Active-mode PNP transistors in circuits

Structure and use of the PNP transistor.

The diagram opposite is a schematic representation of the PNP transistor connected to 2 voltage sources. To make the transistor conduct the appreciable current (on order of 1 mA) from E to C, V_EB should be above a minimum value at time referred to as cut-in voltage. The cut-in voltage is nearly 600 mV for silicon BJTs at the room temperature but can be different depending on the type of transistor and biasing of it. This applied voltage creates the upper P-N junction to 'turn-on' allowing the flow of holes from emitter into base. In active mode, the electric field prevailing among the emitter and the collector (caused by V_CE) causes the majority of these holes to cross the lower P-N junction into the collector to form the collector current I_C. The remainder of holes recombines with the electrons, majority carriers in base, making a current through base connection to form base current, IB. As shown in diagram, the emitter current, I_E, is total transistor current, which is the sum of other terminal currents (that is I_E = I_B + I_C).

In the diagram, arrows representing current point in direction of the conventional current - the flow of holes is in same direction of arrows because holes carry positive  electric charge. In the active mode, the ratio of collector current to the base current is called the DC current gain. This gain is 100 or more, the robust circuit designs do not depend on exact value. The value of this gain for DC signals is referred to as h_FE, and the value of this gain for AC signals is referred as h_fe. However, when there is no particular frequency range of interest, the symbol β is used.

It  should  also  be  noted  that  emitter  current  is  related  to  V_EB  exponentially.  At the  room temperature, an increase in V_EB  by around 60 mV increases emitter current by a factor of 10. Because base current is approximately proportional to collector and emitter currents, they vary in the same manner.

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