Elimination of the guard ring in uniform avalanche photodiodes

The need for a guard ring around the active area of an avalanche diode can be eliminated by the use of a p-n+ (n-p+) planar junction, as opposed to the more conventional p+ -n (n+ -p) planar junction. Breakdown does not occur at the junction corners since the positive curvature of the junction is on the high resistivity side. The p+-n (n-p+) structure significantly reduces both the capacitance and the resistance of small area photodiodes and thereby increases both the available output power and the RC cutoff frequency. A conventional planar avalanche diode with an active area of 10^-4cm²has a total area of cm2because of the guard ring, and a series resistance of ohms. Because of the excess capacitance and resistance multiplications of greater than 10 are necessary in order just to equal the available output power of a conventional nonavalanching p-i-n photodiode. The p-n+ (n-p+) diodes can be designed to have resistances ( ohms), capacitances (C < 1 pf), and RC cutoff frequencies ( GHz) equivalent to those of the p-i-n, and to have uniform multiplication as well.