25.5 A 320GHz subharmonic-mixing coherent imager in 0.13µm SiGe BiCMOS.

Terahertz imaging has been gaining increasing attention for its emerging applications in security, biomedical and material characterization. Previous works have demonstrated terahertz imagers on silicon: in [1], the authors demonstrated a 280GHz 4×4 array and an 860GHz pixel using Schottky-barrier diodes; in [2], a 0.7-to-1.1THz 1k-pixel camera was presented. Unfortunately, most previous works are based on incoherent direct detection (Fig. 25.5.1), which causes low sensitivity due to the output droping quickly with input power (∝VRF2), and, as a result, need exceedingly high power sources for illumination. This problem can be alleviated by utilizing coherent heterodyne detection scheme (Fig. 25.5.1), in which the terahertz input (RF) mixes with a local oscillation (LO) signal and generates an output with the amplitude proportional to VRFVLO. As the LO power is normally significantly higher than RF, the sensitivity is much enhanced. Moreover, as the output also carries the RF phase information (φRF), digital beamforming is achievable. For a better comparison, an NPN transistor is configured both as a direct detector (input power injected from the emitter) and a heterodyne detector (-20dBm LO power pumped into the base). The simulated output current with different input power is shown in Fig. 25.5.1, in which we observe a sensitivity enhancement of more than 40dB. However, this coherent sensing scheme introduces a great challenge of synchronizing the frequencies of the transmitter (TX) radiated signal and the receiver (RX) LO. Fortunately, phase-locked terahertz sources have been demonstrated on silicon [3,4]. In this paper, a fully integrated 320GHz high-sensitivity coherent-imaging transceiver chipset is demonstrated.