How a Semiconductor Junction Works

A diode does not contain a tiny mechanical one-way valve. Its behavior comes from mobile charge carriers, fixed ionized dopants and the electric field that develops where differently doped semiconductor regions meet. The useful circuit rule—forward current rises strongly while reverse current is usually small—emerges from that microscopic structure.

Doping creates controlled majority carriers

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Diagram description

Two simplified silicon bond diagrams compare a phosphorus donor and a boron acceptor. Mobile negative and positive carrier symbols are paired with the corresponding fixed ionized dopant charge.

A hole is a useful description of an unoccupied valence-band state. Neighboring valence electrons can move into that state, so the vacancy behaves as a positive mobile carrier moving in the opposite direction. It is not a free positive particle inserted between silicon atoms. Likewise, an electron's microscopic motion and conventional current direction are opposite descriptions of the same charge transport.

Joining P and N regions creates a depletion-region field

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Diagram description

A horizontal N-to-P junction contains a shaded depletion region with positive fixed ions on the N side and negative fixed ions on the P side. Arrows show diffusion, drift and the built-in field.

Forward bias lowers the barrier and injects carriers

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Diagram description

Two simplified P-to-N bars compare a wider yellow depletion zone at zero external bias with a narrower zone when P is connected positive and N negative.

Applying the positive terminal to P and the negative terminal to N reduces the electrostatic barrier from its equilibrium value. Majority carriers reach the junction more readily and are injected as minority carriers into the opposite quasineutral region. They diffuse, recombine and produce a terminal current that grows strongly with applied voltage.

Ideal junction-diode model

I = I_S[e^(V_D/(nV_T)) − 1]; V_T = kT/q

This compact model captures exponential forward behavior over a useful region. I_S depends strongly on device area, semiconductor, doping and temperature; n is an empirical ideality factor. Series resistance, high-level injection, leakage, generation-recombination, self-heating and breakdown require additional models.

I

junction current under the selected sign convention, A

I_S

reverse saturation-current model parameter, A

V_D

junction voltage, positive for forward bias, V

n

ideality factor in the selected operating region

V_T

thermal voltage, about 25.9 mV at 300 K

k

Boltzmann constant

T

absolute junction temperature, K

q

magnitude of electron charge, C

Example

The equation explains why a small change in forward voltage can produce a large current ratio. It does not establish a universal 0.7 V turn-on threshold.

Translate the material picture into a circuit model carefully

Keep four different directions straight

Quantity	Typical direction in the article's N-to-P drawing
Built-in electric field	From positive donor ions on N side toward negative acceptor ions on P side
Electron diffusion near formation	From high electron concentration in N toward P
Electron drift from built-in field	Opposite the electric field, from P toward N
Conventional forward current	From P/anode toward N/cathode through the external circuit convention
Electron flow in external conductors	Opposite conventional current

The distinction between conventional current and electron motion is introduced in Voltage, Current, Power and Energy Are Different. The junction's nonlinear I–V curve is an application of the model boundaries discussed in Resistance, Nonlinear Loads and Heat.

One-minute questions

Questions to answer before you apply this lesson

What should I select or check first for How a Semiconductor Junction Works?

Doping changes carrier concentration, not bulk charge neutrality. An N-type region has electrons as majority mobile carriers; a P-type region has holes as majority mobile carriers.

Which assumption can change the conclusion?

Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.

What can go wrong if I copy the example directly?

Always provide a current-limiting mechanism. A voltage source applied directly across a forward junction can drive destructive current because current rises steeply and forward voltage changes relatively little.

How do I verify the result in the real system?

Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.

Sources and further reading

1. Lecture Notes — Microelectronic Devices and CircuitsMIT OpenCourseWare · 2026-10-13University course sequence reviewed for intrinsic semiconductors, donor/acceptor doping, carrier concentrations, PN junctions and device models.
2. Lecture 4 — p-n Junctions: ElectrostaticsMIT OpenCourseWare · 2026-10-13University lecture reviewed for depletion approximation, space charge, built-in potential and bias-dependent depletion width.
3. Lecture 5 — p-n Junction Injection and FlowMIT OpenCourseWare · 2026-10-13University lecture reviewed for diffusion and drift, forward-bias minority-carrier injection and junction current.
4. Inhomogeneous SemiconductorsMIT OpenCourseWare · 2026-10-13Independent materials-science lecture reviewed for impurity levels, equilibrium carrier density, depletion charge, built-in voltage and rectification.