Rail-to-Rail Timing Signals Generation Using InGaZnO TFTs For Flexible X-Ray Detector

Pydi Ganga Bahubalindruni,Ana Rovisco,Jorge Martins,Elvira Fortunato,Rodrigo Martins,Pedro Barquinha,Maria Pereira,Vitor Tavares,Ana Santa,Bhawna Tiwari

doi:10.1109/jeds.2020.2971277

Abstract

This paper reports on-chip rail-to-rail timing signals generation thin-film circuits for the first time. These circuits, based on a-IGZO thin-film transistors (TFTs) with a simple staggered bottom gate structure, allow row and column selection of a sensor matrix embedded in a flexible radiation sensing system. They include on-chip clock generator (ring oscillator), column selector (shift register) and row-selector (a frequency divider and a shift register). They are realised with rail-to-rail logic gates with level-shifting ability that can perform inversion and NAND logic operations. These logic gates are capable of providing full output swing between supply rails, V DD and V SS , by introducing a single additional switch for each input in bootstrapping logic gates. These circuits were characterised under normal ambient atmosphere and show an improved performance compared to the conventional logic gates with diode connected load and pseudo CMOS counterparts. By using these high-performance logic gates, a complete rail-to-rail frequency divider is presented from measurements using D-Flip Flop. In order to realize a complete compact system, an on-chip ring oscillator (output clock frequency around 1 kHz) and a shift register are also presented from simulations, where these circuits show a power consumption of 1.5 mW and 0.82 mW at a supply voltage of 8 V, respectively. While the circuit concepts described here were designed for an X-ray sensing system, they can be readily expanded to other domains where flexible on-chip timing signal generation is required, such as, smart packaging, biomedical wearable devices and RFIDs.

Highlights

Design of flexible electronic systems with emerging hybrid technologies is gaining significant interest [1]–[3]
All logic gates were characterised from measurements under typical room environment
As expected a complete rail-to-rail operation together with level shifting ability is being noticed from the voltage transfer characteristics (VTCs) of inverter and NAND gates from Fig. 6

Summary

INTRODUCTION

Design of flexible electronic systems with emerging hybrid technologies is gaining significant interest [1]–[3]. Limited by output swing and is not suitable for clock generation, while the latter uses dual gates, demanding higher mask count and processing steps Both these designs are employing high supply voltages (> 20 V) and the performance is being degraded at low voltages. This work demonstrates high performance logic gates by introducing a switching TFT for each input in capacitive bootstrapping based logic gates They can ensure complete rail-to-rail operation with standard device structure for lower values of VDD without using any external control signal and they have the level shifting ability, as the output logic ‘1’ level can be made equal to VDD, independent of the input signal logic ‘1’ level. Rail-to-rail swing on-chip row and column selectors for a 2 x 2 sensor matrix are demonstrated as a proof of concept, from measurements and simulations, respectively

CIRCUIT FABRICATION

RESULTS AND DISCUSSION

CONCLUSION