PN Junction
Depletion region, built-in V_bi = 0.7 V — apply forward/reverse bias and see width change.
Key Notes
A p-n junction is formed by joining p-type and n-type semiconductors (in a single crystal, not metal contact).
Diffusion: electrons from n-side diffuse to p-side; holes from p-side diffuse to n-side. Diffusion currents flow.
Diffusion leaves IONISED dopants behind — a DEPLETION region (charge-depleted of mobile carriers) forms at the junction.
The ionised donors (positive) and acceptors (negative) create a built-in electric field that OPPOSES further diffusion ⇒ equilibrium.
Built-in potential V₀ (~0.6-0.7 V for Si, ~0.3 V for Ge) is the equilibrium contact potential.
Forward bias (p connected to +): reduces V₀ and depletion width ⇒ large current flows.
Reverse bias (p connected to −): increases V₀ and depletion width ⇒ tiny saturation current.
This asymmetry makes the p-n junction a DIODE — passes current one way, blocks the other.
Formulas
Built-in potential
Depends on doping levels and intrinsic concentration.
Depletion width
Wider for lower doping; shrinks with forward bias.
Diode equation (Shockley)
Forward bias: I grows exponentially. Reverse: I ≈ −I₀ (saturation).
Important Points
The depletion region is essentially insulating — only ionised dopants, no mobile carriers.
Built-in V₀ does NOT appear at the external terminals — internal to the crystal.
Forward bias: current rises steeply once V > V_th ≈ 0.7 V (Si).
Reverse bias: I ≈ −I₀ (~nA to μA); independent of |V| until breakdown.
Reverse breakdown: large V_R causes avalanche or Zener breakdown ⇒ large I; used in voltage regulators (Zener diodes).
p-n junctions are the foundation of ALL semiconductor devices: diodes, BJTs, LEDs, solar cells, photodiodes.
PN Junction notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.