Impact ionization coefficients and excess noise in Al0.55Ga0.45As0.56Sb0.44 lattice matched to InP

The avalanche multiplication and noise characteristics of Al0.55Ga0.45As0.56Sb0.44 p–i–n and n–i–p structures grown lattice matched on InP have been investigated. From measurements undertaken using 530 nm illumination on several devices, the electron (α) and hole (β) impact ionization coefficients have been determined. While α only shows a relatively small increase compared to the higher Al composition alloys of AlxGa1−xAsSb, β is found to increase significantly. Although the β/α ratio is increased to ∼0.125–0.2, higher than the ∼0.003–0.02 seen in the higher-Al alloys, a relatively low excess noise factor of 2.2 was measured in the p-i-n with electron-initiated multiplication of 20. This noise performance is significantly lower than that predicted using a local-field model and comparable to some commercial silicon APDs. This avalanching material with a bandgap of ∼1.24 eV will have the advantages of a smaller band discontinuity with the absorber region and should also operate at a lower voltage.