Now consider what happens when we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction. This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biassed Collector region. As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region. Hence we see a Emitter-Collector current whose magnitude is set by the chosen Emitter-Base voltage we have applied. To maintain the flow through the transistor we have to keep on putting 'fresh' electrons into the emitter and removing the new arrivals from the Collector. Hence we see an external current flowing in the circuit.
The precise value of the chosen Emitter-Base voltage isn't important to our argument here, but it does determine the amount of current we'll see. For the sake of example we've chosen a half a volt. Since the Emitter-Base junction is a PN diode we can expect to see a current when we apply forward voltages of this sort of size. In practice with a Bipolar transistor made using Silicon we can expect to have to use an Emitter-Base voltage in the range from around a half volt up to almost one volt. Higher voltages tend to produce so much current that they can destroy the transistor!
It is worth noting that the magnitude of the current we see isn't really affected by the chosen Base-Collector voltage. This is because the current is mainly set by how easy it is for electrons to get from the Emitter into the Base region. Most (but not all!) the electrons that get into the Base move straight on into the Collector provided the Collector voltage is positive enough to draw them out of the Base region. That said, some of the electrons get 'lost' on the way across the Base. This process is illustrated in Figure 4
Some of the free electrons crossing the Base encounter a hole and 'drop into it'. As a result, the Base region loses one of its positive charges (holes) each time this happens. If we didn't do anything about this we'd find that the Base potential would become more negative (i.e. 'less positive' becuase of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter-Base junction. The current flow would then stop.
To prevent this happening we use the applied Emitter-Base voltage to remove the captured electrons from the Base and maintain the number of holes it contains. This have the overall effect that we see some of the electrons which enter the transistor via the Emitter emerging again from the Base rather than the Collector. For most practical Bipolar Transistors only about 1% of the free electrons which try to cross Base region get caught in this way. Hence we see a Base Current, IB, which is typically around one hundred times smaller than the Emitter Current, IE.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.