Fang Yulong

Influence of the AlGaN barrier thickness on polarization Coulomb field scattering in an AlGaN/AlN/GaN heterostructure field-effect transistor

In this study rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with 22-nm and 12-nm AlGaN barrier layers are fabricated, respectively. Using the measured capacitance–voltage and current–voltage characteristics of the prepared devices with different Schottky areas, it is found that after processing the device, the polarization Coulomb field (PCF) scattering is induced and has an important influence on the two-dimensional electron gas electron mobility. Moreover, the influence of PCF scattering on the electron mobility is enhanced by reducing the AlGaN barrier thickness. This leads to the quite different variation of the electron mobility with gate bias when compared with the AlGaN barrier thickness. This mainly happens because the thinner AlGaN barrier layer suffers from a much stronger electrical field when applying a gate bias, which gives rise to a stronger converse piezoelectric effect.

Extrinsic and intrinsic causes of the electrical degradation of AlGaN/GaN high electron mobility transistors

Electrical stress experiments under different bias configurations for AlGaN/GaN high electron mobility transistors were performed and analyzed. The electric field applied was found to be the extrinsic cause for the device instability, while the traps were recognized as the main intrinsic factor. The effect of the traps on the device degradation was identified by recovery experiments and pulsed I−V measurements. The total degradation of the devices consists of two parts: recoverable degradation and unrecoverable degradation. The electric field induced traps combined with the inherent ones in the device bulk are mainly responsible for the recoverable degradation.

High-Temperature Performance Analysis of AlGaN/GaN Polarization Doped Field Effect Transistors Based on the Quasi-Multi-Channel Model

We report on the temperature-dependent dc performance of AlGaN/GaN polarization doped field effect transistors (PolFETs). The rough decrements of drain current and transconductance with the operation temperature are observed. Compared with the conventional HFETs, the drain current drop of the PolFET is smaller. The transconductance drop of PolFETs at different gate biases shows different temperature dependences. From the aspect of the unique carrier behaviors of graded AlGaN/GaN heterostructure, we propose a quasi-multi-channel model to investigate the physics behind the temperature-dependent performance of AlGaN/GaN PolFETs.

Ti/Al Based Ohmic Contact to As-Grown N-Polar GaN*

Ti/Al based Ohmic contacts to as-grown N-polar GaN are investigated by cross-section transmission electron microscopy and energy dispersive x-ray spectroscopy. Due to the higher oxygen background doping in the N-polar GaN, the Al metal in Ohmic stacks is found to react with background oxygen more easily, resulting in more AlOx. In addition, the formation of AlOx is affected by the Al layer thickness greatly. The AlOx combined with the presence of AlN is detrimental to the Ohmic contacts for N-polar GaN compared with Ga-polar GaN. With the reduction of the Al layer thickness to some extent, less AlOx and AlN are formed, and lower Ohmic contact resistance is obtained. The lowest contact resistivity ρ of 1.97 × 10−6 Ωcm2 is achieved with the Al layer thickness of 80 nm.

High-Frequency AlGaN/GaN High-Electron-Mobility Transistors with Regrown Ohmic Contacts by Metal-Organic Chemical Vapor Deposition*

Nonalloyed ohmic contacts regrown by metal-organic chemical vapor deposition are performed on AlGaN/GaN high-electron-mobility transistors. Low ohmic contact resistance of 0.15ωmm is obtained. It is found that the sidewall obliquity near the regrown interface induced by the plasma dry etching has great influence on the total contact resistance. The fabricated device with a 100-nm T-shaped gate demonstrates a maximum drain current density of 0.95 A/mm at Vgs = 1 V and a maximum peak extrinsic transcondutance Gm of 216 mS/mm. Moreover, a current gain cut-off frequency fT of 115 GHz and a maximum oscillation frequency fmax of 127 GHz are achieved.

Influence of drain bias on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance—voltage curves and current—voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical (LO) phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance (here 4/5), and the polarization Coulomb field scattering is dominant for the sample with a small ratio (here 1/5). However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance (here 20 μm) compared with the one with a large distance (here 100 μm). This is due to the induced strain in the AlGaN layer caused by the drain bias.