Electronic devices form an important and scoring part of the JEE Physics syllabus. This guide covers semiconductors, p-n junction diodes, transistors, logic gates, amplifiers, and more with detailed explanations, formulas, and tips tailored for JEE aspirants. Understanding these topics helps you solve questions related to circuits, digital electronics, and modern applications efficiently.
Semiconductors are materials with conductivity between conductors and insulators. Their electrical properties can be controlled by doping and external influences, which makes them essential for electronic devices.
Types of semiconductors:
Doping:
- n-type doping adds donor impurities (like phosphorus) which provide extra electrons.
- p-type doping adds acceptor impurities (like boron) which create holes (positive charge carriers).
The conductivity in semiconductors increases with temperature and doping.
When p-type and n-type semiconductors are joined, a p-n junction forms. Electrons from n-region diffuse to the p-region and holes from p-region diffuse to n-region, creating a depletion region with an electric field that opposes further diffusion.
This depletion region acts as a potential barrier.
A diode allows current in one direction (forward bias) and blocks in the other (reverse bias).
Forward bias: p-side connected to positive terminal, n-side to negative; diode conducts current.
Reverse bias: p-side connected to negative terminal, n-side to positive; diode blocks current (except a small leakage current).
The current through a diode is approximately given by the Shockley diode equation:
Where:
\(I_0\) = reverse saturation current,
\(q\) = electron charge = \(1.6 \times 10^{-19} \text{C}\),
\(V\) = applied voltage,
\(k\) = Boltzmann constant,\(1.38 \times 10^{-23} \text{J/K}\),
\(T\) = temperature in Kelvin.
Zener diode is designed to operate in reverse bias in breakdown region without damage. It maintains a nearly constant voltage called the Zener voltage, useful for voltage regulation.
BJT is a three-layer semiconductor device (either NPN or PNP) used as an amplifier or switch. The three terminals are emitter (E), base (B), and collector (C).
In an NPN transistor, a small base current controls a much larger current flowing from collector to emitter. The transistor operation depends on the biasing of junctions:
The main currents satisfy:
Where \(I_B\), \(I_C\), and \(I_E\) are base, collector, and emitter currents respectively, and \(\beta\) is the current gain.
Common configurations for BJTs are:
The transistor amplifies current and voltage. Voltage gain is given by:
The input is applied between base and emitter, and output is taken between collector and emitter.
The transistor can act as an electronic switch, working in cutoff (OFF) and saturation (ON) states, controlled by base current.
FETs are voltage-controlled devices used for switching and amplifying signals. Types include:
FETs have high input impedance and are widely used in digital and analog circuits.
Logic gates are basic building blocks of digital circuits, performing logical operations on inputs to produce an output.
| Gate | Symbol | Truth Table | Boolean Expression |
|---|---|---|---|
| AND | ∗ |
A | B | Output 0 | 0 | 0 0 | 1 | 0 1 | 0 | 0 1 | 1 | 1 |
$Y = A \cdot B$ |
| OR | + |
A | B | Output 0 | 0 | 0 0 | 1 | 1 1 | 0 | 1 1 | 1 | 1 |
$Y = A + B$ |
| NOT | ¬ |
A | Output 0 | 1 1 | 0 |
$Y = \overline{A}$ |
| NAND | ⊼ |
A | B | Output 0 | 0 | 1 0 | 1 | 1 1 | 0 | 1 1 | 1 | 0 |
$Y = \overline{A \cdot B}$ |
| NOR | ⊽ |
A | B | Output 0 | 0 | 1 0 | 1 | 0 1 | 0 | 0 1 | 1 | 0 |
$Y = \overline{A + B}$ |
| XOR | ⊕ |
A | B | Output 0 | 0 | 0 0 | 1 | 1 1 | 0 | 1 1 | 1 | 0 |
$Y = A \oplus B = A \overline{B} + \overline{A} B$ |
Amplifiers increase the amplitude of electrical signals. The gain of an amplifier is the ratio of output to input signal.
Voltage gain: $$A_v = \frac{V_{out}}{V_{in}}$$
Types of amplifiers include voltage amplifiers, power amplifiers, and operational amplifiers.
Oscillators generate continuous AC signals without input, used in radio, clocks, and signal generators.
Common types are LC oscillators and RC oscillators.
| Device/Concept | Formula/Relation | Remarks |
|---|---|---|
| Diode Equation | $$I = I_0 \left(e^{\frac{qV}{kT}} - 1\right)$$ | Current through diode |
| Transistor Currents | $$I_C = \beta I_B, \quad I_E = I_B + I_C$$ | Relation between currents in BJT |
| Brewster's Angle | $$\tan \theta_B = \frac{n_2}{n_1}$$ | Angle for polarization by reflection |
A silicon diode has a reverse saturation current \(I_0 = 10^{-12} \text{A}\) and is at room temperature (300K). Calculate the forward current when a voltage of 0.7 V is applied.
Solution:
Substituting,
So,
In an NPN transistor, the collector current is 100 mA and the current gain \(\beta = 50\). Find the base current.
Solution:
This complete guide covers the essential concepts you need for mastering electronic devices in JEE Physics. Regular practice, revision, and problem-solving will help you score confidently on this topic.