FIELD EFFECT TRANSISTOR

CONSTRUCTION AND CHARACTERISTICS OF JFETs

As indicated earlier, the JFET is a three-terminal device with one terminal capable of
controlling the current between the other two. In our discussion of the BJT transistor
the npn transistor was employed through the major part of the analysis and design
sections, with a section devoted to the impact of using a pnp transistor. For the JFET
transistor the n-channel device will appear as the prominent device, with paragraphs
and sections devoted to the impact of using a p-channel JFET.
The basic construction of the n-channel JFET is shown in Fig. 5.2. Note that the
major part of the structure is the n-type material that forms the channel between the
embedded layers of p-type material. The top of the n-type channel is connected through
an ohmic contact to a terminal referred to as the drain (D), while the lower end of
the same material is connected through an ohmic contact to a terminal referred to as
the source (S). The two p-type materials are connected together and to the gate (G)
terminal. In essence, therefore, the drain and source are connected to the ends of the
n-type channel and the gate to the two layers of p-type material. In the absence of
any applied potentials the JFET has two p-n junctions under no-bias conditions. The
result is a depletion region at each junction as shown in Fig. 5.2 that resembles the
same region of a diode under no-bias conditions. Recall also that a depletion region
is that region void of free carriers and therefore unable to support conduction through
the region.

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used for amplifying or switching electronic signals. The basic principle of the device was first proposed by Julius Edgar Lilienfeld in 1925. In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain. The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.

The 'metal' in the name is now often a misnomer because the previously metal gate material is now often a layer of polysilicon (polycrystalline silicon). Aluminium had been the gate material until the mid 1970s, when polysilicon became dominant, due to its capability to form self-aligned gates. Metallic gates are regaining popularity, since it is difficult to increase the speed of operation of transistors without metal gates.

IGFET is a related term meaning insulated-gate field-effect transistor, and is almost synonymous with MOSFET, though it can refer to FETs with a gate insulator that is not oxide. Another synonym is MISFET for metal–insulator–semiconductor FET.