Semiconductor diodes can be defined as diodes that are made up of semiconducting materials (typically, the metals silicon and germanium are used in them). In these diodes, the negatively charged cathode contains a large number of electrons, which is placed adjacent to the anode, which carries an excess of positively charged holes. Common examples of semiconductor diodes include the p-n junction diode and the Zener diode. The junction between the cathode and the anode in a semiconductor diode features the formation of a depletion region, which does not contain electrons or holes. When positive voltages exist at the anode, the depletion region becomes extremely small and the current begins flowing through the semiconductor diode.