NMOS Sanity Check

Discussion in 'Electronics & Electrics' started by imgod22222, May 29, 2013.

  1. imgod22222

    imgod22222 Member

    Joined:
    Jan 4, 2006
    Messages:
    729
    Location:
    Florida, America
    In school, they never taught us failure modes of transistors, so teach me if I'm wrong.

    Assuming we have NMOS transistors, with the bulk attached to the source:
    1. If I have current flow from Drain to Source, will it blow up?
    2. If I have two NMOS, with the gates tied together, with one's source tied to the other's drain, and vice versa, will the pair successfully conduct and go high impedance when the gates are biased?

    Setup for the question being asked by 2:
    VDD
    |
    Resistor
    |---|--Load
    N N
    |-|-|
    GND
    Where GND always = 0VDC and VDD could be +10VDC or -10VDC.
    (my load requires either +10, 0, or -10VDC)
     
  2. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,548
    Location:
    Kalgoorlie, WA
    I've never usually considered the Bulk, but according to Wikipedia it's pretty common for the bulk to be connected to the source, such as with a depletion MOSFET.

    For Q1:
    - Classical current flowing from Drain to Source is completely normal and the intended purpose of a MOSFET transistor. It wont blow up - as long as you stay within the rated current & power limits.
    - If instead you mean electron current, this is also possible. As long as the gate is biased appropriately current can flow in either direction. Also depletion MOSFETS's have built in parasitic diodes that will inherently conduct Source to Drain. Once again, current and power limits apply, double check the parasitic diode rating too however its not uncommon for it to be rated for full current.

    Edit - for Q2 (back to back MOSFETS):
    - As drawn one of the transistors (lets call it #1) has it's source connected to GND, and lets assume the common gate drive is referenced from GND. That means Vg = Vgs1 for that transistor, raise it high enough and it will conduct.
    - Once it transistor #1 conducts, the drain-source voltage goes very low, so Vds1 approx = 0, therefore Vd1 approx = 0.
    - If drain #1 is connected to source #2, the Vs2 = approx 0, therefore Vgs2 approx = Vg = Vgs1. It will also conduct.

    So both transistors will conduct, one slightly better than the other.
     
    Last edited: May 29, 2013

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