A low-noise n+np germanium avalanche photodiode which successfully operates in the wavelength region of0.8-1.5 mum has been designed, fabricated, and tested. Low-excess noise factorF sim 7is experimentally obtained at a multiplication factor ofM sim 10and at a wavelengthlambda sim 1.3 mum, whereas the calculated one is 7.6. Quantum efficiency is as high as 80 percent and a cutoff frequency (-3 dB) is 500 MHz atM sim 10andlambda sim 1.3 mum. Overall performance of an n+np Ge-APD fabricated is estimated atlambda = 1.3 mum, and compared with that of a III-V alloy APD (InGaAs-InP) by calculating minimum detectable power. An n+np Ge-APD has shown minimum detectable power only 1.7 dB larger than that of a III-V APD at a signal-to-noise ratio of 22 dB and at a bandwidth of 100 MHz. An n+np Ge-APD also shows low-multiplication noise and considerably low minimum detectable power in the 0.8 μm region. Breakdown voltage of an n+np Ge-APD is 31 V and is much lower than that of a Si-APD. The n+np Ge-APD is found to be a useful detector in the whole wavelength region0.8-1.5 mum of optical communication systems.