I have the following circuit wired up on a protoboard.
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.