In a common emitter transistor circuit, the emitter base junction and the collector base junction are respectively

Explanation

Forward bias on the emitter base junction injects carriers into the thin lightly doped base, and the reverse biased collector base junction then sweeps almost all of them across, which is what produces the current gain. Biasing both junctions forward drives the transistor into saturation, the fully on switch state, and both reverse gives cut off. Correct biasing is the first requirement of any amplifier design.