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

This work focuses on a systematic analysis of the potential as well as the 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 rectification process at THz frequencies and low-frequency analysis. Considering a strong frequency dependence of the devices' inherent rectification potential, an in-depth understanding and proper modeling of the device internal parasitics in combination with antenna-detector co-design aspects are crucial for successful detector implementation. For that purpose, a simplified nonlinear high-frequency detector model was applied for the device operating not only in the classical forward-active region but also in deep saturation (cold operation). The complete detector was implemented in a modern high-speed 130 nm SiGe HBT technology with
$f_{t}/f_{max}$
of 470/650 GHz. It comprises two independent orthogonal polarization paths within a single dual-polarization lens-coupled on-chip antenna to operate with unpolarized passive illumination. With the aid of an efficient antenna-circuit co-design, a close-to-optimum detector performance in a near-THz fractional bandwidth can be achieved, as experimentally verified in free-space with frequency-tunable coherent CW sources. In particular, the detector optical NEP for each independent polarization path was measured across 200–1000 GHz reporting state-of-the-art values of 2.3–23 pW/
$\mathrm{\sqrt{Hz}}$
and 4.3–45 pW/
$\mathrm{\sqrt{Hz}}$
with the device operating in the forward-active range and in saturation, respectively. This, in combination with the de-embedded equivalent noise bandwidth of 512 GHz centered around 430 GHz, allowed to demonstrate the corresponding 1-Hz defined NETD of 0.86 K and 2 K in a focused measurement setup with a cavity black-body standard chopped mechanically at 1.5 kHz. With a simultaneous dual-channel operation, the NETD scaled down to 0.64 K, indicating near-zero noise correlation between both polarization paths.