AlGaN/GaN High Electron Mobility Transistors with a p-Type GaN Cap Layer
Authors: Tsai Hsin-Chang , Fan Chiang Shao-Chi , Zhong Yi Nan , Hsin Yue-Ming 
Published: February 2020

In the past few years, GaN based materials are widely used for power devices. It seems to be expected to replace silicon in the position of power devices with high operation frequency and power density. Currently in the applications of GaN based power transistors, AlGaN/GaN heterostructurs with high 2DEG concentration and high electron mobility show the low on-resistance. Moreover, due to the wide band-gap in GaN based materials, devices can be operated at high voltage. However, these devices have surface and buffer-related problems which cause current collapse phenomenon. Some publications with analysis models based on the electron trapping by surface states have been proposed. In this study, AlGaN/GaN HEMTs with different p-GaN cap structures are investigated in the gate leakage, breakdown voltage and dynamic on-resistance.

The epitaxial layers of AlGaN/GaN HEMTs were grown by MOCVD including a ~3900-nm buffer layer, a 300-nm GaN channel layer, an 1-nm AlN spacer layer, a 20-nm Al0.25Ga0.75N and a p-GaN cap layer. Three different p-GaN cap layers were investigated. In structure A, the Mg doping concentration was 1´1019 cm-3 and thickness of p-GaN cap layer is 5-nm. In structure B, higher Mg doping concentration (3´1019 cm-3) but the same thickness (5-nm) of p-GaN cap was used. In structure C, thicker p-GaN cap (8-nm) with Mg doping concentration (3´1019 cm-3) was used. To evaluate the effect of p-GaN cap layer on device performance, Schottky gate devices with different p-GaN cap layers were fabricated simultaneously for comparison. Before device fabricating, all samples were activated in nitrogen atmosphere at 700°C for 15 minutes. After activation, mesa isolation was etched by ICP down to the buffer layer. Both source and drain ohmic contacts were made by the deposition of Ti/Al/Ni/Au (25/125/45/55 nm) and annealed at 875°C in a nitrogen atmosphere for 40 s. Ni/Ti/Al/Ti/Au (30/25/250/25/200 nm) metals were used to form Schottky gate. The devices were passivated by a 200-nm SiN. Devices fabricated using structures A, B and C were designated as device A, B and C, respectively.

The Hall measurement results showed that structure A had lowest sheet resistance after activation. The values are similar to those obtained from TLM measurement. The sheet resistance from TLM measurement were 269, 273, and 307 W/sq for the device A, B and C after activation. The drain current (ID) of device A is lower than the device B and device C at VDS = 10 V due to higher power density with self-heating effect. However, lowest on-resistance (RON) is obtained from device A due to its lowest sheet resistance. The measured ID-VGS
and IG-VGS
characteristics show the threshold voltage of three devices are -4.26 V,-4.23 V, -4.09 V, respectively, which are determined at a drain current of 1 mA/mm. The off-state current is dominated by the gate leakage current and the lowest leakage current is observed in device C. Therefore, the highest on/off current ratio is 1.02 × 106 in device C. The measured IG-VGS characteristics in Schottky gate diodes show the turn-on voltages are 0.82, 1.02 and 1.74 V for device A, B, and C, respectively. A higher turn-on voltage was observed in device C due to thicker p-GaN cap layer. The measured off-state breakdown characteristics of three devices at VGS = -6 V showed lower leakage currents in device B and device C and thus higher breakdown voltages. Moreover, pulsed IV measurement was utilized to analyze the effect of p-GaN cap layer on current collapse. The ratio of RON from various quiescent points to the value from quiescent point (VGS ,VDS
) = (0 , 0) showed that device C clearly had the least current collapse and thus lowest RON ratio while increasing VDS bias. The possible reason is due to the improved Schottky barrier in device C. Since the gate leakage current is lowest in device C and thus less electrons injection to the surface to form virtual gate while biasing at higher drain voltages.

AlGaN/GaN HEMTs with three different p-GaN cap layers were fabricated, characterized, and compared. Device C with higher Mg doping concentration and thicker p-GaN cap layer demonstrated a lowest gate leakage current (off-state current), higher on/off current ratio and higher turn-on voltage. Based on the pulsed IV measurement, device C showed less problem in the current collapse, which could be due to lower gate leakage current and less electrons injection to the surface.