Question 3-26C4

When a bipolar junction transistor (BJT) operates in saturation, it acts like a fully "on" switch, allowing maximum current flow between the collector and emitter. For this condition to exist, both junctions within the transistor must be forward biased. Specifically: 1. **Base-Emitter (BE) Junction:** This junction must be forward biased to allow base current to flow, which turns the transistor on and initiates collector current. 2. **Collector-Base (CB) Junction:** In saturation, the collector voltage drops very low, often near the emitter voltage. This voltage condition causes the collector-base junction to also become forward biased. This signifies that the transistor is conducting as much current as possible and can no longer increase its output, even if the base current increases further. Therefore, both junctions being forward biased is the defining characteristic of a transistor in saturation. This is distinct from the active region (where the BE junction is forward biased and the CB junction is reverse biased for amplification) or cutoff (where both are reverse biased).