Heterojunctions as diodes

For now, let us assume a perfect abrupt semiconductor heterojunction, and consider the implieations of applied electric fields on the current conduction process. The 

Transistors are the subject of whole books by themselves. This section presents only the bare outline of transistor types and modes of operation, focusing primarily on materials-related issues. Later chapters make reference to these devices and so it is useful to know the general aspects of their structure and operation. However, virtually all important aspects of the science and engineering of electronic materials can be illustrated through discussion of diodes and capacitors. Consequently, the following description is very brief. 

It is useful to repeat for emphasis that the current emitter to collector is controlled by current emitter to base as much as by the voltage emitter to base. The collector current increases rapidly with base current but the base current (and therefore collector current) increases exponentially, as in any diode, with base voltage. Therefore the emitter-to-base resistance is moderate and decreases with increasing voltage. In other words, a bipolar junction transistor has a low input impedance (resistance) and is a current amplifier by nature. 

Materials issues in bipolar junction transistors have driven massive materials research programs. Through these, the current bipolar junction transistor performances have been achieved. Much of the discussion of circuit technologies has focused on smaller and smaller transistors. The above examples and discussion hopefully convey that advances can be achieved by other means as well. This is useful as at some point the continuing reduction in device scale must end. 

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