Electronics notes

MDCAT Physics

This chapter explains semiconductors, the formation and working of a PN junction, diode biasing, rectification, transistors, logic gates, and common electronic devices. It also covers thermal effects, transistor current relations, oscilloscopes, modulation, electromagnetic waves, and special-purpose diodes.

Semiconductors and Charge Carriers

A semiconductor has electrical conductivity between that of a conductor and an insulator. Silicon and germanium are the commonly studied semiconductor materials. Their conductivity can be increased by adding a small amount of impurity, a process called doping.

In a pure semiconductor, thermal energy produces equal numbers of free electrons and holes. An electron carries negative charge, while a hole behaves as a positive charge carrier. In an N-type semiconductor, electrons are majority carriers. In a P-type semiconductor, holes are majority carriers.

A pure semiconductor is called an intrinsic semiconductor.
An impurity added to a semiconductor is called a dopant.
Pentavalent impurities have five valence electrons and produce N-type semiconductors.
Trivalent impurities have three valence electrons and produce P-type semiconductors.
The valence of an impurity used to convert germanium into a P-type semiconductor is 3.
In N-type material, electrons are majority carriers and holes are minority carriers.
In P-type material, holes are majority carriers and electrons are minority carriers.
When an N-type semiconductor is heated, the number of electrons and holes increases equally because electron-hole pairs are produced.

Formation of a PN Junction

A PN junction is formed when P-type and N-type regions are joined in a single crystal. Just after joining, electrons diffuse from the N-region towards the P-region, while holes diffuse from the P-region towards the N-region. These carriers recombine near the junction.

The region around the junction that contains almost no mobile charge carriers is called the depletion region. Fixed positive donor ions remain on the N-side, and fixed negative acceptor ions remain on the P-side. These ions create an electric field and a potential barrier that opposes further diffusion.

The depletion region is also called the potential barrier region.
A region having almost zero mobile charge particles is the depletion region.
Positive charge is left in the N-region near the junction after electrons diffuse away.
Negative charge is left in the P-region near the junction after holes diffuse away.
The electric field in the depletion region is directed from the N-side towards the P-side.
The potential difference across the depletion region of a silicon diode is about 0.7 V.
The potential difference across the depletion region of a germanium diode is about 0.3 V.
The potential barrier prevents the continuous diffusion of majority carriers across the junction.

Biasing of a PN Junction Diode

Biasing means applying an external voltage to a PN junction. In forward bias, the P-side is connected to the positive terminal and the N-side to the negative terminal of the battery. The applied voltage opposes the barrier potential, so the depletion region becomes narrow.

In reverse bias, the P-side is connected to the negative terminal and the N-side to the positive terminal. The applied voltage supports the barrier potential, so the depletion region becomes wider. A reverse-biased diode allows only a very small reverse saturation current before breakdown.

Forward bias decreases the width of the potential barrier.
Reverse bias increases the width of the potential barrier.
Forward current in an ordinary semiconductor diode is mainly due to majority carriers.
Reverse current before breakdown is mainly due to minority carriers.
A forward-biased diode has low resistance after the barrier potential is overcome.
A reverse-biased diode has very high resistance and acts approximately as an open switch.
The conventional current direction in forward bias is from the P-side to the N-side through the diode.
The diode symbol allows current easily in the forward direction and opposes current in the reverse direction.

V-I Characteristics and Breakdown

The V-I characteristic shows the relation between voltage across a diode and current through it. In forward bias, the current remains very small until the applied voltage reaches the knee or threshold voltage. After this point, a small increase in voltage produces a large increase in current.

In reverse bias, the current is very small and nearly constant until the breakdown voltage is reached. At breakdown, the reverse current increases suddenly. The current must be limited by an external resistor to prevent damage, unless the diode is designed for breakdown operation.

The forward characteristic of a junction diode is non-linear.
The knee voltage is approximately 0.7 V for silicon and 0.3 V for germanium.
The reverse saturation current is small and almost constant before breakdown.
Breakdown is the sudden increase of reverse current at a particular reverse voltage.
When a strong electric field ruptures covalent bonds and produces many electron-hole pairs, the process is called the Zener effect.
Avalanche breakdown occurs when charge carriers gain enough energy to produce further carriers by collision.
A Zener diode is operated in the breakdown region for voltage regulation.
A diode I-V curve has a small reverse current, a breakdown region, and a rapidly increasing forward current after the knee voltage.

Rectification

Rectification is the process of converting alternating current into unidirectional, pulsating direct current. A diode is suitable for rectification because it conducts mainly in one direction. The output of a rectifier is usually passed through a filter to reduce fluctuations called ripple.

A half-wave rectifier uses one diode and conducts during only one half-cycle of the input AC. A full-wave rectifier uses both half-cycles. It may use two diodes with a centre-tapped transformer or four diodes in a bridge arrangement.

A rectifier converts AC into pulsating DC.
A half-wave rectifier uses one diode and gives one output pulse per input cycle.
A full-wave rectifier uses both half-cycles of the AC input.
A bridge rectifier uses four diodes.
A centre-tapped full-wave rectifier uses two diodes and a centre-tapped transformer.
Phase wave rectifier is not a type of rectifier.
Ripple is the unwanted AC component present in rectified output.
The ripple factor is less for full-wave and bridge rectifiers than for a half-wave rectifier.
A filter capacitor connected across the load reduces ripple by storing charge and supplying current when the rectifier output falls.

Transistors and Current Relations

A transistor is a three-layer semiconductor device used for amplification and switching. The three regions are emitter, base, and collector. In an NPN transistor, the layers are N-type, P-type, and N-type. In a PNP transistor, the layers are P-type, N-type, and P-type.

The emitter is heavily doped, the base is very thin and lightly doped, and the collector is moderately doped. In active operation, the emitter-base junction is forward biased and the collector-base junction is reverse biased.

The three transistor terminals are emitter, base, and collector.
The emitter supplies charge carriers, the base controls the current, and the collector collects carriers.
The transistor current relation is Ie = Ic + Ib.
Current gain in common-emitter configuration is beta = Ic/Ib.
Current gain in common-base configuration is alpha = Ic/Ie.
For the given values Ic = 10 mA and Ib = 40 microampere, beta = 250.
If Ie = 8.86 mA and Ic = 6.254 mA, then Ib = Ie - Ic = 2.606 mA.
A transistor can be used as an amplifier or as an electronic switch.
A PN junction diode cannot be used as an amplifier because it has only two terminals and does not provide transistor action.

Heat Production in Conductors and Electronic Effects

When a current flows through a conductor, free electrons move through the material. They repeatedly collide with atoms or ions of the conductor. These collisions transfer electrical energy to the lattice of the material, producing heat.

The electrical energy converted into heat is given by H = I2Rt, where I is current, R is resistance, and t is time. This heating effect is useful in heaters and fuses, but unwanted heating can damage electronic components.

The cause of heat production in a current-carrying conductor is collision of free electrons with atoms or ions.
Joule heating increases with the square of current.
The heating relation is H = I2Rt.
A fuse works because excessive current produces heat and melts its wire.
Semiconductor conductivity generally increases when temperature rises.
Heating an intrinsic or doped semiconductor produces additional electron-hole pairs.
In an N-type semiconductor, heating increases both electron and hole concentrations equally due to thermal generation.

Logic Gates and Boolean Operations

Digital electronics uses two logic levels, usually represented by 0 and 1. A logic gate performs a Boolean operation on one or more inputs and produces an output. The basic operations are AND, OR, and NOT. NAND and NOR gates are called universal gates because they can be used to make other gates.

An AND gate gives output 1 only when all inputs are 1. An OR gate gives output 1 when at least one input is 1. A NOT gate reverses the input. A NAND gate is an AND gate followed by NOT, while a NOR gate is an OR gate followed by NOT.

The basic Boolean operations are AND, OR, and NOT.
YES operation is not a basic Boolean operation.
AND gate output is 1 only when every input is 1.
OR gate output is 0 only when every input is 0.
NOT gate changes 0 to 1 and 1 to 0.
NAND means NOT-AND.
NOR means NOT-OR.
NAND and NOR are universal gates.
Three NAND gates connected in the standard arrangement can form an OR gate by applying De Morgan's theorem.

Special-Purpose Diodes and Conversion Devices

A special-purpose diode is designed to perform a particular function. A Zener diode regulates voltage, an LED emits light, and a photodiode detects light. A solar cell converts light energy into electrical energy. These devices work because the properties of a PN junction can be controlled by material and construction.

An inverter is an electronic device that changes direct current into alternating current. A rectifier performs the opposite basic conversion, changing alternating current into direct or pulsating direct current.

An LED is a light-emitting diode operated in forward bias.
The colour of light emitted by an LED depends on the semiconductor material and its energy band gap.
A photodiode is generally used as a light detector and is commonly operated in reverse bias.
A Zener diode is used for voltage regulation.
A solar cell converts solar energy into electrical energy.
The device used to convert DC into AC is called an inverter.
A rectifier converts AC into DC, while an inverter converts DC into AC.
Common special-purpose diodes include Zener diodes, LEDs, photodiodes, and solar cells.

Oscilloscope, Modulation, and Electromagnetic Waves

A cathode-ray oscilloscope or oscilloscope displays electrical signals as waveforms. The voltage signal is applied to the vertical or Y-input. The time-base circuit moves the spot horizontally with time. A sinusoidal voltage waveform is studied by connecting voltage to the Y-input and switching on the time base.

Modulation is the process of changing a property of a high-frequency carrier wave according to a low-frequency information signal. For a sinusoidal carrier, the three basic types are amplitude modulation, frequency modulation, and phase modulation. Electromagnetic waves consist of mutually perpendicular electric and magnetic fields.

For studying a voltage waveform on an oscilloscope, voltage is connected to the Y-input.
The time base provides horizontal motion proportional to time.
The vertical scale represents voltage, and the horizontal scale represents time.
The three basic modulations of a sinusoidal carrier are amplitude modulation, frequency modulation, and phase modulation.
In amplitude modulation, the amplitude of the carrier changes.
In frequency modulation, the frequency of the carrier changes.
In phase modulation, the phase of the carrier changes.
The electric field and magnetic field of an electromagnetic wave are perpendicular to each other.
The electric field, magnetic field, and direction of propagation are mutually perpendicular.

Key terms

Semiconductor: A material whose conductivity lies between that of a conductor and an insulator.
Doping: The addition of a small amount of impurity to a pure semiconductor to increase its conductivity.
Hole: The absence of a valence electron that behaves as a positive charge carrier.
Majority carrier: The charge carrier present in greater concentration in a doped semiconductor.
PN junction: The boundary formed by joining P-type and N-type semiconductor regions.
Depletion region: The region near a PN junction containing almost no mobile charge carriers.
Potential barrier: The potential difference produced across the depletion region that opposes further diffusion of majority carriers.
Forward bias: The connection of a diode that reduces its barrier and allows substantial current to flow.
Reverse bias: The connection of a diode that increases its barrier and permits only a very small current before breakdown.
Breakdown: The condition in which reverse current increases sharply after the breakdown voltage is reached.
Rectification: The conversion of alternating current into unidirectional or pulsating direct current.
Ripple: The unwanted alternating component remaining in the output of a rectifier.
Transistor: A three-layer semiconductor device used for amplification and switching.
Current gain: The ratio of output current to input current in a transistor configuration.
Logic gate: An electronic circuit that performs a Boolean operation on binary inputs.
Inverter: A device that converts direct current into alternating current.
Modulation: The process of varying a carrier wave according to an information signal.
Electromagnetic wave: A wave consisting of mutually perpendicular oscillating electric and magnetic fields.

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Physics shortcuts

Comparing distance and displacement

Distance equals the magnitude of displacement only when the particle travels along a straight path without reversing direction.

Check whether the path is straight and one-directional.
If yes, distance = |displacement|.
Example: A particle moves 5 m east in a straight line. Distance = 5 m and displacement magnitude = 5 m.

This shortcut does not apply to a curved path or to motion involving a change of direction.

Projectile range and components

For a projectile launched and landing at the same level, use R = u² sin 2θ/g. Resolve the initial velocity into horizontal and vertical components when needed.

Write ux = u cos θ and uy = u sin θ.
For the same launch and landing level, R = u² sin 2θ/g.
Example: u = 20 m/s, θ = 30°, g = 10 m/s². R = 400 sin 60°/10 = 34.6 m.

The range formula does not apply directly when the projectile lands at a different height.

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