Unit – 2Semiconductor Devices

Lecture_2.2

PN Junction Diode & Zener Diode

PN Junction Diode

• Pure semiconductor is doped half with P-type impurity and half with N-type impurity.

• P-type has high concentration of holes and N-type has high concentration of free electrons.

• Diffusion takes place between e- in N-side and holes in P-side at junction.

• At P-side, -ve acceptor ions repels e-.

• At N-side, +ve donor ions repels holes.

• Potential or Junction barrier is set to prevent further movement.• 0.3 V for Germanium.

• 0.72 V for Silicon.

PN junction diode

• This electrostatic field drives away holes and electrons from junction.

• Junction is depleted of mobile charge carriers Depletion Layer.

Biasing of PN diode

• Forward biasing • P-side is connected to positive terminal of battery.• N-side is connected to negative terminal of battery.

• Reverse biasing• P-side is connected to negative terminal of battery.• N-side is connected to positive terminal of battery.

Forward Bias condition

• P-side is connected to positive terminal of battery.

• N-side is connected to negative terminal of battery.

• Applied potential acts opposite to internal potential barrier.

• +ve supply repels holes in P-side towards junction.

• -ve supply repels e- in N-side towards junction.

• As applied potential increases, depletion region and internal potential barrier disappears.

Reverse Bias condition

• P-side is connected to negative terminal of battery.

• N-side is connected to positive terminal of battery.

• Electric field produced by reverse bias is in same direction to potential barrier depletion region width increases.

• Prevents flow of e- & holes.

• Thermally broken covalent bonds small REVERSE SATURATION CURRENT.

• BREAKDOWN VOLTAGE increase in reverse bias produces avalanche of free e- causing breakdown of junction.

Characteristics of PN junction diode

For satisfactory operation…

• Maximum forward current (𝐼𝐹)• Highest instantaneous current through the junction.

• Peak Inverse Voltage (PIV)• Maximum reverse voltage applied to PN junction.

• If 𝑉𝑏𝑖𝑎𝑠 > 𝑃𝐼𝑉, junction damages.

• Maximum power rating• Maximum power dissipated at the junction without damaging it.

• Product of voltage across junction and current through the junction.

Applications

• Rectifiers in dc power supply.

• Switch in logic circuits, demodulation circuits.

• Clipping circuit wave shaping circuits in computers, radars, radio and TV receivers.

• Clamping circuit dc restorer in TV receivers, voltage multipliers.

• Different doping• PIN diode detectors, optical communication, LEDs, Laser diodes.

• Zener diode voltage regulators.

• Varactor diode tuning of radio and TV receivers.

• Tunnel diode relaxation oscillator at microwave frequencies.

Zener Diode

• When reverse bias voltage reaches breakdown, current and power dissipated at the junction increases leading to damage.

• Zener diode• Heavily doped than PN diode.

• Thin depletion region.

• Lower breakdown voltage.

• Designed with adequate power dissipation to operate in breakdown region.

• Sharp increase in current at breakdown is due to1. Avalanche breakdown

2. Zener breakdown

Characteristics of Zener diode

Breakdown in zener diode

Avalanche Breakdown• P & N side lightly doped.

• Thermally generated carriers acquire kinetic energy.

• Disrupt covalent bonds by colliding at high velocity.

• Creation of new e- & hole pairs is cumulative in nature.

• Avalanche multiplication produces large current at same reverse bias value.

• Occurs at higher voltage

Zener Breakdown

• P & N side heavily doped.

• Direct rupture of covalent bonds at strong E field (107 𝑉 𝑚).

• New e- & hole pair, increases reverse current at constant reverse bias (6 V).

• Occurs at lower voltage.

Applications

• Input voltage varies over a range.

• Load resistance needs constant voltage across it.

• As long as zener diode is RB, Vin does not fall below Vz (Zener breakdown voltage).

• Voltage across diode and hence load voltage is constant.

Video segment_2