# Silicon Photomultiplier Structure and Properties

## The P-N Junction

The Silicon Photomultiplier (SiPM) is essentially an array of silicon avalanche p-n photodiodes operating in Geiger-mode. It is necessary to review some basics of the p-n junction to understand how the SiPM operates.
A p-n junction is the fusion of a p-type and n-type semiconductor together. A P-type region contains a majority of hole (positive) carriers and few electrons. A N-type region contains a majority of mobile electrons and few holes. Properties exist that can be exploited for the use of the SiPM when the two regions are brought into contact, diffusion of carrier concentration being one of those properties. Holes from the p-type region diffuse to the n-type region and electrons from n-type region diffuse to the p-type region leaving behind negatively charged ionized acceptor atoms and positively charges ionized donor atoms respectively. As a result of this diffusion, a narrow region on both sides of the p-n junction become nearly depleted of mobile charge carriers, called the depletion layer $$[1]$$. The depletion layer contains only fixed charges of negative ions in the p-region and positively charged ions in the n-region. These fixed charges created an internal electric field that points from n-side to p-side as seen in figure 1. There then exists a built in voltage at the interface preventing electrons moving into p-side and holes into n-side. The built in voltage goes as $$V_{bi}=\frac{kT}{q}ln(\frac{N_{D}N_{A}}{n_{i}^{2}})$$, where $$N_{D}$$ are donor concentrations, $$N_{A}$$ are acceptor concentrations, and $$n_{i}$$ is the intrinsic carrier concentration $$[3]$$. No net current will flow across the junction while unbiased.