Electrical Domains and Sub-millimeter Signal Generation in AlGaN/GaN Superlattices
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Electrical Domains and Sub-millimeter Signal Generation in AlGaN/GaN Superlattices Irina Gordion1, Alexander Manasson2, and Vladimir I. Litvinov1 1 WaveBand Corporation., Irvine, CA 92614, U.S.A. 2 University of Michigan, Department of Applied Physics, 2477 Randall Laboratory, Ann Arbor, MI 48109, U.S.A ABSTRACT The paper discusses the feasibility of a submillimeter-signal source made of AlGaN/GaN superlattice. Negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. We relate the superlattice geometry and conduction band profile, distorted by polarization fields, to the oscillation frequency and power efficiency of the device. We also determine the optimal Al content, superlattice period, and the parameters of external circuit that favor sub-millimeter wave generation. INTRODUCTION Negative differential dc-conductivity (NDC) in semiconductors is a prerequisite for highfrequency instability and signal generation in Gunn diodes. Electric field-induced electron transitions to a valley with a heavy effective mass result in a mobility decrease when the electric field increases (NDC). Another mechanism of NDC found in semiconductor superlattices (SL) is based on the electron Bragg reflections in a narrow SL miniband [1]. The NDC in SLs results in traveling electrical domain formation that has been used in a 147 GHz Gunn-type source made of the InGaAs/GaAs SL [2,3]. Another type of SL source, the Bloch oscillator [1], is projected to oscillate near the Bloch frequency Ω0 = edF h ( F is a dc-electric field, d is the SL period), and exploits the existence of high-frequency negative conductivity. At the moment, no live example of the Bloch-type source exists because the NDC at zero frequency induces electric field domains, thus preventing electrons from oscillating at the Bloch frequency [4]. The output power of the Gunn-type device depends on the current and voltage swings in the NDC region. In this paper we discuss signal generation related to electrical domain formation in GaN/AlGaN SLs. High-power operation requires the use of materials capable of withstanding large current/voltage swings. Therefore, the wide bandgap semiconductor is a material of choice for the active region of the SL sources designed for high power operation. Experimental difficulties justify the importance of simulations and modeling to estimate the achievable device performance. III-Nitride material system for Gunn-type source was proposed in [5], where the oscillation frequency in AlGaN/GaN and InGaN/GaN SLs has been roughly estimated from the expected peak velocity of an idealized simple-cosine miniband electron spectrum without the account for the polarization fields or resonant circuit components. However, the real wurtzite (0001)AlGaN/GaN SL is an intrinsic Stark superlattice where the polarization fields affect the dynamics of miniband electrons. In this paper we discuss realistic the short-period GaN/AlGaN Stark SL in a resonance circuit a
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