Broadband Modeling, Analysis, and Characterization of SiGe HBT Terahertz Direct Detectors

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.