Electronics (ECE) - MCQ Practice Questions

A PN junction diode exhibits rectification property, allowing current to flow easily in the forward direction (low resistance) and blocking it in the reverse direction (high resistance).

Silicon PN junctions have a built-in barrier potential of approximately 0.7 V at room temperature (25°C), while germanium junctions are around 0.3 V.

Zener diodes have low forward resistance (like normal diodes in forward bias) but the unique property is controlled reverse breakdown at a specific voltage for regulation purposes.

BJT operates in three distinct regions: Active region (linear amplification with both junctions forward biased), Saturation (both junctions forward biased, acts as switch ON), and Cutoff (both junctions reverse biased, acts as switch OFF).

In BJT, the current gain (beta or hFE) is defined as the ratio of collector current to base current: β = Ic/Ib, so Ic = Ib × β. Beta typically ranges from 50 to 500 for most transistors.

LEDs emit light through electroluminescence when forward biased with sufficient current. The energy of emitted photons equals the bandgap energy of the semiconductor material.

MOSFETs have fastest switching due to voltage-controlled operation without minority carrier storage effects.

N-channel enhancement mode MOSFETs require positive gate voltage to create inversion layer for conduction.

Zener diodes operate in reverse breakdown region maintaining nearly constant voltage for voltage regulation applications.

Forward bias injects minority carriers; their recombination releases energy as photons (electroluminescence) in direct bandgap semiconductors.

Germanium has Eg ≈ 0.66 eV at 300K, lowest among common semiconductors; causes high intrinsic carrier concentration and leakage current.

Reverse bias widens the depletion region by pulling majority carriers away from the junction, increasing the width of the space charge region.

In intrinsic semiconductors, thermally generated electrons and holes are in equal numbers, making ni = pi at thermal equilibrium.

In saturation, both BE and BC junctions are forward biased, resulting in VCE dropping to approximately 0.2-0.3V for silicon transistors.

VT = kT/q where k=1.38×10⁻²³ J/K, T=300K, q=1.6×10⁻¹⁹C gives approximately 26 mV or 25 mV.

Enhancement-mode NMOS has positive VTh (typically 0.5-2V) because an inversion layer must be created at positive gate voltage.

Reverse bias voltage increases the electric field across the junction, expanding the depletion region width. The depletion width W is proportional to √V_r where V_r is reverse bias.

For silicon at 300K, the built-in potential is typically 0.7 V, determined by V_bi = (kT/q)ln(N_a*N_d/n_i²). This value is fundamental for silicon device calculations.

JFETs are called depletion-type devices because the channel is formed by a depletion region. The p-n junction (gate-channel) creates a depletion layer that widens with reverse bias, pinching off the channel.

The reverse saturation current I_0 doubles for every 5-7°C rise in silicon at room temperature. Since I_CO ≈ I_0, it also doubles per 5-7°C, or roughly per 10°C it increases significantly (by factor of ≈1.5-2).