GaN-based gunn diodes: Their frequency and power performance and experimental considerations

Theoretical and experimental aspects of GaN-based Gunn diodes are reviewed. Since the threshold field for Gunn effect in GaN (F_TH > 150 kV/cm) is reported to be much higher than in GaAs (F_TH = 3.5 kV/cm), the active layer of GaN-based devices can be made thinner (< 3μm) and doped higher (> 10¹⁷ cm^-3) than in conventional Gunn diodes. Consequently, GaN-based devices are expected to offer increased frequency and power capabilities. The advantages of GaN are demonstrated with the help of large-signal simulations of GaN and GaAs Gunn diodes. The simulations revealed that GaN diodes can be operated at a higher frequency (up to 760 GHz vs. 100 GHz) and with larger output power density (10⁵ W/cm² vs. 10³ W/cm²) than GaAs diodes. Epitaxial layers of n⁺/n^-/n⁺ GaN (10¹⁹ cm^-3/10¹⁷ cm^-3/10¹⁹ cm^-13) designed for millimeter-wave operation were grown using MOCVD on SiC substrates. GaN Gunn diodes with 4 μm-thick active layers were fabricated using specially developed dry etching techniques. The RIE was optimized to allow deep low-damage etching and allowed reduction of contact resistivity of etched layers (R_C ≈ 10^-6 Ω·cm²). GaN diodes fabricated on SiC substrates with high thermal conductivity were tested on-wafer and demonstrated high voltage and current capability (60 V and 2.5 A). High frequency testing of these devices requires proper dicing, mounting on efficient heatsinks, and connection to appropriate oscillator cavities.