A 43 µm $\times$ 269 µm Light-Adaptive Optoelectronic Autonomous Microsystem for Neural Recording

We present a 43$μ$m × 269 μm tetherless neural recording microsystem in which the CMOS bulk is forward biased to utilize silicon junctions as a photovoltaic source. Our microsystem, forward-bulk microscale optoelectronic tetherless electrode (FB-MOTE), can operate with as low as 0.2$μ$A at 0.317 V and can withstand light intensity up to 1200$μ$W/mm2, and is power-adaptive: the higher available power increases the system bandwidth while maintaining the input-referred integrated noise. To balance between adaptability and stability, we have designed our amplifier to take up most of the additional power, hence acting like a regulator, while the other circuit blocks are PTAT-biased to remain relatively stable across available power levels. The amplified neural signals are pulse position modulated (PPM) and optically transmitted through an AlGaAs microscale light emitting diode ($μ$LED) for its information per-photon efficiency, where the$μ$LED driver is designed to maximize the emission-to-area ratio. Finally, we discuss various light-induced effects observed in measurements and introduce a simulation methodology to account for such effects and its limitations. Our forward-bulk CMOS microsystem provides an approach that can effectively harness and account for the available light in optoelectronic systems design.