A p–n junction is a combination of two types of semiconductor materials, p-type and n-type, in a single crystal. The "n" (negative) side contains freely-moving electrons, while the "p" (positive) side contains freely-moving electron holes. Connecting the two materials causes creation of a depletion region near the boundary, as the free electrons fill the available holes, which in turn allows electric current to pass through the junction only in one direction. p–n junctions represent the simplest case of a semiconductor electronic device; a p–n junction by itself, when placed in a circuit, is a diode. More complex devices can be created by further combinations of p-type and n-type semiconductors. Transistors such as the BJT, MOSFET, JFET, and IGBT all utilize p–n junctions for operation. Combinations of such semiconductor devices on a single chip allow for the creation of integrated circuits. Solar cells and light-emitting diodes (LEDs) are essentially p–n junctions where the semiconductor materials are chosen, and the component's geometry designed, to maximise the desired effect (light absorption or emission). A Schottky junction is a similar case to a p–n junction, where instead of an n-type semiconductor, a metal directly serves the role of the "negative" charge provider. The invention of the p–n junction is usually attributed to American physicist Russell Ohl of Bell Laboratories in 1939. Two years later (1941), Vadim Lashkaryov reported discovery of p–n junctions in Cu2O and silver sulphide photocells and selenium rectifiers. The modern theory of p–n junctions was elucidated by William Shockley in his classic work Electrons and Holes in Semiconductors (1950).
Properties
A p-doped semiconductor (that is, one where impurities such as Boron are introduced into its crystal lattice) is relatively conductive. The same is true of an n-doped semiconductor, but the junction between them—the boundary where the p-doped and n-doped semiconductor materials meet—can become depleted of charge carriers such as electrons, depending on the relative voltages of the two semiconductor regions. By manipulating the flow of charge carriers across this depleted layer, p–n junctions can be used as diodes: circuit elements that allow a flow of electricity in one direction but not in the opposite direction. This property makes the p–n junction extremely useful in modern semiconductor electronics. Bias is the application of a voltage relative to a p–n junction region:
Forward bias is in the direction in which current readily flows Reverse bias is in the direction of little or no current flow Negative charge carriers (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for positive charge carriers (Electron hole). When the p–n junction is forward-biased, charge carriers flow freely due to the reduction in energy barriers seen by electrons and holes. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal.
Equilibrium (zero bias) In a p–n junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference forms across the junction. This potential difference is called built-in potential V b i {\displaystyle V_{\rm {bi}}} . At the junction, some of the free electrons in the n-type wander into the p-type due to random thermal migration ("diffusion"). As they diffuse into the p-type they combine with electron holes, and cancel each other out. In a similar way, some of the positive holes in the p-type diffuse into the n-type and combine with free electrons and cancel each other out. The positively charged ("donor") dopant atoms in the n-type are part of the crystal, and cannot move. Thus, in the n-type, a region near the junction has a fixed amount of positive charge. The negatively charged ("acceptor") dopant atoms in the p-type are part of the crystal, and cannot move. Thus, in the p-type, a region near the junction becomes negatively charged. The result is a region near the junction that acts to repel the mobile charges away from the junction because of the electric field that these charged regions create. The region near the p–n interface loses electrical neutrality and most of its mobile carriers, forming the depletion layer (see figure A). The electric field created in the space charge then tends to counteract further diffusion, resulting in equilibrium.
The carrier concentration profile at equilibrium is shown in figure A with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium.
The space charge region is a zone with a net charge provided by the fixed ions (donors or acceptors) that have been left uncovered by majority carrier diffusion. When equilibrium is reached, the charge density is approximated by the displayed step function. In fact, since the y-axis of figure A is log-scale, the region is almost completely depleted of majority carriers (leaving a charge density equal to the net doping level), and the edge between the space charge region and the neutral region is quite sharp (see figure B, Q(x) graph). The space charge region has the same magnitude of charge on both sides of the p–n interfaces, thus it extends farther on the less doped side in this example (the n side in figures A and B).
Forward bias
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