Revision: Class 12 >> Electronic Devices NEET (UG) Electronic Devices

The continuous energy band formed by the merging of energy bands of each atom, known as the valence band.

The number of free electrons (nₑ) and the number of holes (nₕ) in an intrinsic semiconductor, where nₑ = nₕ = nᵢ. Here nₑ and nₕ are called the intrinsic carrier concentration.

• A pure semiconductor, such as pure silicon or pure germanium, is called an intrinsic semiconductor.
• A semiconductor free from all types of impurities is called an intrinsic semiconductor.

• The semiconductor with impurity added to it is called a doped semiconductor or extrinsic semiconductor.
• A semiconductor doped with a suitable impurity, so as to possess conductivity much higher than the pure semiconductor is called an extrinsic semiconductor.

The trivalent impurity atom added during doping in a p-type semiconductor is known as an acceptor.

• The semiconductor in which a silicon or germanium crystal is doped with a trivalent impurity (acceptor), making holes the majority charge carriers, is called a p-type semiconductor.
• On doping an intrinsic semiconductor with trivalent impurity like Indium (In) or Gallium (Ga), the semiconductor becomes deficient in electrons, i.e., the number of holes becomes more than the number of electrons. Such a semiconductor is called p-type.

• The semiconductor in which a silicon or germanium crystal is doped with a pentavalent impurity (donor), making electrons the majority charge carriers, is called an n-type semiconductor.
• When a few atoms of the pentavalent elements (phosphorus, arsenic, antimony and bismuth) are added to the pure germanium or silicon crystal, the resulting crystal is called an n-type semiconductor.

The pentavalent impurity atom added during doping in an n-type semiconductor is known as a donor.

A semiconductor diode's depletion zone is the area surrounding the p-n junction where there are no mobile charge carriers, this area generates an electric field that allows the diode to conduct in one direction while blocking in another.

(i) Rectifier: It is a device which converts alternating current into direct current.

(ii) Amplifier: An amplifier is a device which increases the energy of a weak signal by supplying energy from an external source. An amplifier increases the amplitude of a input signal.

(iii) Oscillator: An oscillator is a device which produces electrical oscillations of adjustable frequency and constant amplitude. An oscillator is basically an amplifier. A part of the output energy is fed back into the L-C circuit to produce sustained oscillations.

A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage.

The barrier that the repelling forces use to stop the mobile charge carriers (at the PN junction) is known as the potential barrier.

This results from the concentration of immobile charges close to the junction after electrons and holes diffuse across the function.

The resistance offered by a p-n junction diode when it is in forward biased condition is called static (DC) resistance.

A p-n junction when provided with metallic connectors on each side is called a junction diode.

• The formation of a narrow region on either side of the junction which becomes free from mobile charge carriers is called depletion region.
• The small charge-free region formed near the junction where electrons combine with holes is known as the depletion region.

The difference in potential that prevents charge carriers from moving across the p-n junction is called the potential barrier.

The current flowing from p-side to n-side due to diffusion of electrons and holes because of concentration difference is called diffusion current.

The current flowing from n-side to p-side due to holes and electrons created in the depletion region is called drift current.

When a semiconducting material such as silicon or germanium is doped with a trivalent impurity on one side and pentavalent impurity on the other side, a p-n junction is obtained. The plane separating the two regions is called a junction.

The resistance of a diode at a particular applied voltage is called dynamic (AC) resistance.

When a high reverse voltage causes a sudden and uncontrollable increase in current, the phenomenon is called avalanche breakdown.

• When n-type and p-type semiconductor materials are fused together, the junction formed is called a p-n junction.
• The device obtained by growing a p-type semiconductor over an n-type semiconductor or vice versa is called a p-n junction.

When an external voltage V is applied across a semiconductor diode such that the p-type is connected to +ve terminal and n-type to -ve terminal of battery (in general p-type to high voltage and n-type to low voltage), the diode is said to be forward biased.

When an external voltage V is applied across a semiconductor diode such that the p-type is connected to the -ve terminal and n-type to the +ve terminal of the battery (in general, p-type to low voltage and n-type to high voltage), the diode is said to be reverse biased.

A basic semiconductor device that controls the flow of electric current in a circuit, which when forward biased behaves as a closed circuit and when reverse biased behaves as an open circuit, is called a p-n Junction Diode.

• The electronic circuit which rectifies AC voltage is called a Rectifier.
• The device used to convert an alternating current into a direct current is called a rectifier. 

• The conversion of AC voltage into a DC voltage is called Rectification.
• The process of converting an alternating current into a direct current is called rectification.

• A rectifier that consists of one p-n junction diode in which alternate pulses of AC input are rectified, having a maximum efficiency of 40.6% and output frequency the same as that of input, is called a Half-Wave Rectifier.
• A rectifier which rectifies only one-half of each AC input supply cycle is called a half-wave rectifier.

To rectify AC power for using both half cycles of the sine wave, a different rectification circuit configuration is used, which is known as full-wave rectification.

• A rectifier that consists of two p-n junction diodes in which both the pulses of AC input are rectified, having a maximum efficiency of 81.2% and an output frequency twice that of the input, is called a Full-Wave Rectifier.
• A rectifier which rectifies both halves of each AC input cycle is called a full wave rectifier.

It is a semiconductor device used to convert photons of solar light into electricity. It generates emf when solar radiation falls on the p-n junction. A p-type silicon wafer of about 300 μm is taken over which a thin layer of n-type silicon is grown on one side by the diffusion process.

A unique form of a bipolar device which permits the current flow in the reverse direction when the voltage applied is above a certain characteristic value called Zener voltage or breakdown voltage, most commonly used in voltage regulators to protect other semiconductor devices from fluctuations in voltage, is called a Zener Diode.

A special purpose junction diode that converts light energy into electrical current, works on the principle of the photoelectric effect, operates in reverse bias, and generates a current when exposed to light (proportional to the intensity of incident light), is called a Photodiode.

A heavily doped p-n junction diode used to operate in reverse bias is called a Zener diode.

A signal that has only two states (0 and 1) is called a Digital Signal.

A signal that has continuous values is called an Analog Signal.

A device that acts as a building block for digital circuits and performs basic logical functions that are fundamental to digital circuits is called a Logic Gate.

\[E=\frac{V_b}{d}\]

Where:

• \[V_b\]​ = potential barrier
• d = width of the depletion layer
• E = electric field intensity

r_a = \[\frac {ΔV}{ΔI}\]

It is the reciprocal of the slope of the I-V characteristics at that point.

\[V_{eff}=V_b-V\]

Where:

• \[V_b\] = potential barrier voltage
• V = applied external voltage

\[I=I_e+I_h\]

Where:

• \[I_e\]​ = electron current
• \[I_h\]​ = hole current

\[V_{eff}=V_b+V\]

Where:

• \[V_b\]​ = potential barrier voltage
• \[V\] = applied external voltage

\[I_{Z_{max}}=\frac{P_{max}}{V_Z}\]

Where \[P_{max}\] = power dissipation capability of Zener diode.

\[R=\frac{V_{IN}-V_{OUT}}{I_Z+I_L}\]

\[I_L=\frac{V_s-V_Z}{R_s}\quad\mathrm{or}\quad I_L=I_S+I_Z\]

\[V_{OUT}=V_{IN}-I_R=V_{IN}-(I_Z+I_L)R\]

\[I=I_Z+I_L\]

• Solids are classified as metals, insulators, and semiconductors based on conductivity or band theory.
• Metals have the highest conductivity among the three classes.
• Insulators have the lowest conductivity and the highest resistivity.
• Semiconductors have conductivity intermediate between metals and insulators.
• Higher conductivity corresponds to lower resistivity, and vice versa.

• Solid-state materials are grouped as insulators, semiconductors, and conductors.
• Common semiconductors, silicon and germanium, have 4 outer valence electrons for bonding.
• In a pure crystal, each atom is bonded covalently to another four atoms, with its outer electrons bonded, leaving very few free electrons, making resistance very large.
• A few free electrons arise from imperfections in the crystal lattice and thermal ionisation due to heating.
• Higher temperature results in more free electrons, which increases conductivity and decreases resistance — as seen in a thermistor.

• In Bohr's model, electrons occupy sharp and distinct energy levels, but with many atoms interacting, these levels spread out and broaden into energy bands.
• In a silicon crystal, there are 10²³ atoms per cubic centimetre, so individual energy levels form broad energy ranges.
• Energy band diagrams of semiconductors plot energy as a function of wave number vˉvˉ along crystallographic directions, as the band diagram depends on direction in the crystal.
• Energy band diagrams contain many completely-filled and empty bands along with multiple partially-filled bands.
• When a hole moves, it is actually electrons moving in the opposite direction — the hole appears to move to the right as the electron moves to the left.

• An intrinsic semiconductor is pure — free from all types of impurities.
• At 0 K, an intrinsic semiconductor behaves as an insulator with zero conductivity.
• At temperatures above 0 K, electrons gain energy and move to the conduction band, creating holes in the valence band.
• The number of free electrons always equals the number of holes in an intrinsic semiconductor.
• \[n_e, n_h,\] and \[n_i\]​ are used to denote intrinsic carrier concentrations.

• Electrical properties of semiconductors can be altered by adding small amounts of impurities.
• Doped semiconductors are known as extrinsic semiconductors.
• Two types of dopants are used for tetravalent Si or Ge:
• Pentavalent (valency 5): Arsenic (As), Antimony (Sb), Phosphorous (P)
• Trivalent (valency 3): Indium (In), Boron (B), Aluminium (Al)
• Doping increases conductivity in a controlled manner.
• Extrinsic semiconductors are used in electronic devices like transistors, diodes, and light-dependent resistors (LDRs).

• A p-type semiconductor is formed by doping with a trivalent impurity (e.g., Indium, Gallium, Boron).
• The trivalent impurity has 3 outer electrons, creating a hole in the crystal lattice where no electron is present.
• Due to a lack of electrons, the Fermi level shifts closer to the valence band.
• Holes are majority carriers; electrons are minority carriers — nh≫nenh​≫ne.
• The acceptor energy level lies approximately 0.01 to 0.05 eV above the valence band.

• An n-type semiconductor is formed by doping with a pentavalent impurity (e.g., Phosphorus, Arsenic, Antimony, Bismuth).
• Arsenic has 5 outer electrons — 4 are used in bonding with silicon, and the 5th electron is free to move and conduct.
• The pentavalent impurity atom acts as a donor as it donates a free electron.
• Electrons are majority carriers; holes are minority carriers — ne≫nhne​≫nh.
• The donor energy level lies approximately 0.1 eV below the conduction band.
• The Fermi level shifts closer to the conduction band in n-type semiconductors.

• A semiconductor diode consists of a p-n junction with metallic contacts at both ends.
• It can be made from either Silicon or Germanium, each differing in size and properties.
• Six types of diodes are: Diode, LED, Photodiode, Schottky diode, Tunnel diode, and Zener diode.
• The Anode is the p-side, and the Cathode is the n-side of the diode.
• External voltage is applied through the metallic contacts at the ends.

• At a p-n junction, donor impurity atoms become positively charged ions and acceptor atoms become negatively charged ions — these act like two electrodes forming a p-n junction diode.
• A strong electric field, directed from the n-type to the p-type semiconductor, exists at the junction.
• Within the depletion layer, only immobile positive and negative ions are present; material outside remains neutral.
• The potential barrier is influenced by the type of semiconductor crystal, temperature, and the level of doping.
• If the diode is ON, it has no voltage across it and acts as a short circuit; if OFF, current is zero and acts as an open circuit.
• A diode is a two-terminal device — unlike capacitors (current related to the derivative of voltage) or inductors (derivative of current related to voltage), current in a diode is not linearly related to voltage.
• In a p-n junction, there is a transfer of charge through the junction due to the concentration gradient of charge carriers with the barrier potential.

• In forward bias, the depletion layer becomes thin and the forward current increases strongly after the KNEE point.
• The resistance of an ideal diode in forward bias is zero.
• If external voltage (V) is greater than barrier voltage, majority carriers diffuse across the junction, constituting diffusion current \[I=I_e+I_h\].
• Knee voltage for Germanium (Ge) ≈ 0.3 V and for Silicon (Si) ≈ 0.7 V.
• Electrons and holes freely cross the junction in forward bias, leading to a diffusion current opposite to the reverse saturation current.
• As forward bias increases, the effective barrier reduces to \[V_b - V\], allowing more carriers to cross.

• In reverse bias, the current is quite small and is independent of the external voltage (until breakdown).
• The width of the depletion layer increases, and the p-n junction diode acts as a resistor.
• The width of the potential barrier increases, obstructing the flow of majority carriers in both the n-side and the p-side.
• In reverse bias, the majority charge carriers are attracted away from the depletion layer by their respective battery terminals connected to the p-n junction.
• Positive terminal attracts electrons away from the junction in the n-side; negative terminal attracts holes away from the junction in the p-side.
• Beyond a certain voltage, breakdown occurs via avalanche or Zener mechanism.

• In forward bias, the current is initially very small; it rises sharply only after the knee voltage.
• Knee voltage: Ge = 0.3 V, Si = 0.7 V (also called threshold voltage / forward voltage).
• In reverse bias, current is very low and nearly constant — measured in microamperes (μA).
• At reverse breakdown voltage, current increases sharply due to the avalanche effect.
• Avalanche effect: high-velocity electrons knock bonded electrons → chain reaction → sharp rise in current.
• The V–I graph has four regions: forward bias, knee, reverse bias, and reverse breakdown region.

• A rectifier is a circuit which converts an AC supply into a unidirectional DC supply.
• A p-n junction diode acts as a rectifier because it allows current to flow in one direction only.
• The bridge rectifier circuit uses semiconductor diodes for converting AC, as it allows current to flow in one direction only.
• Input to the rectifier is AC \[(V_{IN})\]; output is DC \[(V_{OUT})\] — shown as a full-wave rectified signal.
• Rectification is the fundamental principle behind power supply circuits in electronic devices.

• A half-wave rectifier uses a single diode, allowing current to flow in one direction, with an AC power source\[V_{ac}\] connected to the diode and a resistor in series.
• Output is discontinuous and pulsating DC - only positive half cycles appear across the load.
• Alternative (negative) half cycles of the AC supply go to waste, making efficiency very low.
• The output waveform shows only the positive side of the sinusoidal cycle, clamping off the negative side.
• Circuit components: transformer (primary & secondary), single diode, and load resistor \[R_L\].
• A transformer is used to step up or step down the AC voltage before rectification.

• A full-wave rectifier rectifies both half-cycles of the AC input; the output is continuous and pulsating.
• Two types: Centre-tap rectifier (uses 2 diodes) and Bridge rectifier (uses 4 diodes).
• The output of a full-wave rectifier is continuous but pulsating — it can be made smooth using a filter circuit.
• A large capacitor in parallel with the output load resistor reduces the ripple from the rectification process.
• Full-wave rectifiers are used in power supplies to convert AC voltages to DC voltages.
• A bridge rectifier uses no centre-tap transformer, making it more commonly used in practice.

• LED emits visible light or invisible infrared light when forward-biased.
• LEDs which emit invisible infrared light are used for remote controls.
• Types of LED materials and their colours:
• GaAs - infrared; GaAsP - red to infrared, orange
• GaP - red, yellow, green; AlGaP - green
• GaN - green, emerald green; GaInN - near ultraviolet, bluish-green, blue
• SiC - blue (as substrate); ZnSe - blue; AlGaN - ultraviolet
• AlGaAsP - high-brightness red, orange-red, orange, yellow
• The energy band diagram shows a forbidden gap between conduction and valence bands - emitted photons correspond to this gap energy.
• LED I–V characteristics show different curves for Red (R), Yellow (Y), Green (G), and Blue (B) - the higher the frequency of light, the higher the threshold voltage.

• A photodiode is specially designed to operate in reverse bias conditions.
• It conducts electric current in a similar proportion to the amount of light falling on it.
• A photodiode has two terminals - anode and cathode - with arrows indicating light rays falling on the diode.
• It is also referred to as a photo-detector, photo-sensor, or light detector.
• A photodiode works in reverse bias and is sensitive to a specific wavelength.
• It can be used as a photodetector to detect optical signals.
• Applications include: sensors, communication systems, and solar cells.

• Solar cells require no biasing — they supply emf like an ordinary cell.
• Sunlight is required for a solar cell to function.
• A solar cell is a sandwich of two different layers of silicon — the lower layer is p-type (fewer electrons), and the upper layer is n-type (more electrons).
• It contains n-type silicon and p-type silicon layers that generate electricity with sunlight by making electrons jump across the junction.
• It is compact in size and bundled with larger units for making solar panels.
• The I–V characteristics show two key parameters:​ \[V_{oc}\](open circuit voltage) and \[I_{sc}\]​ (short circuit current).
• Semiconductors with a band gap close to 1.5 eV are ideal for solar cell fabrication.

• A Zener diode is heavily doped — this causes breakdown at low reverse voltages.
• It is designed to operate in the reverse breakdown region.
• Zener diode is used as a voltage regulator.
• It is commonly used for making reference voltages and to protect electronic devices from voltage surges.
• The breakdown voltage (Zener voltage) is set during manufacturing by controlling the doping level.
• It allows current in both directions — forward like a normal diode, and reverse beyond Zener voltage.

• A Zener diode maintains a constant voltage across the load as long as the supply voltage is more than the Zener voltage.
• If the input voltage increases, the current through the Zener diode increases while the voltage drop remains constant.
• In the Zener regulator circuit,\[R_s \] is used to limit reverse current through the diode to a safer value \[V_s\], and \[R_s \] is selected so the diode operates in the breakdown region.
• When IZIZ​ becomes zero, IZIZ​ reaches its maximum value - at that case \[R=\frac{V_{IN}-V_{OUT}}{I_{Z_{max}}}\]. 
• Voltage regulator IC (e.g. LM7805) is a special three-terminal device: Pin 1 = \[V_{IN}\]​, Pin 2 = GND, Pin 3 = +5V regulated output.
• The voltage regulator has been designed to act as an ideal battery.