Mechanisms of Buffer and Surface Traps in GaN HEMTs for Low Frequency Y21 and Y22 parameters

Characteristics in low frequency Y-parameters for GaN HEMTs is important to investigate device physics of traps and construct equivalent circuit model to improve linearity in power amplifier such as Doherty. We have clarify effects of buffer and surface traps to Im(Y21) and Im(Y22) by device simulation and small signal equivalent circuit model. Results of the device simulation including the buffer trap, surface trap and self-heating model can explain experimental data adequately. Peak at lowest frequency appeared in only Im(Y21) and has positive value. This peak originates from the surface trap. The peaks appeared in both Im(Y21) and Im(Y22) with different polarity originate from the buffer traps. Furthermore, the self-heating effects generate the peak in both Im(Y21) and Im(Y22) with the same polarity. The signals of the traps in Y-parameters induce from drain current changed by ionized traps. In the case of the buffer in Im(Y21), the ionized traps decreased at the drain side in GaN layer as gate voltage change to larger value. These negative charges decrease due to acceptor type trap and the drain current induced by the buffer trap increases. Therefore, the signal for the buffer trap appear as the peak with negative polarity at the frequency determined from trap properties.