A Novel GaN-Based Metal-2DEG-Metal Varactor With Cutoff Frequency of 3.13 THz

A novel metal-2DEG-metal (M-2DEG-M) varactor based on GaN high electron mobility transistor (HEMT) structures was designed and investigated. The novel varactor employed metal-2DEG direct contacts to form two back-to-back Schottky junctions, eliminating the threshold voltage and showing a strong nonlinear
CV
behavior at zero bias, which was more beneficial to terahertz (THz) multiplier circuits design compared with the conventional metal–semiconductor–metal (MSM) 2DEG varactors. The metal-2DEG direct contacts reduced the junction capacitance and the series resistance simultaneously. For the M-2DEG-M varactor with an electrode length of
$1 \mu \text{m}$
, the device capacitance at zero bias was determined to be 3.0 fF (without de-embedding the parasitic component), with the series resistance of
$25.4 \Omega $
, resulting in a record high cutoff frequency of 2.09 THz for GaN-based varactors. After de-embedding the parasitic pad-to-pad capacitance, the cutoff frequency and figure of merit (FOM) would be 3.13 and 7.14 THz, respectively. Additionally, the whole fabrication process of the novel varactor only needed two steps of ordinary UV lithography without elaborate nanoscale T-gates, totally compatible with GaN-based HEMT process.