Physics and design of low noise avalanche photodiodes - LEOS distinguished lecture 2003-2004

Summary form only given. Avalanche photodiodes (APDs) are used in many applications when conventional unity gain photodiodes cannot provide enough sensitivity and the extra amplification provided by the impact ionization process gives it an advantage. Unfortunately this amplification or gain of the incoming optical signal is always accompanied by some ´excess noise´ due to the stochastic nature of the ionization process and this sets a limit to the maximum useful gain. Early work by McIntyre showed that the excess noise depended on the ratio of hole ionization coefficient (β) to electron ionization coefficient (α). α and β are semiconductor material dependent and unfortunately most III-V materials have α≈β, giving rise to relatively high excess noise. Since the ionization coefficients depend on the details of the band structure it is extremely difficult to modify, even using band-gap engineering techniques such as superlattices or MQWs. In recent years, work done at the University of Sheffield and the University of Texas (Austin) has shown that low excess noise can be obtained in homojunction structures simply by utilising thin avalanching regions. Experimental results show that contrary to conventional theory, the excess noise actually decreases as the avalanching width reduces. This behaviour has now been observed in virtually all semiconductor materials including GaAs, AlGaAs, InP, AlInAs and even silicon. The reason for this anomalous behaviour in thin devices is due to the ´dead space´ (d), defined as the minimum distance a carrier has to travel before it is in equilibrium with the electric field. Conventional models of the ionization process ignored. This assumption is generally valid in devices with thick avalanching widths in which the dead space distance, d, is relatively small compared to the avalanching width, w. In thin avalanching width structures, d becomes a significant fraction of w and the ionizing process becomes more deterministic, reducing the stochastic variations that give rise to the excess noise. This talk will review these results and show that in addition to reducing the excess noise, thin avalanching widths offer APDs with other advantages such as lower operating vo- ltages, better temperature stability and predicted enhanced speed of operation.