Electronics · Ch 1 — Field Effect Transistor (FET)

Chapter Summary

An FET is a three-terminal, unipolar, voltage-controlled solid-state device; current is controlled by an electric field and carried by majority carriers only. It shares three terminals and two pn junctions with the BJT, but the BJT is current-controlled and bipolar.
FETs are classified as JFET and MOSFET (IGFET); the JFET is either n-channel or p-channel (Figure 1.1.1).
In a JFET the n-type (or p-type) channel lies between two gate junctions; the terminals are source (supplies carriers), drain (collects carriers) and gate (controls the channel).
The gate junctions are always reverse biased, forming depletion regions that widen with reverse bias and extend more into the lightly doped side; the region is asymmetric, bulging more towards the drain.
With $V_{GS}=0$ the drain current is maximum, $I_{DSS}$ ; increasing the reverse gate voltage narrows the channel until the depletion regions meet and $I_D = 0$ at the pinch-off voltage $V_P$ .
The drain characteristics show ohmic, pinch-off/saturation and breakdown regions; the transfer characteristics and both regions obey Shockley's equation $I_D = I_{DSS}\left(1 - \dfrac{V_{GS}}{V_P}\right)^2$ . …