Ultra-low Quiescent Current, Unipolar TFT-based Low Dropout Regulator on Flexible Substrate for Biomedical Implants

This work presents a low dropout regulator (LDO) with integrated voltage reference implemented using unipolar NMOS thin-film transistors (TFTs) on a mechanically flexible substrate. Utilizing pseudo-resistors operating in the subthreshold region, this design achieves a high-impedance node for increasing the gain of the error amplifier, thus giving stable voltage regulation without using either PMOS devices or high-value resistors. The fabricated LDO consumes ultra-low quiescent current (I_Q) of 150 nA with a current efficiency of 99.99% at a maximum load of 1.3 mA across input voltages from 4 to 7 V with no load capacitor. Measurement results indicate line sensitivity of 0.47 % / V, transient overshoot of 86 mV and undershoot of 39 mV for load change, with a maximum settling time of 180 μ s. The design achieves FoM better than state-of-the-art flexible LDOs and is comparable to silicon-based circuits. This architecture is wellsuited for biomedical applications, including wearable biosensors and implantable electronics, where high energy efficiency, low form-factor, and mechanical flexibility are essential.