Faster response times are available from photodiodes. A photodiode consists of a junction between p-type semiconductor material and n-type semiconductor material. The p type semiconductor material has an excess of holes; the n type material has an excess of electrons. At their junction, the excess holes and electrons interact through recombination, producing ions on each side of the junction. Negative ions are present on the p side of the junction, and positive ions are present on the n side of the junction. This creates an inherent electric field in the junction.