The figure shows two of the many possible structures used for p–n semiconductor diodes, both adapted to increase the voltage the devices can withstand in reverse bias.

The top structure uses a mesa to avoid a sharp curvature of the p+-region next to the adjoining n-layer.

The bottom structure uses a lightly doped p-guard-ring at the edge of the sharp corner of the p+-layer to spread the voltage out over a larger distance and reduce the electric field.

The ideal diode has zero resistance for the forward bias polarity, and infinite resistance (conducts zero current) for the reverse voltage polarity; if connected in an alternating current circuit, the semiconductor diode acts as an electrical rectifier.

As shown in the figure, the on or off resistances are the reciprocal slopes of the current-voltage characteristic at a selected bias point: where

By "abrupt", it is meant that the p- and n-type doping exhibit a step function discontinuity at the plane where they encounter each other.

The objective is to explain the various bias regimes in the figure displaying current-voltage characteristics.

This level ensures that in the field-free bulk on both sides of the junction the hole and electron occupancies are correct.

(So, for example, it is not necessary for an electron to leave the n-side and travel to the p-side through the short circuit to adjust the occupancies.)

Similarly, hole density on the n-side is a Boltzmann factor smaller than on the p-side.

The width of the depletion region adjusts so the negative acceptor charge on the p-side exactly balances the positive donor charge on the n-side, so there is no electric field outside the depletion region on either side.

As shown in the diagram, the step in band edges is reduced by the applied voltage to

(The band bending diagram is made in units of volts, so no electron charge appears to convert

reduces the step in band edges and increases minority carrier densities by a Boltzmann factor

Within the junction, the pn-product is increased above the equilibrium value to:[1] The gradient driving the diffusion is then the difference between the large excess minority carrier densities at the barrier and the low densities in the bulk, and that gradient drives diffusion of minority carriers from the interface into the bulk.

In the light-emitting diode, recombination of electrons and holes is accompanied by emission of light of a wavelength related to the energy gap between valence and conduction bands, so the diode converts a portion of the forward current into light.

Under forward bias, the half-occupancy lines for holes and electrons cannot remain flat throughout the device as they are when in equilibrium, but become quasi-Fermi levels that vary with position.

The reduced step in band edges also means that under forward bias the depletion region narrows as holes are pushed into it from the p-side and electrons from the n-side.

In the simple p–n diode the forward current increases exponentially with forward bias voltage due to the exponential increase in carrier densities, so there is always some current at even very small values of applied voltage.

However, if one is interested in some particular current level, it will require a "knee" voltage before that current level is reached (~0.7 V for silicon diodes, others listed at Diode § Forward threshold voltage for various semiconductors).

Some special diodes, such as some varactors, are designed deliberately to maintain a low current level up to some knee voltage in the forward direction.

so the two bulk occupancy levels are separated again by an energy determined by the applied voltage.

As shown in the diagram, this behavior means the step in band edges is increased to

and the depletion region widens as holes are pulled away from it on the p-side and electrons on the n-side.

When the reverse bias becomes very large, reaching the breakdown voltage, the generation process in the depletion region accelerates leading to an avalanche condition which can cause runaway and destroy the diode.

The DC current-voltage behavior of the ideal p–n diode is governed by the Shockley diode equation:[3] where This equation does not model the non-ideal behavior such as excess reverse leakage or breakdown phenomena.

Note: to refer to differential or time-varying diode current and voltage, lowercase

the depletion width (thickness of the region where mobile carrier density is negligible).

In forward bias, besides the above depletion-layer capacitance, minority carrier charge injection and diffusion occurs.

The diode is a highly non-linear device, but for small-signal variations its response can be analyzed using a small-signal circuit based upon a selected quiescent DC bias point (or Q-point) about which the signal is imagined to vary.