Active-mode NPN transistors in circuits
Figure: Structure and use of NPN transistor. Arrow according to schematic.
The figure opposite is a schematic presentation of an NPN transistor associated to two voltage sources. To construct the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE have to be above a minimum value sometimes considered to as the cut-in voltage. The cut-in voltage is generally about 600 mV for silicon BJTs at room temperature but can be different depending upon the sort of transistor and its biasing. This applied voltage results in the lower P-N junction to 'turn-on' permitting a flow of electrons from the emitter into the base. In active mode, the electric field lay between base and collector (caused by VCE) will cause the majority of these electrons to cross the upper P-N junction into the collector to make the collector current IC. The remainder of the electrons again combines with holes, the majority carriers in the base, making a current by the base connection to form the base current, IB. As displayed in the figure, the emitter current, IE, is the total transistor current that is the sum of the other terminal currents (that is IE=IB+IC).