4.7.1. P-i-N photodiodes P-i-N photodiodes are commonly used in a variety of applications. A typical P-i-N photodiode is shown in Figure 4.7.1. It consists of a highly-doped transparent p-type contact layer on top of an undoped absorbing layer and an n-type highly doped contact layer on the bottom. Discrete photodiodes are fabricated on a conductive substrate as shown in the figure, which facilitates the formation of the n-type contact and reduces the number of process steps. The top contact is typically a metal ring contact, which has a low contact resistance and still allows the light to be absorbed in the semiconductor. An alternative approach uses a transparent conductor such as Indium Tin Oxide (ITO). The active device area is formed by mesa etching or by proton implantation of the adjacent area, which makes it isolating. A dielectric layer is added around the active area to reduce leakage currents and to ensure a low parasitic capacitance of the contact pad.