Discussion in 'General Electronics Chat' started by Webby, Sep 15, 2008.

  1. Webby

    Thread Starter Active Member

    Jun 15, 2008
    Hi, been studying up on transistor theory and now trying to grasp FETs i have looked through the manual eebook but can anyone simplify this for me.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Could you elaborate on what part of FET theory you don't grasp? It's kind of long-winded to go over every bit of theory.
  3. Dave

    Retired Moderator

    Nov 17, 2003
    Put simply: a voltage on the gate (with respect to source) controls a current from source to drain. Other than that we need to be a bit more specific.

  4. Wendy


    Mar 24, 2008
    The problem here is the family of FETs has grown enormously from when they were first developed, and comprise a large family of devices that are only loosely based on the same principles. The original FET was a PN device, but that turned out not to be necessary. MOSFETs are totally different, and have several subtypes in that group.

    The basic principle of a FET, as I understand it, is they use an electric field to pinch off the conductive area between two connected plates (the gate). It's a purely electronic reaction, and because there is no electron flow needed to create the field they are extremely high impedance devices, except for the capacitive effects.

    A MOSFET uses the high input impedance of the basic device and capacitively couples the gate, increasing the impedance even more. Because the layers to create these effects are extremely thin, the ESD sensitivity of these devices is increased.

    I'd like to see the eBook section on MOSFETs finished, but that will depend on someone stepping up to the plate to do so.

    I have a lot to learn about these gadgets too.
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    wikipedia, we wish there was only the MOSFET and JFET :p

    The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body .
    The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.
    The MESFET (Metal–Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
    Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation between gate and body.
    The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.
    The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It has a structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These are commonly used for the 200-3000 V drain-to-source voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V.
    The FREDFET(Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode.
    The DNAFET is a specialized FET that acts as a biosensor, by using a gate made of single-strand DNA molecules to detect matching DNA strands.