Atlanta Fiber System Experiment: Planar Epitaxial Silicon Avalanche Photodiode

Solid-state photodetectors are particularly well suited to optical communications. The detectors, fabricated from semiconducting materials, are small, fast, highly sensitive and relatively inexpensive. 1 Two widely employed detectors are the PIN photodiode, which in reverse bias collects the photogenerated minority carriers, and the avalanche photodiode, which has a high field region t h a t multiplies the photocurrent through the avalanche generation of additional electron-hole pairs. The avalanche photodiode (APD) designed specifically for the FT3 optical communications system 2 will be described in this paper. At a bit 1791 rate of 44.7 Mb/s, the APD allows an increase in system sensitivity by approximately 15 dB over t h e same receiver with a PIN detector. 3 T h i s improvement is possible because t h e shot noise in the photocurrent of the PIN diode is much smaller t h a n t h e equivalent input noise of a receiver amplifier having sufficient bandwidth to carry the 45 Mb/s signal. 4 T h e gain in the APD increases the signal intensity and system sensitivity until the noise in the multiplied current is comparable to the noise of the amplifier. T h e avalanche photodiode is not, however, an ideal multiplier. It adds a substantial a m o u n t of noise because of fluctuations in the avalanche gain. T h e excess noise depends on t h e device structure and the electron and hole ionization rates of the material chosen for the detector. 5 T h e avalanche photodiode for the FT3 application was designed to minimize excess noise and maximize the q u a n t u m efficiency without compromising manufacturability or reliability.