Revision: Electronic Devices JEE Main Electronic Devices

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.

• 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.

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.

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

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.

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.

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.

(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 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. 

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 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 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 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.

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.

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

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.

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

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

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

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

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

\[I=I_Z+I_L\]

• 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.

• 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).

• 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.

• 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.

• 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 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.