We present the enhanced performances of a vertical-illumination-type Ge-on-Si avalanche photodetector based on internal RF-gain effects operating up to 50 Gb/s. A fabricated Ge-on-Si avalanche photodetector (APD) exhibits three operational voltage regions associated with different aspects of the current (DC) gain and bandwidth characteristics. The measured current-voltage (I-V) curve of a Ge-on-Si APD exhibits a negative photoconductance (negative differential resistance [NDR]) in a high bias region beyond the avalanche breakdown voltage (V br ), where a device shows good eye openings up to 50 Gb/s (non-return-to-zero [NRZ] signal) with further improved signal-to-noise ratios and signal amplitudes. A ROSA packaged module, wherein a fabricated Ge-on-Si APD is wire-bonded to a commercial TIA with a ∼75% optical alignment for λ∼1310 nm and biased at a lower voltage than the V br , exhibits the sensitivities of -18.9 and -15.3 dBm for 30 and 35 Gb/s, respectively, and -13.9 dBm for 40 Gb/s at a 10-12 bit error rate. The experimental results indicate that considerable improvement in a module performance can be expected by utilizing the Ge-on-Si APD operated in the NDR region with a properly customized TIA.