An extrinsic semiconductor, or doped semiconductor, is a semiconductor, that was intentionally doped for the purpose of modulating its electrical, optical and structural properties. In case of semiconductor detectors of ionizing radiation, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of changes in their electrical properties. Therefore, intrinsic semiconductors are also known as pure semiconductors or i-type semiconductors.
The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, since these foreign atoms incorporated into the crystal structure of the semiconductor provide free charge carriers (electrons or electron holes) in the semiconductor. In an extrinsic semiconductor it is these foreign dopant atoms in the crystal lattice that mainly provide the charge carriers which carry electric current through the crystal. In general, there are two types of dopant atoms resulting in two types of extrinsic semiconductors. These dopants that produce the desired controlled changes are classified as either electron acceptors or donors and the corresponding doped semiconductors are known as:
- n-type Semiconductors.
- p-type Semiconductors.
It must be noted, increased doping leads to increased conductivity due to the higher concentration of charge carriers. Highly doped semiconductors behave metallically and these semiconductors are known as degenerate (very highly doped) semiconductors. Degenerate semiconductors are often used in integrated circuits as a replacement for metal. Often superscript plus and minus symbols are used to denote relative doping concentration in semiconductors. For example, n+ denotes an n-type semiconductor with a high doping concentration. Similarly, p− would indicate a very lightly doped p-type material.