Thin-All films have been utilized to boost the quantum efficiency of SiGe/Si multi-quantum well avalanche photodiodes (MQW-A-PDs), which are fabricated by using ultrahigh-vacuum chemical vapor deposition system. In this structure, in addition to the conventional absorption layer of MQW, an undoped strained-Si0.8Ge0.2 layer is inserted to enhance the absorption of MQW-APDs. It is found that the MQW-APDs with a thin-Al coating have a better performance than those with thin-Al-coating ones. In the fabricated APDs, an avalanche multiplication occurred at about 2 V reverse-bias voltages. At unmultiplied voltage of 1 V, the responsivity is only 0.41 A/W. When bias is increased to 4 V, a significant avalanche multiplication is achieved and the responsivity is as high as 3.5 A/W for the APDs with thin-Al coating. As compared to the APD without thin-At coating, the one with thin-At coating has a higher responsivity and quantum efficiency by a magnitude of 2.1 under a 4-V reverse-bias voltage for 850-nm light source. (c) 2007 Elsevier B.V. All rights reserved.