The surface of a CCD is packed with photodiodes, each of which senses light and accumulates electrical charge in accordance with the strength of light that strikes it. Let's take a look at what happens in each photodiode. Photodiodes are, in fact, semiconductors, the most basic of which is a pn pair made up of a p-type and an n-type semiconductor. If a plus electrode (anode) is attached to the p-type side, and a minus electrode (cathode) is attached to the n-type side of the pn pair, and electric current is then passed through this circuit, current flows through the semiconductor. This is known as forward bias. If you create a reversed circuit by attaching a plus electrode to the n-type side, and a minus electrode to the p-type side of the pn pair, then electrical current will be unable to flow. This is known as reverse bias. Photodiodes possess this reverse bias structure. The main difference from standard semiconductors is the way in which they accumulate electrical charge in direct proportion to the amount of light that strikes them.