Improving RF characteristic and suppress gate leakage in normally-off GaN-HEMTs using negative polarization effect and floating gate for millimeter-wave systems

This article demonstrates the effect of reverse gradient barrier layer and floating gate structure on DC and RF performance of GaN-based HEMTs. In terms of power characteristics, using reverse gradient barrier and floating gate, the GaN-HEMTs with Lg of 240 nm and S-D spacing of 8.4 μm demonstrated the maximum drain current and peak transconductance are increased by 70 % and 15 % respectively, and the linearity of GaN based HEMT is greatly improved. Meanwhile, using the reverse gradient barrier layer, the gate leakage current is significantly reduced by nearly 1 to 5 orders of magnitude, and it is proved that the negative polarization effect has a certain effect on increasing the threshold voltage and breakdown voltage. In terms of RF characteristics, due to the existence of the floating gate, the parasitic capacitance is reduced and the frequency performance is improved. In addition, for both conventional and proposed HEMTs, this article proposes two equivalent capacitance models to optimize RF performance. As a result, the optimal structure offers approximately 10 and 7 times improvement in the current gain cutoff frequency (ft) and unilateral power gain cutoff frequency (fmax) (from 5.3 and 9.8 to 52 and 73.8 GHz, respectively). Additionally, the optimal structure exhibits outstanding scaling factors of 12.48 GHz·μm.