Saturday, July 21, 2012
JFET,MOSFET biasing-Explanatin and Animation
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.
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.
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).
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)
Insulated-gate FET (IGFET)
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.
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.
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
i)N-Channel MOSFET (Depletion Mode)
ii)N-Channel MOSFET (Enhancement Mode)
iv)NPN BJT (Bipolar Junction Transistor)
Hardwork Can Never Ever Fails...