JFET are three-terminal semiconductor devices that can be used as electronically controlled switches, amplifiers or electrically controlled resistors. Junction gate field effect transistor (JFET or JUGFET) is the simplest type of field effect transistor.
Unlike bipolar transistors, JFETs are only powered live because they do not need a polarization current. The electric charge flows through a semiconductor channel between the source and drain terminals. By applying an inverse polarization voltage to a gate terminal, the channel is “pinched”, so that the electrical current is prevented or switched off completely.
A JFET is usually turned on when there is no potential difference between its gate and source terminals. If a potential difference in the correct polarity between its gate and source terminals is applied, the JFET will be more current-resistant, which means less current will flow into the channel between the source and drain terminals. Therefore, JFETs are sometimes termed outdated devices.
JFETs may have a type n or p channel. In type n, if the voltage applied to the gate is less than that applied to the source, the current will be reduced (similarly to type p, if the voltage applied to the gate is greater than that applied to the source). A JFET has a large input impedance (sometimes at 1010 ohms), which means it has a negligible effect on components or external circuits connected to its port.
The junction field effect transistor
We have previously seen that a bipolar junction transistor is constructed using two PN junctions in the main current transport path between the emitter and the collector terminals. Junction field effect transistor (JUGFET or JFET) has no PN junctions but has a narrow piece of high resistivity semiconductor material that forms a N or P type “channel” to slide the major carriers with two The ohmic electrical connections at either end called Drain and Source respectively.
There are two basic configurations of junction field effect transistor, JFET to channel N and JFET to channel P. Channel JFET channel N is doped with donor impurities indicating that the current flow through the channel is negative (from here the term N-channel) in the form of electrons.
Likewise, channel JFET channel P is doped with impurity of the transducer meaning that the current flow through the channel is positive (hence the channel term P) in the form of holes. The N-channel JFETs have a greater channel conductivity (lower resistance) than their P-type channel types, since electrons have greater mobility through a conductor than the holes. This makes the JFET to channel N a conductor more efficient than the P-channel counterparts.
We have previously said that there are two ohmic electrical connections at either end of the channel called Drain and Source. But inside this channel there is a third electrical connection called the gate terminal and this can also be a type P or type N material that forms a PN junction with the main channel.