Govt. Exams
Entrance Exams
Wide bandgap emitter (e.g., AlGaAs) blocks minority carrier injection from base, improving γE and allowing lower base doping for higher β.
MESFET uses Schottky (metal-semiconductor) junction for gate control instead of p-n junction, enabling high-frequency GaAs implementation.
In modern sub-28nm nodes, static power dominates due to increased leakage from subthreshold conduction (IOFF) and gate-induced drain leakage (GIDL).
Negative differential resistance occurs in the region between peak and valley current (Ip to Iv), where current decreases with increasing voltage.
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)).
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)).
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.
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.
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.
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.
