Download J-FET (Junction Field Effect Transistor)

J-FET (Junction Field Effect Transistor)
Introduction
The field-effect transistor (FET) controls
the current between two points but does
so differently than the bipolar
transistor. The FET operates by the
effects of an electric field on the flow of
electrons through a single type of
semiconductor material. This is why the
FET is sometimes called a unipolar
transistor.
J-FET (Junction Field Effect Transistor)
Introduction…cont
Current moves within the FET in a channel, from
the source (S) connection to the drain (D)
connection. A gate (G) terminal generates an
electric field that controls the current .
The channel is made of either N-type or P-type
semiconductor material; an FET is specified as
either an N-channel or P-channel device
Majority carriers flow from source to drain.
In N-channel devices, electrons flow so the drain
potential must be higher than that of the Source
(VDS > O)- In P-channel devices, the flow of
holes requires that VDS < 0
JFET Construction
A schematic representation of an n
channel JFET is shown in Figure 118.
An n-type channel is formed between
two p-type layers which are connected
to the gate. Majority carrier electrons
flow from the source and exit the drain,
forming the drain current. The pn
junction is reverse biased during
normal operation, and this widens the
depletion layers which extend into the n
channel only (since the doping of the p
regions is much larger than that of the
n channel). As the depletion layers
widen, the channel narrows, restricting
current flow.
J-FET (Junction Field Effect Transistor)
Introduction…contd.
The behavior of a JFET can be
described in terms of a set of
Characteristic Curves shown here. In
the region shown with a green
background the drain-source voltage is
small and the channel behaves like a
fairly ordinary conductor. In this region
the current varies roughly in proportion
to the drain-source voltage as if the
JFET obeys Ohm's law. However, as
we increase the drain-source voltage
and move into the region with a light
background we increase the drainchannel voltage so much that we start
to ‘squeeze down’ the channel.
Gate Voltage control S-D currents
J-FET (Junction Field Effect Transistor)
Hence a large increase in drain-source:



‘pulls
harder’, trying to drag the electrons
more quickly from source to drain.
‘squeezes down’ the channel making it harder
for the electrons to get through.
These effects tend to cancel out, leaving the
current the same at all high drain-source
voltage.