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

Abstract

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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</inf> , owing to outstanding breakdown field and good electron mobility <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> . 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . We report in this work the DC and s-parameter characterization results and discuss the potential of this varactor technology.