Towards Passive Imaging With Uncooled, Low-NEP SiGe HBT Terahertz Direct Detectors

This work focuses on a systematic analysis of the potential and limitations of modern SiGe HBT devices for broadband passive room-temperature detection in the lower THz range. Multiple necessary conditions need to be fulfilled to facilitate broadband passive imaging with a sufficiently low in-band NEP, which refer to various technology-driven device operation aspects, including the THz rectification process and low-frequency analysis. To properly understand and model the devices' internal parasitics in combination with antenna-detector co-design aspects, a simplified nonlinear high-frequency detector model was applied for the devices operating in the forward-active and saturation region (cold operation). The complete detector was implemented in a modern high-speed 130 nm SiGe HBT technology with
$f_{t}/f_{\text{max}}$
of 470/650 GHz. It comprises two orthogonal polarization paths within a single dual-polarization lens-coupled on-chip antenna to operate with unpolarized passive illumination. Due to an efficient antenna-circuit co-design, a close-to-optimum detector performance in a near-THz fractional bandwidth was achieved, as experimentally verified in free-space measurements with frequency-tunable coherent CW sources. The detector optical NEP for each polarization path was measured across 200–1000 GHz reporting state-of-the-art values of 2.3–23 pW/
$\mathrm{\sqrt{Hz}}$
(forward-active) and 4.3–45 pW/
$\mathrm{\sqrt{Hz}}$
(saturation). This, combined with the de-embedded equivalent noise bandwidth of 512 GHz around 430 GHz, allowed to demonstrate a 1-Hz defined NETD of 0.86 K and 2 K with a focussed cavity black-body standard chopped mechanically at 1.5 kHz. By dual-channel operation, the NETD scaled down to 0.64 K, indicating near-zero noise correlation between both polarization paths.