Lumped-Element Model of THz HEB Mixer Based on Sputtered MgB2 Thin Film

TL;DR

This paper develops a lumped-element model for THz MgB2-based HEB mixers, demonstrating how optical losses and Johnson noise limit sensitivity, and providing insights for device optimization.

Contribution

It introduces a comprehensive bolometric model for MgB2 HEB mixers that aligns well with experimental data and highlights key noise sources affecting sensitivity.

Findings

Strong agreement between model and measurements of gain and noise.

Optical losses and Johnson noise are primary sensitivity limitations.

Simulations suggest further noise reduction requires intrinsic gain improvements.

Abstract

We present a comprehensive analysis and experimental study of THz hot-electron bolometer (HEB) mixers made from 40-nm-thick sputtered magnesium diboride (MgB2) thin films on high-resistivity silicon substrates. Using a lumped-element bolometric model, we achieve strong quantitative agreement with measurements of conversion gain, noise temperature, and local-oscillator (LO) coupling to the HEB devices. Our analysis shows that the sensitivity of current HEB devices is primarily limited by on-chip optical losses, with both Johnson and thermal-fluctuation noise contributing significantly to the overall noise temperature. Simulations of an optimized device with near-ideal optical coupling suggest that Johnson noise remains a substantial factor even with improved coupling. Further reduction of the noise temperature may require additional suppression of Johnson noise (via improved intrinsic conversion gain) beyond optimizing optical coupling efficiency. We emphasize the importance of accurate modeling to achieve good numerical agreement with experiments, thereby enabling understanding of the causes of sensitivity loss.