Channel Engineering of III-Nitride HEMTs for Enhanced Device Performance

Since the first demonstration of the AlGaN/GaN high electron mobility transistor (HEMT) over a decade ago, there has been rapid development in wide bandgap GaN-based materials and devices. Owing to their unique capabilities of achieving high breakdown voltage and high current density at microwave frequencies, AlGaN/GaN HEMTs are emerging as the promising candidates for radio-frequency (RF) and microwave power amplifiers. With tremendous progresses made during the last decade in material quality and device processing, AlGaN/GaN HEMTs have been improved significantly in both DC and RF performances. Meanwhile, more advanced device structures are being explored for further performance improvement. For employing GaN-based HEMTs as the high frequency and high power microwave devices, the limitations of the conventional AlGaN/GaN HEMTs emerge quickly. Two of the most important limitations need to be overcome are the device linearity and buffer isolation. Up to now, significant efforts have been made to optimize or modify the gate barrier layer and improve the crystal quality of the buffer layer. The optimization of another important part of the HEMT, namely the channel region, only starts to attract attention recently.;In this work, several novel GaN-based HEMT structures were proposed by explicitly focusing on the channel region. Based on extensive band profile simulation, two kinds of new HEMT structures, AlxGa1-xN/Al yGa1-yN/GaN composite-channel HEMTs (CC-HEMTs) and AlGaN/GaN/InGaN/GaN double-heterojunction HEMTs (DH-HEMTs), were designed and fabricated. The composite-channel HEMT shows significantly improved linearity. They exhibited a peak transconductance of 150 mS/mm, a peak current gain cutoff frequency (fT) of 12 GHz and a peak power gain cutoff frequency (fmax) of 30 GHz. For devices grown on sapphire substrate, a maximum power density of 3.4 W/mm and a power-added efficiency (PAE) of 43% were obtained at 2GHz. The output third-order intercept point (OIP3) was 33.2 dBm from two-tone measurement at 2GHz. The AlGaN/GaN/InGaN/GaN DH-HEMT features an InGaN-notch embedded in the channel region. Assisted by the InGaN layer's polarization field, an additional potential barrier is introduced between the channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (fT) of 14.5 GHz, and a peak power gain cutoff frequency (fmax) of 45.4 GHz were achieved. The off-state source-drain leakage current was as low as ∼5 muA/mm at VDS = 10 V. For the devices on sapphire substrate, a maximum power density of 3.4 W/mm and a PAE of 44% were obtained at 2GHz. The DH-HEMT was further implemented in enhancement-mode (E-mode) operation, required for single-polarity supply voltage RFICs and MMICs. State-of-the-art RF power performance was demonstrated in E-mode GaN-based HEMTs for the first time. Noise characteristics of E-mode GaN-based HEMTs were also explored for the first time.