Sangsik Park

The effect of size on photodiode pinch-off voltage for small pixel CMOS image sensors

The electron potential of a photodiode in a CMOS image sensor should be designed precisely since the charge capacity of the photodiode decreases as the pixel area shrinks. The pinch-off voltage of a photodiode, which also affects the electron capacity, is dependant on the doping profile of the pn junction as well as the size of the photodiode. The pinch-off voltage is lower in a smaller photodiode. A simple method that uses the lateral depletion of a photodiode for an estimate of the pinch-off voltage in small photodiodes is proposed, and is compared to the measured experimental data. Two constants are used to account for the doping profile and photodiode size. The measurement data shows the error of the estimation of the pinch-off voltage to be <0.05V.

A Subthreshold Slope and Low-frequency Noise Characteristics in Charge Trap Flash Memories with Gate-All-Around and Planar Structure

The causes of showing different subthreshold slopes (SS) in programmed and erased states for two different charge trap flash (CTF) memory devices, SONOS type flash memory with gate-all-around (GAA) structure and TANOS type NAND flash memory with planar structure were investigated. To analyze the difference in SSs, TCAD simulation and low-frequency noise (LFN) measurement were fulfilled. The device simulation was performed to compare SSs considering the gate electric field effect to the channel and to check the localized trapped charge distribution effect in nitride layer while the comparison of noise power spectrum was carried out to inspect the generation of interface traps (NIT). When each cell in the measured two memory devices is erased, the normalized LFN power is increased by one order of magnitude, which is attributed to the generation of NIT originated by the movement of hydrogen species (h * ) from the interface. As a result, the SS is degraded for the GAA SONOS memory device when erased where the N IT generation is a prominent factor. However, the TANOS memory cell is relatively immune to the SS degradation effect induced by the generated N IT .

A high-sensitivity CCD image sensor using source follower circuit with actively controlled gain characteristics

A high-sensitivity charge coupled device (CCD) image sensor that utilizes a new output structure has been developed. The structure utilizes an enhancement inverter, a coupling capacitor, and resistors in addition to the conventional source follower. The output of the source follower is inverted and ac component of that signal is fed back to the gate of the load transistor of source follower to modulate the impedance. The overall gain improvement can be estimated from simple small-signal analysis and the results match well to the circuit simulation. A 1/4 in. 250,000 pixels color CCD image sensor was fabricated employing the new output circuit. The fabrication process is same to that for conventional CCD image sensors since the inverter in this circuit consists of enhancement mode transistors. It is observed that 14% of sensitivity improvement in the CCD image sensor is achieved by the new structure and the linearity shows no distortion compared to conventional image sensors in the full range of imaging illumination.

Effect of Emitter Layer on the Signal Generation of the Active Pixel Sensor

The image sensor in the optical mouse reads the scene of the bottom, and compares it with the last sceneto acquire the moving data. The scene should be read with high frame rate to make the moving of the cursor soft and exact. The high frame rate means the short time to integrate the signal charge. The weak signal causes malfunctioning of the moving of the cursor. The signal should be amplified to ensure the signal to noise ratio. Some type of the pixel in the optical mouse uses a bipolar transistor to amplify the signal. The conventional bipolar transistor used in the optical mouse has full covering emitter layer on the base layer. The emitter layer helps the bipolar action, but it decreases the signal electron-hole pair generation since the generation occurs in the collector-base junction. We propose the emitter structure which covers only the boundary of the base layer. The photo current of the new structure increased 20% compared to the conventional structure.

Effect of guard ring on sensitivity of the photo-transistor used in an optical mouse

In the operation of an optical mouse sensor, amplification of signal charge is necessary since low power dissipation and high speed are important. 3 kinds of structure of photo transistor were proposed to get high amplification gain. The performances were verified using 18pixel × 18pixel image sensor. The image sensor was manufactured using 0.18 μm design rule CMOS process. The highest sensitivity was appeared in structure which has no emitter layer on entire surface but has guard ring on boundary of the base. The guard ring maintains base voltage more uniform to get strong bipolar action. This image sensor was mounted in the mouse and showed sampling rate of motion detector up to 50 kH.

Studies on High Speed Addressing Driving Scheme using the Priming Effect in Plasma Display Panel

This study is related to the realization of high speed address driving method for Full-HD PDP. The new self-priming addressing(SPA) driving scheme was proposed to improve an address discharge time lag, which utilizes the priming effect maintaining the priming discharge during an address period. In this study, the basic characteristics of the priming ramp discharge were investigated and optimize the reset pulse and priming pulse. It is noted that the address discharge time lag is significantly improved from 1.2[] to 0.8[] when the slope of the priming ramp pulse is below 0.1[].