Electronics (ECE) - MCQ Practice Questions

Tunnel diodes show negative differential resistance due to quantum tunneling through the narrow depletion region created by heavy doping. This occurs in forward bias and enables their use as oscillators and amplifiers.

In saturation, both junctions are forward biased. The collector current becomes less than β × Ib because the transistor is no longer in linear region. Excess base current doesn't proportionally increase collector current.

CMOS (Complementary MOS) has very low static power dissipation because complementary P and N transistors prevent steady-state current paths. However, dynamic power dissipation increases with switching frequency due to capacitive charging/discharging.

RDS_on consists of channel resistance and drift region resistance. For power MOSFETs, drift region (epitaxial layer) dominates. Minimizing drift region thickness reduces RDS_on but decreases breakdown voltage, creating a design trade-off.

Schottky diodes have metal-semiconductor junctions with lower forward voltage (0.3-0.5V vs 0.7V for silicon) and faster switching due to absence of minority carrier storage. This makes them ideal for high-frequency and high-speed switching applications.

Early voltage (VA) represents the output resistance (ro = VA/Ic) of a BJT. Larger VA indicates steeper output characteristics, meaning collector current is more independent of collector-emitter voltage, indicating better current source behavior.

Aspect ratio (W/L) is a fundamental design parameter in MOSFETs affecting current capability and switching speed. Higher W/L provides larger current capacity and faster transitions, crucial for circuit optimization and performance tuning.

Transconductance gm = ∂ID/∂VGS = ID/(VGS - VT) at saturation, fundamental small-signal parameter.

Punch-through occurs when CBJ depletion width extends to base contact, causing base resistance to vanish and uncontrolled current flow.

Body effect (substrate bias) increases VT due to back-bias effect and decreases output impedance; VT = VT0 + √(2εqNa/Cox)·φt term.

Gunn effect: electrons transfer from high-mobility Γ valley to low-mobility L valley at threshold field, causing dI/dV < 0 (negative resistance).

Miller effect: CBE capacitance reflected to input = CμC·(1+|Av|); dominates high-frequency response and limits bandwidth in CE configuration.

Breakdown voltage BV ∝ ND^(-$\frac{3}{4}$) approximately; lower doping increases depletion width, requiring higher field and higher voltage for breakdown.

Overdrive factor β = IB(actual) / IB(min) ensures transistor stays saturated even with parameter variations.

At pinch-off, VGS reaches VP (pinch-off voltage), channel closes, and further VDS increase has minimal effect on ID (saturation region).

The gate-drain capacitance (Cgd) creates Miller effect, which effectively multiplies the capacitive reactance in feedback, severely limiting high-frequency gain.

At low frequencies, shot noise associated with base current (IB) dominates, following Poisson statistics: i²n = 2q×IB×Δf.

Current mirror loads increase output impedance, while source degeneration resistors provide common-mode feedback, both reducing common-mode gain.

The avalanche factor M = 1/(1-(VR/BV)ⁿ) shows multiplication increases exponentially as reverse voltage approaches breakdown voltage BV, where n≈3-6.

In saturation, I_D = ($\frac{1}{2}$)μ_n C_ox (W/L)(V_GS - V_T)², so g_m = ∂I_D/∂V_GS = μ_n C_ox (W/L)(V_GS - V_T) = √(2μ_n C_ox (W/L)I_D).