The PN Junction
Depletion region, built-in potential, and forward/reverse bias
How a PN Junction Forms
How a PN Junction Forms
When P-type and N-type silicon are brought together, something remarkable happens at the boundary:
- Electrons from the N-side diffuse into the P-side (where there are fewer electrons)
- Holes from the P-side diffuse into the N-side
- This creates a region depleted of free carriers — the depletion region
- Exposed dopant ions create an electric field that opposes further diffusion
- Equilibrium is reached with a built-in potential (~0.7V for silicon)
Analogy: Two Rooms with Different Temperatures
Imagine two rooms separated by an open door — one hot, one cold. Heat (carriers) flows from hot to cold until both sides of the doorway reach a balance. The "doorway zone" is like the depletion region, and the temperature difference is like the built-in potential.
Key Concept: The Depletion Region
The depletion region is typically 0.1–1 µm wide and contains no free carriers — only fixed, ionized dopant atoms. It acts as a natural insulating barrier that can be modulated by applying voltage.
Forward and Reverse Bias
Forward and Reverse Bias
Applying voltage to a PN junction dramatically changes its behavior:
- Forward bias (positive voltage to P-side): Shrinks the depletion region, reduces the barrier, and allows current to flow. Current increases exponentially with voltage.
- Reverse bias (positive voltage to N-side): Widens the depletion region, increases the barrier, and blocks current. Only a tiny leakage current flows.
This asymmetric behavior — conducting in one direction but blocking in the other — is rectification, and it's what makes a diode.
Key Concept: The Diode Equation
Current through a PN junction follows: I = I₀(e^(V/Vt) - 1), where Vt ≈ 26mV at room temperature. This exponential relationship means a small voltage change produces a large current change — the basis for amplification.
Knowledge Check
Knowledge Check
1 / 2What is the built-in potential of a silicon PN junction?