Lei Pang

High-

This letter reports a 0.2- μm gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on an Si substrate passivated with an AlN/SiN_x (4/20 nm) stack layer. The 4-nm-thick AlN was grown by plasma-enhanced atomic-layer-deposition. The AlN/SiN_x-passivated HEMTs exhibit a high maximum drain current of 930 mA/mm, an three-terminal OFF-state breakdown voltage (BV_DS) of 119 V, and a small threshold voltage shift of 130 mV in a wide drain bias range (V_DS=3-24 V). Owing to the additional positive polarization charge in the AlN passivation layer, the access resistance R_s in the GaN-on-Si HEMTs is significantly reduced while maintaining small parasitic gate-drain capacitance C_gd, contributing to a high power-gain cutoff frequency f_MAX of 182 GHz and a high Johnsons figure of merit of BV_DS × f T of 6.43 × 10¹² V/s simultaneously. The accuracy of the RF performance is verified by a small signal modeling based on measured S-parameters.

f_{{\rm MAX}}

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.

High Johnson's Figure-of-Merit 0.2-

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.

\mu{\rm m}

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.

Gate AlGaN/GaN HEMTs on Silicon Substrate With

Thermal performance of AlGaN/GaN HEMT is a critical issue during the design stage, since it significantly affect the lifetime of the device. This paper introduces a three-dimensional modeling technique including electro-thermal coupling effects for investing the thermal characteristic of the AlGaN/GaN HEMT. The method achieves a good balance between simulation time and accuracy through iterative calculation between the 2D and 3D model.

{\rm AlN}/{\rm SiN}_{{\rm x}}

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.

Passivation

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.

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

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.