The output characteristic of the transistor s studied for different emitter current . The curves have three regions, saturation, active and cut-off. When is less then the barrier potential (0.7 V for silicon), both junctions are forward biased resulting in the exponential change in with small change in . As is further increased, the base emitter junction is reversed biased and a constant current flows. When is further increased, the base emitter junction breaks down and a very large current flows in the circuit. The resultant curves for different is shown in the figure.