| An avalanche photodiode (APD) is frequently the photodetector of choice in high-bit-rate, long-haul fiber optic communication systems due to its higher sensitivity, relative to a PIN photodiode, afforded by its internal gain. However, this can only be accomplished given that the multiplication noise is low. Impact-ionization-engineering (I2E) is a novel approach that incorporates materials with different impact ionization threshold energies (Eth) into the multiplication region of APDs for low noise, high gain, and low dark current. A series of multiplication region structures with record-low multiplication noise were developed on both GaAs and InP substrates; an excess noise level comparable to silicon APDs was achieved on I 2E structures grown on GaAs. Unlike "superlattice" or "staircase" structures, the band gap continuities are not involved in the working mechanism of I2E APDs. Monte Carlo simulation has revealed the spatial modulation effect of the impact ionization events in these heterostructure devices, which makes the ionization process more deterministic than in homojunctions, thus yielding lower noise. These low-noise I2E multiplication region structures are promising in improving APD performance once they are implemented into SACM structures, with working wavelengths including 800--900nm, 1.3mum, and 1.55mum. |
