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, Qmin, owing to outstanding breakdown field and good electron mobility1. 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 diodes2, 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 layer3. 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-Si4. We report in this work the DC and s-parameter characterization results and discuss the potential of this varactor technology.