One of the most important characteristics of the FET is its high input
impedance. At a level of 1 to several hundred mega ohms it far exceeds the
typical input resistance levels of the BJT transistor configurations—a very
important characteristic in the design of linear ac amplifier systems. On the
other hand, the BJT transistor has a much higher sensitivity to changes in the
applied signal. In other words, the variation in output current is typically a great
deal more for BJTs than FETs for the same change in applied voltage. For this
reason, typical ac voltage gains for BJT amplifiers are a great deal more than for
FETs. In general, FETs are more temperature stable than BJTs, and FETs are
usually smaller in construction than BJTs, making them particularly useful in
integrated-circuit (IC) chips. The construction characteristics of some FETs,
however, can make them more sensitive to handling than BJTs.
Two types of FETs are there: the junction field-effect transistor (JFET) and the
metal-oxide-semiconductor field-effect transistor (MOSFET).
The MOSFET category is further broken down into depletion and enhancement
types. The MOSFET transistor has become one of the most important devices
used in the design and construction of integrated circuits for digital computers.
Its thermal stability and other general characteristics make it extremely popular
in computer circuit design.
1.2 CONSTRUCTION AND CHARACTERISTICS OF JFETs
The JFET is a three-terminal device with one terminal capable of controlling the
current between the other two.
The basic construction of the n-channel JFET is shown in Fig. 1.2. 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. 1.2 that resembles the same region of a diode
under no-bias conditions. A depletion region is that region void of free carriers
and therefore unable to support conduction through the region. The drain and
source terminals are at opposite ends of the n-channel as introduced in Fig. 1.2
because the terminology is defined for electron flow.
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