Summary
6.1 Three-terminal active devices capable of power gain are key to the switching and amplification of electronic signals. The input impedance is (desirably) high for voltage-controlled FETs and low for current-controlled bipolar junction transistors. The power gain of the a-c signal comes from the d-c power supply. It is also desirable to have a high output impedance and a linear dependence of output current with respect to input voltage (or current).
6.2 Field effect devices such as the JFETs (or MESFETs and HEMTs) use a reverse-biased p-n junction (or Schottky diode) as a gate to control the undepleted part of a channel and thereby modulate the current flow between a source (S) of carriers and a drain (D) of carriers.
6.3 The MOSFET
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