The contacts of metal Al with p-Ge,n-Ge and n-Si are comparatively studied ba sed on circular transmission line model by analyzing the specific contact resist ance and current-voltage curve.The results show that large contact resistance between metal and n-Ge due to Fermi level pining effect at the interface strongly makes it difficult to ge t low specific contact resistance.When the concentration of p-Ge is 4.2×10¹⁸ cm－3,the specific contact resistance is 4.0×10－7 Ω·cm^2after annealing.Compared with Al/n-Ge,the Al/n- Si can obtain good ohmic contact,and the specific contact resistance of Al/n-Si reduced by one order of magnitude.The specific contact resistivity of the fab ricated Al/n⁺Si can reach 5.21×10－5 Ω·cm^2after annealing.These parameters are helpful to achieve high performance Ge photodetectors.