Hot-electron bolometric mixer with negative differential resistance

TL;DR

This paper shows that biasing superconducting hot-electron bolometer mixers in the negative differential resistance region can enhance conversion gain, with redesigned circuits suppressing oscillations for stable operation.

Contribution

It introduces a method to utilize negative differential resistance biasing in HEB mixers, overcoming oscillation issues and improving performance.

Findings

NDR biasing increases HEB mixer gain.

Oscillations are caused by LC resonance, not intrinsic instability.

Redesigned circuits suppress oscillations and enable stable NDR operation.

Abstract

We demonstrate that the conversion gain of a superconducting hot-electron bolometer (HEB) mixer can be increased by biasing the device within the negative differential resistance (NDR) region of its current-voltage characteristic. Although NDR biasing has historically been avoided due to MHz-range resistive oscillations, we show that these oscillations arise from an LC resonance formed by the bias-T inductance and the effective thermal capacitance of the HEB. By applying stability criteria analogous to those developed for tunnel diodes, we redesigned the embedding circuit to suppress this resonance and achieve stable NDR operation. Direct measurements using two monochromatic 2.5-THz sources confirm the predicted gain enhancement. These results establish NDR biasing as a viable method for improving HEB mixer performance and motivate further studies of noise behavior and circuit optimization.