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Base diodes in FETs

  1. Aug 26, 2005 #1

    es

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    I was curious if anyone knew why some FETs have base diodes. The only purpose I can imagine is wanting current to flow in a direction when the FET is off because when the diode is on current flows through the channel bypassing the diode and when the FET is off there is no current flow at all so preventing current from flowing in any direction is intrinsic.
     
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  3. Aug 27, 2005 #2

    berkeman

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    Staff: Mentor

    The main reason that I'm aware of is to protect the gate oxide against high gate-to-channel voltages, especially ESD transients. So an enhancement mode n-channel MOSFET (like the Fairchild FDG6301N dual) may have a zener diode structure between its gate and source, with the cathode of the zener connected to the gate. When you raise the G-S voltage to turn on the FET, the zener is out of the picture. But if you raise it too far, or take a positive ESD hit on the gate, the zener breaks down to protect the gate oxide. If the gate us pulled below the source for some reason, the zener turns on like a regular diode.

    Hope that helps, -Mike-
     
  4. Aug 27, 2005 #3

    es

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    Thanks for the reply Berkeman. I wasn't aware of transistor packages with a zener across the gate and source and I understand the purpose of that diode now.

    Using the Fairchild FDG6301N dual as an example.
    http://www.fairchildsemiconductor.com/ds/FD/FDG6301N.pdf

    One can see a second diode across the source and drain of the FET. Why is that diode there? Is it really in parallel with the source and drain of the FET? Or is it that the FET is not really built symmetrical horizontally like one usually assumes?
     
  5. Aug 27, 2005 #4

    berkeman

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    Oh, the D-S diode. Good question. AFAIK, it is part of the construction of the FET, and is known as the "body drain diode". You get it when you tie the Source to the Body of the die. That's the asymmetrical part of these FETs -- it's done inside the package, and that's why the symbol for these FETs shows the body tie explicitly. So for the enhancement mode N-channel FET we are talking about, the wells under the D and S are N+, and the Body is P-. When you tie the S to the Body inside the FET chip, that forms the P-N junction diode from the Source (and Body) to the Drain. With some MOSFET devices, the body is not connected to the Source internal to the device, and is brought out on a separate pin. See the ALD1101 duals, for example. If you don't explicitly tie the Source to the Body when using those devices, then you won't get just the one body diode -- you get one each from the Body to the Drain and Source. For N-channel FETs, the Body usually should be tied to the lowest voltage in the system that the FET can see, so for typical use of an N-channel FET, that means that the Body gets tied to the Source, and many power FETs are connected that way internally. For P-channel MOSFETs, the Body is connected to the highest voltage, which is still usually the Source.

    Here are a couple links for more info:

    IRF App note on power MOSFETs talks about body-drain diode:
    http://www.irf.com/technical-info/appnotes/an-936.pdf

    Fairchild Semi app note about FETs
    http://www.fairchildsemi.com/an/AN/AN-9010.pdf#page=16
     
    Last edited: Aug 27, 2005
  6. Aug 27, 2005 #5
    The source to drain diode(also found in some Power BJT's) is sometimes called a flyback diode. Mosfets are really sensative to overvoltage conditions so, to prevent reactive load spikes from destroying your MOSFET(or darlington pair) a diode is commonly added across the source to drain(or C to E). Essentially, when you turn off the power to an inductor you can get a HUGE voltage spike---fuel injectors operate with 10->14V but will spike up to 30 to 60V limited by the value of flyback Zener used. All solenoids and motors exhibit these huge voltage spikes when switched so these little diodes are there to save you money. Image having to use 500V mosfets for a small motorized toy powered from a 9V battery. The diode allows you to use a 24V(safety margine) mosfet to switch that 9V inductive load.
     
  7. Aug 27, 2005 #6
    Your internatiional Rectifier reference describes the clamping zener in section 5. Not all MOSFETS show the clamping diode as part of the schematic symbol because it was not a standard feature. Most MOSFETS made today include a clamping diode to protect the MOSFET substrait now though. In the good old days of the early 90's when I first started playing wit electronics, MOSFETS where actually quite exponsive; however, today they are much cheaper and much much much more efficient(lower Rds), so adding a clamping doide to the die is a cost effective solution now.

    My 2 cents enjoy.
     
  8. Aug 28, 2005 #7

    es

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    Thanks guys! That information is really helpfull and I think I learned something new on every page of that fairchild app note. Thanks again...
     
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