Progress In Electromagnetics Research Symposium, Cambridge, USA, July 2–6, 2008 889 Wide Band Gap Semiconductor High-power Coherent THz Source V. I. Litvinov 1 , H. Morkoc 2 , and J. Xie 2 1 WaveBand Division, Sierra Nevada Corporartion, 15245 Alton Pkwy, Irvine, CA 92618, USA 2 Department of Electrical and Computer Engineering, Virginia Commonwealth University 601 West Main Street, Room 338, P.O. Box 843072, Richmond, Virginia 23284, USA Abstract— Formation of the electrical domains in semiconductor superlattices prevents Bloch oscillations to occur. Despite this, it is practical to explore a microwave source where the domains themselves may provide high-frequency operation. Negative differential dc-conductivity (NDC) in semiconductor superlattices has proven to cause traveling electrical domains oscillations at 147 GHz in InGaAs/GaAs device. High-power operation requires the use of materials capable of withstanding large current/voltage swings. Therefore, the wide band gap semiconductor is a material of choice for the active region of the device. We study III-Nitride material system, a wurtzite (0001) AlGaN/GaN superlattice, where the polarization fields affect the dynamics of miniband electrons. Polarization fields stem from the bulk spontaneous polarization and the lattice-mismatch-induced piezoelectric component. We explore the short-period GaN/AlGaN Stark superlattice as a potential high-power sub-millimeter wave source. We calculate the electron energy, width of the first miniband, and the mobility-field relation. These results create a base for simulation of the source performance using the Atlas-Silvaco package capable of simulating Gunn-type devices. Superlattice source performance (oscillation frequency and power efficiency) depends on the material parameters of its active region and could be of (500–600) GHz with the intrinsic power efficiency of (9–18)%. The frequency of the output signal is tunable by an applied voltage and a series resistance. The GaN superlattices have been grown and characterized. It is shown that the dc-current-voltage characteristics have NDC region that is the prerequisite for the formation of electrical domains. 1. INTRODUCTION Since the negative differential dc-conductivity (NDC) in semiconductor superlattices (SL) was found [1], the study of traveling electrical domains in SLs [2–4] resulted in fabrication of a 147 GHz InGaAs/GaAs microwave source [5]. The output power of this device depends on the current and voltage swings in the NDC region. High-power operation requires the use of materials capable of withstanding large current/voltage swings. Therefore, the wide bandgap semiconductor, for exam- ple GaN, is a material of choice for the active region of the SL sources designed for high power operation. In addition, the performance of GaN-based electronic and optoelectronic devices is less sensitive to a high dislocation density as compared to their GaAs-InAs counterparts. III-Nitride material system for high-frequency source was proposed in [6, 7]. In this paper we further explore the short-period GaN/AlGaN Stark SL as a perspective high-power sub-millimeter wave source. We calculate the electron band dispersion, the width of the first miniband, and the mobility-field relation in GaN-based SL. These results create a base for simulation of the source using the Atlas-Silvaco semiconductor simulator. The SLs have been grown, characterized, and their dc current-voltage characteristics are reported. 2. MOBILITY-FIELD RELATION AND OSCILLATION SPECTRA The electron mobility in a dc-biased SL is given in Ref. [8, 9]. It can be approximated by a standard model used for Gunn diode simulations: μ(F )= μ 0 +(v sat /F )(F/F crit ) 2 1+(F/F crit ) 2 . (1) The structure under study comprises fifty 24 ˚ A-long periods (L =0.12 μm) of AlGaN/GaN su- perlattice. The SL with Al-content of x =0.42 has the following characteristics: F crit = 100 kV/cm, μ 0 = 50 cm 2 /Vs, v sat = 10 cm/s. Complete screening of the polarization fields (flat-band SL) leads to a 10% decrease in the miniband width (low-field mobility). That slow change allows neglecting the carrier concentration dependence of the SL parameters. Both sides of the sample are connected to highly doped (10 19 cm -3 ) 0.01 μm-thick layers to provide good ohmic contacts. Additional p- doped layer (p =1.5 * 10 17 cm -3 in Fig. 1) prevents spillover of electrons from the metal contact.