N or P doping

to improve the capacity of a semiconductor such as silicon to either give up electrons or absorb them, impurities are added. This is called doping. The semiconductor is doped with phosphorus or arsenic to become an electron donor and with boron or gallium to become an electron receiver. The junction of an n-doped region with a p-doped region creates a diode. The electric current that crosses the junction can be increased by adding light energy. This is the principle of the photodiode.