What is the functional difference between bipolar transistors and FETs in electronics applications?

In our simplified models, here is how we treat these different devices functionally:

• A bipolar transistor in its operating regime (with $V_{ce}$ greater than about 2 V) has $V_{be}=V_{pn}$ and the current from collector to emitter is given by the Transistor Man equation, $I_c = \beta I_b$ (with AC-equivalent expression $i_c = \beta i_b$). We tend to think of the collector current as being controlled by the base current. (For $V_{ce}$ less than about 2 V, the $I_c$ behavior is nonlinear.)

• A JFET in its operating regime draws approximately zero ($\sim$pA) current at the gate. In its operating (``pinch-off'') regime, where $V_{ds}$ is greater than about 2 V, we tend to think of the drain-to-source current as being controlled by the gate-to-source voltage. (For $V_{ds}$ less than about 2 V, the JFET acts like a variable, but linear, resistor, with resistance controlled by $V_{gs}$.)