Revision: Electronic Devices JEE Main Electronic Devices

A pure semiconductor in which no impurity is added intentionally.

The concentration of charge carriers in an intrinsic semiconductor, where the number of electrons equals the number of holes.

The vacancy left in the valence band when an electron leaves it behaves like a positive charge carrier in semiconductor theory.

Intrinsic semiconductors have very low conductivity at room temperature. Therefore, they are not useful for constructing electronic devices. Their electrical conductivity can be increased by adding a suitable impurity. This process is called doping.

A doped semiconductor is called an extrinsic semiconductor or impurity semiconductor.

The impurity added is called a dopant.

The semiconductor to which the dopant is added is called the host.

The region around the junction that is free from mobile charge carriers is called the depletion region.

The potential difference developed across the depletion layer due to immobile ions is called the barrier potential.

The two-terminal semiconductor device that allows current mainly in one direction is called a p-n junction diode.

The motion of charge carriers under the influence of an electric field is called drift.

The process in which charge carriers move from a higher concentration to a lower concentration is called diffusion.

The minimum forward voltage after which the current rises sharply is called the knee voltage.

The small current flowing in reverse bias due to minority carriers is called the reverse saturation current.

The reverse voltage at which the current suddenly increases rapidly is called the breakdown voltage.

The ratio of voltage to current at any operating point of a diode is called static resistance.

The ratio of a small change in voltage to the corresponding small change in current is called dynamic resistance.

The boundary formed when p-type and n-type semiconductor regions are joined in a single crystal is called a p-n junction.

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 a two-terminal p-n junction device that allows current to pass easily in one direction and offers high resistance in the opposite direction.

• 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.

R = \[\frac {V}{I}\]

\[r_d=\frac{\Delta V}{\Delta I}\]

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

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

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

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

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

\[I=I_Z+I_L\]

Formation of Energy Bands

In a single isolated atom, electrons occupy discrete, well-defined energy levels (like rungs of a ladder).

When atoms are packed together in a crystal lattice, electrons are shared among neighbouring atoms. This causes:

• Each discrete energy level splits into closely spaced levels
• These closely spaced levels collectively form an energy band

Analogy: Think of a single tuning fork producing one frequency. When thousands of tuning forks are placed close together, they create a continuous range (band) of frequencies, not a single note.

Key Energy Bands

• Intrinsic semiconductors have very low conductivity at room temperature.
• Doping increases conductivity.
• A doped semiconductor is called an extrinsic semiconductor.
• The impurity added is called a dopant.
• The semiconductor receiving the impurity is called the host.
• The dopant size should be nearly the same as that of the host atom.
• Pentavalent and trivalent impurities are used as dopants.
• Extrinsic semiconductors are of two types: n-type and p-type.

• A p-n junction is formed by joining p-type and n-type semiconductor regions in a single crystal.
• Diffusion of carriers creates a depletion region and barrier potential.
• A p-n junction diode conducts mainly in one direction.
• In forward bias, the barrier potential decreases, and the current becomes large.
• In reverse bias, the barrier potential increases and only a small minority-carrier current flows.
• In zero bias, the diffusion and drift currents balance, so the net current is zero.
• The knee voltage is about 0.3 V for germanium and 0.7 V for silicon.
• Static resistance is given by R = V/I, and dynamic resistance is given by r_d = ΔV/ΔI.

• 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.