An Over 200 dB Dynamic Range Image Capture using a CMOS Image Sensor with Lateral Overflow Integration Capacitor and Current Readout Circuit in a Pixel

An image capturing solution extending dynamic range (DR) over 200 dB has been demonstrated by 64 × 64 pixels 20 μm pixel pitch CMOS image sensor with the lateral overflow integration capacitor and the current readout circuit in each pixel. The operation combining the four times of the voltage readouts and the one time of the current readout has achieved a high S/N ratio performance in the incident light ranging from about 10 to 10 lx. Introduction A DR enhancement is strongly required in addition to a high sensitivity, a high S/N ratio and a high resolution in order to realize the ultimate performance of the image-sensing device for the digital camera, the security, the automotive and the medical applications. Several recent papers have reported the various approaches to extend DR; such as, the multiple exposures in a frame [1-6], the logarithmic compression [7-16], the dual photodiode in a pixel [17], the lateral overflow integration capacitor [18-20] and the combination of the linear and the logarithmic responses [21-24]. These approaches have improved DRs, however still remain in 100 to 140 dB of DRs. In order to extend DR furthermore, the CMOS image sensor featuring the lateral overflow integration capacitor and the current readout circuit in each pixel has been previously demonstrated [25]. This paper describes the over 200 dB DR image capturing method combining the four times of the voltage readout operations and the one time of the current readout operation to keep a high S/N ratio in the incident light ranging from about 10 to 10 lx. Device structure and circuit diagram Fig. 1 shows a schematic diagram of a pixel and a column circuit [25]. The pixel circuit consists of a fully depleted photodiode (PD), a transfer switch (T), a floating diffusion to convert the charge to the voltage (FD), a reset switch (R), a source follower amplifier (SF), a row select switch (X), an overflow photoelectron integration capacitor (CS), a connection switch between FD and CS (S), another transfer switch (T’), current mirror circuits amplifying the photocurrent from PD (CM1, CM2), a reference current source (IREF) and another row select switch (X1’). The column line component consists of a current source driven by the voltage readout operation (Ivol) and a current readout circuit of the current readout operation, which includes current mirror circuits comprising CM3 and CM4, a common-gate transistor (X2’) and a logarithmic compression circuit (LOG), in each column. The voltage readout circuit with a lateral overflow integration capacitor in a pixel is as same as described in the previous paper [20] and leads to the extension of the DR keeping a high sensitivity and a high S/N ratio without losing the overflow charges from PD. The current readout circuit achieves further extension of the DR to the bright side by reading out the logarithmic compression voltage of the photocurrent amplified in each pixel and column.