Comparison of Charge Collection in a Si Avalanche Photodiode for Alpha-Particle and Photon Irradiation

Abstract

Silicon avalanche photodiodes (Si APDs) are semiconductor p-n junction devices that convert photons into amplified electrical current by enhancing generated charge via avalanche. They are intrinsically immune to magnetic fields and achieve an internal avalanche gain greater than 50 at low noise levels, which is a function of the APD’s doping profile. This profile is created using spreading resistance measurements. These features give APDs a size, weight, and power advantage over non-avalanche photodiodes. This is partially because APDs do not need amplifiers to observe pulses. An APD (a Hamamatsu Photonics device with type number S8664-1010) is irradiated with ~5.5 MeV alpha particles from a 10 μCi Am241. The APD’s charge collection as a function of bias is found to differ from that from photons. This study looks at this difference by comparing the Si APD’s charge collection mechanisms. Charge enhancement for the alpha particles is found to be reduced because the field where most of the charge is generated is not capable of providing significant generated charge via avalanche. Meanwhile, the field can generate a larger number of carriers sufficient for avalanche for a continuous photon source. Temporal characteristics (e.g., rise time) is used to compare the two sources’ responses.