Avalanche Noise Characteristics of Single Al Ga As -GaAs Heterojunction APDs

Abstract— Avalanche multiplication and excess noisehave been measured on a series of Al Ga 1 As–GaAs andGaAs–Al Ga 1 As ( =03 0 45 , and 0 6 ) single hetero-junction p + -i-n + diodes. In some devices excess noise is lowerthan in equivalent homojunction devices with avalanche regionscomposed of either of the constituent materials, the heterojunctionwith =0 3 showing the greatest improvement. Excess noisedeteriorates with higher values of because of the associatedincrease in hole ionization in the Al Ga 1 As layer. It alsodepends critically upon the carrier injection conditions and MonteCarlo simulations show that this dependence results from thevariation in the degree of noisy feedback processes on the positionof the injected carriers. Index Terms— Avalanche photodiodes (APDs), heterojunctions,impact ionization, noise. I. I NTRODUCTION A VALANCHE photodiodes (APDs) are key componentsof optical datacommunications systems because of theirhigh sensitivity which results from their internal avalanche gain.However,thisinternalgainisprovidedviathestochasticprocessof impact ionization and therefore contributes its own excessnoise.It is well known that excess avalanche noise can be reducedbelow the level predicted by the local model of McIntyre [1] byreducing the avalanche region width . This is because the deadspace, the distance travelled by carriers before their ionizationcoefficient reaches equilibrium with the electric field, becomesa larger fraction of their mean ionization path lengths. The as-sociated reduced uncertainty in ionization position reduces thefluctuations in multiplication and hence in excess noise.The use of heterojunctions in avalanche regions as a meansof reducing excess noise was first proposed by Chin