Al0.8Ga0.2As: a very low excess noise multiplication medium for avalanche photodiodes

The maximum useful gain of an avalanche photodiode is limited by the excess noise introduced by the stochastic nature of avalanche multiplication. Low avalanche excess noise can only be achieved in bulk materials with disparate electron and hole ionisation coefficients (α and β). The avalanche excess noise of bulk AlGa_1-x As (x⩽0.6) is relatively large as a result of α/β ratios that are close to unity. On the other hand, there is no information on the excess noise characteristics of bulk Al_xGa _1-xAs with x>0.6. A series of bulk Al_0.8Ga_0.2As p-i-n and n-i-p diodes were grown and characterised to investigate avalanche behaviour. Measurements indicate that bulk Al_0.8Ga_0.2As exhibit very low avalanche excess noise as a result of the significantly larger α/β ratio. This silicon-like characteristic is in stark contrast to that of almost all other III-V semiconductors, including InP. The results suggest that low-noise GaAs-based APDs can be realised using Al_0.8 Ga_0.2As as the multiplication medium