Bipolar Junction Transistors (BJT) — A/L Physics

What must I remember about Bipolar Junction Transistors (BJT)?

• BJT Architecture: Three-terminal current-controlled device composed of two back-to-back p-n junctions: Emitter (E, heavily doped to inject carriers), Base (B, extremely thin and lightly doped to minimize recombination), Collector (C, moderately doped and physically largest to dissipate heat).
• Transistor Types: npn (majority electrons injected from emitter to collector; standard configuration) and pnp (majority holes injected from emitter).
• Current Relationships: By Kirchhoff's Current Law: Emitter current Ie = Base current Ib + Collector current Ic; DC Current Gain β = hFE = Ic / Ib (typically 50-300); Alpha α = Ic / Ie = β / (β + 1) ≈ 0.98 - 0.998.
• Transistor Operating Modes: 1) Cut-off (both junctions reverse-biased, Ic = 0, acts as open switch); 2) Active (Emitter-Base junction forward-biased, Collector-Base junction reverse-biased, acts as linear amplifier where Ic = β Ib); 3) Saturation (both junctions forward-biased, Vce ≈ 0.2 V, acts as closed switch).

Test yourself (1 minute)

1. Common Emitter (CE) Voltage Amplifier

   Input applied between base and emitter, output taken between collector and emitter; provides high voltage gain Av = - (Rc / r_in) * β and 180° (π radians) phase inversion between input and output waveforms.

2. Base-Emitter forward drop

   In active/saturation mode for Silicon BJT, Vbe remains strictly constant at approximately 0.7 V.

3. Transistor as a Switch

   Driven between Cut-off (V_in = 0, Ib = 0, Vce = Vcc, OFF state) and Saturation (V_in high, Ib > Ic_sat / β, Vce = Vce_sat ≈ 0.2 V, ON state).

Practise Bipolar Junction Transistors (BJT) questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 9: Electronics

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Where does Bipolar Junction Transistors (BJT) fit in the syllabus?

Unit 9.4 · Year 2 · Term 2