P-N junction transistor

P-N junction transistor

1. What It Is

P-N junction transistor (BJT — Bipolar Junction Transistor) = three-layer sandwich p-n-p or n-p-n forming two p-n junctions (emitter-base and collector-base). Here syllabus phrase "P-N junction transistor" likely means BJT operation based on p-n junctions (vs FET).

Structure n-p-n (most common): emitter (n heavily doped) → base (p thin, lightly doped) → collector (n). Symbols with arrow on emitter indicating conventional current.

2. Operation — Active Mode (amplification)

Bias: Emitter-base forward, Collector-base reverse.

• Emitter injects electrons (n-p-n) into base.
• Base thin (~µm) so most electrons diffuse across base without recombining, are swept by collector-base reverse field into collector.
• Current relations:

with

(common-base current gain),

(common-emitter gain).

$I_C$ controlled by $I_B$ (small base current controls large collector current) ⇒ amplification.

3. Characteristics

• Input: $I_B$ vs $V_{BE}$ forward diode-like exponential.
• Output: $I_C$ vs $V_{CE}$ shows saturation (both junctions forward) and active (flat $I_C$ vs $V_{CE}$, Early effect) and cutoff.

4. Uses

Switch (saturation/cutoff as ON/OFF) and amplifier (active). In ICs, now often MOSFET, but BJT still for analog/high speed.

5. Worked Note

β=100, I_B=10 µA ⇒ I_C=1 mA ⇒ power gain.
N-p-n vs p-n-p: hole vs electron as majority in emitter, but same physics.

6. Exam Points

• Sketch n-p-n/p-n-p structure and bias for active mode — 5 marks, very frequent.
• Explain emitter injection, base diffusion, collector sweep — 5-7 marks, very frequent.
• Current relations $I_E=I_C+I_B$, $β=I_C/I_B$ — 3-5 marks, frequent.
• Input/output characteristics sketch — 5 marks, frequent.