| Though microdisplay has small physical dimensions that about several inches, it can realize high-quality and large-screen image display by the optical system. Microdisplay has advantages such as light weight, small size, portable, high image resolution, low power consumption, which widely used in near-eye display and portable mini projectors and other devices, it also has a broad application prospects in aerospace, military, medical, entertainment and other fields.Organic Light-emitting Device on Silicon in which OLED as display devices, it means that the top-emitting OLED made on the monocrystalline silicon substrate that with CMOS driving circuits, so OLEDoS not only has the advantages of OLED, but also using of the mature CMOS technology. However, pixel quantity of microdisplay are the same as ordinary large-size display, and the display area is much smaller, the individual pixel area is small, such as only15×15μm2. At the same time, the current efficiency of OLED is relatively high, therefore the current of the pixel would be a tiny one, may be only several dozens Picoamperes or hundreds Picoamperes. As we all know, the mobility of monocrystalline silicon is great, resulting that the saturation current of regular size driving MOS transistor is in order of mocroamperes, in order to realize the smaller current in the conventional2T-1C pixel circuit, the only way is to increase the channel length of the driving transistor, but the longer channel and smaller pixel areas are irreconcilable conflict.To solve this problem, eMagin corporate successfully produced the first OLEDoS in2002using the driving MOS transistor working in sub-threshold region, however the sub-threshold current is very sensitive with mismatch threshold voltage and gate-source voltage of the driving transistor. Paralleling another MOS transistor across the driving MOS proposed by institute of microelectronics may generate great useless power consumption, and easy to rise the working temperature of microdisplay. Based on the principle of the luminance that eyes can perception is depends on the luminous intensity and its time, our group had proposed the PWM pixel driving circuit that can applied for monochrome OLEDoS, the proposed circuit realized the low average brightness, but the poor data holding causing low gray scale. After analyzed the leakage of MOS transistor, using switches series and dual capacitor structure effectively improved the data holding result, and even under the worst-case analysis, the gray scale can as high as75, at the same time the improved pixel driving circuit can compensate for the OLED degradation effectively.We also proposed the circuit applied for color OLEDoS, which is based on pixel multiplexing. Many neighboring pixels share the same driving circuit unit that composes by switching transistors, driving transistors and storage capacitors, and each pixel corresponding with an small size switching transistor controlling the current on or not. The method of adjacent pixels using the same driving circuit unit can increase the channel length of the driving MOS transistor and the area of storage capacitors, improving the data holding effect effectively. When twelve sub-pixel using the same driving circuit unit, analysis under the worst-case show that the average luminance vary is so little that about the brightness difference of neighboring Just Noticeable Difference Index. Also the longer channel of driving MOS transistor effectively suppressed the channel length modulation effect, that is to say the current of the MOS don’t dependence the voltage difference of the source and drain so much, and can compensate for the OLED degradation effectively. The simulation results show that if the channel length is50μm and twelve sub-pixel multiplexing, even the degradation is as high as30%, the luminance decay is still less than1.0%. |
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