| dc.description.abstract | This study presents the gate characteristics of p-GaN gate HEMT. Discussion has been divided into the following two parts: (1) Measurement and analysis of ohmic p-GaN gate HEMT and Schottky p-GaN gate HEMT; (2) Simulation of p-GaN gate HEMT with new gate structure.
For the analysis of ohmic p-GaN gate HEMT and Schottky p-GaN gate HEMT, several measurements are proposed. Compare to Schottky p-GaN gate HEMT, large hole injection from p-GaN occurs in ohmic p-GaN gate HEMT under positive gate bias, which can be observed in characteristics of gate leakage current, second Gm peak in the transfer curve and negative drain on-state current under low drain bias, etc. In the short-time gate-stress-induced threshold voltage instability test, the negative and positive threshold voltage shift have shown in ohmic p-GaN gate HEMT and Schottky p-GaN gate HEMT. Different equivalent models of gate stack of the two devices result in different characteristic of the gate capacitance under positive gate bias. As for dynamic switching measurement, Schottky p-GaN gate HEMT shows higher increase in dynamic on-resistance with off-state drain applied bias change from 100 V to 600 V. However, ohmic p-GaN gate HEMT turns out to be larger switching loss, which may contribute to lower switching speed.
For the simulation of p-GaN gate HEMT with new gate structure, three hybrid Schottky-ohmic gate structures are proposed for normally-off p-GaN/AlGaN/GaN HEMT. One with Schottky-gate cover on the ohmic-gate and has part of area contact to the p-GaN surface at left side and right side of ohmic-gate (structure A) and two others only has the Schottky-gate contact to the p-GaN surface at left side or right side of ohmic-gate (structure B and C). The new devices demostrate the characteristic between ohmic p-GaN gate HEMT and Schottky p-GaN gate HEMT. when compare to ohmic p-GaN gate HEMT, gate leakage current of the new devices shows about two orders of magnitude smaller. As for Schottky p-GaN gate HEMT, on-state drain current can have over 60 mA/mm improvement in the new devices. After summarize the on-state drain current, second Gm, peak and gate leakage current, better performance shows in the new device with the optimized ratio of ohmic-gate and Schottky-gate, which are Schottky-gate contact length between 0.8 to 1.8 μm for structure B. | en_US |