Drain-bias dependence of low-frequency Y22 signals for Fe-related GaN traps in GaN HEMTs with different Fe doping concentrations

Abstract

The bias dependence of trap properties for GaN HEMTs with Fe-doped GaN layers was investigated using low-frequency Y-parameters measurement in the two-port network. This measurement technique can be estimated quasi-equilibrium trap properties and location by changing the combination between gate/drain current response and gate/drain input voltage with small AC signal overlapped fixed DC bias. We focused on Y22 which can be detected only in GaN traps. Activation energies for the GaN traps were estimated bias-by-bias for wide drain voltage (VDS) ranging from 2 V below the knee voltage to 15 V for the saturation region with high current at the gate voltage of 0 V (on-state condition). Fe-related GaN traps with low-frequency Y22 characteristics were identified experimentally using GaN HEMTs including GaN layers with different Fe doping concentrations. The Fe-related GaN trap signals in the imaginary part of Y22 (Im(Y22)) appeared at a peak near 200 Hz at a low VDS of 3 V under on-state conditions and moved to a higher frequency with an increase of VDS. The amplitudes of the trap peaks enhanced around VDS of 3 V. The maximum value in the amplitude of the trap peaks was large for the high-concentration Fe doping. On the other hand, the peak frequency depending on VDS had a similar trend for both the low- and high-concentration Fe doping. The activation energy for the Fe-related trap decreased for VDS up to 7 V and saturated for both Fe doping concentrations. Device simulation was performed to analyze the location in GaN HEMTs for GaN trap response and bias dependence of the activation energy. To simulate the characteristics of Im(Y22), drain current (ID) responses against AC (sine wave) input signal of VDS were calculated at the frequency corresponding to the trap peak. Because ID led VDS in phase, the trap effect is considered to change the trapped charge. From two-dimensional plots for the difference in ionized trap density, trap response regions were located under the gate edge at the drain side. Because the imaginary part of the admittance vector between ID and VDS corresponded with Im(Y22), the Fe-related traps under the gate of the drain side changed the charge condition according to the AC signal and generated the peak in Im(Y22) depending on the frequency. Furthermore, the saturation of the activation energy in the bias-dependence characteristics occurred due to decreasing the thermal conductivity at high temperatures. The temperature in GaN HEMTs at high VDS became high because of the self-heating effect in addition to the high ambient temperature. The internal temperature increase induced the trap peak frequency increase, and the activation energies were saturated.