AN101
Siliconix
10-Mar-97
1
An Introduction to FETs
Introduction
The basic principle of the field-effect transistor (FET) has
been known since J. E. Lilienfeld’s patent of 1925. The
theoretical description of a FET made by Shockley in
1952 paved the way for development of a classic electron-
ic device which provides the designer the means to ac-
complish nearly every circuit function. At one time, the
field-effect transistor was known as a “unipolar” transis-
tor. The term refers to the fact that current is transported
by carriers of one polarity (majority), whereas in the con-
ventional bipolar transistor carriers of both polarities
(majority and minority) are involved.
This Application Note provides an insight into the nature of
the FET, and touches briefly on its basic characteristics, ter-
minology, parameters, and typical applications.
The following list of FET applications indicates the ver-
satility of the FET family:
Amplifiers Switches
Small Signal Chopper-Type
Low Distortion Analog Gate
High Gain Communicator
Low Noise Protection Diodes
Selectivity Low-leakage
DC
High-Frequency
Current Limiters
Voltage-Controlled Resistors
Mixers
Oscillators
The family tree of FET devices (Figure 1) may be divided
into two main branches, Junction FETs (JFETs) and Insu-
lated Gate FETs (or MOSFETs, metal-oxide- semicon-
ductor field-effect transistors). Junction FETs are in-
herently depletion-mode devices, and are available in
both n- and p-channel configurations. MOSFETs are
available in both enhancement and depletion modes, and
also exist as both n- and p-channel devices. The two main
FET groups depend on different phenomena for their op-
eration, and will be discussed separately.
Junction FETs
In its most elementary form, this transistor consists of a
piece of high-resistivity semiconductor material (usually
silicon) which constitutes a channel for the majority carri-
er flow. The magnitude of this current is controlled by a
voltage applied to a gate, which is a reverse-biased pn
junction formed along the channel. Implicit in this de-
scription is the fundamental difference between JFET and
bipolar devices: when the JFET junction is reverse-biased
the gate current is practically zero, whereas the base cur-
rent of the bipolar transistor is always some value greater
than zero. The JFET is a high-input resistance device,
while the input resistance of the bipolar transistor is com-
paratively low. If the channel is doped with a donor impu-
rity, n-type material is formed and the channel current
will consist of electrons. If the channel is doped with an
acceptor impurity, p-type material will be formed and the
channel current will consist of holes. N-channel devices
have greater conductivity than p-channel types, since
electrons have higher mobility than do holes; thus n-chan-
nel JFETs are approximately twice as efficient conductors
compared to their p-channel counterparts.
FETs
Junction MOS
Depletion
Enhancement
Not Possible
Depletion Enhancement
n p n p n p
Figure 1. FET Family Tree
Updates to this app note may be obtained via facsimile by calling Siliconix FaxBack, 1-408-970-5600. Please request FaxBack document #70594.