For normal operation of a junction transistor, the emitter-base junction is always forward biased and the collector-base junction is always reverse biased. Below figure shows the biasing of the junctions for an npn transistor connected as an amplifier with the commonbase configuration, that is, the base lead is common to the input and output circuits. The emitter-base junction is forward biased by the battery VBB while the collector-base junction is reverse biased by the battery VCC . VBB should be greater than the emitter-base barrier potential (the threshold voltage). The arrows of the various currents indicate the direction of current under normal operating conditions (also called the active mode).
Active mode biasing of an npn transistor and transistor action. Shaded regions show the depletion regions under biasing conditions.
Since the emitter-base junction is forward biased, majority carriers electrons in the n emitter are injected into the base and holes (majority carriers in the p-type base) are injected from the base into the emitter. Under the ideal-diode condition, these two current components constitute the total emitter current IE.
The emitter is a very heavily doped n-type region. Hence, the current between emitter E and base B is almost entirely electron current from E into B across the forward-biased emitter junction.
The p-type base is narrow and the hole density in the base is very low. Therefore, virtually all the injected electrons (more than 95%) diffuse right across the base to the collector junction without recombining with holes. Since the collector junction is reverse biased, the electrons on reaching the collector junction are quickly swept by the strong electric field there into the n-type collector region, where they constitute the collector current IC.
In practice, about 1% to 5% of the holes from the emitter recombine with holes in the base layer and cause a small current IB in the base lead. Therefore,
IE = IB + IC ≈ IC
Therefore, carriers injected from a nearby emitter junction can result in a large current flow in a reverse-biased collector junction. This is the transistor action, and it can be realized only when the two junctions are physically close enough to interact as described.
If a pnp transistor is used, the battery connections must be reversed to give the correct bias. The conduction process is similar but takes place instead by migration of holes from emitter to collector. A few of these holes recombine with electrons in the base.
[Notes : (1) If, the two junctions are so far apart that all the injected electrons are recombined in the base before reaching the base-collector junction, then the transistor action is lost and the p-n-p structure becomes merely two diodes connected back to back. (2) Use of double-subscripted voltage notation in transistor circuits : same subscripts (viz., Vm and Vcc) represent the voltage of a biasing battery; different subscripts (viz, VBB and VCC) are used to indicate voltage between two points. Single subscripts are used for a node voltage, that is, the voltage between the subscripted point and ground.]
