Why does conductivity of a semiconductor increase with temperature?

More electron-hole pairs are thermally generated, increasing the number of charge carriers; this dominates over the small reduction in mobility.

A vacant state in the valence band that behaves as a positively charged particle for purposes of conduction.

Why is the base of a transistor thin and lightly doped?

So that most injected carriers from the emitter diffuse across the base without recombining, ensuring high current gain.

What makes NAND gates universal?

Any Boolean function can be implemented using only NAND gates, simplifying integrated-circuit fabrication.

What is virtual ground in op-amps?

In an inverting configuration with non-inverting input grounded, negative feedback forces the inverting input to 0 V — although no real connection to ground exists.

An n-type semiconductor is formed by doping silicon with:

Phosphorus. Pentavalent atoms (P, As, Sb) donate electrons, producing n-type.

Knee voltage of a silicon diode is approximately:

0.7 V. Silicon junction barrier ≈ 0.7 V; germanium ≈ 0.3 V.

A transistor with β = 200 has a base current of 20 μA. Collector current is:

4 mA. I_C = β·I_B = 200·20 μA = 4 mA.

Output of a NAND gate is 0 only when:

All inputs are 1. NAND = NOT-AND; AND is 1 only when all inputs are 1, so NAND is 0 then.

Inverting op-amp with R_f = 100 kΩ and R_in = 10 kΩ has voltage gain:

−10. Gain = −R_f/R_in = −10.

Electronics MDCAT 2026 — Notes, Key Concepts & MCQs (Physics)

Electronics

Electronics averages 2 MCQs per paper — semiconductors, diodes, transistors, and logic gates are the steady picks.

Electronics is a Physics chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 36-MCQ Physics section. Mastering the core concepts below typically secures the full chapter weightage.

Semiconductors and doping

Silicon and germanium are tetravalent. At 0 K they are insulators; at room temperature thermal excitation creates electron-hole pairs. Doping with pentavalent atoms (P, As) gives n-type (extra electrons); trivalent atoms (B, Al) gives p-type (holes are majority). The energy gap E_g for Si ≈ 1.1 eV, Ge ≈ 0.7 eV. Conductivity rises with temperature in semiconductors (opposite of metals).

The p-n junction diode

A p-n junction develops a depletion region with built-in potential ≈ 0.7 V (Si) or 0.3 V (Ge). Forward-biased (p to + terminal): current flows freely above the threshold. Reverse-biased: only a tiny saturation current; beyond the breakdown voltage Zener or avalanche conduction occurs. Half-wave rectification uses one diode; full-wave (centre-tap or bridge) uses two or four. Smoothing capacitors reduce ripple.

Bipolar junction transistors

NPN or PNP, with three regions: emitter (heavily doped), base (thin, lightly doped), collector. Common-emitter configuration is most popular. Current gain β = I_C/I_B; α = I_C/I_E; relation β = α/(1−α). A transistor with β = 100 amplifies a 10 μA base current to 1 mA collector current. Modes of operation: cutoff, active (amplifier), saturation (switch).

Operational amplifiers

Op-amps have very high gain (~10⁵), high input impedance, and low output impedance. With negative feedback, two golden rules apply: (1) input voltages are equal (virtual short); (2) input currents are zero. Inverting amplifier gain = −R_f/R_in; non-inverting gain = 1 + R_f/R_in. Op-amps build summers, integrators, differentiators, and comparators.

Logic gates and Boolean algebra

AND (output 1 only if all inputs 1), OR (output 1 if any input 1), NOT (inverter), NAND, NOR, XOR. NAND and NOR are universal — any logic function can be built from either alone. Boolean identities: A + A = A, A·A = A, A + Ā = 1, A·Ā = 0. De Morgan's laws: A+B = Ā·B̄, AB = Ā+B̄. Truth tables for two-input gates have 4 rows; for three-input, 8 rows. References: FSc Punjab Chapter 18, Serway Chapter 43, and supplementary chapters on digital electronics in Boylestad & Nashelsky.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Electronics Is Tested

MDCAT questions on Electronics are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.