Non-linear charge collection mechanisms in high-speed communication avalanche photodiodes

High-speed avalanche photodiodes used in harsh radiation environments such as space or close to HEP experiments suffer from background Single Event Transients due to the generation of high-energy heavy ion secondary recoils and nuclear reactions. These transients degrade the Bit Error Rate of an optical receiver introducing spurious noise. For small high-speed devices, the electron-hole pair density introduced by an MeV ion well exceeds background doping levels in the top active layers leading to the possibility of an anomalous like gain mechanism due to the internal dipolar field generated by the high-injection plasma. In this paper, we examine this possibility and its spatial dependence using a high-energy focussed ion microbeam and the Transient Ion Beam Induced Current technique to measure the Single Event Transient data collected on an InP InGaAs APD device using 6 MeV N and 7 MeV O ions.