Unit 4: Fundamentals of semiconductor devices - Subjective Questions

Semiconductor: A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Its conductivity can be controlled by temperature, light, electric field, or the addition of impurities. Silicon and germanium are common semiconductor materials.

Intrinsic semiconductor:

• It is a chemically pure semiconductor without intentional impurities.
• Thermal energy breaks some covalent bonds and produces equal numbers of free electrons and holes.
• Therefore, the electron and hole concentrations are equal: .
• Both electrons and holes participate in conduction.

Extrinsic semiconductor: It is formed by adding a controlled amount of impurity to an intrinsic semiconductor. This process is called doping.

• N-type semiconductor: A pentavalent impurity such as phosphorus is added. Electrons are the majority carriers, while holes are the minority carriers.
• P-type semiconductor: A trivalent impurity such as boron is added. Holes are the majority carriers, while electrons are the minority carriers.

Thus, doping substantially increases the conductivity of a semiconductor.

A PN junction diode is formed by joining P-type and N-type semiconductor regions.

Formation of the junction:

• Electrons diffuse from the N-side to the P-side, while holes diffuse from the P-side to the N-side.
• They recombine near the junction, leaving fixed ions behind.
• This creates a region without mobile charge carriers called the depletion region.
• The fixed ions produce an internal electric field and a potential barrier.

Unbiased condition:

• No external voltage is applied.
• Diffusion current and drift current are equal and opposite.
• Hence, the net current is zero.

Forward bias:

• The P-side is connected to the positive terminal and the N-side to the negative terminal.
• The external voltage reduces the barrier potential and narrows the depletion region.
• A large current flows after the applied voltage exceeds the cut-in voltage, approximately for silicon and for germanium.

Reverse bias:

• The P-side is connected to the negative terminal and the N-side to the positive terminal.
• The barrier potential and depletion width increase.
• Only a very small reverse saturation current flows because of minority carriers.
• At a sufficiently high reverse voltage, breakdown occurs and the reverse current rises sharply.

The V-I characteristic of a PN junction diode shows the relationship between diode voltage and diode current.

Forward-bias characteristic:

• Initially, the current is extremely small because the applied voltage is insufficient to overcome the junction barrier.
• At the cut-in or knee voltage, the current begins to increase rapidly.
• Typical cut-in voltages are about for silicon and for germanium.
• Beyond the knee voltage, a small increase in voltage produces a large increase in current.

The diode current is approximately described by:

where is reverse saturation current, is thermal voltage, and is the ideality factor.

Reverse-bias characteristic:

• A small, nearly constant reverse saturation current flows because of minority carriers.
• When the reverse voltage reaches the breakdown voltage , reverse current increases sharply.
• Current must be limited externally to prevent damage in an ordinary diode.

The forward resistance is low after conduction begins, whereas the reverse resistance is very high before breakdown.

A Zener diode is a heavily doped PN junction diode designed to operate safely in the reverse-breakdown region.

Construction and operation:

• Heavy doping creates a thin depletion region.
• In forward bias, it behaves like an ordinary diode.
• In reverse bias, only a small current flows until the reverse voltage reaches the Zener voltage .
• At , the current increases sharply while the voltage across the diode remains nearly constant.
• Breakdown may occur by the Zener effect at lower voltages or the avalanche effect at higher voltages.

Voltage regulation:

• The Zener diode is connected in reverse bias across the load.
• A series resistor is connected between the supply and the load to limit current.
• The output voltage is approximately:

• The series current is:

• This current divides as .
• If input voltage rises or load current falls, the Zener current increases and absorbs the change.
• If input voltage falls or load current rises, the Zener current decreases.

As long as the Zener current stays within its permitted range, the output voltage remains nearly constant.

Comparison of PN junction and Zener diodes:

Similarity: Both are two-terminal PN junction semiconductor devices and conduct current in forward bias after their cut-in voltage is reached.

A series resistor is especially important with a Zener diode because it limits current in the breakdown region.

Logic gates perform Boolean operations on binary inputs.

Boolean expressions:

• AND:
• OR:
• NOT:
• NAND:
• NOR:
• XOR:
• XNOR:

Two-input truth table:

NOT gate truth table:

NAND and NOR are called universal gates because any Boolean function can be implemented using only NAND gates or only NOR gates.

A varactor diode, also called a varicap diode, is a PN junction diode designed to act as a voltage-controlled capacitor.

Operation:

• It is always operated in reverse bias.
• The P and N regions act as capacitor plates, while the depletion layer acts as the dielectric.
• Increasing the reverse voltage widens the depletion region.
• Since capacitance is inversely proportional to separation, the junction capacitance decreases as reverse voltage increases.

The junction capacitance is commonly represented by:

where is zero-bias capacitance, is reverse voltage, is junction potential, and depends on the junction profile.

Characteristics:

• Capacitance is maximum at low reverse voltage.
• Capacitance decreases nonlinearly as reverse voltage increases.
• Ideally, negligible reverse current flows.

Applications:

• Electronic tuning circuits
• Voltage-controlled oscillators
• Frequency modulators
• Phase-locked loops
• Television and radio tuners
• RF filters and frequency multipliers

Testing procedure using diode-test mode:

1. Switch off the circuit and discharge all capacitors.
2. Isolate at least one diode terminal from the circuit if necessary.
3. Select the diode-test mode on the digital multimeter.
4. Connect the red probe to the anode and the black probe to the cathode.
5. Note the forward-voltage reading.
6. Reverse the probes and note the reverse-bias indication.

Expected results:

• Good silicon diode: Approximately to in forward bias and an open-loop or very high reading in reverse bias.
• Good germanium diode: Approximately to in forward bias and a high reading in reverse bias.
• Short-circuited diode: Nearly or very low resistance in both directions.
• Open-circuited diode: Open-loop or infinite resistance in both directions.
• Leaky diode: An abnormally low reverse resistance or measurable reverse conduction.

The diode should not be touched across both leads during resistance testing because body resistance may affect the measurement.

A BJT has two PN junctions: the base-emitter junction and the base-collector junction. It can therefore be tested like two diodes sharing the base terminal.

Procedure:

1. Switch off the circuit and identify the transistor terminals if possible.
2. Select diode-test mode on the multimeter.
3. Test all terminal pairs in both directions.

For an NPN transistor:

• Place the red probe on the base.
• Touch the black probe to the emitter and then to the collector.
• Both readings should normally be about to for a silicon transistor.
• Reversing the probes should give an open indication.

For a PNP transistor:

• Place the black probe on the base.
• Touch the red probe to the emitter and then to the collector.
• Both junctions should show a normal forward-voltage drop.
• Reversing the probes should give an open indication.

Collector-emitter test:

• The collector-emitter path should normally indicate open in both directions when the base is not driven.

A transistor is defective if a junction conducts in both directions, remains open in both directions, or the collector-emitter terminals are shorted.

A half-wave rectifier consists of a diode and load resistance connected to an AC source. The diode conducts during one half-cycle and blocks the other.

For an input :

• During the positive half-cycle, .
• During the negative half-cycle, .

Average or DC current:

Therefore,

RMS current and voltage:

Ripple factor:

Maximum rectification efficiency:

Thus, the maximum efficiency is .

Peak inverse voltage:

The ripple frequency equals the supply frequency . The circuit is simple but has high ripple and poor efficiency.

A centre-tapped full-wave rectifier uses a centre-tapped transformer and two diodes. Each diode conducts during one half-cycle, but the load current always flows in the same direction.

Operation:

• During the positive half-cycle, the upper end of the secondary is positive, so diode conducts and remains off.
• During the negative half-cycle, the lower end becomes positive with respect to the centre tap, so conducts and remains off.
• Both input half-cycles therefore produce positive load voltage.

For a peak load current :

Average current and voltage:

RMS current and voltage:

Ripple factor:

Maximum rectification efficiency:

Thus, the maximum efficiency is .

Peak inverse voltage per diode:

The ripple frequency is . Its disadvantages are the need for a centre-tapped transformer and the high PIV rating of each diode.

A full-wave bridge rectifier uses four diodes arranged in a bridge. The AC supply is applied across one diagonal, and the load is connected across the other.

Operation:

• During the positive half-cycle, one diagonal pair of diodes conducts.
• During the negative half-cycle, the other diagonal pair conducts.
• In both cases, current through the load flows in the same direction.
• Two diode forward-voltage drops occur in each conducting path.

For ideal diodes:

The ripple factor is approximately , maximum efficiency is , and ripple frequency is .

Peak inverse voltage per diode:

Advantages:

• No centre-tapped transformer is required.
• It uses both half-cycles of the input.
• Lower PIV rating is required per diode.
• Transformer utilization is better than in a centre-tapped circuit.

Disadvantages:

• Four diodes are required.
• Two diodes conduct at a time, producing a larger total forward-voltage drop.
• It is less suitable for very low-voltage outputs unless low-drop diodes are used.

Comparison of rectifiers:

Conclusion:

• The half-wave rectifier is simple but inefficient and has high ripple.
• The centre-tapped rectifier has good efficiency but requires a special transformer and high-PIV diodes.
• The bridge rectifier is widely used because it needs no centre tap and imposes lower PIV on each diode, although two diode drops occur in the conducting path.

A rectifier produces pulsating DC, which contains an AC component called ripple. A filter is connected after the rectifier to reduce this ripple and obtain smoother DC.

Common filters:

• Capacitor filter: A capacitor is connected across the load. It charges near the waveform peak and discharges slowly through the load between peaks.
• Inductor filter: An inductor connected in series opposes changes in current and reduces AC ripple.
• LC filter: Combines a series inductor and shunt capacitor for improved smoothing.
• Pi filter: Uses a capacitor-inductor-capacitor arrangement and provides effective ripple reduction.

For a capacitor-filtered full-wave rectifier, ripple decreases when capacitance, load resistance, or frequency increases. An approximate relation is:

Applications of rectifiers:

• DC power supplies for electronic equipment
• Battery chargers
• DC motor drives
• Electroplating and electrolysis
• Welding power supplies
• Uninterruptible power supplies
• Signal detection and demodulation
• High-voltage DC transmission converter systems

A voltage regulator may be added after the filter to maintain a constant output voltage.

A metal-oxide-semiconductor field-effect transistor, or MOSFET, is a three-terminal or four-terminal semiconductor device. Its main terminals are gate, drain, and source; the body terminal is often internally connected to the source.

Representation and types:

• N-channel enhancement MOSFET
• P-channel enhancement MOSFET
• N-channel depletion MOSFET
• P-channel depletion MOSFET

In circuit symbols, the insulated gate is shown separated from the channel. A broken channel commonly represents enhancement type, while a solid channel represents depletion type. The arrow identifies the channel or body polarity according to the symbol convention used.

Construction:

• Source and drain regions are formed in a semiconductor substrate.
• A thin silicon-dioxide insulating layer separates the gate from the substrate.
• The insulated gate causes extremely high input resistance.

Working of an N-channel enhancement MOSFET:

• At , no conducting channel exists and drain current is nearly zero.
• A positive gate voltage attracts electrons toward the gate region.
• When reaches the threshold voltage , an inversion channel forms.
• Applying then causes drain current to flow.
• Increasing strengthens the channel and increases drain current.

It is called a voltage-controlled device because the drain current is controlled by gate-to-source voltage, while the steady gate current is ideally zero.

The characteristics of an N-channel enhancement MOSFET describe how drain current varies with and .

1. Cut-off region:

• When , no strong channel is formed.
• Ideally:

2. Ohmic or triode region:

• Conditions are and .
• The MOSFET behaves approximately as a voltage-controlled resistance.

3. Saturation region:

• The condition is .
• The channel pinches off near the drain, and current is mainly controlled by .

With channel-length modulation:

Output characteristics: These are plots of against for different fixed values of . They show the triode and saturation regions.

Transfer characteristic: This is the plot of against in saturation. Current begins at and then rises approximately according to the square law.

MOSFETs are used as switches in cut-off and triode regions, and as amplifiers primarily in the saturation region.

Major applications of diodes include:

• Rectification: PN diodes convert AC into pulsating DC in half-wave and full-wave rectifiers.
• Voltage regulation: Zener diodes maintain an approximately constant output voltage in the reverse-breakdown region.
• Clipping circuits: Diodes remove or limit selected portions of a voltage waveform.
• Clamping circuits: Diodes shift the DC level of a waveform without significantly changing its shape.
• Signal detection: Diodes recover information from amplitude-modulated signals.
• Electronic switching: A forward-biased diode acts approximately as a closed switch, while a reverse-biased diode acts as an open switch.
• Protection: Flyback diodes protect transistor switches from inductive voltage spikes. Diodes also provide reverse-polarity and transient protection.
• Light emission: LEDs convert electrical energy into light.
• Light detection: Photodiodes convert incident light into current.
• Variable capacitance: Varactor diodes provide voltage-controlled capacitance in tuning circuits.
• Voltage multiplication: Diode-capacitor networks produce output voltages greater than the AC input peak.

The choice of diode depends on current rating, reverse-voltage rating, switching speed, power dissipation, and forward-voltage drop.

A bipolar junction transistor is widely used for amplification, switching, oscillation, and signal processing.

BJT as an amplifier:

• The transistor is biased in the active region.
• The base-emitter junction is forward biased and the base-collector junction is reverse biased.
• A small variation in base current produces a larger variation in collector current.
• The current relation is approximately:

• A collector resistor converts collector-current variations into voltage variations, producing voltage amplification.

BJT as a switch:

• In the cut-off region, and , so the transistor behaves like an open switch.
• In the saturation region, sufficient base current is supplied and becomes small, so it behaves like a closed switch.

Applications:

• Audio and radio-frequency amplifiers
• Digital switching circuits
• Relay, motor, LED, and solenoid drivers
• Oscillators and waveform generators
• Current sources and current mirrors
• Voltage-regulator circuits
• Signal modulation and demodulation
• Temperature and sensor interface circuits

A base resistor is required in switching circuits to limit the base current.

Applications of MOSFETs:

• Switching-mode power supplies
• DC-DC converters and inverters
• Motor speed controllers
• Battery protection and power-management systems
• Class-D audio amplifiers
• Analog amplifiers and source followers
• CMOS digital logic circuits
• Microprocessors and memory circuits
• Electronic load switches
• Automotive and renewable-energy power converters

Reasons for preference:

• Very high input resistance: The insulated gate requires negligible steady-state current.
• Low drive power: Gate voltage, rather than continuous gate current, controls the device.
• Fast switching: Majority-carrier operation allows rapid turn-on and turn-off.
• Low on-state resistance: Power MOSFETs can provide low conduction loss when fully enhanced.
• Easy integration: MOSFETs occupy small chip area and are suitable for very-large-scale integration.
• Good thermal behavior: Power MOSFETs can be connected in parallel more easily than BJTs in many applications.
• CMOS efficiency: Complementary MOS circuits consume very low static power.

MOSFET gates are sensitive to electrostatic discharge because the oxide layer is thin. Proper gate protection and handling procedures are therefore necessary.

The regulated output is approximately equal to the Zener voltage:

Step 1: Calculate load current.

Step 2: Select the series resistance at minimum input voltage.

At , the series current must supply both the load current and minimum Zener current:

Therefore,

Thus, a suitable series resistance is .

Step 3: Find maximum Zener current.

At :

Since the load still takes :

Step 4: Calculate maximum Zener power.

Therefore, the design requires approximately , and the Zener diode should have a power rating safely above , such as or higher.