A Field Effect Transistor (FET) is a three-terminal Active semiconductor device, where the output current is controlled by an electric field generated by the input voltage. FETs are also known as unipolar transistors because, unlike bipolar transistors, FETs only have either electrons or holes operating as charge carriers. FET uses the voltage applied to its input terminal (called the Gate), to control the current flowing from the source to drain, making the Field Effect Transistor a “Voltage” operated device.
FETs are extensively used in Integrated Circuits (ICs) due to their compact size and significantly lower power consumption. Apart from that, FETs are also used in high power switching applications, as voltage-variable resistors (VVRs) in operational amplifiers (Op-Amps), and tone controls, etc., for mixer operation on FM and TV receivers and in logic circuits.
A FET has four terminals named Source, Drain, Gate, and Body.
Source: Source is the terminal through which the majority charge carriers are entered in the FET. Drain: Drain is the terminal through which the majority charge carriers exit from the FET. Gate: The gate terminal is formed by diffusion of an N-type semiconductor with a P-type semiconductor. This creates a heavily doped PN junction region that controls the flow of the carrier from source to drain. Body: This is the substrate on which the FET is built. In discrete applications, it is internally tied to the source pin allowing its effects to be ignored entirely. However, in integrated circuits, this pin will be typically connected to the most negative power supply in an NMOS circuit(most positive in a PMOS circuit) because many transistors will share it. Careful connections and design are critical to maintaining FET performance when the Body connection is involved.
Channel: This is the region in which the majority carriers pass from the source terminal to the drain terminal.
FET transistors are classified into Junction Field Effect transistors (JFET) and Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) transistors.
JFET (Junction-Field Effect Transistor)
The Junction Field Effect transistor (JFET) is the earliest type of FET. The current flows through an active channel between sources to drain terminals. The voltage applied between gate and source controls the flow of electric current between the source and drain of the JFET. By applying a reverse bias voltage to the gate terminal, the channel is strained, so the electric current is switched off completely. That is why JFETs are referred to as “normally on” devices. The JFET transistors are available in both N-channel and P-channel types.
In the N-channel JFET, the channel is doped with donor impurities, making it an N-type semiconductor. Therefore the flow of current through the channel is negative in the form of Electrons. Hence the name N-channel JFET. Two P-type substrates doped at the opposite sides of its middle part. Thus two PN junctions are formed by those heavily doped P-type regions and the N-type channel in between. The gate (G) lead is connected internally to both the P-type terminals, while the drain (D) and source (S) leads are connected to either end of the N-type channel.
How does it work?
When no voltage is applied across the gate terminal, the channel becomes a wide-open path for electrons to flow. Therefore maximum currentContinue reading