Govt. Exams
Entrance Exams
Early voltage characterizes the output resistance of a BJT. V_A is inversely proportional to the base width modulation effect (Early effect), which becomes significant in short-base transistors.
Heavy doping narrows the depletion region, allowing quantum tunneling at lower voltages. This shifts breakdown mechanism from avalanche (Zener ~5-6V) to tunneling (~3-4V) in heavily doped junctions.
In Zener breakdown, as the reverse bias increases slightly, the breakdown mechanism (avalanche or tunneling) generates more current, but the voltage across the junction decreases due to voltage regulation.
Is ∝ ni²/(NA·ND), depending on intrinsic carrier concentration squared and inversely on doping concentrations.
Minimum noise figure occurs at optimum source impedance that matches the device's noise characteristics, typically provided in device specifications.
β varies with temperature and VBE changes, following the Ebers-Moll model. Early voltage causes slight VCE dependence.
In tunnel diode's NDR region, as voltage increases, tunneling current decreases (fewer states to tunnel into) while diffusion current increases, causing net decrease in total current.
ni = √(Nc·Nv)·exp(-Eg/2kT) where Nc, Nv depend on effective masses. Small Eg and high density of states both exponentially increase ni. GaAs has higher ni than Si due to smaller Eg.
Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.
APD operates in high reverse bias where impact ionization (collision ionization) produces secondary electron-hole pairs, creating avalanche multiplication and signal amplification internally.
