Q. 1
Electronics Engineering
Difficulty: Easy
(1 Mark)
In an intrinsic (pure) semiconductor at room temperature (300 K), the relationship between electron concentration (n) and hole concentration (p) is:
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Step-by-Step Explanation & Concept Rationale
In an intrinsic semiconductor, thermal energy excites valence electrons across the bandgap, creating equal numbers of free conduction electrons and holes. Thus, n = p = ni. For Silicon at 300 K, ni ≈ 1.5 * 10^10 cm^-3.
Q. 2
Electronics Engineering
Difficulty: Easy
(1 Mark)
What is the forbidden energy bandgap (Eg) at 300 K for Silicon (Si) and Germanium (Ge), respectively?
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Step-by-Step Explanation & Concept Rationale
At room temperature (300 K), the forbidden energy bandgap of Silicon is approximately 1.12 eV, while for Germanium it is approximately 0.66 eV (0.67 eV). Note that 0.7 V and 0.3 V represent the forward threshold/barrier voltages of their respective PN junctions.
Q. 3
Electronics Engineering
Difficulty: Easy
(1 Mark)
Doping intrinsic Silicon with a pentavalent impurity (such as Phosphorus, Arsenic, or Antimony) creates a(n):
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Step-by-Step Explanation & Concept Rationale
Pentavalent atoms have 5 valence electrons. Four form covalent bonds with adjacent Si atoms, while the fifth electron is loosely bound (donor level ~0.05 eV below conduction band) and easily liberated at room temperature, creating an N-type semiconductor where electrons are majority carriers.
Q. 4
Electronics Engineering
Difficulty: Easy
(1 Mark)
Doping Silicon with a trivalent impurity (such as Boron, Gallium, or Indium) creates a(n):
💡
Step-by-Step Explanation & Concept Rationale
Trivalent atoms have 3 valence electrons, leaving one covalent bond vacancy (a hole). They readily accept electrons from nearby bonds, creating mobile holes as majority carriers and fixed negative acceptor ions, forming a P-type semiconductor.
Q. 5
Electronics Engineering
Difficulty: Easy
(1 Mark)
The Mass Action Law for a semiconductor under thermal equilibrium states that:
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Step-by-Step Explanation & Concept Rationale
Under thermal equilibrium, the product of electron concentration n and hole concentration p is constant and equals the square of the intrinsic carrier concentration: n * p = ni^2. If donor doping increases n, hole concentration p decreases proportionately through recombination.
Q. 6
Electronics Engineering
Difficulty: Medium
(1 Mark)
The Fermi-Dirac distribution function f(E) gives the probability that an energy state E is occupied by an electron. At energy E = Ef (Fermi energy level), the probability f(Ef) is:
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Step-by-Step Explanation & Concept Rationale
f(E) = 1 / [1 + exp((E - Ef) / (k * T))]. When E = Ef, exp(0) = 1, so f(Ef) = 1 / (1 + 1) = 1/2 = 0.5 (or 50%) at all temperatures above absolute zero.
Q. 7
Electronics Engineering
Difficulty: Medium
(1 Mark)
In an N-type semiconductor as donor doping concentration Nd increases, the Fermi level (Ef):
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Step-by-Step Explanation & Concept Rationale
Ef - Ei = k * T * ln(Nd / ni). As donor concentration Nd increases, the density of electrons in the conduction band rises, shifting the Fermi level closer to the conduction band edge Ec. In degenerate semiconductors, Ef enters the conduction band.
Q. 8
Electronics Engineering
Difficulty: Medium
(1 Mark)
Einstein's relation relating carrier diffusion coefficient (D) to mobility (mu) in a semiconductor is:
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Step-by-Step Explanation & Concept Rationale
Einstein's relation states that Dn / mun = Dp / mup = V_T = k * T / q, where thermal voltage V_T ≈ 25.86 mV (approx. 26 mV) at room temperature (300 K).
Q. 9
Electronics Engineering
Difficulty: Easy
(1 Mark)
The depletion region (space-charge layer) formed at an unbiased PN junction consists of:
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Step-by-Step Explanation & Concept Rationale
When P and N regions meet, electrons diffuse from N to P and holes from P to N. This leaves behind uncovered, fixed positive donor ions on the N-side and fixed negative acceptor ions on the P-side, creating a built-in electric field that opposes further carrier diffusion.
Q. 10
Electronics Engineering
Difficulty: Easy
(1 Mark)
The built-in contact potential barrier (V0) of a Silicon PN junction at 300 K is typically around:
💡
Step-by-Step Explanation & Concept Rationale
V0 = V_T * ln[(Na * Nd) / ni^2]. For typical doping levels in Silicon at 300 K, the contact barrier potential is approximately 0.7 V. For Germanium, it is approximately 0.3 V.
Q. 11
Electronics Engineering
Difficulty: Easy
(1 Mark)
When a PN junction is forward-biased (P-side positive, N-side negative):
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Step-by-Step Explanation & Concept Rationale
Forward bias opposes the built-in electric field, reducing barrier height to (V0 - Vf) and narrowing the space-charge width. This allows majority carriers to overcome the barrier, resulting in exponential diffusion current: I = Is * [exp(V / (eta*V_T)) - 1].
Q. 12
Electronics Engineering
Difficulty: Easy
(1 Mark)
When a PN junction is reverse-biased (P-side negative, N-side positive):
💡
Step-by-Step Explanation & Concept Rationale
Reverse bias aids the built-in field, pulling majority carriers away from the junction and widening the depletion layer. Only thermally generated minority carriers swept across by the field constitute the reverse saturation current Is.
Q. 13
Electronics Engineering
Difficulty: Easy
(1 Mark)
The reverse saturation current (Is) of a Silicon PN diode approximately doubles for every:
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Step-by-Step Explanation & Concept Rationale
Because thermal generation of electron-hole pairs increases exponentially with temperature, the reverse saturation current Is of a diode approximately doubles for every 10 °C rise in operating junction temperature: Is(T2) = Is(T1) * 2^((T2 - T1) / 10).
Q. 14
Electronics Engineering
Difficulty: Medium
(1 Mark)
The forward voltage drop (Vf) of a Silicon PN diode at constant current decreases with temperature at a rate of approximately:
💡
Step-by-Step Explanation & Concept Rationale
Due to the rapid increase in intrinsic carrier concentration ni with temperature, the forward diode voltage at fixed current decreases by roughly -2 mV/°C to -2.5 mV/°C. This negative temperature coefficient is frequently exploited in semiconductor temperature sensors.
Q. 15
Electronics Engineering
Difficulty: Medium
(1 Mark)
Transition (or depletion layer) capacitance (CT) in a reverse-biased PN junction dominates over diffusion capacitance because:
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Step-by-Step Explanation & Concept Rationale
Under reverse bias, diffusion current is negligible. The immobile ions on either side of the depletion layer act as charged plates separated by a dielectric of width W, giving transition capacitance CT = epsilon * A / W. Because W ∝ sqrt(V0 + Vr), CT varies inversely with reverse voltage.
Q. 16
Electronics Engineering
Difficulty: Easy
(1 Mark)
A Varactor diode (varicap) operates in which region and is primarily used for:
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Step-by-Step Explanation & Concept Rationale
A Varactor diode is specifically designed to maximize the voltage-dependent transition capacitance under reverse bias. Varying reverse voltage controls CT smoothly, providing electronic tuning for LC resonant tanks without mechanical variable capacitors.
Q. 17
Electronics Engineering
Difficulty: Medium
(1 Mark)
Zener breakdown occurs in PN junctions that are:
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Step-by-Step Explanation & Concept Rationale
Zener breakdown occurs in heavily doped junctions at low reverse voltages (< 5-6 V). The narrow depletion width creates an intense electric field that directly ruptures covalent bonds via quantum tunneling. It exhibits a negative temperature coefficient of breakdown voltage.
Q. 18
Electronics Engineering
Difficulty: Medium
(1 Mark)
Avalanche breakdown in a reverse-biased PN junction occurs in:
💡
Step-by-Step Explanation & Concept Rationale
In lightly doped diodes, the depletion layer is relatively wide. High reverse voltage accelerates minority carriers to high velocities, dislodging additional valence electrons upon impact. These secondary carriers cause cumulative carrier multiplication (avalanche), having a positive temperature coefficient of breakdown voltage.
Q. 19
Electronics Engineering
Difficulty: Medium
(1 Mark)
A Zener diode with a nominal breakdown voltage of 5.6 V has a temperature coefficient that is:
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Step-by-Step Explanation & Concept Rationale
Below 5 V, Zener breakdown (negative temp coeff) dominates. Above 6 V, Avalanche breakdown (positive temp coeff) dominates. Around 5.6 V, both mechanisms contribute equally, cancelling each other's temperature drift to yield superior voltage reference stability.
Q. 20
Electronics Engineering
Difficulty: Medium
(1 Mark)
A Tunnel diode (Esaki diode) exhibits negative differential resistance (dV/dI < 0) over a portion of its forward characteristic due to:
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Step-by-Step Explanation & Concept Rationale
Degenerate doping causes the conduction band on the N-side to overlap the valence band on the P-side. Under small forward bias, electrons tunnel directly through the thin barrier (~10 nm). As forward bias increases, band overlap diminishes, causing forward current to drop from peak current (Ip) to valley current (Iv), creating negative resistance used in high-frequency microwave oscillators.
Q. 21
Electronics Engineering
Difficulty: Medium
(1 Mark)
A Schottky barrier diode is formed by a metal-semiconductor junction (e.g., gold or platinum on N-type silicon). Its key operational advantages are:
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Step-by-Step Explanation & Concept Rationale
Because conduction is by majority carriers (electrons in N-type silicon), there is no minority carrier charge storage in the depletion region. Reverse recovery time is virtually zero (picoseconds), and forward drop is only 0.2-0.3 V, making Schottky diodes ideal for high-efficiency switch-mode power supplies (SMPS) and RF mixers.
Q. 22
Electronics Engineering
Difficulty: Hard
(1 Mark)
A PIN diode contains an undoped intrinsic (I) semiconductor layer sandwiched between heavily doped P and N layers. At microwave radio frequencies, it behaves as a:
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Step-by-Step Explanation & Concept Rationale
At low frequencies, a PIN diode acts as a standard rectifier. At RF/microwave frequencies, carriers cannot follow fast AC cycles; the intrinsic layer behaves as an almost pure resistance whose RF value varies continuously from thousands of ohms to under 1 ohm depending on applied DC forward bias current.
Q. 23
Electronics Engineering
Difficulty: Easy
(1 Mark)
A Photodiode is always operated in which mode to detect incident optical radiation with linear response and high bandwidth?
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Step-by-Step Explanation & Concept Rationale
In reverse bias, dark current is minimal. Incident photons of energy h*nu >= Eg generate electron-hole pairs in the wide depletion layer, which are swept by the strong built-in electric field. Reverse photocurrent is strictly linear with optical power and switching speed is maximal.
Q. 24
Electronics Engineering
Difficulty: Medium
(1 Mark)
Light Emitting Diodes (LEDs) are fabricated from direct bandgap compound semiconductors (such as GaAs, GaN, InGaN) rather than indirect bandgap semiconductors like Silicon because:
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Step-by-Step Explanation & Concept Rationale
In direct bandgap semiconductors, the conduction band minimum and valence band maximum align at the same crystal momentum (k = 0). Radiative electron-hole recombination is direct and highly efficient, producing photons: lambda = h * c / Eg. In Silicon (indirect bandgap), recombination requires a phonon (lattice vibration), dissipating energy as heat.
Q. 25
Electronics Engineering
Difficulty: Easy
(1 Mark)
What is the peak inverse voltage (PIV) rating across each diode in a single-phase center-tapped full-wave transformer rectifier supplying peak secondary voltage Vm across each half?
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Step-by-Step Explanation & Concept Rationale
In a center-tapped full-wave rectifier, during the non-conducting half cycle, the reverse-biased diode must withstand its own half-winding peak voltage Vm plus the peak voltage delivered by the other conducting half to the load, resulting in PIV = 2 * Vm.
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