p–n junction

A p–n junction is a combination of two types of semiconductor materials, p-type and n-type, in a single crystal.

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.

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).

The invention of the p–n junction is usually attributed to American physicist Russell Ohl of Bell Laboratories in 1939.

[1] Two years later (1941), Vadim Lashkaryov reported discovery of p–n junctions in Cu2O and silver sulphide photocells and selenium rectifiers.

[2] The modern theory of p-n junctions was elucidated by William Shockley in his classic work Electrons and Holes in Semiconductors (1950).

[3] A p-doped semiconductor (that is, one where impurities such as Boron are introduced into its crystal lattice) is relatively conductive.

Bias is the application of a voltage relative to a p–n junction region: 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.

At the junction, some of the free electrons in the n-type wander into the p-type due to random thermal migration ("diffusion").

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.

The negatively charged ("acceptor") dopant atoms in the p-type are part of the crystal, and cannot move.

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.

When equilibrium is reached, the charge density is approximated by the displayed step function.

The panels show energy band diagram, electric field, and net charge density.

The built-in potential of the semiconductor varies, depending on the concentration of doping atoms.

Only majority carriers (electrons in n-type material or holes in p-type) can flow through a semiconductor for a macroscopic length.

With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the p-type material.

However, they do not continue to flow through the p-type material indefinitely, because it is energetically favorable for them to recombine with holes.

[5] Although the electrons penetrate only a short distance into the p-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction.

The Shockley diode equation models the forward-bias operational characteristics of a p–n junction outside the avalanche (reverse-biased conducting) region.

Likewise, because the n-type region is connected to the positive terminal, the electrons are pulled away from the junction, with similar effect.

This increases the voltage barrier causing a high resistance to the flow of charge carriers, thus allowing minimal electric current to cross the p–n junction.

Once the electric field intensity increases beyond a critical level, the p–n junction depletion zone breaks down and current begins to flow, usually by either the Zener or the avalanche breakdown processes.

[6] The Shockley ideal diode equation characterizes the current across a p–n junction as a function of external voltage and ambient conditions (temperature, choice of semiconductor, etc.).

The convention is that the forward (+) direction be pointed against the diode's built-in potential gradient at equilibrium.

A p–n junction diode . The circuit symbol is also shown.
Silicon atoms (Si) enlarged about 45,000,000x (Image size approximately 955 pm × 955 pm)
Figure A. A p–n junction in thermal equilibrium with zero-bias voltage applied. Electron and hole concentration are reported with blue and red lines, respectively. Gray regions are charge-neutral. Light-red zone is positively charged. Light-blue zone is negatively charged. The electric field is shown on the bottom, the electrostatic force on electrons and holes and the direction in which the diffusion tends to move electrons and holes. (The log concentration curves should actually be smoother with slope varying with field strength.)
Figure B. A p–n junction in thermal equilibrium with zero-bias voltage applied. Under the junction, plots for the charge density, the electric field, and the voltage are reported. (The log concentration curves should actually be smoother, like the voltage.)
PN junction operation in forward-bias mode, showing reducing depletion width.
A silicon p–n junction in reverse bias