- Extrinsic Semiconductor Definition: An extrinsic semiconductor is defined as a semiconductor with added impurities to increase its conductivity.
- Doping Process: Doping involves adding impurities to a pure semiconductor to create more free charge carriers.
- n-Type Semiconductors: n-Type semiconductors are created by adding Group V elements, which donate extra electrons.
- p-Type Semiconductors: p-Type semiconductors are formed by adding Group III elements, which create holes by accepting electrons.
- Conductivity in Extrinsic Semiconductors: The added impurities in extrinsic semiconductors increase their conductivity by creating more free electrons or holes.
An extrinsic semiconductor is a pure crystal that has been doped. Intrinsic Semiconductors have no added atoms. Extrinsic semiconductors do. Doping adds donors or acceptors so more electrons or holes are free to conduct.
Silicon and germanium each have four valence electrons. To add free electrons they are doped with Group V atoms such as phosphorus (P) or arsenic (As). Those pentavalent atoms form four bonds and leave one electron spare.
Those pentavalent atoms are donors. The spare electron is only loosely bound to its parent atom and a little heat frees it. It then leaves its current donor level ED (green line in Figure 2a) for the conduction band. It joins the electrons already freed by the breaking of host bonds. Total electrons equal thermal electrons plus donor electrons.
Holes in that same crystal still equal only the thermal pairs. Electrons therefore outnumber holes, so electrons are majority carriers and holes are minority carriers. The material is an n type semiconductors.
The same Group IV hosts can be doped with Group III atoms such as boron (B) or aluminium (Al). Each trivalent atom forms three bonds and leaves one bond empty. That hole can accept an electron and the crystal conducts by hole motion.
Each added acceptor atom adds one hole at energy EA. Those holes take electrons easily, which is why Group III atoms are called acceptors.
When an electron fills a hole it leaves a new hole behind, so hole motion is opposite to electron motion. Total holes equal acceptor holes plus thermal holes. Electrons come only from thermal pairs, so holes outnumber electrons. Holes are majority carriers and electrons are minority carriers. That extrinsic semiconductor is a p type semiconductors.
