Semiconductor PN Junction

PN Junction Structure

P-Type (Acceptors)

N-Type (Donors)

Depletion Region

Built-in Potential: 0.70 V

Depletion Width: 0.50 μm

Electric Field: 1.40×10⁴ V/cm

Energy Band Diagram

Conduction Band (Ec)

Valence Band (Ev)

Fermi Level (Ef)

Band Gap (Eg): 1.12 eV (Si)

Barrier Height: 0.70 eV

I-V Characteristic Curve

Current (I): 0.00 mA

Voltage (V): 0.00 V

Operating Mode: Equilibrium

Carrier Concentration Profile

PN Junction Parameters

Acceptor Concentration (N_A): 1e16 cm⁻³

Donor Concentration (N_D): 1e16 cm⁻³

Applied Voltage (V): 0.00 V

Temperature (T): 300 K

Band Gap (Eg): 1.12 eV

Show Carriers

Show Electric Field

Show Depletion Region

PN Junction Equations

Intrinsic Carriers nᵢ²(T) = N_c·N_v·exp(-E_g/kT)

Built-in Potential V_bi = (kT/e)·ln(N_A·N_D/nᵢ²)

Depletion Width W = √(2ε·V_bi/e·(1/N_A + 1/N_D))

Diode Equation I = I₀(e^(eV/kT) - 1)

Saturation Current I₀ ∝ nᵢ² ∝ T³·exp(-E_g/kT)

Constants: k = 8.617×10⁻⁵ eV/K, e = 1.602×10⁻¹⁹ C, ε_Si = 11.7ε₀

What is a PN Junction?

A PN junction is formed by joining P-type and N-type semiconductors. The P-type region has an excess of holes (positive charge carriers) from acceptor doping, while the N-type region has an excess of electrons (negative charge carriers) from donor doping. At the junction, carriers diffuse across the boundary, creating a depletion region with an built-in electric field that opposes further diffusion.

Junction Formation

Carrier Diffusion: When P and N materials are joined, concentration gradients cause holes to diffuse from P to N and electrons from N to P.

Depletion Region: As carriers diffuse, they leave behind ionized dopants (negative acceptors in P, positive donors in N), creating a space charge region depleted of mobile carriers.

Built-in Potential: The space charge creates an electric field and a potential barrier (V_bi ≈ 0.7V for Si) that balances diffusion and drift currents at equilibrium.

Energy Bands: The bands bend near the junction, with conduction and valence bands higher in the N-region by e·V_bi, keeping the Fermi level constant.

Biasing Modes

Forward Bias (V > 0): Positive voltage applied to P-region reduces the barrier height, allowing large currents to flow exponentially (I ∝ e^(eV/kT)). Used in LEDs, rectifiers.

Reverse Bias (V < 0): Negative voltage on P-region increases the barrier, widening the depletion region. Only small leakage current flows (minority carrier drift).

Breakdown: At large reverse voltage, avalanche multiplication or Zener tunneling causes sudden current increase. Avalanche breakdown occurs in higher-doped junctions (> 6V), Zener in heavily-doped (< 4V).

Applications

Rectifier Diodes: Convert AC to DC by allowing current in only one direction. Essential in power supplies

Light Emitting Diodes (LEDs): Forward bias causes electron-hole recombination, emitting photons with energy ≈ band gap

Solar Cells: Reverse biased junctions convert absorbed photon energy into electrical current through the photovoltaic effect

Photodiodes: Reverse biased diodes generate current when exposed to light, used in optical communication and sensors

Zener Diodes: Designed to operate in breakdown region for voltage regulation and reference applications

Bipolar Junction Transistors (BJTs): Two PN junctions (NPN or PNP) used for amplification and switching in integrated circuits

How to Use This Visualization

Adjust Doping: Change acceptor (N_A) and donor (N_D) concentrations to see their effect on built-in potential and depletion width

Apply Voltage: Use the voltage slider or preset buttons to see forward/reverse bias effects on energy bands and current

Observe Bands: Watch how energy bands bend and the barrier changes with bias voltage

I-V Curve: Note the exponential current increase in forward bias and saturation in reverse bias

Carrier Profiles: View how electron and hole concentrations vary across the junction