Alternatively, a metal-semiconductor junction (Schottky barrier) can be used but is less common. Since an avalanche photodiode is operate under a large reverse bias, it is critical to incorporate a guard ring at the perimeter of the junction to prevent surface breakdown mechanisms. Common material are Si,Ge and III-V compound semiconductors. The majority of commercial avalanche photodiodes are ,]made of Si.As discussed later, for minimum noise, a large difference in ionization coefficients for electron (αn )and hole (αp ) is critical. Silicon has a large αn/αp ratio of 50,while Ge and III-V compounds have ratios less than ~ 2. Despite higher noise, the latter material are used for longer wavelengths that require smaller energy gap.