BJT Fundamentals: PNP and NPN transistors

BJT Fundamentals: PNP and NPN transistors

1. What a BJT Is

BJT (Bipolar Junction Transistor) = three-layer, two-junction semiconductor device with two p-n junctions back-to-back, forming n-p-n or p-n-p. Bipolar = both electrons and holes participate; Transistor = transfer resistor (transferring resistance from low input to high output for amplification).

2. Structure

Type	Layers	Symbol (arrow on emitter)	Conventional Current Direction
NPN	n (emitter) - p (base) - n (collector)	Emitter arrow outward (from base)	$I_C$ into collector, out of emitter
PNP	p - n - p	Emitter arrow inward (toward base)	$I_C$ out of collector (opposite)

Terminals: Emitter (E) — heavily doped, emits carriers; Base (B) — thin, lightly doped, controls; Collector (C) — larger, collects carriers.

3. Bias for Active Mode (amplification)

• Emitter-base junction forward biased (injects carriers).
• Collector-base junction reverse biased (sweeps carriers, creates field to collect).

NPN active: $V_{BE}≈0.7V$ forward (B higher than E? Actually E lower for NPN? $V_B > V_E$ by 0.7, $V_C > V_B$). PNP active: $V_{EB}≈0.7V$ (E higher than B), $V_{EC}$ positive but polarities opposite.

If both junctions forward → saturation (switch ON); both reverse → cutoff (OFF); one forward one reverse → active.

4. Current Relations (ideal, active)

(KCL at transistor)

Current gains:

(common-base)

(common-emitter)

So small $I_B$ controls large $I_C$:

(amplification).

For PNP, currents reverse direction but magnitudes same relations.

5. Why NPN Dominant

NPN uses electrons as majority in emitter-collector path ⇒ higher mobility ⇒ faster than PNP (holes slower), so most circuits use NPN (PNP as complementary).

6. Exam Points

• Sketch n-p-n vs p-n-p structure and symbol (arrow on emitter) — 5-7 marks, very frequent (almost certain to ask symbol).
• State active bias: EB forward, CB reverse — 5 marks, very frequent.
• Give $I_E=I_C+I_B$, $β=I_C/I_B$, $α≈0.99$ — 5 marks, frequent.
• Distinguish NPN ($V_{BE}≈0.7$ positive) vs PNP opposite — 3-5 marks, frequent.