RBSE Class 12 Physics Chapter 14 — Semiconductor Electronics: Most Repeated Questions (2013–2026)

The process of converting alternating current (AC) into direct current (DC) is called rectification

A half-wave rectifier converts alternating current (AC) to pulsating direct current (DC) by allowing only one-half cycle of the input waveform to pass to the output while blocking the other half

It typically consists of a transformer, a single diode, and a load resistor

Working Principle

The operation is based on the one-way conduction property of a diode:

Positive Half-Cycle:

The AC input voltage makes the anode of the diode positive with respect to the cathode

The diode is forward biased and acts like a closed switch (it conducts)

Current flows through the diode and the load resistor, producing an output voltage across the load

Negative Half-Cycle:

The AC input voltage reverses polarity, making the anode of the diode negative with respect to the cathode

The diode is reverse biased and acts like an open switch (it does not conduct)

No significant current flows through the circuit, resulting in zero output voltage across the load

The output voltage is a pulsating DC signal, consisting of only the positive half-cycles of the original AC input, separated by periods of zero voltage. This output requires further filtering (using capacitors or inductors) to become a smooth, usable DC voltage

All solved questions for this chapter are in the full RBSE Physics PYQ book.

Connect the p-side to the battery's positive terminal and the n-side to the negative terminal through a variable voltage source ( V_F ) and a milli-ammeter ( I_F )

Gradually increase the applied voltage ( V_F ) from zero, recording the corresponding forward current ( I_F ) at each step

Initially, the current is negligible until V_F overcomes the potential barrier (Knee voltage)

Past the Knee voltage, the diode conducts, and the current ( I_F ) increases sharply; plotting the measured V_F vs. I_F gives the V-I characteristic curve

All solved questions for this chapter are in the full RBSE Physics PYQ book.

An extrinsic semiconductor is a material that has been intentionally doped with impurities to significantly increase its conductivity

There are two types of extrinsic semiconductors:

​N-type Semiconductor

​P-type Semiconductor

​Processes in P-N Junction Formation

​When a P-type semiconductor is brought into contact with an N-type semiconductor to form a P-N junction, two primary processes occur: Diffusion and Drift

​Diffusion: Due to the large concentration difference, the majority carriers spontaneously move across the junction:

​Electrons from the N-side (high concentration) diffuse into the P-side (low concentration)

​Holes from the P-side (high concentration) diffuse into the N-side (low concentration)

​Formation of Depletion Layer & Drift:

​As electrons move from N to P, they leave behind immobile positively charged donor ions on the N-side

​As holes move from P to N, they leave behind immobile negatively charged acceptor ions on the P-side

​The region around the junction, now devoid of mobile charge carriers, is called the Depletion Layer or Space-Charge Region

​The net charge separation creates an internal electric field ( E⃗ ) directed from the positive N-side to the negative P-side

​This electric field creates a potential barrier ( V_b ) that opposes further diffusion. The force exerted by this field on mobile carriers causes the drift process, where minority carriers are swept across the junction, balancing the diffusion current in equilibrium

E=V_B/W

Barrier Potential (V_B)=0.7 V

Width of Depletion Layer (W)=1 micrometer

Convert to meters: W=1×10^-6 m

E=0.7 V/(1×10^-6 m)

E=0.7×10⁶ V/m

E=7×10⁵ V/m

All solved questions for this chapter are in the full RBSE Physics PYQ book.

(A)

Feature

Conductor (Metal)

Semiconductor

Insulator

Band Gap ( E_g )

Zero ( E_g= 0 eV). The valence band and conduction band overlap

Small (typically 1 eV, generally E_g < 3 eV)

Large (typically E_g > 3 eV, often 5 - 15 eV)

Band Structure

The conduction band is partially filled, or the valence band overlaps the conduction band

A small gap separates the filled valence band from the empty conduction band at absolute zero (0 K)

A large gap separates the filled valence band from the empty conduction band

Electron Availability for Conduction

Abundant free electrons are available in the conduction band even at room temperature because no energy is required to move them

Few electrons are available at room temperature, but the number increases significantly with temperature or doping, as a small amount of energy can excite electrons across the gap

Virtually none are available at room temperature, as the large energy gap prevents electrons from easily jumping to the conduction band

Electrical Conductivity ( σ )

Very High (High conductivity, low resistivity)

Moderate

(B)

All solved questions for this chapter are in the full RBSE Physics PYQ book.

i)

The depletion region is the narrow zone around a p-n junction, depleted of mobile charge carriers, containing only fixed immobile donor and acceptor ions, formed by diffusion of majority carriers across the junction

The barrier potential is the potential difference that develops across the depletion region (due to the separated fixed charges) which opposes further diffusion of majority carriers across the junction

ii) Energy band diagram of a p-type semiconductor (T > 0 K): the acceptor energy level (Eᴀ) lies just above the valence band edge (Eᵛ), close enough that at T > 0 K electrons from the valence band are thermally excited up into it, leaving holes behind in the valence band; the Fermi level sits between Eᵛ and Eᴀ, close to the valence band

All solved questions for this chapter are in the full RBSE Physics PYQ book.