Electronic Devices

Conductivity classification:

Silicon (Si) band gap: 1.1 eV. Germanium (Ge): 0.7 eV.

Intrinsic semiconductor: pure Si or Ge. Conductivity rises sharply with temperature (more thermal electrons jump to conduction band).

Doping — adding small amounts of impurities (~1 in 10⁶ atoms) to control conductivity.

n-type: dope with pentavalent (P, As, Sb). Extra electron in conduction band. Electrons are majority carriers, holes minority.

p-type: dope with trivalent (B, Al, Ga). Creates "holes" in valence band. Holes are majority carriers, electrons minority.

Conductivity: σ = n_e e μ_e + n_h e μ_h (where μ = mobility, n = carrier density).

p-n junction. When p-type and n-type are joined:

• Near the junction, electrons from n-side diffuse to p-side; holes diffuse the other way.
• This creates a depletion region (no free carriers, only ionized donors/acceptors).
• An electric field builds up across the depletion region, opposing further diffusion. Equilibrium reached when drift = diffusion.
• Barrier potential ~0.7 V for Si, ~0.3 V for Ge.

Forward bias (p to +, n to −):

• Reduces barrier → current flows.
• I rises exponentially with V (Shockley equation: I = I₀(e^(V/V_T) − 1)).
• Used in rectifiers.

Reverse bias (p to −, n to +):

• Increases barrier → essentially no current (just a tiny reverse saturation current).
• At very high reverse voltage → breakdown (Zener or avalanche).

Diode applications:

1. Rectifier.

• Half-wave rectifier: one diode passes only positive half. Ripple frequency = input frequency.
• Full-wave rectifier: uses 2 or 4 diodes (bridge). Both halves used. Ripple frequency = 2× input.

Filter capacitor + transformer smooths the rectified DC. Output: nearly constant DC.

2. Zener diode. Designed to operate in reverse breakdown. Used as a voltage regulator.

3. LED (Light Emitting Diode). Forward-biased junction with direct band gap (GaAs, GaP). Recombination produces photons of energy hf ≈ E_g.

4. Photodiode. Reverse-biased; light generates electron-hole pairs → current proportional to light intensity. Used in light sensors.

5. Solar cell. Photodiode without external power; the light-generated EMF drives current through external load.

BJT (Bipolar Junction Transistor).

Three regions: emitter (E, heavily doped), base (B, lightly doped, very thin), collector (C, moderate).

Two types: npn and pnp.

Common-emitter (CE) amplifier:

• Input applied between base and emitter.
• Output taken between collector and emitter.
• Small base current controls large collector current.
• Current gain β = I_C / I_B (typically 50-500).

The output is amplified (in voltage and power) and phase-inverted by 180°.

Cutoff: B-E junction reverse biased → no I_C → output high → logic 0 or off state.
Saturation: B-E forward biased, B-C also forward → I_C maxed → output low → logic 1 or on.
Active: between cutoff and saturation; used for analog amplification.

Logic gates (from transistors): AND, OR, NOT, NAND, NOR, XOR. NAND and NOR are universal — any logic function can be built from them alone.

Conductivity classification:

Silicon (Si) band gap: 1.1 eV. Germanium (Ge): 0.7 eV.

Intrinsic semiconductor: pure Si or Ge. Conductivity rises sharply with temperature (more thermal electrons jump to conduction band).

Doping — adding small amounts of impurities (~1 in 10⁶ atoms) to control conductivity.

n-type: dope with pentavalent (P, As, Sb). Extra electron in conduction band. Electrons are majority carriers, holes minority.

p-type: dope with trivalent (B, Al, Ga). Creates "holes" in valence band. Holes are majority carriers, electrons minority.

Conductivity: σ = n_e e μ_e + n_h e μ_h (where μ = mobility, n = carrier density).

p-n junction. When p-type and n-type are joined:

• Near the junction, electrons from n-side diffuse to p-side; holes diffuse the other way.
• This creates a depletion region (no free carriers, only ionized donors/acceptors).
• An electric field builds up across the depletion region, opposing further diffusion. Equilibrium reached when drift = diffusion.
• Barrier potential ~0.7 V for Si, ~0.3 V for Ge.

Forward bias (p to +, n to −):

• Reduces barrier → current flows.
• I rises exponentially with V (Shockley equation: I = I₀(e^(V/V_T) − 1)).
• Used in rectifiers.

Reverse bias (p to −, n to +):

• Increases barrier → essentially no current (just a tiny reverse saturation current).
• At very high reverse voltage → breakdown (Zener or avalanche).

Diode applications:

1. Rectifier.

• Half-wave rectifier: one diode passes only positive half. Ripple frequency = input frequency.
• Full-wave rectifier: uses 2 or 4 diodes (bridge). Both halves used. Ripple frequency = 2× input.

Filter capacitor + transformer smooths the rectified DC. Output: nearly constant DC.

2. Zener diode. Designed to operate in reverse breakdown. Used as a voltage regulator.

3. LED (Light Emitting Diode). Forward-biased junction with direct band gap (GaAs, GaP). Recombination produces photons of energy hf ≈ E_g.

4. Photodiode. Reverse-biased; light generates electron-hole pairs → current proportional to light intensity. Used in light sensors.

5. Solar cell. Photodiode without external power; the light-generated EMF drives current through external load.

BJT (Bipolar Junction Transistor).

Three regions: emitter (E, heavily doped), base (B, lightly doped, very thin), collector (C, moderate).

Two types: npn and pnp.

Common-emitter (CE) amplifier:

• Input applied between base and emitter.
• Output taken between collector and emitter.
• Small base current controls large collector current.
• Current gain β = I_C / I_B (typically 50-500).

The output is amplified (in voltage and power) and phase-inverted by 180°.

Cutoff: B-E junction reverse biased → no I_C → output high → logic 0 or off state.
Saturation: B-E forward biased, B-C also forward → I_C maxed → output low → logic 1 or on.
Active: between cutoff and saturation; used for analog amplification.

Logic gates (from transistors): AND, OR, NOT, NAND, NOR, XOR. NAND and NOR are universal — any logic function can be built from them alone.