Electronics >> Junction Field Effect Transistors (JFETs)

Field Effect Transistors (FETs) are unipolar devices. Their operation depends only on one type of charge, free electrons (N-type material) or holes (P-type material).

The FETs are the preferred device for switching application, because there is no minority carrier in an FET (junction), therefore it can switch off faster, since no stored charge has to be removed from the junction.

(FETs have majority carriers, but not minority carriers)

N-type material are majority carriers
P-type material are minority carriers

The two types of unipolar transistors are:

• JFETs (junction field effect transistors)
• MOSFETs (metal-oxide semiconductor FETs)

View MOSFETs: Metal Oxide Semiconductor FETs (MOSFETs)

Junction Field Effect Transistors - JFETs

To create a JFET a manufacturer diffuses two area of P-type semiconductor material into the N-type semiconductor. (The two parts of P-type material are connected internally in order for the device to have only one gate lead.)

The term field effect is linked to the depletion layer around each P-material region. The depletion layer exists because free electrons diffuse from the N region into the P region.

The drain supply voltage is positive, and the gate supply voltage is negative.

The gate-source diode of a JFET is always connected in reverse-bias mode. Because of this, the gate current is zero, which is the same as saying that the JFET has an extremely large input resistance.

A JFET has an input resistance in hundreds of Mega Ohm. This is the large benefit that a JFET has over a bipolar transistor, and is the reason that JFETs do extremely well in applications in which high impedance is required.

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