Freestanding High-Power GaN Multi-Fin Camel Diode Varactors for Wideband Telecom Tunable Filters

Abstract

The quality, resolution, and quantity of information transmitted in RF front end modules favor compact tunable filters with varactor elements that can handle high power and that are highly linear. Gallium nitride (GaN) varactors can theoretically reach the highest quality factor figure of merit (QFOM), owing to the GaN outstanding breakdown electric field and good electron mobility. Popularly pursued GaN Schottky diode varactors suffer from leakage currents at high reverse voltage biases at their junctions and because of GaN inherent threading dislocations in thin GaN layers on foreign substrates. We devised a novel device architecture to overcome these limitations. First, we employ an interdigitated anti-series multi-Fin camel diode structure composed of a thin <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{p}^{+}$ </tex-math></inline-formula> GaN top layer situated between the Schottky metal and an n-type GaN drift layer. The anti-series symmetric Fin structure leads to excellent linearity and minimizes device parasitics to achieve measured record high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factors. The p-GaN layer raises the barrier height to suppress reverse leakage current to maintain a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor at higher reverse biases than Schottky diodes. Second, we utilize GaN on Qromis Substrate Technology (QST) wafers that permit the growth of thick GaN layers with lower dislocation densities and lower leakage currents than GaN-on-Si and that are comparable to GaN-on-GaN. We report in this work the fabrication and dc, RF <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -parameters of these diodes, and assess their potential with empirical models. Breakdown voltage of 146 V and extrapolated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor of 84, 33, and 9 at 2, 5, and 18 GHz, respectively, are obtained in multi-Fin back-to-back varactor device configuration.