Field Effect Transistors
The field effect transistor (FET) is a transistor with a fundamentally different control mechanism from the BJT you studied in the last lesson. Instead of using a base current to control a larger collector current, a FET uses an electric field — a voltage — to control current flow through a channel. Because the control terminal (the gate) draws virtually no current, FETs have an extremely high input impedance. This makes them ideal for sensitive measuring instruments, audio preamps, and any application where loading the signal source would be unacceptable.
- FET families: JFET and MOSFET
- JFET construction and operation
- N-channel and P-channel JFETs
- Key JFET parameters: pinch-off voltage and transconductance
- Depletion mode operation
- JFETs in practice
FET Families: JFET and MOSFET
There are two main families of field effect transistors:
- JFET (Junction FET): Uses a reverse-biased PN junction as the gate. Always operates in depletion mode — current flows when no gate voltage is applied and is reduced by applying gate voltage. Good noise performance at audio and high frequencies. Covered in this lesson.
- MOSFET (Metal-Oxide-Semiconductor FET): Uses a thin insulating oxide layer to separate the gate from the channel. Can operate in depletion or enhancement mode. Dominant in digital logic and power switching applications. Covered in the next lesson.
JFET Construction and Operation
An N-channel JFET consists of a bar of N-type semiconductor (the channel) with two P-type regions diffused into the sides to form the gate junctions. Current flows from one end of the channel (the drain, connected to the positive supply) to the other end (the source, connected to ground or a lower voltage). The gate junction is always reverse-biased, so essentially no gate current flows — the gate input impedance is in the range of hundreds of megaohms.
When the gate is made more negative than the source, the depletion layers around the gate junctions grow inward and begin to squeeze the conducting channel. This is like pinching a hose — less channel area means less current can flow for a given drain-to-source voltage. When the gate voltage becomes negative enough, the depletion layers meet in the middle, completely pinching off the channel and reducing drain current to nearly zero.
N-channel JFET (left) and P-channel JFET (right). The arrow on the gate indicates the direction of the PN junction — pointing inward for N-channel, outward for P-channel. Gate draws negligible current.
View LargerN-Channel and P-Channel JFETs
Just as with BJTs, JFETs come in two polarities:
- N-channel JFET: Current flows through an N-type channel from drain to source. Gate must be made negative relative to source to reduce channel current. The 2N5457, 2N3819, and J310 are common N-channel JFETs.
- P-channel JFET: Current flows through a P-type channel from source to drain. Gate must be made positive relative to source to reduce channel current. Less common than N-channel.
Key JFET Parameters
The two most important JFET parameters for circuit design are:
| Parameter | Symbol | Meaning |
|---|---|---|
| Pinch-off voltage | VP or VGS(off) | Gate-source voltage at which drain current falls to nearly zero. Negative for N-channel. Typical range: −0.5 V to −10 V. |
| Drain current at zero bias | IDSS | Drain current when VGS = 0. Maximum drain current for the device at a given drain-source voltage. Typical range: 1–30 mA. |
| Transconductance | gm | The gain parameter: change in drain current per unit change in gate voltage. Units: mA/V or millisiemens. Typical range: 1–15 mA/V. |
Transconductance is the JFET equivalent of the BJT's current gain. A higher gm means more drain current change for a given gate voltage swing, which translates to higher voltage gain in an amplifier stage (Voltage gain Av = gm × RD).
Depletion Mode Operation
The JFET is a depletion mode device: it is normally on with VGS = 0 and is turned off (or reduced) by applying a gate voltage. This is the opposite of enhancement-mode MOSFETs, which are normally off and require a gate voltage to turn them on. The depletion mode characteristic means JFETs can be self-biased with a simple source resistor — no separate negative supply is needed for the gate.
VGS = − ID × RS
A source resistor RS develops a voltage drop equal to the drain current times RS, making the source more positive than the gate (which is held at ground through the gate resistor). This automatically reverse-biases the gate-source junction and sets the operating point without any negative supply rail.
JFETs in Practice
- Sensor and instrument preamplifiers: The extremely high input impedance and low noise of JFETs make them ideal for the first stage of measuring instruments and for high-impedance sensors such as piezo pickups and pH probes. The J201, 2N5457 and MPF102 are widely used in this role.
- Crystal oscillators: JFETs in common-gate or common-drain configuration provide stable, low-noise oscillator circuits with good frequency stability.
- Audio preamplifiers: Microphone and guitar preamplifiers benefit from the JFET's high input impedance — electret microphone capsules contain a JFET, and many guitar effects pedals use JFET input stages.
- Electronic switching: A JFET with the gate driven to pinch-off acts as a very high-impedance switch, used for audio muting, sample-and-hold circuits and analog switching.
- Constant-current sources: A JFET with its gate tied to its source passes a nearly constant current, a simple way to bias an LED or a precision circuit.
Frequently Asked Questions
Why does a JFET draw virtually no gate current?
The gate of a JFET is a reverse-biased PN junction. A reverse-biased junction has an extremely high impedance — only the tiny leakage current flows. This is why JFET gate input impedance is measured in hundreds of megaohms, compared to the few kilohms input impedance of a BJT base. The consequence is that the JFET does not load the signal source — it takes virtually no power from the source to control the drain current.
What is the difference between depletion mode and enhancement mode?
A depletion-mode device (like the JFET) is fully conducting with zero gate voltage and is turned off by applying gate voltage. An enhancement-mode device (like most power MOSFETs and digital logic transistors) is off with zero gate voltage and is turned on by applying gate voltage above a threshold. For a sensitive preamplifier, depletion mode is often preferred because the transistor can be biased with a simple self-bias resistor and does not need an active gate driver circuit.
Why are JFETs preferred over BJTs in some preamplifiers?
Two reasons: input impedance and noise. The JFET's near-infinite input impedance allows a high-impedance source, such as a piezo pickup or a sensor, to deliver its full signal without being loaded by the amplifier input. The JFET also has an intrinsically lower noise contribution at lower frequencies than a BJT in the same role, because there is no shot noise from base current. With high-impedance sources, a well-chosen JFET often gives lower noise than a BJT, improving the quality of weak signals.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.