High-Gain Common-Source Voltage Amplifier with Intrinsic Temperature Compensation for Biosensing

Printed and flexible electronics is rapidly gaining traction as technologies become increasingly manufacturable and sustainable. Thin-film transistors (TFTs), based on the conventional field-effect architecture, are at the core of many large area electronic (LAE) devices and systems for flexible applications and sensors. However, conventional TFT technology faces challenges, particularly variations in manufacturing processes that degrades performance and reduces yield[1]. These issues are exacerbated in larger circuits that rely on additional components. One solution is to design simpler circuits comprising fewer components [2].<br/>Here, we have implemented common-source amplifiers with a zero-V_GS topology, using low temperature polysilicon (LTPS) source-gated transistors (SGTs). This approach allows for high gain amplification (over 250x reported [2]) in a simple layout using only two transistors (2T). In addition, we exploit circuit topology benefits to compensate for the SGT’s inherent temperature dependence of the drain current. Since the temperature behaviour of SGTs depends on the dimensions of the source injection area [3], we have used driver and load devices with identical source geometry. This allows the amplifier to automatically compensate variations in temperature. When the transconductance of the driver increases, the effective resistance of the load reduces proportionally, thus maintaining a constant gain over a relatively stable input voltage range.<br/>As fabricated, the circuit draws µW of power, but the choice of source contact barrier[4–6] can reduce this to the nW range. To demonstrate potential functionality, we used a flexible PVDF-based piezoelectric sensor [7], in a textile form factor, as an input to the amplifier. This compact and low-power voltage amplifier, including such textile sensors, could be embedded in wearables to capture very small input signals, irrespective of temperature fluctuations.<br/>The final system, based on this high gain, temperature-compensated zero-V_GS topology, is highly suited to the healthcare domain and biosensing applications, including: pregnancy monitoring by capturing fetal movements; compact fitness trackers; and environmental sensing.<br/>The usage of piezoelectric sensors is just an example of how broad can be the applications of these simple circuits in many domains of the market mainly devoted to healthcare devices, wearables, etc.<br/>[1] A.F. Paterson and T.D. Anthopoulos, <i>Nat. Commun.</i>, <b>9</b>, 1–4 (2018).<br/>[2] E. Bestelink, K.M. Niang, G. Bairaktaris, L. Maiolo, F. Maita, K. Ali, A.J. Flewitt, S.R.P. Silva, and R.A. Sporea, <i>IEEE Sens. J.</i>, <b>20</b>, 1–11 (2020).<br/>[3] R.A. Sporea, M. Overy, J.M. Shannon, and S.R.P. Silva, <i>J. Appl. Phys.</i>, <b>117</b>, (2015).<br/>[4] R.A. Sporea, K.M. Niang, A.J. Flewitt, and S.R.P. Silva, <i>Adv. Mater.</i>, <b>31</b>, (2019).<br/>[5] J. Zhang, J. Wilson, G. Auton, Y. Wang, M. Xu, Q. Xin, and A. Song, <i>Proc. Natl. Acad. Sci. U. S. A.</i>, <b>116</b>, 4843–4848 (2019).<br/>[6] R.A. Sporea and S.R.P. Silva, Design considerations for the source region of Schottky-barrier source-gated transistors, in: Proc. Int. Semicond. Conf. CAS, (2017), pp. 155–158.<br/>[7] M. Forouharshad, S.G. King, W. Buxton, P. Kunovski, and V. Stolojan, <i>Macromol. Chem. Phys.</i>, <b>220</b>, 1900364 (2019).