JUNCTION-GATE FIELD-EFFECT TRANSISTORS (JFETS)

Junction-Gate Field-Effect Transistors (JFETs)

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1. Overview

A junction-gate field-effect transistor (JFET) is a voltage controlled, three terminal transistor. It is closely related to the more popular MOSFET (another form of field-effect transistor).

The main differences between a MOSFET and a JFET is that the JFET has more gate current due to the absence of the gate oxide layer, and JFETs only come in depletion mode (JFET is on at $$V_{GS} = 0V$$, and turns off as $$V_{GS}$$ goes negative), whilst MOSFETs come in both enhancement mode and depletion mode types.

2. Schematic Symbols

Figure 1 shows the commonly used schematic symbols for JFETs. As with MOSFETs, there is an arrow pointing in towards the body for the N-channel, and an arrow pointing outwards for the P-channel.

Figure 1. Schematic symbols of asymmetric and symmetric JFETs, both having N-channel and P-channel variants.

Interestingly, when you learn about the basic construction of a JFET, you would assume that the drain and source pins are interchangeable due to the symmetric nature of it’s design. That leads you to being able to use a schematic symbol with the gate pin in the middle of the gate. However, not all JFET pins are symmetric, and in that case it’s better to use the symbols shown which move the gate pin closer to the source (similar to how a MOSFET symbol is drawn).

3. Uses

JFETs are used for:

• Simple current sources

• Low noise op-amp input circuitry

• As a switch (although MOSFETs are more commonly used for this)

• Amplifiers

4. Datasheet Parameters

4.1. Gate-Source Cutoff Voltage ($$V_{gs(off)}$$)

This is the gate-source voltage at which the JFET is pretty much "turned off", i.e. the current flowing through the drain has dropped to almost 0. Typically it is specified as the voltage at which the drain current drops down to a current of $$10nA$$. Unfortunately for JFETs, this value varies significantly due to manufacturing tolerances and typically a Vgs(off) min. to max. range of >5V can be given! Taking the OnSemi 2N5457 for example, it’s datasheet specifies a $$V_{GS(off)}$$ min. of $$-0.5V$$ and a max. of $$-6.0V$$ (ignore that because we are dealing with negative numbers, the max. is actually a "smaller" number than the min.)[1].

4.2. Zero Gate Voltage Drain Current ($$I_{dss}$$)

This is the current through the drain when $$V_{GS} = 0V$$.

4.3. Gate-Source Breakdown Voltage ($$V_{(br)gss}$$)

The gate-source breakdown voltage $$V_{(BR)GSS}$$ is the maximum voltage the gate can withstand (relative to the source) before breakdown occurs.

4.4. Transconductance ($$g_m$$ or $$|Y_{fs}|$$)

The transconductance $$g_m$$ (also called small-signal transconductance, forward transfer admittance $$|Y_{fs}|$$, or $$g_{fs}$$) is the ratio of change in $$I_D$$ to a change in $$V_{GS}$$ at a constant drain-to-source voltage $$V_{DS}$$ (see $$Eq.\ \ref{eq-yfs-gm-di-dv}$$). It is typically given in units of $$\mu S$$ (or $$\mu mhos$$, which is equivalent). It can be thought of as the small-signal gain, when the JFET is operating at a fixed DC bias point.

\begin{align} \label{eq-yfs-gm-di-dv} g_m = |Y_{fs}| = \frac{\Delta I_D}{\Delta V_{GS}} \end{align}

The transconductance is maximum at $$V_{GS} = 0V$$, which is called $$g_{mo}$$ and is typically specified in the JFET’s datasheet[2].

Figure 2. The transconductance is the gradient of the $$I_D$$ vs. $$V_{GS}$$ graph at the DC operating point. DC operating point shown by dotted blue lines. This is the source transfer characteristics of the 2N5456 N-channel JFET[1].

The transconductance for most JFETs ranges from $$1-100mS$$. They are typically specified at the bias point $$V_{GS} = 0V.$$

4.5. Output Conductance

The output conductance $$g_{oss}$$ is the ratio of change in $$I_D$$ to a change in $$V_{DS}$$[3].

\begin{align} \label{eq-goss-id-vds} g_{oss} = \frac{\Delta I_D}{\Delta V_{DS}} \end{align}

$$g_{oss}$$ ranges from $$1\mu S$$ to $$50\mu S$$ for most JFETs[3].

The output conductance is an important consideration for the stability/accuracy of a JFET current source[3]. Another way of expressing output conductance is the dynamic resistance.

4.6. Dynamic Resistance

The dynamic resistance $$r_d$$ is just the reciprocal of the output conductance, and is typically expressed in units of $$M\Omega$$.

\begin{align} r_d = \frac{1}{g_{oss}} \end{align}

5. Current Source

JFETs can be used to make two-terminal current sources which can be useful in circuit design due to their simplicity. See Current Sources And Sinks: Constant-Current Diode (JFET Current Source) for schematics, how they work, and equations for calculating the values. They can also be used to make low-noise current sources.

6. Common Components

• 2N5457: Common "general purpose" N-channel JFET.

• J202: N-channel JFET by ON Semiconductor that originally came in a TO-92 package, but now comes in a SOT-23-3 package.

Table 1. Parameters for popular JFET parts.
Part Number $$g_m (typ)$$ Package

2N5457[1]

$$3000\mu S$$

TO-92

Authors

Geoffrey Hunter

Dude making stuff.