Semiconductor Basics

A semiconductor is a material whose ability to conduct electricity lies between that of a conductor, like copper, and an insulator, like glass. Silicon is the most common example. It has a small energy band gap of about 1.1 electron volts, so at room temperature only a few electrons can move and it conducts weakly. What makes semiconductors so useful is that their conductivity can be precisely controlled by adding impurities, called doping, and by applying voltage, light or heat, which is the basis of every diode, transistor and integrated circuit.

Silicon is used because it is abundant and cheap (it comes from sand), it has four valence electrons that form a stable covalent crystal, it has a band gap of about 1.1 electron volts that suits room-temperature electronics, and it grows a high-quality natural oxide, silicon dioxide, that is essential for making transistors and integrated circuits. Germanium was used in early devices but silicon works over a wider temperature range and is far easier to manufacture, so it dominates the industry.

A conductor has overlapping valence and conduction bands, so electrons move freely and resistivity is very low. An insulator has a large energy band gap of more than about five electron volts, so almost no electrons can cross it and resistivity is very high. A semiconductor has a small band gap of around one electron volt, so it conducts weakly on its own but its conductivity can be increased dramatically by heat, light or doping, placing it between the two.

The energy band gap, or forbidden gap Eg, is the energy difference between the top of the valence band, where electrons are bound in covalent bonds, and the bottom of the conduction band, where electrons are free to move and carry current. An electron must gain at least this much energy to jump the gap and conduct. Silicon has a band gap of about 1.1 electron volts and germanium about 0.7 electron volts, while insulators have gaps larger than five electron volts.

An intrinsic semiconductor is a pure semiconductor with no added impurities, such as pure silicon. At absolute zero it behaves like an insulator, but at room temperature thermal energy breaks a few covalent bonds and creates electron-hole pairs. In an intrinsic semiconductor the number of free electrons always equals the number of holes, and its conductivity is low but rises as temperature increases.

Doping is the deliberate addition of a tiny, controlled amount of impurity atoms to a pure semiconductor to increase and control its conductivity. Adding a pentavalent impurity such as phosphorus, which has five valence electrons, donates extra free electrons and makes N-type material. Adding a trivalent impurity such as boron, which has three valence electrons, creates extra holes and makes P-type material. Even a few impurity atoms per million silicon atoms change the conductivity enormously, while the crystal stays electrically neutral.

An N-type semiconductor is doped with a pentavalent donor impurity such as phosphorus or arsenic, so it has extra free electrons; electrons are the majority carriers and holes are the minority carriers. A P-type semiconductor is doped with a trivalent acceptor impurity such as boron, so it has extra holes; holes are the majority carriers and electrons are the minority carriers. Both types are electrically neutral overall, and joining them forms a P-N junction.

Charge carriers are the moving particles that carry current in a semiconductor: negatively charged electrons and positively charged holes. Majority carriers are the more numerous type set by doping: electrons in N-type material and holes in P-type material. Minority carriers are the less numerous type: holes in N-type material and electrons in P-type material. The behaviour of diodes and transistors depends on both.

A hole is the empty position left in a covalent bond when an electron leaves it. Because a neighbouring electron can move in to fill the hole, the hole appears to move in the opposite direction to the electron, so it behaves like a positive charge carrier. Holes and electrons together carry current in a semiconductor; in P-type material holes are the majority carriers.

A P-N junction is the boundary formed when P-type and N-type semiconductors are joined in a single crystal. Near the junction, free electrons from the N side and holes from the P side diffuse across and recombine, leaving fixed positive donor ions on the N side and fixed negative acceptor ions on the P side. This creates a depletion region that has no mobile carriers and a built-in potential barrier and electric field that oppose further diffusion. The P-N junction is the basis of the diode, transistor and solar cell.

Raising the temperature gives more electrons enough energy to break covalent bonds and jump the band gap, creating more electron-hole pairs. So unlike a metal, a semiconductor's resistance decreases as temperature rises, meaning it has a negative temperature coefficient of resistance. This is why semiconductor devices must manage heat, since excessive temperature can lead to more current, more heating and thermal runaway.

An intrinsic semiconductor is pure, with equal numbers of electrons and holes and low conductivity that depends only on temperature. An extrinsic semiconductor has been doped with impurities to become N-type or P-type, giving it many more of one type of carrier and much higher, controllable conductivity. Practically all real semiconductor devices use extrinsic material because doping lets engineers design their exact electrical behaviour.

Semiconductors are the foundation of all modern electronics. The P-N junction and doped material build diodes and rectifiers, bipolar and MOSFET transistors, integrated circuits and microprocessors, memory chips, light-emitting diodes and lasers, photodiodes and solar cells, and power devices such as IGBTs and thyristors. Materials beyond silicon, such as gallium arsenide, silicon carbide and gallium nitride, are used for high-frequency, high-power and optical applications.