Unit 4: Fundamentals of semiconductor devices - Subjective Questions

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator, typically having a band gap of around (e.g., Silicon , Germanium ).

Classification based on conductivity:

• Conductors: Very low resistivity, overlapping valence and conduction bands (e.g., copper).
• Insulators: Very high resistivity, large band gap () (e.g., diamond).
• Semiconductors: Moderate resistivity, small band gap ().

Intrinsic Semiconductor:

• Pure semiconductor with no impurities.
• Number of electrons equals number of holes: .
• Conductivity increases with temperature.

Extrinsic Semiconductor:

• Doped with impurities to increase conductivity.
• N-type: doped with pentavalent impurity (e.g., Phosphorus); electrons are majority carriers.
• P-type: doped with trivalent impurity (e.g., Boron); holes are majority carriers.

A PN junction diode is formed by joining a P-type and an N-type semiconductor. At the junction, a depletion region forms due to diffusion of carriers, creating a barrier potential ( for Si, for Ge).

Forward Bias:

• P-side connected to positive terminal, N-side to negative terminal.
• The applied voltage opposes the barrier potential, reducing the depletion width.
• When the applied voltage exceeds the barrier potential, large current flows.
• The diode acts as a closed switch (ON).

Reverse Bias:

• P-side connected to negative terminal, N-side to positive terminal.
• The depletion region widens, increasing the barrier.
• Only a small reverse saturation current (due to minority carriers) flows.
• The diode acts as an open switch (OFF).

The diode thus allows current flow predominantly in one direction, making it a rectifying device.

The V-I characteristics describe the relationship between diode voltage and current.

Forward Characteristics:

• Below the knee/cut-in voltage ( for Si), current is negligible.
• Beyond this, current rises exponentially.

Reverse Characteristics:

• A small reverse saturation current flows.
• At the breakdown voltage, current increases sharply.

Diode Current Equation (Shockley Equation):

Where:

• = diode current
• = reverse saturation current
• = applied voltage
• = ideality factor (1 for Ge, 2 for Si)
• at room temperature

Key points:

• The curve is non-linear.
• Forward resistance is low; reverse resistance is high.

A Zener diode is a specially doped PN junction diode designed to operate in the reverse breakdown region without being damaged.

Working:

• It is heavily doped, resulting in a thin depletion region.
• Under reverse bias, when the voltage reaches the Zener voltage (), breakdown occurs.
• Two mechanisms cause breakdown:
  • Zener breakdown: dominant below , due to strong electric field pulling electrons from covalent bonds.
  • Avalanche breakdown: dominant above , due to carrier collisions.
• During breakdown, voltage remains nearly constant while current varies, enabling voltage regulation.

Differences from ordinary diode:

• Operates in reverse breakdown region (ordinary diode operates in forward region).
• Heavily doped vs. lightly doped.
• Used as a voltage regulator; ordinary diode used as a rectifier.
• Does not get damaged in breakdown (within rated power).

V-I Characteristics of Zener Diode:

• Forward region: behaves like an ordinary diode (conducts above ).
• Reverse region: small leakage current until the Zener voltage is reached.
• At , breakdown occurs and the voltage stays nearly constant while current changes sharply (steep vertical line).

Zener as a Voltage Regulator:

• The Zener diode is connected in reverse bias parallel to the load.
• A series resistor limits the current.

Operation:

• If input voltage increases, extra current flows through the Zener, keeping constant.
• If load current increases, Zener current decreases to maintain constant output.

Current relation:

This provides a stable output voltage despite variations in input voltage or load.

Logic gates are the fundamental building blocks of digital circuits that perform logical operations on binary inputs.

1. AND Gate:

• Output is HIGH only when all inputs are HIGH.
• Boolean:

2. OR Gate:

• Output is HIGH when at least one input is HIGH.
• Boolean:

3. NOT Gate (Inverter):

• Output is the complement of the input.
• Boolean:

Universal gates are gates from which any other logic gate can be constructed.

NAND Gate:

• Combination of AND followed by NOT.
• Boolean:

NOR Gate:

• Combination of OR followed by NOT.
• Boolean:

Why Universal?

• Using only NAND gates, we can realize NOT, AND, OR, and all other gates.
• Similarly, NOR gates alone can realize all logic functions.
• This reduces design complexity and manufacturing cost, hence they are called universal gates.

A Varactor diode (also called a varicap or voltage-variable capacitor) is a diode that uses the variable capacitance of the depletion region under reverse bias.

Working:

• The diode is always operated in reverse bias.
• The depletion region acts as a dielectric, while the P and N regions act as plates of a capacitor.
• As the reverse voltage increases, the depletion width increases, decreasing the capacitance.

Capacitance relation:

Where (depletion width) increases with reverse voltage, so decreases.

Characteristics:

• Capacitance is inversely related to reverse voltage.

Applications:

• Tuning circuits in radio and TV receivers.
• Voltage-controlled oscillators (VCO).
• Frequency modulators.
• Automatic frequency control (AFC) circuits.
• RF filters and parametric amplifiers.

A diode can be tested using the diode test mode of a digital multimeter (DMM).

Procedure:

1. Set the multimeter to the diode mode (symbol: ▷|).
2. Forward test: Connect red (+) probe to anode and black (–) probe to cathode.
3. Reverse test: Reverse the probe connections.

Interpreting results:

• Good diode:
  • Forward: reads – (Si) or (Ge).
  • Reverse: reads OL (open/overload).
• Open (faulty) diode:
  • Reads OL in both directions.
• Shorted (faulty) diode:
  • Reads in both directions.

Resistance mode method:

• Low resistance in forward, high resistance in reverse ⇒ good diode.

This helps verify the diode's rectifying behavior and junction health.

A BJT has two PN junctions: base-emitter (BE) and base-collector (BC). It can be tested using the diode mode of a multimeter.

Testing Procedure:

1. Set the multimeter to diode mode.
2. Identify the base terminal: the terminal that shows conduction (0.6–0.7 V) with the other two terminals.

For NPN transistor:

• Place red (+) probe on base.
• Black probe on emitter and collector ⇒ both show (conduction).
• Reversing shows OL.

For PNP transistor:

• Place black (–) probe on base.
• Red probe on emitter and collector ⇒ both show .
• Reversing shows OL.

Identifying faults:

• Good: BE and BC junctions conduct in one direction, OL in reverse.
• Shorted: reads ~0 V in both directions.
• Open: reads OL in both directions.

Note: The collector-emitter path should normally read OL in both directions for a healthy transistor.

A Half Wave Rectifier (HWR) converts AC into pulsating DC using a single diode, conducting only during one half-cycle.

Working:

• During the positive half-cycle, the diode is forward biased and conducts; current flows through the load.
• During the negative half-cycle, the diode is reverse biased and does not conduct.
• Output appears only for positive half-cycles.

Average (DC) value:

RMS value:

Ripple Factor:

Rectifier Efficiency:

Peak Inverse Voltage (PIV):

Disadvantages: Low efficiency, high ripple, poor transformer utilization.

A Full Wave Rectifier (FWR) using a center-tapped transformer uses two diodes to rectify both halves of the AC cycle.

Working:

• The transformer secondary is center-tapped, providing two equal voltages of opposite polarity.
• During the positive half-cycle, diode conducts.
• During the negative half-cycle, diode conducts.
• Current flows through the load in the same direction in both half-cycles.

Average (DC) value:

RMS value:

Ripple Factor:

Rectifier Efficiency:

Peak Inverse Voltage (PIV):

Advantages: Higher efficiency and lower ripple than HWR.
Disadvantage: Requires a center-tapped transformer; high PIV.

A Full Wave Bridge Rectifier uses four diodes arranged in a bridge configuration and does not require a center-tapped transformer.

Working:

• During the positive half-cycle, diodes and conduct, while and are off.
• During the negative half-cycle, diodes and conduct, while and are off.
• In both cases, current flows through the load in the same direction.

Key parameters:

• Ripple factor
• Efficiency
• PIV (half that of center-tapped type)

Comparison with Center-Tapped FWR:

Advantage: No center tap needed and lower PIV, hence widely used.

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a voltage-controlled, three-terminal device (Gate, Drain, Source) used for switching and amplification. The gate is insulated from the channel by a thin oxide () layer, giving very high input impedance.

Construction:

• A substrate (P or N type) with two heavily doped regions forming source and drain.
• A metal gate separated from the channel by an insulating oxide layer.

Types and Representation:

1. Depletion-type MOSFET (D-MOSFET):

• A physical channel exists even at .
• Can operate in both depletion and enhancement modes.
• Symbol: continuous (solid) line between drain and source.

2. Enhancement-type MOSFET (E-MOSFET):

• No channel at ; channel forms only when exceeds threshold voltage .
• Symbol: broken (dashed) line between drain and source.

Each type is further classified as N-channel or P-channel, indicated by the direction of the arrow on the substrate.

An Enhancement MOSFET conducts only when gate-source voltage exceeds the threshold voltage ().

Drain Characteristics ( vs ):

• Plotted for different constant values of .
• Three regions:
  • Cut-off region: , .
  • Ohmic (Triode) region: increases linearly with ; device acts like a resistor.
  • Saturation region: remains almost constant; used for amplification.

Transfer Characteristics ( vs ):

• No current until reaches .
• Beyond , drain current increases following:
  

Where is a constant depending on device dimensions.

Key points:

• Only enhancement mode operation.
• High input impedance due to insulated gate.
• Widely used in digital switching (CMOS logic).

PN Junction Diode vs Zener Diode:

Avalanche vs Zener Breakdown:

Both mechanisms limit reverse current and are used in voltage regulation, but they occur under different doping and voltage conditions.

A detailed comparison of the three rectifier types is given below.

Observations:

• Full wave rectifiers are twice as efficient as the half wave type.
• The bridge rectifier combines low PIV with no need for a center tap, making it the most practical choice.
• Output ripple is lowest in full wave rectifiers, requiring smaller filters.

Applications of Diodes:

• Rectification (converting AC to DC).
• Clipping and clamping circuits.
• Voltage multipliers.
• Zener diode for voltage regulation.
• LEDs for display and indication.
• Varactor diode for tuning.

Applications of Transistors (BJT):

• Amplification of weak signals.
• Switching circuits.
• Oscillators and signal generators.
• Logic gates and digital circuits.
• Voltage regulators.

Applications of Rectifiers:

• DC power supplies (SMPS, adapters).
• Battery chargers.
• Welding equipment.
• Signal demodulation.
• Converting AC mains to usable DC.

Applications of MOSFETs:

• High-speed switching in digital circuits (CMOS logic).
• Power electronics (inverters, converters).
• Microprocessors and memory (billions of MOSFETs).
• Audio and RF amplifiers.
• Motor control and power management.

These devices form the backbone of modern analog and digital electronic systems.

Shockley Diode Equation:

The current through a PN junction diode is given by:

Derivation outline:

• Under equilibrium, diffusion and drift currents balance.
• When a forward voltage is applied, the minority carrier concentration at the junction increases exponentially by factor (Boltzmann relation).
• The net diode current is the difference between the injected current and the reverse saturation current , giving the above equation.

Where:

• at
• = ideality factor (1 for Ge, 2 for Si)
• = reverse saturation current

Effect of Temperature:

• Reverse saturation current doubles for every rise.
• Barrier/cut-in voltage decreases by about .
• Reverse leakage current increases with temperature.
• Overall conductivity increases with temperature, which can lead to thermal runaway if not controlled.

Thus, temperature significantly affects both forward and reverse characteristics.

Semiconductor devices are electronic components made from semiconductor materials (mainly Silicon and Germanium) whose conduction properties can be controlled by doping, voltage, light, or temperature.

Why semiconductors?

• They can be made to conduct selectively, enabling switching and amplification.
• Small size, low power consumption, long life, and high reliability.
• Have replaced bulky, power-hungry vacuum tubes.

Major Types of Semiconductor Devices:

• Diodes: PN junction, Zener, LED, photodiode, varactor — used for rectification, regulation, and tuning.
• Transistors (BJT): used for amplification and switching.
• Field Effect Transistors (JFET, MOSFET): voltage-controlled devices for high-speed switching and VLSI.
• Thyristors (SCR, TRIAC): used in power control.
• Integrated Circuits (ICs): millions of devices on a single chip.

Importance:

• Form the foundation of computers, smartphones, communication systems, and power electronics.
• Enable miniaturization and the entire digital revolution.
• Essential in both analog and digital electronic systems.