Field-effect transistors ( FETs). Prepared by Engr Sarfaraz Khan Turk Lecturer at I BT LUMHS Jamshoro. The Field Effect Transistor (FET). In 1945, Shockley had an idea for making a solid state device out of semiconductors.
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Engr Sarfaraz Khan Turk
Lecturer at I BT LUMHS Jamshoro
The family of FET devices may be divided into :
The "Metal-Oxide-Silicon FET" represents an evolution of the JFET in its technology and construction. Although its principle of operation is similar to the FET, it has a different structure. A thin layer of insulating oxide is placed between the Gate and the Drain-Source channel. For this reason, it is sometimes called an "Insulated Gate FET" (IGFET). There are two kinds of MOSFET. One type operates on the principle of carrier depletion, and the other on the principle of carrier enhancement. Their symbols are shown in figure B 14.8.
The structure of an N channel DEPLETION MOSFET is as shown in figure B14.9. As in the FET, the channel between D and S is continuous. It is supported by a lightly doped semiconductor base (P type), called the "Substrate". In the absence of Gate biasing, the MOSFET conducts with the carriers available in the channel. If the Gate is reverse biased, the channel is depleted of its carriers and conduction decreases. The "Drain current" / "Drain-Source voltage" output characteristic is shown in figure B14.10
The N channel Enhancement type MOSFET is shown in figure B14.11. It does not have a continuous channel between Drain and Source, and so it cannot conduct when there is no Gate biasing. However for VGS > 0 negative carriers are attracted by the Gate into the area between S and D. An N-channel is created and the device can then conduct. This is the only kind of FET which is cut off with VGS = 0, and which controls the Drain current with a positive V GS. This behavior is similar to a bipolar transistor. . Figure BI4.12. shows the "Drain current" / "Drain-Source voltage" output characteristic.
The advantages of the MOSFET compared to the JFET are:
• as the Gate is insulated, these devices present an even higher input impedance than the JFET
• the Gate usually has a lower input capacitance, so the MOSFET shows a better response to high frequencies.
• The disadvantage of MOS technology is that the insulating coating of SiO2 can be damaged permanently by electrostatic discharges. They must be handled with care