Germanium reachthrough avalanche photodiodes for optical communication systems at 1.55-µm wavelength region

Germanium reachthrough avalanche photodiodes (Ge RAPD´s) with high-frequency response have been designed, fabricated, and tested. In the calculation of frequency response, optimum depletion layer width of 21 µm has been found for 1.55-µm wavelength with the highest cutoff frequency of 830 MHz. The diodes fabricated by this design showed frequency degradation of less than 2 dB at 500 MHz and at 1.55 µm. This response has been unchanged up to 1.58 µm, indicating useful spectral limit lies at more than 1.58 µm. The diodes exhibited quantum efficiency of 80 percent and excess noise factor of 6.1 at a multiplication of 10 both for 1.55 µm. The breakdown voltage was 60- 90 v. The sensitivity of the diodes was measured at 100 Mb/s and 1.55 µm. The minimum detectable power of -44.3 dBm which is by 5.2 dB better than the conventional p⁺-n Ge APD has been achieved for 10^-11error rate. Comparison with InGaAs APD and p-i-n/FET receiver has been made by calculating minimum detectable power of RAPD at 500 Mb/s. Calculated sensitivity of RAPD is 1-2 dB worse than InGaAS APD and comparable to that of InGaAs p-i-n/FET receiver estimated from the reported experimental results.