JFET
What if JFET?
In the junction FET (JFET),
the gate material is made of the opposite polarity semiconductor to the
channel material (for a P-channel FET the gate is made of N-type semiconductor
material). The gate-channel junction is similar to a diode's PN junction.
As with the diode, current is high if the junction is forward biased and
is extremely small when the junction is reverse biased. The latter
case is the way that JFETs are used, since any current in the gate is undesirable.
The magnitude of the reverse bias at the junction is proportional to the
size of the electric field that 11 pinches" the channel. Thus, the
current in the channel is reduced for higher reverse gate bias voltage.
working:
Because the gate-channel junction in a JFET is similar to a bipolar
junction diode, this junction must never be forward biased, otherwise large
currents will pass through the gate and into the channel.
N-channel and P-channel JFET:
For an
N-channel JFET, the gate must always be at a lower potential than the source
(Vcs < 0). The channel is as fully open as it can get when the
gate and source voltages are equal (VGS = 0). The prohibited condition
is when VGS > 0.
For P-channel JFETs these conditions are reversed (in
normal operation VGS 0 and the prohibited condition is when VGS < 0).
MOSFET
What is MOSFET?
Placing an insulating layer between the gate and the channel allows for
a wider range of control (gate) voltages and further decreases the gate
current (and thus increases the device input resistance). The insulator
is typically made of an oxide (such as silicon dioxide, SiO2),
This type of device is called a metal-oxide-semiconductor FET (MOSFET)
Other Name of MOSFET:
Insulated-gate FET (IGFET)
Operation:
The substrate is often connected to
the source internally. The insulated gate is on the opposite side
of the channel from the substrate . The bias voltage on the gate
terminal either attracts or repels the majority carriers of the substrate
across the PN junction with the channel.This narrows (depletes)
or widens (enhances) the channel, respectively, as VGS
changes polarity.
N-channel and P-channel:
For N-channel MOSFETs, positive gate voltages with
respect to the substrate and the source (VGS > 0) repel
holes from the channel into the substrate, thereby widening the channel
and decreasing channel resistance. Conversely, VGS
< 0 causes holes to be attracted from the substrate, narrowing the channel
and increasing the channel resistance. Once again, the polarities
discussed in this example are reversed for P-channel devices. The common
abbreviation for an N-channel MOSFET is NMOS, and for a P-channel
MOSFET, PMOS.
Modes of Transistor:
A depletion
mode, device (also called a normally on MOSFET) has a channel
in resting state that gets smaller as a reverse bias s applied, this device
conducts current with no bias applied.
An enhancement
mode device (also called a normally off MOSFET) is built without
a channel and does not conduct current when VGS = 0;
increasing forward bias forms a channel that conducts current
Diagram:
i)N-Channel MOSFET (Depletion Mode)
ii)N-Channel MOSFET
(Enhancement Mode)
iii)N-Channel JFET
iv)NPN BJT
(Bipolar Junction Transistor)
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