User:John R. Brews/CZ psychology authors

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(PD) Image: John R. Brews
JFET with n-type semiconductor body and p-type gate. Ohmic contacts are source (S), drain (D), and gate (G)

A junction field-effect transistor or JFET is a three-terminal device that conducts a current that can be controlled by an applied voltage. It is made of two semiconductor layers.

Operation

The figure shows a JFET with an n-type body and a p-type gate region. For a discussion of dopant impurities and the terminology p- and n-type. see dopant impurities.

The dark-colored portion of the body is conducting, due to the electrons in this region, and is called the channel. The light-colored portion of the body is depleted of electrons and is not electrically conducting.

The n-type body in the figure conducts electricity when a voltage drop is applied between source and drain. The amount of current depends, among other things, upon the cross-section of the conducting channel. This cross section is controlled by the gate, which makes ohmic contact to the p-type region. If the gate-source pn-junction is reverse biased by applying a negative voltage to the gate and holding the source at ground, the depletion region within the pn-junction widens, restricting the cross section of the conducting channel.

The insulating portion of the body varies in width from source to drain because the current through the body causes a voltage drop as suggested by Ohm's law that varies with distance and increases the amount of reverse bias between the gate and body as a function of distance down the channel.