A top-illuminated circular mesa uni-traveling-carrier photodetector(UTC-PD) is proposed in this paper. By employing Gaussian graded doping in In Ga As absorption layer and In P depleted layer, the responsivity and high speed response characteristics of the device are optimized simultaneously. The responsivity up to 1.071 A/W(the external quantum efficiency of 86%) is obtained at 1550 nm with a 40-μm diameter device under 10-V reverse bias condition. Meanwhile, the dark current of 7.874 n A and the 3-d B bandwidth of 11 GHz are obtained with the same device at a reverse bias voltage of3 V. A top-illuminated circular mesa uni-traveling-carrier photodetector (UTC-PD) is proposed in this paper. By employing Gaussian graded doping in Ga As absorption layer and In P depleted layer, the responsivity and high speed response characteristics of the device are the same as those for the 40-μm diameter device under 10-V reverse bias condition. However, the dark current of 7.874 n A and the 3-dB bandwidth of 11 GHz are obtained with the same device at a reverse bias voltage of 3 V. FIG.