Frequency improvement and asymmetry characteristics of tri-stripe AlGaN/GaN MSM-2DEG varactors

This study presents an AlGaN/GaN heterojunction varactor employing a tri-stripe metal–semiconductor–metal (MSM) structure that exploits the two-dimensional electron gas (2DEG). The device exhibits a strong dependence of its electrical characteristics on the device width (W): both series resistance and capacitance decrease monotonically with reducing W. Through dimensional scaling, the R0·Cj product of the device decreases significantly, thereby improving the cutoff frequency (fc). Although the capacitance switching ratio (Cratio) shows a slight decrease with reduced W, the overall cutoff frequency figure-of-merit (FOM) of the device is effectively improved. Specifically, using a tri-stripe MSM-2DEG structure with a width of 10 μm and a gate gap of 450 nm, a high FOM of 8.27 THz and a Cratio of 3.42 are obtained. The study further reveals that the capacitance characteristics become asymmetric for smaller W values. Concurrently, the shift voltage (Vshift) shifts toward lower voltages with decreasing W, corresponding to the point of most rapid capacitance change, where the Fermi level (Ef) aligns with the conduction band (Ec) minimum. This phenomenon is closely related to the reduction in 2DEG concentration at the AlGaN/GaN interface, likely attributed to enhanced etching-induced damage during size scaling. The 2DEG concentration vs bias exhibits a transition from linear decay on both sides when Ef is far from Ec minimum, to exponential decay near Vshift region—representing a “linear–exponential–linear” evolution. Furthermore, the fabrication process of the device is significantly simplified—all electrodes can be formed in a single step via electron beam lithography.