For a transistor to conduct current, what must happen at the emitter base junction?

For a transistor to conduct current, the emitter-base junction must be forward-biased. In a bipolar junction transistor (BJT), this means that the voltage at the base must be higher than the voltage at the emitter for an NPN transistor (or lower for a PNP transistor). This forward bias condition allows charge carriers (electrons or holes) to flow across the junction, which is essential for transistor operation.

When the junction is forward-biased, it reduces the potential barrier, enabling the flow of carriers from the emitter into the base and allowing a larger current to flow from the collector to the emitter as a result. This process is fundamental for the transistor to function as an amplifier or a switch, facilitating controlled current flow through the device.

In contrast, a reverse-biased condition would prevent current flow through the emitter-base junction, making it impossible for the transistor to operate in the active region. Similarly, if the junction were unconnected or open-circuited, there wouldn't be any path for current to flow, which would also impede the transistor's function. Thus, the requirement for the emitter-base junction to be forward-biased is critical to the transistor's ability to conduct current effectively.