Semiconductor Devices

What is a JFET transistor and how it works

JFET transistor

This post is about structure, parameters and properties of JFET transistor.

JFET transistor is a three-terminal device, where one of the terminal can control current between two others. JFET transistor terminals are drain (D), source (S) and gate (G). Here current between D and S can be controlled by gate-source voltage.

Construction of JFET transistor is depicted on the figure below. N-type JFET transistor consist of n-type semiconductor with heavily doped p-type regions as shown on the figure. P-type areas are forming the gate, both p-type areas and n-type area are equipped with thin contacts layers.

JFET n-type

JFET p-type

JFET electrical symbols

Let’s consider n-type JFET transistor. Here n-type semiconductor is connected to drain and source with ohmic contacts, p-type semiconductors are connected to the gate and connected to each other.

JFET transistor is in cut-off mode, and does not conduct any current when both source-gate and drain-source potentials are zero. There is thin depletion regions are formed around p-type regions of JFET. Depletion regions are free from free careers, so there is no current through depletion area.

Let’s keep vGS=0 and apply drain-source voltage vDS>0. Current ID will flow from drain and source (note that electrons will move in the opposite direction). Here JFET transistor operates in ohmic mode. Depletion region around p-type areas is wider close to drain because of the voltage distribution  between drain and source. When vDS is growing, width of depletion areas close of drain is growing as well. At some point drain-source voltage will reach vP level, when these depletion areas will get very close to each other. vP voltage is called pinch-off voltage. After this point JFET transistor will go to the saturation mode. Here small channel between two depletion areas will still exist with constant current through is IDSS.

If vGS<0, depletion regions still grows around p-type areas of JFET transistor. In order to maintain depletion regions vGS should be smaller than vDS. Lower vGS. lower pinch-off voltage vP. Finally when vGS=vPIDSS=0. JFET is off.


JFET transistor
JFET transistor
JFET transistor
JFET transistor

Ohmic mode

JFET transistor
JFET transistor

Saturation mode

When JFET transistor operates in ohmic mode, the resistance of n-channel can  be controlled by vGS voltage, so JFET behaves like a voltage-controlled resistor.  First approximation of JFET resistance in this case is rDS=v2P2IDSS(vGS+Vp)2.

Finally equations, describing JFET transistor behaviour are:

VGS<Vpfor cut-off region;

vDS>Vbreak for breakdown voltage;

vDS <vGS+vp4, vGS>vpiD=iDSRDSRDS=vp22IDsS(vGS+vp) for ohmic region;

iD=IDSSvp2(vGS+vp)2, vDS>vGS+vp, vGS>vpfor saturation region. Here IDSS is a maximum current, when vGS=0 and vDS>vP.

If vGS < vPID=0.

If 0<vGS<vP, then 0<i<IDSS.

JFET transistor transfer function

Transfer characteristic is the relation between output current ID and controlling voltage VGS.  The easiest way to obtain transfer characteristic is to apply Shockley equation ID=IDS(1vGSvP)2=f(vGS).  It gives us ID=IDSS, when vGS=0 and ID=0, when vGS=vp. Here we are having  important relationship between vGS and vP is vGS=vP(1IDIDSS).

In order to outline the transfer function, we must calculate ID current for different key levels of vGS.

Transfer function can be obtained from the ID(vDS) characteristics like on the figure below.

JFET transistor
JFET transistor

Fixed-bias JFET configuration

Three most important relationships for operation of FET transistor devices are IG=0, ID=IS and Shockley equation ID=IDSS(1VGSvP)2.

Here  vin and vout are ac levels.

JFET transistor
JFET transistor


vDS=vDDIDRDvG=vGSvD=vDS, because vS=0V.


Self-biased JFET transistor configuration

JFET transistor
JFET transistor


Here vRS=vGS, and from the other side vRS=ISRSID=IS then vGS=IDRS and  vDS=vDDIDRSIDRDvG=0, vS=IDRS, and vD=VDD+IDRS,

In accordance to Shockley equation ID=IDSS(1+IDRSvP)2, then we are having second order equation that will lead us to quadratic function.