Built-in potential

Built-in potential is the voltage that forms across a p-n junction when p-type and n-type semiconductors meet. In Intro to Electrical Engineering, it explains why the junction resists carrier flow at equilibrium.

What is built-in potential?

Built-in potential is the internal voltage that appears across a p-n junction after p-type and n-type semiconductor regions are joined in Intro to Electrical Engineering. It is not a battery you add from the outside. It forms on its own because carriers diffuse from high concentration to low concentration right after the junction is made.

Here is the basic sequence. Electrons on the n-side and holes on the p-side move toward each other and recombine near the boundary. That leaves behind fixed ionized donors and acceptors, which cannot move. The result is the depletion region, a zone with very few free carriers and an electric field pointing across the junction.

That electric field creates the built-in potential. Its direction pushes back against further diffusion, so the junction reaches thermal equilibrium. At equilibrium, diffusion current and drift current balance each other, which is why the net current is zero even though carriers are still moving microscopically.

The size of the built-in potential depends on the semiconductor material and on carrier concentration on each side. A stronger difference in doping levels usually means a larger barrier. In a circuit problem, you will often connect this idea to how wide the depletion region is and how hard it is for majority carriers to cross the junction.

When you apply external bias, you do not remove the built-in potential, you change the effective barrier seen by carriers. Forward bias lowers the barrier and makes current flow easier. Reverse bias raises the effective barrier and widens the depletion region, which is why the junction conducts so little in that direction until breakdown conditions appear.

A common confusion is thinking built-in potential means the diode always has that much voltage across it like a fixed source. It does not. It is an internal electrostatic barrier tied to charge separation inside the junction. You usually infer it from the device behavior, band diagram, or junction equations rather than measuring it as a simple external drop.

Why built-in potential matters in Intro to Electrical Engineering

Built-in potential shows up any time you analyze how a diode or transistor junction actually behaves instead of just memorizing symbols. It is the reason a p-n junction does not act like two ordinary pieces of wire stuck together. Without that internal barrier, there would be no depletion region and no clean rectifying behavior.

In Intro to Electrical Engineering, this concept connects the microscopic picture of carriers to the circuit-level picture you see in diode I-V curves. If you know why the barrier exists, forward bias makes more sense, reverse bias makes more sense, and the idea of a threshold-like turn-on becomes less mysterious. It also gives you a better handle on why temperature changes device behavior, since carrier activity affects the barrier.

You will also see built-in potential again when studying transistors, junction capacitance, and semiconductor devices that depend on controlled carrier movement. It is one of those ideas that keeps coming back in slightly different forms, so getting the mechanism straight early makes later material easier to read and easier to calculate.

How built-in potential connects across the course

depletion region

The depletion region is the physical zone where mobile carriers have mostly left the junction. Built-in potential comes from the charge separation in this region, and the electric field across it is what creates the barrier that resists further diffusion. If the depletion region changes width, the effective junction barrier changes too.

forward bias

Forward bias lowers the barrier created by the built-in potential. When you connect the p-side to a higher potential than the n-side, the external voltage opposes the internal field and lets more carriers cross the junction. That is why diode current rises quickly once the junction is sufficiently forward biased.

drift current

Drift current is the motion of carriers caused by the electric field inside the junction. In thermal equilibrium, drift current balances diffusion current, and that balance is what keeps the net current at zero. Built-in potential is the voltage associated with the field that makes this drift happen.

current-voltage (i-v) characteristics

The diode I-V curve is the circuit-level result of the built-in potential and the depletion region. The curve stays low in reverse bias because the barrier is still blocking carriers, then rises sharply in forward bias when the barrier is reduced. Reading the curve is really reading how the junction responds to that internal voltage.

Is built-in potential on the Intro to Electrical Engineering exam?

A quiz problem might give you a p-n junction diagram and ask why current is small at equilibrium, why the depletion region forms, or what changes under forward bias. Your job is to connect the internal field to carrier motion, not just label the junction. You may also be asked to interpret a band diagram, identify which side is p-type or n-type, or explain why a higher doping level changes the barrier.

In calculation questions, built-in potential often appears when comparing junctions, predicting barrier changes, or explaining why a diode starts conducting more strongly after bias is applied. On lab-style questions, you might read an I-V curve and describe how the built-in potential shows up indirectly in the flat reverse region and the turn-on behavior in forward bias.

Key things to remember about built-in potential

• Built-in potential is the internal voltage barrier that forms naturally across a p-n junction after the two semiconductor regions are joined.

• It comes from carrier diffusion and the fixed ions left behind in the depletion region, not from an external power supply.

• At equilibrium, the built-in potential creates an electric field that balances diffusion current with drift current, so the net current is zero.

• Forward bias reduces the effective barrier, while reverse bias increases it and makes the junction harder to conduct through.

• You can think of it as the junction's built-in resistance to carrier flow, tied directly to doping, carrier concentration, and device behavior.

Frequently asked questions about built-in potential

What is built-in potential in Intro to Electrical Engineering?

Built-in potential is the voltage barrier that forms automatically across a p-n junction when electrons and holes diffuse and leave behind fixed charges. In Intro to Electrical Engineering, it explains why a diode has a depletion region and why the junction does not conduct freely at equilibrium.

How is built-in potential different from forward bias?

Built-in potential is the internal barrier that exists on its own inside the junction. Forward bias is an external voltage you apply to reduce that barrier. So forward bias does not create the junction's voltage, it changes how strong the junction's internal barrier feels to carriers.

Why does the depletion region form because of built-in potential?

The depletion region forms first, then the charge separation in that region creates the built-in potential. As carriers diffuse and recombine near the junction, they leave fixed ions behind. That uncovered charge creates the electric field and voltage barrier across the depleted zone.

How do I use built-in potential in diode problems?

Use it to explain carrier motion, junction equilibrium, and how bias changes the current. If a problem asks why the diode blocks current one way or why the depletion region widens under reverse bias, built-in potential is the reason. It is often part of the physical explanation even when the math focuses on I-V behavior.