Highly Uniform Low Gray AMOLED Pixel Using Stable Circuit and Duty Ratio Modulation Driving

Abstract

In this paper, a new pixel circuit for active matrix organic light-emitting diode (AMOLED) display that can achieve high uniformity in low gray levels and its driving method are proposed. The proposed circuit compensates for threshold voltage variation of thin-film-transistors (TFTs), with the structure that minimizes the loss of sensed threshold voltage. However, the high current error rate in extremely low gray level is unavoidable, as the driving TFT (DRT) operates in subthreshold region, where the current difference caused by the threshold voltage variation can be severe. To suppress high error rates in low gray levels, the operation region of DRT is restricted to the saturation region, by adopting duty ratio modulation (DRM) method. With the DRM method, low gray is expressed with high current value and short emission time. The viability of the proposed circuit and its operation are analyzed with HSPICE. Compared to the conventional driving method, DRM significantly reduces the current error rate in low gray area. The proposed circuit is fabricated within 220 $\mu {\mathrm {m}} \times 440 \mu {\mathrm {m}}$ . The measurement of the circuit also verified the capability of the proposed circuit and the DRM method.

Keywords

• Active matrix organic light-emitting diode (AMOLED)
• amorphous In-Sn-Zn-oxide (a-ITZO) thin-film transistor (TFT)
• duty ratio modulation
• pixel circuit
• threshold voltage compensation