Junction field effect transistors (JFET) are a type of FET in which the conducting channel lies between one or more p-n junctions.

Like all transistors, JFETs have three terminals: source (S), drain (D), and gate (G). All JFETs are constructed with a conductive channel running from the source to the drain. The channel is sandwiched between two regions of contrasting polarity; for example, in an n-channel JFET — the most common type — the channel is composed of n-type material, while the two outer regions are p-type materials. Each p-region is surrounded by a thin depletion layer. The gate contact is situated on one of the p-regions. The image below shows a schematic symbol and basic diagram of an n-channel JFET.

Image credit: CircuitsToday

JFETs are often used as switches or voltage-controlled resistors, and their operation can easily be compared to that of a faucet. If we consider that we can control the flow of water through a faucet by adjusting a valve, we can then compare a JFET by assigning the source terminal as the water source, the gate as the valve, and the drain as the physical drain. This means that the source contact provides the electrons which flow through the conductive channel to the drain terminal. By applying voltage to the gate, the channel becomes smaller and effectively limits the flow of electrons; this shows the JFET's use as a voltage-controlled resistor, an operation which will be described in more detail below.

Image credit: Bolestad & Nashewski; Pearson Publishing

JFETs, like the related bipolar junction transistors (BJT), are used in a variety of modern electronic devices, such as amplifiers, switching devices, and impedance matching networks. Field effect transistors (FET) do have a number of differences when compared to BJTs, including:

• FETs are voltage-controlled; BJTs are current-controlled
• FETs have higher input impedance; BJTs have higher gain
• FETs are less sensitive to temperature variation
• FETs are unipolar, while BJTs are bipolar

Operating Principles

A JFET's operating conditions are determined by the values and changes in two different voltages: the gate-source voltage (V_GS) and the drain-source voltage (V_DS).

V_GS = 0; V_DS Increasing

If the gate-source voltage is zero (V_GS=0) and the drain-source voltage is increased, three changes occur within the transistor:

• The depletion region between the n- and p-regions grows in size.
• The n-channel becomes smaller and resistance is increased.
• Despite the increased resistance, the current from the source to the drain increases due to the increasing drain-source voltage.

If the gate voltage remains at zero and the drain-source voltage continues to increase the transistor will eventually experience a pinch-off, in which the depletion layers become so thick that they close off the channel. Based on the images below, it might seem that a pinch-off would result in the drain current dropping to zero, but in this condition the drain current simply remains constant despite further increases in the drain voltage. At the pinch-off point, the drain current may be referred to as saturated (or maximized) and can be specified using the value I_DSS.

The two conditions described above: VGS=0 (left) and pinch-off.

Image credit: Bolestad and Nashewski; Pearson Publishing

Several other specifications are helpful when considering JFET operation:

• V_P represents the pinch-off voltage, or the voltage necessary to induce a pinch-off condition. JFETs typically have relatively low pinch-off voltages.

• If V_DS continues to increase after drain saturation has occurred, the drain current (I_D) eventually begins to decrease. The gate-source voltage necessary to reduce the drain current to zero is referred to as V_GS(Off).

• V_DSmax specifies the maximum drain-source voltage. If this value is exceeded the drain current will increase uncontrollably, resulting in a breakdown condition and likely physical damage or destruction.

Operation as a Voltage-Controlled Resistor

Referring back to the comparison between JFETs and water faucets, the gate-source voltage can be altered to adjust the channel resistance. The graph below shows the relationship between all the variables that have been previously discussed. Note that the graph region to the left of the pinch-off voltage (V_P) is known as the ohmic region.

Image credit: Bolestad and Nashewski; Pearson Publishing

Applications

JFETs are used in many of the same applications — those involving switching or amplifying — as other field effect transistors, including metal oxide semiconductor FETs (MOSFET). Due to their relatively higher transconductance properties, JFETs are especially suitable for low noise operational amplifiers.

Standards

SMD 5962-98636 - JFET Op-Amp

SMD 5962-87718  - JFET Multiplexer

References

UC Berkeley - JFET Circuits