The thermal runaway condition in a BJT occurs because:
Answer: A
Thermal runaway is a positive feedback mechanism: temperature ↑ → V_BE ↓ (≈ -2mV/°C) → I_B ↑ → I_C ↑ → P_dissipated ↑ → T ↑, creating instability. β also increases with temperature.
Q.22Hard
The charge storage time in a BJT switch is primarily due to:
Answer: C
Storage time (t_s) occurs because excess minority carriers accumulate in the base during saturation. When the input is removed, these carriers must recombine or diffuse out before reverse current can flow, causing delay in switching off.
Q.23Hard
In a PIN diode used as an RF switch, the operating principle is based on:
Answer: C
PIN diodes have wide intrinsic regions. When forward biased, high carrier concentration (conductivity modulation) is established in the I-region, giving low impedance. When reverse biased, carriers are swept out, giving high impedance—ideal for RF switching.
Q.24Hard
The noise figure of an amplifier is defined as:
Answer: C
Noise Figure F = (SNR_in)/(SNR_out) = (input noise power × gain)/(output noise power). It represents the degradation in SNR due to the amplifier's internal noise contributions.
Q.25Hard
The substrate effect in a MOSFET causes:
Answer: D
Body bias effect: reverse substrate bias increases depletion width, increasing V_T (back-gate bias effect). This reduces g_m and changes I_D characteristics. The effect follows V_T = V_T0 + γ(√(2φ_F + V_SB) - √(2φ_F)).
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Q.26Hard
The frequency response of a BJT amplifier is limited at high frequencies primarily by:
Answer: B
The base-collector capacitance C_bc exhibits the Miller effect, appearing as (1+|A_v|)C_bc at the base, creating a dominant pole that limits bandwidth. This is the primary high-frequency limitation in BJT amplifiers, with f_T ≈ g_m/(2π(C_bc(1+A_v) + C_be)).
Q.27Hard
A tunnel diode exhibits negative differential resistance. At what operating point is the dynamic resistance most negative?
Answer: C
Negative differential resistance occurs in the region between peak and valley current (Ip to Iv), where current decreases with increasing voltage.
Q.28Hard
A CMOS logic gate has static power dissipation. Which phenomenon is primarily responsible for this in 2024 technology nodes?
Answer: A
In modern sub-28nm nodes, static power dominates due to increased leakage from subthreshold conduction (IOFF) and gate-induced drain leakage (GIDL).
Q.29Hard
A MESFET (Metal-Semiconductor FET) differs from a JFET primarily in which aspect?
Answer: A
MESFET uses Schottky (metal-semiconductor) junction for gate control instead of p-n junction, enabling high-frequency GaAs implementation.
Q.30Hard
In a heterostructure bipolar transistor (HBT), the wide bandgap material in the emitter provides which advantage?
Answer: A
Wide bandgap emitter (e.g., AlGaAs) blocks minority carrier injection from base, improving γE and allowing lower base doping for higher β.
