Whether a transistor is biased or unbiased decides if it can do anything useful. This page explains what happens inside an unbiased transistor, why biasing is necessary, and how the depletion regions behave in each case.
What is an unbiased transistor?
An unbiased transistor is one with no external voltage applied to either junction. Nothing is connected — it simply sits there. Understanding this state matters because it shows why a transistor cannot amplify until it is biased.
What happens inside an unbiased transistor
A transistor has two junctions: the emitter-base junction and the collector-base junction. With no applied voltage, both behave like isolated PN junctions.
At each junction, free electrons from the n-region diffuse across into the p-region and recombine with holes. This leaves positive ions on the n-side and negative ions on the p-side. The region left behind has no free charge carriers and is called the depletion region.
The ions create an electric field that opposes further diffusion. Once this field is strong enough, diffusion stops and the junction reaches equilibrium. The potential across the depletion region is the barrier potential — about 0.7 V for silicon and 0.3 V for germanium at room temperature.
Depletion region widths
The two depletion regions in an unbiased transistor are not the same size, and this is a common exam question.
- The emitter region is heavily doped, so its depletion region penetrates only a short distance into it.
- The base is lightly doped and very thin, so depletion regions extend relatively further into the base.
- The collector is moderately doped and physically largest, so the collector-base depletion region is wider than the emitter-base one.
The collector-base depletion region being wider is what allows the collector to withstand a higher reverse voltage than the emitter-base junction.
Why biasing is necessary
An unbiased transistor carries no useful current — the barrier potential blocks carrier movement across both junctions. To make it conduct and amplify, the junctions must be biased so that carriers flow from emitter to collector.
For a transistor working in the active region, which is what amplification requires:
- The emitter-base junction is forward biased, which narrows that depletion region and lets the emitter inject carriers into the base.
- The collector-base junction is reverse biased, which widens that depletion region and sweeps the arriving carriers across to the collector.
Effect of bias on the depletion region
- Forward bias opposes the barrier potential, so the depletion region narrows and current flows readily.
- Reverse bias adds to the barrier potential, so the depletion region widens and only a very small leakage current flows.
The four operating regions
- Active — emitter-base forward, collector-base reverse. Used for amplification.
- Saturation — both junctions forward biased. The transistor behaves as a closed switch.
- Cut-off — both junctions reverse biased. The transistor behaves as an open switch.
- Reverse active — emitter-base reverse, collector-base forward. Rarely used, and gives poor gain.
Frequently asked questions
What is an unbiased transistor?
A transistor with no external supply connected to either junction, so only the built-in barrier potential exists across each depletion region.
Which depletion region is wider in an unbiased transistor?
The collector-base depletion region, because the collector is less heavily doped than the emitter and physically larger.
What is the barrier potential?
The voltage across a depletion region at equilibrium — approximately 0.7 V for silicon and 0.3 V for germanium at room temperature.
Why can an unbiased transistor not amplify?
No useful current flows. Amplification requires the emitter-base junction to be forward biased and the collector-base junction reverse biased, so carriers move from emitter to collector in a controlled way.