15 extra multiple-choice questions for Electronics and Communication (12th Standard Physics, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
At room temperature, what are the forbidden energy gap values for Silicon and Germanium respectively?
- A. 0.7 eV and 1.1 eV
- B. 1.1 eV and 0.7 eVCorrect
- C. 6 eV and 1 eV
- D. 1 eV and 6 eV
Explanation. Silicon has a higher energy gap of 1.1 eV compared to Germanium's 0.7 eV at room temperature. These values determine how easily electrons move from the valence band to the conduction band in these specific semiconductor materials.
Q2
Why are semiconductors described as having a negative temperature coefficient of resistance?
- A. Resistance increases with temperature
- B. Resistance decreases as temperature risesCorrect
- C. Resistance remains constant regardless of temperature
- D. Conductivity decreases as temperature rises
Explanation. As the temperature of a semiconductor increases, more covalent bonds break, releasing charge carriers. This increase in the number of available electrons and holes lowers the material's resistance, leading to a negative coefficient.
Q3
In an intrinsic semiconductor at room temperature, what is the relationship between the number of electrons in the conduction band and holes in the valence band?
- A. More electrons than holes
- B. More holes than electrons
- C. Equal number of electrons and holesCorrect
- D. No holes are present
Explanation. In pure semiconductors, every electron that gains enough thermal energy to jump to the conduction band leaves behind a single vacancy or hole in the valence band. Thus, the concentrations of these charge carriers are always identical.
Q4
Which group of elements is used as dopants to create an n-type extrinsic semiconductor from pure silicon?
- A. Group III trivalent elements
- B. Group V pentavalent elementsCorrect
- C. Group IV tetravalent elements
- D. Noble gases
Explanation. To create n-type material, silicon is doped with pentavalent atoms like phosphorus or arsenic. Four valence electrons form bonds with silicon, while the fifth electron remains free to contribute to electrical conduction.
Q5
What is the primary cause for the formation of a depletion region near a p-n junction?
- A. Application of forward bias
- B. Diffusion of majority charge carriersCorrect
- C. Application of reverse bias
- D. Doping with neutral atoms
Explanation. When p-type and n-type materials contact, electrons from the n-side and holes from the p-side diffuse across the boundary. This recombination process near the junction creates a region devoid of mobile charge carriers, known as the depletion layer.
Q6
By what factor does the reverse saturation current of a silicon diode increase for every 10 degree Celsius rise in temperature?
- A. It stays the same
- B. It is halved
- C. It doublesCorrect
- D. It triples
Explanation. Reverse saturation current is highly sensitive to thermal changes because it depends on minority carriers generated by heat. In silicon diodes, this leakage current effectively doubles for every ten-degree increase in the operating temperature.
Q7
What is the theoretical maximum efficiency of a full wave rectifier circuit?
- A. 40.6 percent
- B. 50 percent
- C. 81.2 percentCorrect
- D. 100 percent
Explanation. A full wave rectifier processes both halves of the AC input signal. Its maximum theoretical efficiency is double that of a half wave rectifier, reaching 81.2 percent, which represents the conversion of AC power to DC.
Q8
In which region of its V-I characteristics is a Zener diode primarily operated for voltage regulation?
- A. Forward bias region
- B. Breakdown regionCorrect
- C. Saturation region
- D. Cut-off region
Explanation. Zener diodes are heavily doped to withstand operation in the reverse breakdown region. In this state, the voltage across the diode remains remarkably constant even as the current changes significantly, allowing it to regulate output voltage.
Q9
What physical property of the semiconductor material determines the specific color of light emitted by a Light Emitting Diode (LED)?
- A. The size of the diode
- B. The amount of forward current
- C. The energy band gap of the materialCorrect
- D. The type of casing used
Explanation. When charge carriers recombine in an LED, they release energy as photons. The wavelength, and thus the color, of this light is directly related to the specific energy band gap of the semiconductor material being used.
Q10
According to Kirchhoff's law applied to a bipolar junction transistor, how are the emitter (IE), base (IB), and collector (IC) currents related?
- A. IB = IE + IC
- B. IC = IE + IB
- C. IE = IB + ICCorrect
- D. IE = IB - IC
Explanation. The emitter current represents the total flow of charge carriers entering the transistor. This current splits, with a tiny portion forming the base current and the majority forming the collector current as they exit the device.
Q11
When a transistor is used as an amplifier in a common emitter configuration, what is the phase relationship between the input signal and the output signal?
- A. They are in phase
- B. Output lags by 90 degrees
- C. Output is reversed by 180 degreesCorrect
- D. Output leads by 270 degrees
Explanation. In a common emitter amplifier, an increase in input signal increases collector current, which subsequently lowers the output collector-emitter voltage. This inverse relationship results in a 180-degree phase shift between the input and amplified output signals.
Q12
Which Barkhausen condition must be satisfied for a transistor circuit to maintain sustained oscillations?
- A. Negative feedback must be used
- B. The loop gain must be unityCorrect
- C. The phase shift must be 90 degrees
- D. The loop gain must be zero
Explanation. For an oscillator to provide a constant output without an external input, the product of the amplifier gain and the feedback ratio must equal one. This loop gain condition ensures that losses are perfectly compensated.
Q13
In digital electronics, which logic gate produces a high output (1) only when all of its inputs are high?
- A. OR gate
- B. AND gateCorrect
- C. NAND gate
- D. XOR gate
Explanation. The AND gate performs logical multiplication. Its truth table shows that the output remains low for any combination involving a zero input, and only switches to high when every input signal is at logic level one.
Q14
According to De Morgan's first theorem, a NOR gate is logically equivalent to which other arrangement?
- A. Bubbled OR gate
- B. Bubbled AND gateCorrect
- C. NAND gate followed by NOT
- D. XOR gate
Explanation. De Morgan's first theorem states that the complement of a sum equals the product of the complements. Therefore, the operation of a NOR gate is identical to an AND gate whose individual inputs have been inverted.
Q15
Which mode of electromagnetic wave propagation is typically used for radio frequencies between 3 MHz and 30 MHz by reflecting signals off the ionosphere?
- A. Ground wave propagation
- B. Sky wave propagationCorrect
- C. Space wave propagation
- D. Satellite communication
Explanation. Frequencies in the high-frequency range can travel long distances by reflecting between the Earth's surface and the ionized layers of the atmosphere. This ionospheric reflection method is known as sky wave propagation.
