Bipolar Transistor Structure

The device structure, sign convention and circuit diagram are shown in figure below. The device consists of three alternating n-type and p-type regions. A NPN structure is shown which will be used throughout this text to explain the device operation and to derive the device equations. Alternatively one can also construct and analyze a PNP structure.

Fig. Structure (left) and circuit symbol (upper right) of an NPN bipolar junction transistor (BJT). Also shown are the two p-n diodes (lower right) within the bipolar transistor.

The physical widths of the emitter, base, and collector are indicated on the figure with the associated symbols, w_E, w_B and w_C. Two p-n diodes exist in the three layer structure, namely the base-emitter diode and the base-collector diode. The diodes are shown in the lower right corner of figure 5.1. It should be noted that the two diodes do not represent a complete equivalent circuit of a bipolar transistor since the transport of minority carriers through the base is not included.

The two diodes are biased by the respective voltage sources. The sign convention indicated on the figure is such that the diodes are forward based when a positive voltage is applied. The collector and base currents are considered to be positive when current flows into the terminals while the emitter current is considered to be positive if the current flows out of the emitter terminal.

These "quasi-neutral" regions are neutral only in thermal equilibrium. However, when voltages are applied one finds that the charge densities and the electric field in these regions are significantly smaller than in the depletion regions. We will therefore treat these regions as if they were neutral and refer to them as being "quasi-neutral".