Study On Reset Operation Of VPS

Today, solid-state image sensors play an increasingly important role in people’s working lives. The popular sensors of them are CCD and CMOS image sensor. In addition people are also studying new types of sensors. The Vertically-charge-transferring Pixel Sensor (VPS) is one of them. The pixel structure and array architecture are similar to NOR FLASH. Reset of VPS is an important thing, directly affect the imaging window size and dynamic range.The main contents of this paper include research flash reset mode and reset mechanism, VPS feasibility of these pixel reset mechanism and propose an improved method to reset VPS 5K×5K VPS array.Channel hot electron injection (CHE) can be a good method to VPS pixel array reset. Because of the low CHE injection efficiency, power consumption, CHE reset of chips is prone to reliability problem. CHE for VPS chip of high-resolution is too slow. Drain avalanche hot carrier injection (DAHC) is also a hot electron injection method, which injection characteristics are similar to CHE. FN tunneling reset has advantages of high efficiency and low power consumption. It can simultaneously reset all pixels in the VPS array, so the reset time is short.It was found that the 3-step-reset method of FN tunneling has high efficiency and high speed. Power consumption of this method is very low. After the reset, threshold voltages of pixels have a compact distribution. By FN tunneling method, the cells in the array are programmed into the threshold voltage of about 3V. Then they are erased to about 0.5V. Finally, they are programmed to about IV. Although it can be programmed to 3V and then erased to about 1V,however, this method is worse than the former. The reason is that cells are programmed by last step to avoid that the threshold voltages of the cells are negative. Moreover, the interfaces of the oxide layer on both sides are different.Another important point is that the substrate voltage and the source voltage are consistent when the array is resetting. The reason is that it can have a compact distribution of threshold voltages. The substrate capacitances of cells are different. The differences will affect the threshold voltage of the pixels in VPS array.The first two steps of the three steps can be relatively short (lms), the last step of the reset should be longer (100ms), so that the result of reset is better.