Ullrich R. Pfeiffer

This letter presents a 206.5–220.5-GHz fundamental differential Colpitts oscillator in a cascode topology implemented in a 130-nm SiGe HBT technology with f_t/fmax of 350/450 GHz. Base inductors at the common-base (CB) stages are used to provide an inductive load at the output of the common-emitter (CE) stage, hence, boosting the output power by 27%. The resonant tank is embedded straight on top of the devices, reducing layout parasitics and resulting in a compact and efficient oscillator core layout. The oscillator provides a peak output power of 7 dBm at 208 GHz, 7.4% peak dc-to-RF efficiency at 220 GHz, and 6.55% tuning range (TR). At peak efficiency, the oscillator delivers an output power of 5.7 dBm with 50-mW dc power consumption and a phase noise (PN) of -90.2/-110 dBc/Hz at 1-/10-MHz offset, respectively. To the best of the authors’ knowledge, the presented oscillator has the best PN figure-of-merit (FoM) of -182.1 dBc/Hz at 1-MHz offset in a SiGe/CMOS technology above 200 GHz. It occupies a total area of 0.086 mm², including the RF pad, and an ultracompact core size of 0.0049 mm².

Terahertz (THz) light field imaging inherently allows capturing the 3-D geometry of a target but at the cost of an increased data volume. Compressive sensing techniques are instrumental in minimizing data acquisition requirements. However, they often rely on computationally expensive sparse reconstruction approaches with high memory footprint. This research introduces an advanced coarse-to-fine (CTF) sparse 3-D reconstruction strategy aimed at enhancing the precision of reconstructed images while significantly reducing computational load and memory footprint. By employing a sequence of sensing matrices of increasing resolution, our approach avoids falling into an ill-conditioned inversion and strikes a balance between reconstruction quality and computational efficiency. We demonstrate the effectiveness of this CTF strategy through its integration with several established algorithms, including basis pursuit (BP), fast iterative shrinkage-threshold algorithm (FISTA), and others. The results showcase the potential of the CTF approach to improve 3-D image reconstruction accuracy and processing speed in THz light field imaging.

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.

The terahertz (THz) frequency range is widely considered as the most challenging and under-developed frequency range due to the lack of technologies to effectively bridge the transition region between microwaves (below 100 GHz) and optics (above 10,000 GHz). Although THz radiation would be perfect for material identification and as a safe alternative to X-rays for producing high resolution images of the interior of opaque objects, first a fundamentally new approach is needed to establish novel devices and techniques.

This paper presents a multi-finger doubler based on amorphous-indium gallium zinc oxide (a-IGZO) thin film transistors (TFT) operating at GHz frequency. The doubler and the TFTs have been fabricated in-house with chromium (Cr) gate electrodes. We have used the Rensselaer Polytechnic Institute amorphous silicon (RPI-a) TFT DC model and a Null-Bias method extracted AC model for simulation. The device output power, the threshold voltage mismatch, and breakdown characteristics are explained with the help of the model. The device yield reduces as the number of fingers in the TFTs increase. The model helps explain the doubler’s non-idealities and compensate for these transistors’ low yield. The peak second harmonic conversion gain of the doubler is measured to be -32 dB at a gate overdrive voltage of around 2.8V for a 500MHz input signal. This second harmonic output is beyond the TFT’s transit and maximum oscillation frequencies.

Terahertz (THz) systems open up the possibility of new applications of electromagnetic waves. Enormous bandwidths up to several terahertz enable the development of powerful spectroscopy systems that can provide insights into the structure and material of objects with micrometer resolution. New high power and compact THz time-domain spectroscopy (TDS) systems will be presented. The wide bandwidth also enables high-resolution imaging under far-field conditions to analyze arbitrary objects from a distance. In addition, a near-field system-on-a-chip imaging system is presented that achieves a resolution of 10 μm. Additionally, the application of terahertz waves presents challenges due to the low transmit power of the sources, high attenuation in free space, and the high noise figures of the receivers. These challenges can be overcome by antennas with high gain. Since - depending on the application - spatial scanning or focusing in a time-varying direction is required, beam steering is an essential component of many THz systems. In terms of communications, terahertz carrier frequencies offer wide bandwidths, enabling correspondingly high data rates of 100 Gbit/s and more. As a result, the frequency bands between 250 GHz and 450 GHz have already been identified and/or allocated for communication services and are being discussed as a component of 6G mobile communications. Concepts and demonstrations for 6G terahertz mobile communications will be presented. The high bandwidth of terahertz waves can also be used for high-accuracy indoor localization.

A broadband three-stage pseudo-differential SiGe-interconnection bipolar transistor (HBT) power amplifier (PA) for high-speed communication at around 300 GHz is presented. The amplifier is fabricated in an experimental 130-nm SiGe BiCMOS technology with an $f_{t}/f_{\max }$ of 470/650 GHz. The use of asymmetric coupled line transformers is proposed to facilitate broadband impedance transformation with device reactance compensation at all amplifier interfaces. The amplifier achieves a maximum small-signal power gain of 23.0 dB and a $P_{\mathrm{ sat}}/{\mathrm{ OP}}_{\mathrm{ 1\,dB}}$ up to 9.7/6.7 dBm, respectively. It shows a 3-dB bandwidth of 63 GHz (239–302 GHz) in small-signal operation and 94 GHz (223–317 GHz) when saturated. The amplifier consumes about 360 mW at a 3-V supply voltage yielding a peak power-added efficiency (PAE) of 1.95% at 260 GHz.

This article presents a comprehensive analysis of the terahertz (THz) rectification process with modern high-speed silicon–germanium (SiGe) heterojunction bipolar transistor (HBT) devices to enable low-power and close-to-optimum detector sensitivity in a near-THz fractional bandwidth. The influence of all major device internal parasitics on the detector responsivity and noise equivalent power (NEP) is analyzed in detail as a function of the device bias point and the operating frequency. By analogy to the cold MOSFET operation, the fundamentals of the detector operating in weak and deep saturation are studied and compared with the forward-active region, indicating its potentially superior NEP performance with a suitable readout path. The antenna’s broadband radio frequency (RF) power coupling aspects are further elaborated and related to the device bias conditions to maximize the internal base–emitter driving voltage. A set of single- and dual-polarized antenna-coupled detectors with close-to-optimum detector operation in a near-THz fractional bandwidth and a state-of-the-art optical performance was implemented in a 0.13- $\mu \text{m}$ SiGe HBT technology with $f_{t}/f_{\text {max}}$ of 350/550 GHz and extensively characterized across 200–1000 GHz to support the underlying analysis. For the forward-active operation, the following maximum optical current responsivity and minimum optical NEP values have been demonstrated: 5 A/W, 1.9 pW/ $\sqrt {\text {Hz}}$ at 300 GHz and 0.65 A/W, 19 pW/ $\sqrt {\text {Hz}}$ at 900 GHz. The corresponding NEP values for deep saturation (with unbiased collector) are 5.1 pW/ $\sqrt {\text {Hz}}$ and 45 pW/ $\sqrt {\text {Hz}}$ but they are largely limited by the noise floor of the consecutive external amplifier.