Semiconductor Electronics Class 12 Notes

• Intrinsic semiconductors (pure Si, Ge) have equal numbers of electrons and holes.
• n-type doping adds pentavalent atoms (P, As, Sb): electrons become majority carriers.
• p-type doping adds trivalent atoms (B, Al, In): holes become majority carriers.
• At ordinary temperature the conductivity of a semiconductor rises sharply — unlike metals, which conduct better when cooler.

• Forward bias (p side positive): the barrier lowers and current flows once the applied voltage exceeds ~0.7 V (Si).
• Reverse bias (n side positive): the barrier thickens and only a tiny leakage current flows until breakdown.
• Diode current:
• $I=I_0\left(e^{eV/kT}-1\right)$
• .
• An ammeter in forward bias shows milliamps; reverse bias shows microamps or less.

• LED: emits light when forward biased — energy released as a photon, so the material must have a direct band gap (e.g. GaAs, GaN).
• Photodiode: operated in reverse bias; incident light increases the reverse current.
• Solar cell: a p-n junction that converts light to electricity, operating in the photovoltaic (fourth-quadrant) region with no external bias.

• In the common-emitter (CE) configuration the emitter is common to input and output.
• Voltage gain:
• $A_v=-\beta \frac{R_C}{R_{\text{in}}}$
• — the minus sign means the output is 180° out of phase with the input.
• The base region is thin and lightly doped; the emitter is heavily doped.

• AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts.
• NAND = AND followed by NOT; NOR = OR followed by NOT.
• NAND and NOR are universal gates — every other gate can be built from either one alone.
• XOR outputs 1 when the inputs differ.

Example: In a common-emitter amplifier, $\beta =100$, $R_C=2\text{k}\Omega$ and $R_{\text{in}}=1\text{k}\Omega$. Find the voltage gain.

Solution: $A_v=-\beta \frac{R_C}{R_{\text{in}}}=-100\times \frac{2000}{1000}=-200$. The magnitude is 200 and the negative sign shows the output is an inverted (180° phase-shifted) copy of the input.

Exercises (NCERT Q14.1–Q14.6)

• 14.1) In an n-type silicon, which of the following statement is true: (a) Electrons are majority carriers and trivalent atoms are the dopants. (b) Electrons are minority carriers and pentavalent atoms are the dopants. (c) Holes are minority carriers and pentavalent atoms are the dopants. (d) Holes are majority carriers and trivalent atoms are the dopants.
• 14.2) Which of the statements given in Exercise 14.1 is true for p-type semiconductors.
• 14.3) Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy band gap respectively equal to (Eg)C, (Eg)Si and (Eg)Ge. Which of the following statements is true? (a) (Eg)Si < (Eg)Ge < (Eg)C (b) (Eg)C < (Eg)Ge > (Eg)Si (c) (Eg)C > (Eg)Si > (Eg)Ge (d) (Eg)C = (Eg)Si = (Eg)Ge
• 14.4) In an unbiased p-n junction, holes diffuse from the p-region to n-region because (a) free electrons in the n-region attract them. (b) they move across the junction by the potential difference. (c) hole concentration in p-region is more as compared to n-region. (d) All the above.
• 14.5) When a forward bias is applied to a p-n junction, it (a) raises the potential barrier. (b) reduces the majority carrier current to zero. (c) lowers the potential barrier. (d) None of the above.
• 14.6) In half-wave rectification, what is the output frequency if the input frequency is 50 Hz. What is the output frequency of a full-wave rectifier for the same input frequency.

Answers at a glance

1. 1
   Q14.1 → (c) Holes are minority carriers and pentavalent atoms are the dopants.
2. 2
   Q14.2 → (d) Holes are majority carriers and trivalent atoms are the dopants.
3. 3
   Q14.3 → (c) (Eg)C > (Eg)Si > (Eg)Ge.
4. 4
   Q14.4 → (c) hole concentration in p-region is more as compared to n-region.
5. 5
   Q14.5 → (c) lowers the potential barrier.
6. 6
   Q14.6 → Half-wave: 50 Hz. Full-wave: 100 Hz.

MCQ Questions

1 Mark Questions

1. 1
   Define doping and name typical donor and acceptor impurities for silicon.
2. 2
   What is the barrier potential of a silicon pn junction at room temperature?
3. 3
   In which bias (forward or reverse) does the width of the depletion region increase?
4. 4
   What is meant by the potential barrier at a pn junction?

2 Mark Questions

1. 1
   Classify the following as n-type or p-type: silicon doped with phosphorus; silicon doped with boron. (Ans: n-type; p-type)
2. 2
   Draw the circuit symbol of a pn junction diode and label its anode and cathode.
3. 3
   Write the truth table of a two-input NOR gate.

3 Mark Questions

1. 1
   Explain the formation of the depletion layer and barrier potential at a pn junction, and describe the current in forward and reverse bias with the help of I–V characteristics.
2. 2
   Describe the action of a half-wave rectifier with a circuit diagram, waveforms, and its efficiency. (Ans: η = 40.6%)
3. 3
   Explain how a Zener diode is used as a voltage regulator with a circuit diagram and its characteristic curve.

5 Mark Questions

1. 1
   Explain the working of a transistor as a common-emitter amplifier: derive the relation between input and output and define voltage, current and power gain.
2. 2
   Explain the construction and working of a full-wave rectifier with a transformer centre-tap, giving output waveforms, ripple factor and efficiency (81.2%), and compare it with the half-wave rectifier.