A Low-Power Photon-Counter Front-End Dedicated to NIRS Brain Imaging

This paper introduces a new miniaturized on-chip photodetector front-end targeted for portable near infrared spectroscopy as a noninvasive tool for real-time brain imaging. It includes silicon avalanche photodiodes (SiAPDs) with dual detection modes using a transimpedance amplifier (TIA) with on-chip gain/bias control, and a controllable mixed (active-passive) quench circuit, with tunable hold-off time, and a novel gated quench-reset technique. This integrated photoreceiver front-end has been fabricated using submicrometer standard CMOS technologies with a minimum fill-factor of 95%.

Fabricated SiAPDs exhibit avalanche gains of 35 and 22 at 10 and 18 V bias voltages with red-shifted peak photon-detection efficiency and dark count-rates of 114 and 4 kHz (at 1 V excess bias voltage). The TIA consumes 1-mW power, and offers a transimpedance gain of 250 MV/A, a tunable bandwidth (1 kHz-1 GHz), and an input current referred noise <;10 fA/√Hz at 1 kHz. The photon-counter exhibits a quench-time of 10 ns with a 0.4-mW power-consumption with an adaptive hold-off time control. The on-chip integration of SiAPDs and front-end circuit, reduced the power-consumption and after-pulsing, and increased the sensitivity.