Resonant properties of resistively shunted tunnel junctions dominate the high-frequency performance of Josephson array oscillators. To improve the operating frequency, we have developed resistively shunted Nb/AlOx/Nb tunnel junctions with a small parasitic inductance. The inductance was minimized by reducing the inductive length between the tunnel junction and the contact hole to be about 1µm. By fitting the measured I-V characteristics of the shunted tunnel junction to the simulated characteristics, we estimated the inductance to be about 105 fH. The analysis of resonant properties showed that the shunted tunnel junctions with the small parasitic inductance have a high-frequency performance up to the Nb gap frequency. Josephson array oscillators using 11 such junctions were designed and fabricated to operate at 650 GHz and 1 THz. Shapiro steps induced by Josephson oscillation were clearly observed up to 1 THz. By fitting the step heights to the simulated results, we estimated the output power of the Josephson oscillator delivered to the load resistor to be about 10 µW at 625 GHz and 50 nW at 1 THz.

Resonant properties of resistance shunted tunnel junctions have been investigated using the RLCSJ model. We found that an increase in dc current resulted from an increase in impedance of the shunted tunnel junctions. The static and dynamic properties of the shunted tunnel junctions were described in detail by numerical simulations and experiments. The simulated and measured results showed good agreement in I-V characteristics. A Josephson array oscillator has been proposed using the resonant properties for increasing oscillator output impedance. We designed and fabricated the oscillator with 20 shunted tunnel junctions. The output power of the oscillator delivered to the load resistor was estimated to be about 0.5µW at 312 GHz.