In one line
A JFET is a bar of n-type (or p-type) semiconductor forming a channel between source and drain, with heavily doped regions of the opposite type on its sides forming the gate; current is controlled by gate voltage, not gate current.
Explained simply
Think of a garden hose (the channel) with water flowing from tap (source) to nozzle (drain). Squeeze the hose from both sides and less water flows. In a JFET, the "squeezing hands" are depletion regions created by reverse-biasing the gate, and the squeeze is set by a voltage, so almost no gate current is needed.
Technically, a JFET is a unipolar, voltage-controlled device: only majority carriers (electrons in an n-channel JFET) carry current. Its gate-channel p-n junction is always reverse biased, which gives a very high input resistance.
How it works
- Channel: a lightly doped n-type bar.
- Source (S): ohmic contact at one end, where carriers enter the channel.
- Drain (D): ohmic contact at the other end, where carriers leave.
- Gate (G): heavily doped \( p^+ \) regions diffused on both sides of the channel and connected together, forming two p-n junctions with the channel.
- Symbol: a vertical line for the channel with the gate arrow pointing in for n-channel and out for p-channel.
| BJT | FET |
|---|---|
| bipolar (electrons and holes) | unipolar (majority carriers only) |
| current controlled | voltage controlled |
| low to moderate input resistance | very high input resistance |
| more noise | less noise |
| higher voltage gain in similar circuits | lower voltage gain |
| prone to thermal runaway | better thermal stability |
| larger size in ICs | smaller, easier to fabricate in ICs (MOSFET) |
Quick example
The BFW10 and 2N3819 are common n-channel JFETs used in college labs. Their high input resistance makes them ideal as the first stage of instruments (e.g. a voltmeter input), because they hardly load the circuit being measured.
Common mistakes
- Saying the JFET gate is forward biased in normal use.
- Calling the FET bipolar.
- Drawing the gate as an insulated plate (that is a MOSFET).
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the construction of an n-channel JFET with symbol" (5 marks) or "Compare BJT and FET" (5 marks). For full marks draw the bar with source, drain, the two gate regions and depletion layers, draw both n- and p-channel symbols, and give a comparison table of at least five points.