High-Frequency Generation in Low-Mobility Superlattices
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E11.9.1
High-Frequency Generation in Low-Mobility Superlattices Vladimir Litvinov1 and Alexander Manasson2 1 WaveBand Corporation, 17152 Armstrong Ave, Irvine, CA 92614 2 University of Michigan, Department of Applied Physics, 2477 Randall Lab, Ann Arbor, MI 48109-1120 ABSTRACT We study the transport properties of miniband electrons in an electromagnetic field as related to the high-frequency generation in a semiconductor superlattice (SL). We study the regimes where the external ac-field is in resonance and out of resonance with the two lowest SL minibands. In the diffusive quasi-static regime Bloch oscillations and electrical domains do not develop and high-frequency generation may occur as a result of the negative high-harmonic nonlinear conductivity. This regime is different from the frequency conversion in a stable nonlinear media. We point out that AlGaN/GaN material system has a great potential for the high-power SL terahertz (THz) source. INTRODUCTION Negative differential conductivity (NDC) in semiconductor SLs [1] is a topic of interest for various approaches to compact sub-millimeter wave sources. Currently, two types of oscillators are subject of intensive studies. First is the Gunn-type source where the NDC in dc-biased SLs results in the formation of traveling electrical domains. Another type of an SL source is the Bloch oscillator [1] that exploits the existence of high-frequency dynamic NDC and is projected to oscillate close to the Bloch frequency. So far, continuous wave Bloch-type source was not demonstrated, because the dc-NDC makes the system unstable to the formation of traveling electric field domains interfering with Bloch oscillations [2]. GaN-based SLs are expected to have benefits from high output power, high breakdown voltage, high temperature operation, and lower sensitivity to dislocations as compared to GaAs SLs. However, GaN-based material system demonstrates significantly lower mobility compared to GaAs. In a low-mobility SL the THz-range frequency falls into the region of < 1. In this paper, we explore a new possibility of sub-millimeter wave generation in the diffusive regime of SL operation ( < 1, where τ is the momentum relaxation time). We also consider the regime of dc-NDC in an SL driven by an inter-miniband resonant acfield. We show that the strong resonant ac-field changes the dispersion relation of SL minibands, consequently affecting the transport properties of the miniband electrons. We demonstrate that such resonant field can substantially reduce the critical field necessary for the onset of NDC in Gunn-type mode of operation of the AlGaN/GaN SLs. Reduction of the critical field is of great interest in large band gap material systems like GaN due to dc-power dissipation issues.
E11.9.2
THEORY AND RESULTS SL in a diffusive regime Below we calculate the nonlinear conductivity at frequencies 2 and 3 , where ω is the frequency of an ac-driving force. We show that the large-signal ac-driving force with frequency < -1 makes the nonlinear conductivity at frequencies 2 and 3
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