How it works
A BJT has four operating regions defined by the bias states of its two junctions. In the active region, the emitter-base junction is forward biased (VBE ≈ 0.7 V for silicon) and the collector-base junction is reverse biased; here IC = βIB and the transistor amplifies. In saturation, both junctions are forward biased, VCE(sat) ≈ 0.2 V, and increasing IB no longer increases IC — the transistor is fully ON, used as a switch. In cutoff, both junctions are reverse biased, IC ≈ ICEO (leakage only), and the transistor is fully OFF. The reverse active region flips emitter and collector roles; it is used only in special circuits like ECL gates. The condition IC = βIB is strictly true only in the active region.
Key points to remember
Four regions: active (amplification, VBE ≈ 0.7 V, VCB reverse biased), saturation (switch ON, VCE(sat) ≈ 0.2 V), cutoff (switch OFF, both junctions reverse biased), and reverse active (emitter and collector swapped). In saturation, the forced beta βforced = IC/IB must be less than hFE to confirm saturation — this check is mandatory in switch design problems. VCE(sat) ≈ 0.2 V for silicon NPN transistors. ICEO, collector-to-emitter leakage in cutoff, is typically in the nanoampere range for silicon but significantly higher for germanium, which is why silicon dominates modern design.
Exam tip
The examiner always asks you to determine the region of operation given VBE and VCE values — check VBE against 0.7 V first, then check VCE against 0.2 V, and state your conclusion explicitly.