and 187/spl deg/C, respectively.

AlGaN/GaN Heterostructure Metal-Insulator-Semiconductor High-Electron-Mobility Transistors with Si3N4 Gate Insulator

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) using Si3N4 film as a gate insulator have been successfully fabricated. The gate leakage current decreased by about three orders of magnitude compared to that of the conventional AlGaN/GaN HEMTs without the Si3N4 gate insulator. The low-frequency noise of the MIS-HEMTs was smaller than that of the conventional HEMTs. Current collapse has also been suppressed in the MIS-HEMTs.

Current collapse in AlGaN/GaN HEMTs investigated by electrical and optical characterizations

Drain current collapse in AlGaN/GaN HEMTs has been studied in detail. Applying negative gate bias stress to the device caused a serious current collapse. Spatially resolved light illumination on the device combined with the measurement of series source and drain resistances revealed that the collapse was caused by the electrons trapped at the surface states between the gate and drain electrodes. Passivation of the device surface with Si 3 N 4 film eliminated the current collapse.

Temperature distribution measurement in AlGaN/GaN high-electron-mobility transistors by micro-raman scattering spectroscopy

Temperature distributions in AlGaN/GaN high-electron-mobility transistors under bias voltage have been measured by micro-Raman scattering spectroscopy. The temperature was estimated from the Raman shift of E2 phonons of GaN. The spatial and temperature resolutions are 1 µm and 10 K, respectively. When the power dissipation was 248 mW at a drain voltage of 40 V, the peak temperature of 443 K was observed at the gate edge on the drain side at the center of the channel. This position corresponds to the high-field region at the gate edge.

Electroluminescence in AlGaN/GaN High Electron Mobility Transistors under High Bias Voltage

Electroluminescence (EL) in GaN high electron mobility transistors (HEMTs) biased at high drain-source voltages was investigated. The electroluminescence was observed at the drain edge where the high-field region is formed. This is quite different from the case of GaAs HEMTs where luminescence was observed at the gate edge. EL spectroscopy was also performed. The luminescence with the wavelength of visible to near-infrared light, the energy of which was less than the band gap energy, was observed with polarization in the direction parallel to the drain current.

Measurement of Frequency Dispersion of AlGaN/GaN High Electron Mobility Transistors

Frequency dispersion of the drain conductance was observed in AlGaN/GaN high electron mobility transistors (HEMTs). The transition frequency shifted to higher frequencies with increasing temperature. The activation energy for the change of the transition frequency was 0.47 eV, which was almost the same as that obtained by the measurement of the temperature dependence of the low-frequency noise.

Temperature dependence of high-frequency performances of AlGaN/GaN HEMTs

High-frequency measurements of AlGaN/GaN HEMTs (high electron mobility transistors) with 1.3 μm long gate have been performed at temperatures between 23 and 187 °C. The cut-off frequency f T decreased monotonically with increasing temperature. It was 13.7 and 8.7 GHz at 23 and 187 °C, respectively. The effective electron velocities ν eff in the channel, evaluated from the relation of total delay time versus the I D -inverse, were 1.2 x 10 7 and 0.8 x 10 7 cm/s at 23 and 187 °C, respectively. It has been shown that the present device is in an intermediate state between mobility-dominant and peak-velocity-dominant operations.