Research On AlGaN/GaN High Electron Mobility Transistor’s Reliability And New Device Structure Design

Gallium Nitride-based high electrom mobility transistors paly an important role in microwave application and power device application. Great progress has been achieved in GaN HEMT technology for the last twenty years. However, there still remain some issues making their performance below theoretical expectations. Two of these issues are concerned in this paper. One issue is about the non-linear behavior of the transconductance of Al GaN/GaN HEMT. The other is about the concentration of electricfield line near the gate, which leads to a low breakdown voltage of the device.In order to understand the non-linear behavior of the transconductance of AlGaN/GaN HEMT, DC and pulsed transfer characteristics of the AlGaN/GaN HEMT smaples have been systematically investigated. A significant difference of transconductance(gm) linearity between DC and gate-pulsed measurements is clearly observed. The acceptor-like traps in the barrier layer under the gate is the main cause of non-linear behavior of AlGaN/GaN HEMTs transconductance. A physical model has been constructed to explain the phenomenon. In the modeling, an acceptor-like trap concentration of 1.2×1019 cm-3 with an energy level of 0.5eV below the conduction band minimum shows the best fit to measurement results.To solve the problem of the electricfield line concentration near the gate, a novel high breakdown voltage AlGaN/GaN HEMT with a high-K/low-K compound passivation layer is proposed. The compound passivation layer is formed by blocks of low-K dielectric(Si3N4) embedded in a high-K passivation layer(La2O3). Because of their different dielectric constants, there is a discontinuity of horizontal electrical field at the high-K/low-K interface, which can introduce a new electric field peak in the nearby channel in the semiconductor, and modulate the distribution of electric field along the channel. Hence an enhancement of breakdown voltage can be achieved. Compared to typical field-plate structure, high-K/low-K passivation introduces no parasitic capacitance. Based on the physical mechanism, several design principles for high-K/low-K passivation layer are presented. Numerical simulation demonstrates a breakdown voltage of 1400 V for the proposed device with four blocks of low-K dielectric embedded in a high-K passivation, compared to the breakdown voltages of 917 V and 288 V for the device with high-K passivation and the device with low-K passivation respectively.