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Abstract-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.