Channel-to-Channel Coupling in Normally-Off GaN Double-Channel MOS-HEMT

A previously reported normally-off GaN double-channel (DC-) MOS-HEMT with a gate recess into the upper channel layer has achieved a remarkably low R-on. In this letter, we found that the double-channel structure itself does not guarantee a low Ron without careful consideration of the electrical coupling between the two channels. A strong channel-to-channel (C2C) coupling between the two channels is critical to reduce Ron in the DC-MOS-HEMT by exploiting both channels at the access region while allowing currents to converge into the heterojunction lower channel at the gate region. Otherwise, with a weak C2C coupling, the conduction through the upper channel is comprised due to the high-resistivity MOS-channel section at the gate region. The fabricated DC-MOS-HEMT with C2C distance (t(C2C)) of 7.5 nm exhibits an appreciably lower R-on than that with t(C2C) = 11.5 nm. Further analysis reveals the latter suffers from an increased sheet resistance at access region compared with the sheet resistance of the DC-heterostructure itself. A modified TLM characterization is proposed to analyze the C2C coupling in the DC-heterostructure. It is confirmed the DC-heterostructure with t(C2C) = 7.5 nm boasts a strong C2C coupling, while that with t(C2C) = 11.5 nm features relatively weak C2C coupling.