EC-304 Electronic Devices - Jun 2024 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the working of tunnel diode and its VI characteristics. What is the sufficient conditions for tunnelling?
   How to answer

   • Principle of quantum mechanical tunneling in heavily doped p-n junctions
   • Conditions for tunneling: heavily doped degenerate semiconductors, very thin depletion region (< 100 Å), filled states in conduction band aligned with empty states in valence band
   • Energy band diagrams showing tunneling process at various bias conditions
   • V-I characteristic curve showing peak voltage/current, valley voltage/current, and negative dynamic resistance region
2. 7 Marks
   bExplain the construction and working of Zener diode.
   How to answer

   • Construction: heavily doped p-n junction diode, thin depletion layer
   • Working mechanism under reverse bias condition
   • Explanation of Zener breakdown (quantum tunneling) vs Avalanche breakdown
   • V-I characteristics showing sharp breakdown at Zener voltage ($V_Z$)
   • Application as a reference voltage / voltage regulator
3. 7 Marks
   aFind the concentration of holes and electrons in a P-type germanium at $300^{\circ}\text{ K}$, if the conductivity is $100\,\Omega\text{cm}$, mobility of holes in germanium $\mu_p = 1800\text{ cm}^2/\text{volt-sec}$.
   How to answer

   • Recall relation for p-type semiconductor conductivity: $\sigma \approx q \cdot p \cdot \mu_p$
   • Calculate hole concentration $p = \sigma / (q \cdot \mu_p)$ using $q = 1.6 \times 10^{-19}\text{ C}$
   • Use intrinsic carrier concentration for Ge at $300\text{ K}$ ($n_i \approx 2.5 \times 10^{13}\text{ cm}^{-3}$)
   • Calculate electron concentration using mass-action law: $n = n_i^2 / p$
4. 7 Marks
   bWith a neat sketch explain the working of half wave rectifier. Derive the characteristics of HWR.
   How to answer

   • Draw circuit diagram of half-wave rectifier and input/output waveforms
   • Explain working during positive and negative half-cycles
   • Derive $I_{dc} = I_m / \pi$ and $I_{rms} = I_m / 2$
   • Derive ripple factor $\gamma = \sqrt{(I_{rms}/I_{dc})^2 - 1} = 1.21$
   • Derive efficiency $\eta = P_{dc} / P_{ac} = 40.6\%$ and state $\text{PIV} = V_m$
5. 7 Marks
   aDetermine the rating of a transformer to deliver 125watts of DC power to a load for the following. i) Half wave rectifier ii) Full wave rectifier
   How to answer

   • Define Transformer Utilization Factor: $\text{TUF} = P_{dc} / \text{Transformer Rating (VA)}$
   • For Half Wave Rectifier: $\text{TUF} = 0.287$, hence Transformer Rating $= 125 / 0.287 \approx 435.54\text{ VA}$
   • For Full Wave Rectifier: for center-tap $\text{TUF} = 0.693$, Rating $= 125 / 0.693 \approx 180.38\text{ VA}$; or for bridge $\text{TUF} = 0.812$, Rating $= 125 / 0.812 \approx 153.94\text{ VA}$
6. 7 Marks
   bAn AC supply of 220V is applied to half wave rectifier circuit through a transformer with a turns ratio of 10:1. Find: i) DC output voltage ii) PIV Assume the diode to be an ideal one.
   How to answer

   • Secondary RMS voltage: $V_{s(rms)} = 220 / 10 = 22\text{ V}$
   • Peak secondary voltage: $V_m = \sqrt{2} \times 22 = 31.11\text{ V}$
   • Calculate DC output voltage: $V_{dc} = V_m / \pi = 31.11 / 3.1416 \approx 9.9\text{ V}$
   • Calculate PIV: $\text{PIV} = V_m \approx 31.11\text{ V}$
7. 7 Marks
   aFrom the transistor current components deduce the current equation of transistor.
   How to answer

   • Draw diagram of current components across emitter, base, and collector junctions for a pnp or npn transistor
   • Identify components: emitter current ($I_{pE} + I_{nE}$), base recombination current, collected current, reverse saturation current $I_{CBO}$
   • Define emitter efficiency $\gamma$ and base transport factor $\beta^*$
   • Derive $I_C = \alpha I_E + I_{CBO}$
   • Relate $I_E = I_B + I_C$ to obtain $I_C = \frac{\alpha}{1-\alpha}I_B + \frac{I_{CBO}}{1-\alpha} = \beta I_B + I_{CEO}$
8. 7 Marks
   bCalculate the values of $I_D$ and $g_m$ for $V_{GS} = -0.8\text{V}$. If $I_{DSS}$ and $V_p$ are given as $12\text{ Amp}$ and $-6\text{V}$ respectively.
   How to answer

   • State Shockley's equation: $I_D = I_{DSS} \left(1 - \frac{V_{GS}}{V_p}\right)^2$
   • Calculate $I_D = 12 \left(1 - \frac{-0.8}{-6}\right)^2 = 12 \left(1 - \frac{0.8}{6}\right)^2 = 12 \left(\frac{5.2}{6}\right)^2 = 12 \times 0.7511 \approx 9.013\text{ A}$
   • Calculate transconductance at $V_{GS} = 0$: $g_{m0} = \frac{2 I_{DSS}}{|V_p|} = \frac{24}{6} = 4\text{ S (or A/V)}$
   • Calculate transconductance: $g_m = g_{m0} \left(1 - \frac{V_{GS}}{V_p}\right) = 4 \times \left(\frac{5.2}{6}\right) \approx 3.467\text{ S}$
9. 7 Marks
   aWhat is biasing? Explain the need of it. List the different types of biasing circuits.
   How to answer

   • Definition of transistor biasing (establishing suitable DC operating point)
   • Need for biasing: fixing Q-point in active region, preventing distortion, ensuring stability against temperature changes and $\beta$ variations
   • List of biasing circuits: Fixed bias / base bias, Collector-to-base feedback bias, Voltage divider bias (self-bias), Emitter bias
10. 7 Marks
    bIn a self-bias circuit containing $R_1 = 50\text{K}\Omega$, $R_2 = 25\text{K}\Omega$, $R_E = 1\text{K}\Omega$, $R_C = 3\text{K}\Omega$, $\beta = 90$, $V_{CC} = 12\text{V}$, $V_{BE} = 0.7\text{V}$. Find the operating point, $S, S'$.
    How to answer

    • Find Thevenin parameters: $V_{th} = 12 \times \frac{25}{75} = 4\text{ V}$, $R_{th} = \frac{50 \times 25}{75} = 16.67\text{ k}\Omega$
    • Calculate base current: $I_B = \frac{4 - 0.7}{16.67 + 91 \times 1} = \frac{3.3}{107.67\text{ k}\Omega} \approx 30.65\,\mu\text{A}$
    • Calculate collector current: $I_C = \beta I_B = 90 \times 30.65\,\mu\text{A} \approx 2.76\text{ mA}$
    • Calculate $V_{CE} = V_{CC} - I_C(R_C + R_E) = 12 - 2.76\text{ mA} \times 4\text{ k}\Omega = 0.96\text{ V}$ (Q-point: $(2.76\text{ mA}, 0.96\text{ V})$)
    • Calculate stability factor $S = \frac{1+\beta}{1 + \beta \frac{R_E}{R_{th}+R_E}} = \frac{91}{1 + 90 \times \frac{1}{17.67}} \approx 14.93$
    • Calculate stability factor $S' = \frac{\partial I_C}{\partial V_{BE}} = \frac{-\beta}{R_{th} + (1+\beta)R_E} = \frac{-90}{107.67\text{ k}\Omega} \approx -0.836\text{ mA/V}$
11. 7 Marks
    aWith the help of neat diagram explain class A and class B amplifiers.
    How to answer

    • Define Class A amplifier: transistor conducts for full input cycle ($360^\circ$), Q-point centered on load line, maximum theoretical efficiency $25\%$ (direct-coupled) or $50\%$ (transformer-coupled)
    • Define Class B amplifier: transistor conducts for half input cycle ($180^\circ$), Q-point located at cut-off, maximum theoretical efficiency $78.5\%$
    • Draw circuit schematics and output collector current waveforms showing conduction angles for both classes
    • Mention crossover distortion occurring in Class B push-pull amplifiers
12. 7 Marks
    bWhat is the need of bootstrapping circuit? Explain bootstrapping circuit with neat diagram.
    How to answer

    • Need: In an emitter follower circuit, biasing resistors severely reduce the effective input impedance; bootstrapping overcomes this shunting effect
    • Principle: Use of a capacitor to feed back a portion of the in-phase output to maintain nearly identical potential on both sides of the resistor, effectively making AC current minimal
    • Circuit diagram of Darlington pair or emitter follower with bootstrapping capacitor and resistor
    • Mathematical explanation showing $R_{in}$ effectively increased by factor $1/(1-A_v)$
13. 7 Marks
    aExplain the construction and working of JFET with the help of neat diagram.
    How to answer

    • Construction: n-channel (or p-channel) semiconductor bar with ohmic contacts (source, drain) and heavily doped p+ regions forming the gate
    • Working: application of $V_{DS}$ causes carrier flow; reverse-biasing the gate-source junction creates widening depletion regions
    • Pinch-off mechanism: depletion regions meet, channel resistance becomes high, current saturates at $I_{DSS}$
    • Diagram showing channel structure, depletion layers, and external voltage connections
14. 7 Marks
    bWhat is UJT? Explain UJT as relaxation oscillator.
    How to answer

    • Definition of UJT: three-terminal semiconductor device with one junction and two base contacts ($B_1, B_2$)
    • Circuit diagram of UJT relaxation oscillator with charging resistor $R_T$ and capacitor $C_T$
    • Working: charging of capacitor through $R_T$ up to peak point voltage $V_P = \eta V_{BB} + V_D$
    • Discharge of capacitor through UJT emitter into $R_1$, generating sawtooth voltage at capacitor and pulse voltage at $B_1$
    • Oscillation frequency formula: $f = \frac{1}{T} \approx \frac{1}{R_T C_T \ln\left(\frac{1}{1-\eta}\right)}$
15. 14 Marks
    Write short notes on any two: i) Schottky diode ii) Voltage regulation using IC iii) Thermal Runaway iv) Push-pull amplifier
    How to answer

    • Schottky diode: metal-semiconductor junction, majority carrier device, zero reverse recovery time, low forward drop, used in high-frequency switching
    • Voltage regulation using IC: fixed regulators (78XX, 79XX) and adjustable (LM317), pin configurations, connection with filter capacitors, line/load regulation
    • Thermal Runaway: self-destruction of transistor due to positive feedback loop ($T \uparrow \to I_{CBO} \uparrow \to I_C \uparrow \to P_D \uparrow \to T \uparrow$), condition to prevent: $\frac{\partial P_C}{\partial T_j} < \frac{1}{\theta}$
    • Push-pull amplifier: pair of complementary transistors conducting alternate half cycles, cancellation of even harmonics, high efficiency, crossover distortion