Sunday, October 3, 2010

FET Concepts

1. The drain and source may be doped of opposite type to the channel, in the case of depletion mode FETs, or doped of similar type to the channel as in enhancement mode FETs.

2. The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.

3. The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body.

4. The most commonly used FET is the MOSFET. The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off.


6. A FET is a "voltage-controlled" device.

7. Complementary MOS technology employs MOS transistors of both polarites PMOS and NMOS, thus many powerful circuits are possible using CMOS.

8. Depletion-type MOSFET has a physically implanted channel. Thus, a n-type depletion-type MOSFET has already a n-type channel between drain and source.



Schematic symbol for N channel MOSFET  

The transistor's "turn on" (a.k.a. threshold) voltage varies from one FET to another but is approximately 3.3 volts with respect to the source.

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