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.