Semiconductor Physics
Semiconductors are materials whose modest band gap lets their conductivity be tuned by temperature, doping, and applied fields, making them the physical foundation of modern electronics.
Definition
Semiconductor physics is the application of electronic band theory to materials with a band gap small enough that thermal excitation and doping populate the conduction band and valence band with mobile electrons and holes, whose concentrations and motion can be controlled to build electronic devices.
Scope
This area covers the physics of semiconducting solids: intrinsic and extrinsic (doped) behavior, electron and hole carrier statistics, the position of the Fermi level, the formation of p-n junctions and the band bending at interfaces, and the optical absorption and transport properties that govern devices. It applies band theory to materials with a small gap and connects the microscopic electronic structure to the operation of diodes, transistors, and optoelectronic devices, while leaving device engineering details to applied fields.
Sub-topics
Core questions
- How does the small band gap of a semiconductor make its carrier concentration sensitive to temperature and doping?
- What is the role of holes, and how do donor and acceptor impurities create n-type and p-type material?
- How does a p-n junction rectify current through band bending and a built-in potential?
- What determines the optical absorption and carrier mobility that govern semiconductor devices?
Key concepts
- Band gap, conduction band, and valence band
- Electrons and holes as charge carriers
- Donor and acceptor doping (n-type and p-type)
- Fermi level and carrier statistics
- p-n junction, built-in potential, and rectification
Key theories
- Carrier statistics and the law of mass action
- The equilibrium electron and hole concentrations follow from the density of states and Fermi-Dirac statistics; their product is fixed at a given temperature, so doping that raises one carrier suppresses the other.
- p-n junction rectification
- Joining p-type and n-type material aligns the Fermi level, bending the bands and creating a depletion region with a built-in field that allows current to flow easily in one direction only, the basis of the diode.
Clinical relevance
Semiconductor physics is the foundation of the entire electronics and information-technology industry: diodes, transistors, integrated circuits, solar cells, light-emitting diodes, lasers, and photodetectors all rest on the carrier and junction physics developed here.
History
The quantum theory of bands explained semiconducting behavior in the 1930s, and the invention of the point-contact and junction transistors by Bardeen, Brattain, and Shockley at Bell Labs in 1947-1948 turned semiconductor physics into the basis of modern electronics and the subsequent microelectronics revolution.
Key figures
- William Shockley
- John Bardeen
- Walter Brattain
Related topics
Seminal works
- sze2007
- ashcroft1976
Frequently asked questions
- What is a hole?
- A hole is the absence of an electron in an otherwise filled valence band; it behaves like a positively charged mobile carrier, and tracking holes is far simpler than tracking the many electrons that move to fill them.
- Why does adding tiny amounts of impurity change conductivity so dramatically?
- Donor or acceptor atoms introduce energy levels just inside the gap that are easily ionized at room temperature, so even parts-per-million doping can change the free-carrier concentration by many orders of magnitude.