Final answer:
The diode between the transistor base and emitter controls the current flow through the transistor.
Step-by-step explanation:
The purpose of the diode between the transistor base and emitter is to control the current flow through the transistor. When a positive voltage is applied to the diode, it allows current to flow from the base to the emitter, which activates the transistor. This allows a larger collector current to flow through the collector.