Investigation on the Influence of Ohmic Structure on Channel-to-Channel Coupling Effect in InAlN/GaN Double Channel HEMTs

In this paper, the impact of ohmic structure on channel-to-channel (C2C) coupling effect in InAlN/GaN double channel (DC) HEMTs is systematically analyzed and studied. For the un-recessed ohmic structure, the electrons in the upper channel can easily inject into the bottom channel due to the ultra-thin InAlN back barrier layer. Therefore, the maximum drain current and transconductance peak of the bottom channel significantly increase. For recessed ohmic structure, the reduced vertical electric field strength of the upper channel can effectively weaken the coupling effect between the two GaN channels. Benefiting from the suppressed vertical transport of electrons in the upper channel, higher drain current and transconductance of the upper channel are obtained in the recessed ohmic structure. In addition, the transmission electron microscope (TEM) microstructural analysis of the DC HEMTs with recessed ohmic structure was also performed. This work shows that the recessed ohmic structure can modulate the electron transport mode in the InAlN/GaN DC HEMTs. The coupling effect of the two channels will play a major role in influencing the characteristics of current gain cutoff frequency ( $\,f_{\mathrm{ T}}$ ) / maximum power gain cutoff frequency ( $\,f_{\mathrm{ max}}$ ) versus $V_{\mathrm{ GS}}$ and have a significant effect on large-signal characteristics, which is quite attractive for the fabrication of power microwave GaN-based HEMTs with wide gate swing.