Free-Standing High Power GaN Multi-Fin Camel Diode Varactors

To achieve wideband tunable filters that can handle high power in RF front end modules, high voltage varactors with a high quality factor, Q, are required. Among candidate semiconductor materials for high breakdown voltages exceeding 100V, gallium nitride (GaN) varactors can theoretically reach the highest figure of merit, Q
min
, owing to outstanding breakdown field and good electron mobility
¹
. However, high reverse bias leakage currents lower the breakdown voltage and restrict Q of conventional vertical GaN -based Schottky diodes due to (i) limited barrier heights attained on GaN, and (ii) leakage through threading dislocations. Furthermore, generally high contact resistance of p-type GaN Ohmic contact limits Q of vertical GaN pn junction diodes
²
, Here, we report devices that overcome these limitations by combining novel material and device architectures. First, we employ a camel diode structure composed of a thin and fully depleted p+ GaN top layer situated between the Schottky metal and an n-type GaN drift layer
³
. This raises the barrier height to suppress electron tunneling when compared to Schottky diodes (Fig. 1 a) and reduces the overlap of states for band-to-band tunneling in pn diodes. Second, we utilize GaN on a QST (Qromis Substrate Technology) wafer that permits the growth of thick GaN layers with lower dislocation densities and lower leakage than GaN-on-Si
⁴
. We report in this work the DC and s-parameter characterization results and discuss the potential of this varactor technology.