Fundamentals of BJT operation : PN junction
The bipolar transistor is basically a simple device, and this section is devoted to a simple and largely qualitative view of BJT operation. We will deal with
the details of these transistors in following sections, but first we must define some terms and gain physical understanding of how carriers are transported
through the device. Then we can discuss how the current through two terminals can be controlled by small changes in the current at a third terminal.
Operation of PN junction:
- Let us begin the discussion of bipolar transistors by considering the reverse-biased p-n junction diode of Fig.(a).
- According to the theory, the reverse saturation current through this diode depends on the rate at which minority carriers are generated in the neighborhood of the junction.
- For example, that the reverse current due to holes being swept from n to p is essentially independent of the size of the junction ξ field and hence is independent of the reverse bias.
- The reason given was that the hole current depends on how often minority holes are generated by EHP creation within a diffusion length of the junction—not upon how fast a particular hole is swept across the depletion layer by the field.
- As a result, it is possible to increase the reverse current through the diode by increasing the rate of EHP generation (Fig.b).
- One convenient method for accomplishing this is optical excitation of EHPs with light (hv > Eg).
- With steady photoexcitation the reverse current will still be essentially independent of bias voltage, and if the dark saturation current is negligible, the reverse current is directly proportional to the optical generation rate gop.
The example of external control of current through a junction by optical generation raises an interesting question: Is it possible to inject minority
carriers in to the neighborhood of the junction electrically instead of optically?
- If so, we could control the junction reverse current simply by varying the rate of minority carrier injection.
- For example, let us consider a hypothetical hole injection device as in Fig.(c).
- If we can inject holes at a predetermined rate into the n side of the junction, the effect on the junction current will resemble the effects of optical generation.
- The current from n to p will depend on the hole injection rate and will be essentially independent of the bias voltage. There are several obvious advantages to such external control of a current;
For example, the current through the reverse-biased junction would vary very little if the load resistor RL were changed, since the magnitude of the
junction voltage is relatively unimportant.
- Therefore, such an arrangement should be a good approximation to a controllable constant current source.