The characteristic can be divided into three main region.

1) Cut-off region: When the emitter voltage $V_E$ is less than $V_P$, the p-n junction is reverse biased.A small amaount of reverse saturation current $I_{EO}$ flows through the device.

2) Negative resistance region: When the emitter voltage $V_E$ becomes equal to $V_P$,the p-n junction becomes forward biased and $I_E$ starts flowing. The voltage across the device decreases in this region,though the current through the devices increases.Hence the region is called negative resistance region.This decreases the resistance $R_{B1}$.This regions continues till valley point.

3) Saturation region: The further increase in the $I_E$ beyond the valley point current $I_v$ drives the device in the saturation region.The voltage corresponding to the valley point denoted as $V_v$.