Fundamental Oscillation up to 1.42 THz in Resonant Tunneling Diodes by Optimized Collector Spacer Thickness

We report an increase in the oscillation frequency of terahertz oscillators using AlAs/InGaAs double-barrier resonant tunneling diodes (RTDs) by optimizing the collector spacer thickness. For high-frequency oscillation of RTDs, the electron delay time, which is composed of the dwell time in the resonance region and the transit time in the collector depletion region, must be reduced. Although the transit time is reduced by a thin collector spacer, the capacitance increases. Thus, an optimum thickness of collector spacer layer exists. In this report, we investigate the dependence of oscillation frequency on the collector spacer thickness. The RTDs were integrated with 20-μm-long slot antennas, and oscillations up to 1.1, 1.42, and 1.29 THz were obtained for spacer thicknesses of 25, 12, and 6 nm, respectively. The optimum spacer thickness for high-frequency oscillation was around 12 nm. The highest frequency in this experiment was 1.42 THz oscillation, with an output power of ~1 μW. We also extracted the electron velocity in the collector depletion region and the dwell time from the dependence of the delay time on the collector spacer thickness.