An Integrator and Schmitt Trigger Based Voltage-to-Frequency Converter Using Unipolar Metal-Oxide Thin Film Transistors

Bin Li,Wei Zhong,Hoi Sing Kwok,Lei Lu,Yuming Xu,Houbo Fan,Zhaohui Wu,Fion Sze Yan Yeung,Sunbin Deng,Rongsheng Chen

doi:10.1109/jeds.2020.3045160

Abstract

This article proposes a voltage-to-frequency converter (VFC) design using unipolar metal-oxide thin film transistor (TFT) technology. The proposed VFC has an integrator and Schmitt trigger based structure. This structure has the advantages of constant power consumption, full-swing output, and low circuit complexity compared to the early designs. To verify the proposed design, SmartSpice simulation based on a Rensselaer Polytechnic Institute (RPI) model whose parameters are turned to fix the measured characteristics of our indium tin oxide(ITO-) stabilized ZnO TFTs is carried out. The ITO-stabilized ZnO TFT has a single-gate staggered structure. Its typical field-effect mobility, threshold voltage, on/off current ratio, and subthreshold-slope are 14.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs, 0.5 V, 1.2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> , and 77 mV/decade, respectively. Simulation results show that the proposed VFC has maximum linearity error less than 1.8%, tuning sensitivity about 1 kHz/V, and power consumption less than 130 μW even under device variations. These performances are competitive compared to the state-of-the-arts. When configured to an analog-to-digital converter (ADC), 6 bit resolution and 14 S/s sampling rate can be realized. These results indicate that the VFC can find potential applications in flexible large-area low-voltage sensor interfaces for quasi-static signals.

Highlights

Thin film technologies have attractive characteristics including flexibility, transparency, light weight, ultra-thin dimensions, stretchability, and capability for large-area low-cost fabrication
The gain bandwidth product (GBW) is frequently used as a figure of merit for the comparison of different amplifiers, and it is mainly limited by the thin film transistor (TFT) technologies
Up to 6 bit within a sufficient long conversion time (1/max linearity error, here is about 71 ms). These results suggest that our voltage-to-frequency converter (VFC) is suitable for some applications that require high resolution and low speed, such as sensor interfaces for quasi-static signals

Summary

INTRODUCTION

Thin film technologies have attractive characteristics including flexibility, transparency, light weight, ultra-thin dimensions, stretchability, and capability for large-area low-cost fabrication. Several thin film transistor (TFT)-based VFC designs have been reported, including the back gate control structure [5], the ring oscillator (RO) structure [6], [7], the relaxation oscillator structure [8], and the LC oscillator based structure [9]. For the RO and LC oscillator based structure, the supply voltage is directly controlled by the input signal This means a large power changes under different inputs, which is not conducive to reducing power consumption. The complexity of manufacturing process and circuit topology is reduced since no dual-gate transistors and external clock signals are required

DEVICE AND MODEL

RESULTS AND DISCUSSION

CONCLUSION