I have the following circuit wired up on a protoboard.

enter image description here

The datasheet for the BSS138 MOSFET is here. I'm puzzled regarding what I'm seeing happen with this circuit - when I apply 3.3V at the gate resistor, the MOSFET turns on fully and I see 3mV at the output. This, of course, is expected.

However, if I remove 3.3V from the gate resistor, the pull-down resistor turns the gate off. I expected to see approximately 3.3V at the output, yet I only see 2.7V. If I replace the 3.3V on R1 with 5V, the output shows 4V. In other words, a volt is being dropped on R1 when the MOSFET is off. Is this expected? Somehow, I expected the MOSFET to have an immensely high resistance when off and hence expected approximately 5V being dropped on it when it's off.

Are my expectations not in line for this MOSFET?

Test 1: Passed.

Test 2: Drain to Source Vf = 0.515V, Source to Drain = 0.09V, Gate to Source = 0.07V.

This was quite bizarre. Please note, I have done test done multiple times. I always get a consist result. I have not seen a Open-Loop anywhere. This leads me to believe I indeed destroyed this MOSFET while handling it. A colleague told me that he destroyed another MOSFET from the same reel yesterday. This leads me to Test 4.

Test 4: Incomplete. I'm a bit more wary in handling these MOSFETs now. I didn't realize that the smaller the device, the more likely it is to damage it. I have handled MOSFETs before, but they were much bigger in size: TO-220. I did bring my anti-static wrist strap from home to work, but the room I work in does not have an Earth terminal(see note!!. But I'm working on getting this fixed ASAP. I don't think I'm gonna even bother doing anything till I'm properly grounded. I'm also ordering a anti-static mat. The environment here is fairly dry. But there are obviously no carpets anywhere in the building, let alone the room, this leads me to think it's either my clothing or my work desk.

I have made sure that the circuit is OK. I have gotten it checked by another person as well, this makes me think I'm OK here.

Note: These are just some of the things you have to put up with in a 3rd world country! Fortunately, at least the building has an earth connection. So getting it my room shouldn't be too difficult.


3 Answers 3


Your results are inconsistent with expected operation.
Either you are not doing what you think you are doing or the MOSFET is damaged or your test meter is of very low "ohms per volt".

Test1: Connect test meter with -ve probe to ground and +ve probe via 1 k to 3V3.
What is the voltage reading?

This should read 3V3 to a very close approximation.
If it does not give or throw the meter away and get a slightly better one :-).

Any meter that reads wrongly in that situation is a VERY poor one and useful only for eg battery testing.

Test2: Set meter to diode test range.
Measure Drain - Source.
With Source = +ve you should see a diode with Vf higher than a usual silicon diode.
With +ve on Drain you should see O/C.
With metetr connected either way G-D and G-S you should get open circuit.

Test 3 Ask Olin for advice.

Test 4: Check your circuit carefully.
Recheck MOSFET pinouts.

Try a new FET.

Note that MOSFETS are VERY prone to ESD damage - especially gate to D or S.
Handle with proper electrostatic precautions.

Report back.

  • 2
    \$\begingroup\$ I agree that the "results are inconsistent with expected operation". However, the gate resistors R2 and R3 have unusually high values. Try something in the range of 10...100 Ohms for R2 and something below 10k for R3. Unlike bipolar transisors that require a current limiting resistor towards the conducting base connection, a MOSFET's gate acts just like a small capacitance that needs to be charged or discharged so you are safe with small resistor values. High resistor values will just make your MOSFET slow. \$\endgroup\$
    – zebonaut
    Commented Nov 30, 2011 at 13:25
  • 2
    \$\begingroup\$ @zebonaut: Even 27 kOhms discharging the gate should turn off the FET instantaneously in human time. \$\endgroup\$ Commented Nov 30, 2011 at 14:54
  • 3
    \$\begingroup\$ @Zebonaut - A MOSFET needs only a whsiper of current carrying capability in a grounded gate reistor to keep it turnhed off. As ling as the breadboard is not leaking then even a 10 megohm to ground would work well to turn it off. \$\endgroup\$
    – Russell McMahon
    Commented Nov 30, 2011 at 15:45
  • 3
    \$\begingroup\$ @Russell, I think your Test 3 is a particularly clever idea. \$\endgroup\$ Commented Nov 30, 2011 at 16:09
  • 1
    \$\begingroup\$ @OlinLathrop - Better? \$\endgroup\$
    – Russell McMahon
    Commented Dec 1, 2011 at 7:57

I agree with Russell. I think the most likely answer is that the FET was previously damaged. Excessive leakage is one of the symptoms of abuse. For example, did you ever use that FET to switch a inductive load and forget to put a catch diode accross the inductor? Remeber that indidividual devices don't have protection dioedes like whole chips do. Even a little static discharge onto the gate can damage the FET.

  • \$\begingroup\$ The datasheet shows a diode across Drain and Source. It also tells that it is rated at 0.22A continious forward current. Could you give information on the function of that diode, if it is not for inductive kickback? \$\endgroup\$ Commented Nov 30, 2011 at 14:28
  • 3
    \$\begingroup\$ @abdullah: That diode is a byproduct of the FET construction, although it is really there and can be deliberately used if desired. However, it is not in the right place in the circuit to provide inductive kickback protection. \$\endgroup\$ Commented Nov 30, 2011 at 14:52

It's quite possible the FET is damaged. If you have another then give it a try.
As well as what Russell mentioned, I suggest checking the pulldown resistor is connected properly. If the gate if floating it's possible you might see such behaviour.

  • \$\begingroup\$ Oli - That's my test 4 :-) \$\endgroup\$
    – Russell McMahon
    Commented Nov 30, 2011 at 15:44
  • \$\begingroup\$ Yeh, I noticed that. Thought it was worth mentioning the resistor specifically though, as if the FET is still alive then my bet would be on this being the problem. \$\endgroup\$
    – Oli Glaser
    Commented Nov 30, 2011 at 15:59

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