MOSFET Leakage current issue at Vgs = 0 volts

Discussion in 'General Electronics Chat' started by GopherT, Mar 15, 2015.

  1. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    I just made a simple motor switch for a friend. Essentially a tactile switch that pulls the gate voltage up to turn on the motor (high side) with MOSFET Source pin to ground. The gate is tied to ground with a 33k resistor.

    Supply voltage is 6V.

    My problem, I measured the leakage current without pressing the switch and I get about 300 microamps (0.3 mA). It climbs from 120 uA when first energized and starts to hold steady at about 360 uA

    The Vgs is 0.000 volts.

    There is no leakage across the switch. Switch was also disconnected from the circuit and same happened.

    MOSFET is STP55NF06L
    It should have a leakage current of less than 1 uA.

    Any ideas what may cause this? Static, no-reverse bias diode while testing, ? Unfortunatly I do not have another logic level MOSFET to check other theories for now.

  2. madsi


    Feb 13, 2015
    Two possibilities:
    (1) The MOSFET is kaput.
    (2) There is some conductive contamination on the MOSFET/PCB/Breadboard or whatever you've built the circuit on.
    Hypatia's Protege likes this.
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    Test another from the same lot to rule out manufacturing defect...

    For a 'proper' postmortem check for leakage G-S and G-D --- As a practical matter only the gate exhibits ESD liability -- thus D-S 'leakage' may, in fact, owe to damaged gate insulation --
    Otherwise it's probably down to a leaky (internal) 'snubber' diode -- Tho' I don't suppose that's of much help... ;)

    Best regards
    Last edited: Mar 15, 2015
  4. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    Thanks, HP. A small motor was connected and tested before the external protection diode across the D-S was in place. It is possible that that killed the internal diode in that attempt.

  5. crutschow


    Mar 14, 2008
    The protection diode doesn't go across the D-S, it goes across the motor windings (unless the diode is a zener).
  6. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    My mistake, yes, motor winding.