Simple Pixel Circuits for High Resolution and High Image Quality OLED Microdisplay

Abstract

Microdisplay is a small display which is used for virtual reality (VR) and augmented reality (AR) applications such as head mounted displays (HMDs) and head up displays (HUDs). As the demand for more realistic VR and AR increases, many researches have been conducted to achieve high resolution and high image quality microdisplays. There are several microdisplay technologies such as digital micromirror device (DMD), liquid crystal-on-silicon (LCoS), and organic light emitting diode-on-silicon (OLEDoS). Among these technologies, the OLEDoS has several advantages such as high contrast ratio, fast optical response time, and low power consumption. Moreover, it enables a light and thin display system due to no need for backlight. The OLED microdisplay using the OLEDoS is fabricated on a single crystalline silicon backplane using a CMOS process, which differs from conventional displays which are fabricated on a polycrystalline silicon or amorphous silicon backplane using a thin film transistor (TFT) process. Thus, the CMOS process with fine design rules enables a display system including pixel, driving, power management, and interface circuits to be integrated into a single chip. However, there are several constraints on designing the pixel circuit to achieve a high resolution and high image quality OLED microdisplay. For high resolution, a pixel circuit with simple structure is required so as to be integrated into a unit subpixel area of tens of μm2. Accordingly, the size of driving transistor should be reduced to increase the resolution of the OLED microdisplay so that the pixel circuit can be integrated into a restricted unit subpixel area. However, as the size of driving transistor decreases, the threshold voltage variation of driving transistor increases due to process variation, which results in emission current deviation error. In addition, since the OLED is evaporated on top of the pixel circuit, it has an area of tens of μm2. Moreover, the luminance of the OLED is proportional to the current density, the emission current to emit the target luminance is very low, ranging from a few pA to a few nA. Due to the high transconductance of the MOSFETs and low emission current, the pixel circuit suffers from narrow data voltage range. Since the emission current is vulnerable to the programmed data voltage distortion caused by the charge injection, kickback voltage, and leakage current, the narrow data voltage range makes the pixel circuit hard to accurately control the low emission current. To solve the aforementioned problems, two pixel circuits for OLED microdisplays are proposed; one with the negative feedback and the other with the threshold voltage cancellation. Using a simple pixel circuit structure, the proposed pixel circuits are fabricated using a 90 nm standard CMOS process with 6 V high-voltage devices and have a unit subpixel area of 3 × 9 μm2. In addition, the proposed pixel circuits compensate for the threshold voltage variation of driving transistor and extend the data voltage range. The measurement results of 24 pixel circuits show that the emission current deviation errors of the proposed pixel circuits with the negative feedback and with the threshold voltage cancellation range from -2.59% to +2.78% and from -1.86% to +1.84%, respectively. In addition, the data voltage ranges of the proposed pixel circuits with the negative feedback and with the threshold voltage cancellation are 1.193 V and 1.792 V, respectively.