Tingting Yuan

Robust SiN x /AlGaN Interface in GaN HEMTs Passivated by Thick LPCVD-Grown SiN x Layer

Low-pressure chemical vapor deposition (LPCVD) technique is utilized for SiNx passivation of AlGaN/GaN high-electron-mobility transistors (HEMTs). A robust SiNx/ AlGaN interface featuring high thermal stability and well-ordered crystalline structure is achieved by a processing strategy of “passivation-prior-to-ohmic” in HEMTs fabrication. Effective suppression of surface-trap-induced current collapse and lateral interface leakage current are demonstrated in the LPCVD-SiNx passivated HEMTs, as compared with conventional plasma-enhanced chemical vapor deposition-SiNx passivated ones. Energy dispersive X-ray spectroscopy mapping analysis of SiNx/AlGaN interfaces suggests the interface traps are likely to stem from amorphous oxide/oxynitride interfacial layer.

A 22W Ku band power amplifier based on internal-matched 6mm GaN HEMTs single chip

A Ku-band power amplifier is successfully developed with an internal-matched single chip 6mm AlGaN/GaN high electron mobility transistors (HEMTs). LCL network together with microstrip circuits are used to directly match the impedance of the 6mm GaN HEMTs to 50Ohm without using power combiner. Under the pulsed condition (100μs, 10%), the developed GaN HEMTs power amplifier delivers a 22W saturated output power with 7.6dB linear gain and 26.8% maximum power-added efficiency (PAE) with a drain voltage of 32V and at the frequency of 13.7GHz. To our best knowledge, the achieved high performance power amplifier is the state-of-the-art result ever reported for internal-matched 6mm gate width single chip GaN-based hybrid microwave integrated power amplifier at Ku-band.

Degradation of GaN high-electron mobility transistors in voltage step stress

Voltage step-stress tests on GaN-on-SiC HEMT showed that electric field is a driving factor for degradation. The position of localized damage is corresponding to the high electric field region. A degradation mode different from previous reports is observed, which led to an increase of drain current after stress in certain conditions. We attribute this to the collection of the positive mobile charge under the gate during the stress.

Design of an X-Band 6-Bit Phase Shifter

The design and performance of X-band (8-12 GHz) 6-bit MMIC phase shifter is presented. The phase shifter is fabricated in 0.25um GaAs pHEMT technology. Each bit of this phase shifter design is based on an optimum topology to reach balanced and optimal insertion loss, phase shifting accuracy and return loss. A new type of loaded-line topology for small phase shift states is also relevant. The adoption of the parallel transistors and a resonance inductor in parallel with the series transistors is to improve the isolation of the SPDT switch. Ordering the sequence of bits is optimized so as to minimize the overall return and insertion loss. The proposed phase shifter, with a chip size of 4.2mm*2.2mm, exhibits better than 10dB return loss, no more than 7.6 dB insertion loss, better than 3.3° RMS phase error and 0.7dB RMS amplitude error over the operating frequency range.

X-band 11.7-W, 29-dB gain, 42% PAE three-stage pHEMT MMIC power amplifier

This paper presents an X-band high gain and high power three-stage PHEMT monolithic microwave integrated circuit (MMIC) power amplifier (PA). Based on 0.15-μm GaAs power PHEMT technology, this PA is fabricated on a 2-mil thick wafer. While operating under 7.2 V and 3300 mA dc bias condition, the characteristics of 29.2-dB small signal gain, 11.7-W output power, and 42.2% power added efficiency at 9.2 GHz can be achieved.

Electric field dependent drain current drift of AlGaN/GaN HEMT

We have performed constant voltage stress (CVS) tests on GaN-on-SiC HEMT to investigate the drain current drift. Two kinds of current drift behavior are observed in CVS. The off-state drain voltage step stress tests are carried out to confirm the electric field dependent current drift. A critical voltage for drain current recovery is observed. We suggest that the recovery of drain current is due to holes generation near the heterojunction interface or the detrapping of acceptors in the barrier layer. The drain current drift is balanced by the current degradation and recovery mechanism.

High voltage degradation of GaN high electron mobility transistors with AlGaN back barrier on SiC substrate

We have stressed GaN High Electron Mobility Transistors (HEMTs) with AlGaN back barrier on SiC substrate at high voltages at normal and high temperatures. A pattern of device degradation differs from what occurred in reported GaN-on-SiC HEMTs and GaN-on-Si HEMTs. The recoverable degradation of drain current is observed under Vds=0 step stress condition, but no degradation is observed under off-state condition. We attribute the results to electron injection into the back quantum well, other than electron trapping. Raman scattering indicates planar strain is greatly enhanced under Vds=0V condition, which is likely to assist electron in tunneling the back barrier.