TABLE OF CONTENTS Figure 18.1 shows the geometry, representation, and the symbol for an NPN BJT. Figure 18.1(a) shows the cross-section of a planar BJT. This drawing shows a central volume of n-type doped Si called the emitter, a p-type Si layer around the emitter volume called the base, and an n-type volume of doped Si called the collector. Metallic leads are connected to each of these semiconductor sections to form the transistor.
Figure 18.1(b) is another representation of an NPN BJT, showing the n-type emitter, the p-type base, and the n-type collector stacked one above the other, with leads exiting from each region. This representation shows the junctions between the elements.
The symbol for the NPN BJT used in schematic diagrams is shown in Figure 18.1(c). The arrow points in the direction of conventional current flow.
The operation of the Si NPN transistor is as follows. This device acts as a current-controlled valve, controlling the flow of electrons between the emitter and the collector. The collector is biased very positive with respect to the base. The combination of base and collector thus acts like a back-biased pn junction diode.
The base may be biased positive or negative with respect to the emitter, depending on whether the device is turned on or off. The combination of base and emitter also acts as a junction diode.
