Design of a Low-Voltage and Reduced Programming cycle AMOLED Pixel Circuit using IGZO TFTs

This work presents a 5T −2C pixel circuit based on amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) for flexible displays utilizing improved stacked voltage-programmed pixel circuit topology. Due to a low operating voltage of 5 V and reduced programming time of 8 $\mu\mathrm{s}$, the proposed circuit finds its application in large screen HD displays as it can provide a very high frame rate of 120 Hz. Moreover, a good compensation ability of the proposed circuit against the threshold voltage variations of the driving TFT in the range of −0.2 volts to 2 volts from the nominal voltage of 0.7 volts, makes it suitable for flexible AMOLED displays. The error in organic light emitting diode (OLE D) current is within 0.4% over the range of data voltage (3.8 V to 6 V) when the substrate is subjected to both compressive and tensile strains of $\pm 0.3$ % and within 10% due to threshold voltage variations under electrical stress. The adapted SPICE level-3 flexible TFT model efficiently captures the variations in threshold voltage due to mechanical as well as electrical stress. As a result, the proposed 5$T$2C pixel circuit reveals good performance for applications in low-voltage flexible displays.

I. Introduction

AMOLED displays are widely used today in the display industry as they provide multitudinous advantages such as wide-viewing angle displays, high resolution, high contrast ratio, better response time, low cost, and low power requirement. The backplane technology used for the pixel circuit employs various materials such as hydrogenated amorphous silicon (a-Si:H), poly-silicon (poly-Si), organic semiconductors, oxide semiconductors, etc. Among these, oxide semiconductors such as amorphous indium gallium zinc oxide (a-IGZO), provide the most promising backplane technology for pixel-driving circuits owing to their excellent attributes for driving large-sized AMOLED (Active-Matrix Organic Light Emitting Diode) displays. These attributes include a wide energy bandgap (3.4 eV for a-IGZO), high mobility, higher temporal stability, etc. [1]. Due to the wide energy bandgap, a-IGZO-based thin film transistors (TFTs) are transparent. A low programming time is achievable using a-IGZO TFTs as their mobility is high and thus it can supply high driving current.