Electronics

Modern electronics is built on the semiconductor PN junction and the diode action it provides. The PMDC MDCAT 2026 syllabus restricts this chapter to two essentials: the PN junction itself and rectification (converting AC into DC) using half-wave and full-wave rectifiers. Expect 1-2 MCQs.

PMC Table of Specifications. Two subtopics: PN Junction and Rectification (Half-Wave and Full-Wave).

PN Junction

A PN junction is formed when a single semiconductor crystal has one region doped with a trivalent impurity (P-type, holes are majority carriers) and the adjacent region doped with a pentavalent impurity (N-type, electrons are majority carriers).

Depletion region

At the junction, electrons from the N-side diffuse into the P-side and recombine with holes; holes diffuse the other way. This leaves a thin region near the junction with no free carriers — the depletion region. The fixed donor (positive) and acceptor (negative) ions create a built-in electric field that opposes further diffusion. The corresponding potential is the barrier potential:

Silicon: ~ 0.7 V
Germanium: ~ 0.3 V

Forward and reverse bias

Forward bias: P-side connected to + and N-side to − of the external supply. The applied voltage opposes the built-in field, narrowing the depletion region. Once V exceeds the barrier (~0.7 V for Si), large current flows.
Reverse bias: P to − and N to +. The applied voltage adds to the built-in field, widening the depletion region. Only a tiny reverse saturation current flows (due to minority carriers). Beyond breakdown voltage, the junction conducts heavily.

The diode

A PN junction packaged as a two-terminal device is a diode. It acts as a one-way valve: it conducts when forward biased, blocks when reverse biased. This unidirectional behaviour is the basis of rectification.

Common trap. In an N-type semiconductor, electrons are majority but it remains electrically neutral overall — the donor atoms supply both the electron and a fixed positive ion. Don't confuse "majority carrier type" with "net charge."

Rectification (Half and Full Wave)

Rectification is the conversion of alternating current into unidirectional (pulsating DC) current using one or more diodes.

Half-wave rectifier

Uses a single diode in series with a load resistor across the AC source (often via a transformer).
During the positive half-cycle the diode is forward biased — current flows through the load.
During the negative half-cycle the diode is reverse biased — no current.
Output: only the positive halves of the AC waveform appear across the load.
Output (ripple) frequency = input frequency f. For 50 Hz mains, ripple = 50 Hz.
Average DC: V_dc = V₀/π. Maximum theoretical efficiency ~ 40.6%. Inefficient because half the input power is wasted.

Full-wave rectifier (centre-tap)

Uses a centre-tapped transformer and two diodes.
The two halves of the secondary feed two diodes alternately, so current always flows in the same direction through the load.
Both halves of the AC are utilised.
Output (ripple) frequency = 2f. For 50 Hz mains, ripple = 100 Hz — easier to filter to a smooth DC.
Average DC: V_dc = 2V₀/π. Maximum theoretical efficiency ~ 81.2%.

Bridge rectifier (full-wave)

Uses four diodes in a bridge configuration; no centre-tapped transformer needed.
During each half-cycle two of the four diodes conduct, sending current through the load in the same direction.
Output frequency 2f, like the centre-tap full-wave; same V_dc = 2V₀/π.
Most common rectifier in real power supplies because the transformer is cheaper and smaller.

Half-wave vs Full-wave (centre-tap) vs Bridge rectifier
Property	Half-wave	Full-wave (centre-tap)	Bridge (full-wave)
Diodes	1	2 (with centre-tapped secondary)	4
Conducts on	Positive half-cycle only	Both half-cycles (alternating diodes)	Both half-cycles (2 diodes per cycle)
Output ripple frequency	= f (50 Hz from mains)	= 2f (100 Hz)	= 2f (100 Hz)
V_dc (average)	V₀ / π	2V₀ / π	2V₀ / π
Max efficiency	~40.6%	~81.2%	~81.2%
Transformer	Simple secondary	Centre-tapped secondary required	Simple secondary — cheaper
Peak inverse voltage (PIV)	V₀	2V₀	V₀
Use	Low-power, simple supplies	Older designs	Most modern power supplies

Smoothing the output

The pulsating DC from a rectifier is smoothed using a filter capacitor across the load, often combined with an inductor (LC or π-filter). The capacitor charges near the peaks and discharges through the load between peaks, reducing ripple.

Quick mnemonic. "1 diode, 1 hump — 4 diodes, 2 humps." Half-wave (1 diode) yields only one hump per cycle — ripple = f. Full-wave (4 diodes in a bridge, or 2 with centre-tap) yields two humps per cycle — ripple = 2f, easier to filter.

Worked MCQs

Five MCQs that capture the high-yield testing patterns for this chapter. Read the explanation even when you get the answer right — it's where the deeper concept lives.

Q1. The barrier potential of a silicon PN junction at room temperature is approximately:

0.3 V
0.7 V
1.5 V
5 V

Si has a barrier of about 0.7 V and Ge about 0.3 V. The diode does not conduct appreciably until forward bias exceeds this barrier — this is why a Si diode is said to "turn on" at ~0.7 V.

Q2. A bridge rectifier uses how many diodes?

One
Two
Four
Six

A bridge rectifier uses four diodes arranged so that two conduct on each half-cycle and route the current through the load in the same direction. It avoids the need for a centre-tapped transformer.

Q3. If the input AC frequency is 50 Hz, the ripple frequency at the output of a full-wave rectifier is:

25 Hz
50 Hz
100 Hz
200 Hz

Full-wave rectifiers process both half-cycles, doubling the ripple frequency to 2f = 100 Hz. Half-wave keeps it at 50 Hz. Higher ripple frequency is easier to smooth with a small capacitor.

Q4. When a PN junction is reverse biased:

Depletion region narrows
Majority carriers cross the junction freely
Depletion region widens and only a small leakage current flows
Resistance becomes very low

Reverse bias adds to the built-in field, sweeping carriers away from the junction. The depletion region widens and only minority carriers contribute a tiny reverse saturation current. Resistance is high.

Q5. In a half-wave rectifier circuit, the diode conducts during:

Both half-cycles
Only the half-cycle in which it is forward biased
Only the negative half-cycle
Neither half-cycle

A diode is unidirectional. In a half-wave rectifier it is forward biased during one half-cycle (output present) and reverse biased during the other (no output). Because half the input is discarded the efficiency is low (~40.6%).

Quick Recap

PN junction: diffusion creates a depletion region and a barrier potential (~0.7 V Si, ~0.3 V Ge).
Forward bias narrows depletion → conducts; reverse bias widens depletion → blocks.
Half-wave: 1 diode, ripple frequency = f, V_dc = V₀/π, ~40.6% efficient.
Full-wave (centre-tap): 2 diodes; (bridge): 4 diodes — ripple = 2f, V_dc = 2V₀/π, ~81.2% efficient.
Bridge rectifier is preferred — no centre-tapped transformer required.
Filter capacitor smooths pulsating DC into nearly steady DC.

Test yourself. Take a timed Electronics quiz or browse all Physics MCQs to lock these concepts in.