2N7000; how sensitive to ESD?

In Electronics A2, we were playing around with 2N7000 MOSFETs. We had a very high failure rate with only me and two other people getting good results. We were trying to measure their transconductance by varying the gate voltage and measuring the current, up to 22mA (through a 220 ohm resistor.) However most people had the problem where the MOSFET was always conducting current no matter what the gate voltage was.

I got mine working, but then I disconnected the gate lead briefly with my fingers; when I plugged it back in, it was dead (shorted.) My teacher is convinced that we are exceeding the power dissipation of the MOSFETs with a 22mA current, but I showed him that it was unlikely as it would only heat up by about 5°C. I'm thinking ESD as that is the only thing which explains why my handling of the gate lead blew it up.

Anyone else experienced these problems?

The way you describe it sure sounds like ESD, you touch a floating gate lead and after you reconnect it the part is shorted. However I just don't find FETs overly sensitive to ESD, though I only use power devices, not small signal. Both share miniscule gate regions.

What I have seen kill a FET instantly dead is an excess of gate voltage, which is something above 20V for this device.

I usually don't take any ESD precautions when breadboarding things and don't see a lot of failures anyway, but my bench does have a grounded mat. I only put on a strap when I'm working on production parts, and then only because "its the rule."

In my entire career I have only seen consistent failures attributable to ESD one time: we were "chip on board" wiring a 4000 series CMOS gate, meaning we had a bare chip and were attaching .001 inch gold wires from it to the board. They were failing badly until I had the person wiring do the power pins first, to attach the ESD diodes, then wire the other pins.

The failure rate went to near zero the first time we tried that.

They are very sensitive to ESD, and the ambient atmosphere will affect that dramatically. A counter top that is an ESD generative material for example, along with the kind of clothes you wear, even the shoes make a difference. Sometimes you can get by without protections, but it is never a sure bet. I used to have the same problems with a professional repair shop and BJT transistors.

Have you read this?

Chapter 9: PRACTICAL ANALOG SEMICONDUCTOR CIRCUITS

• ElectroStatic Discharge

When I was mounting some large mosfets, I was so paranoid that I first shorted all leads, then soldered back to back zeners straight to the gate and then I continued, but all that was beacause they cost a lot and I didn´t have any spares.

When working with small signal and cmos, I never had a causulty due to ESD, and I don´t have any protection like mats and straps. The only thing I do is touch the steel frame of my table before touching sensitive parts, and trying not to touch the leads too much.

All the ESD boils down to what clothes you wear and what chair youre sitting on, as some materials make much more static charge and less conductivity than others, generally wooden/metal chair with natural fabric being the best, the more synthetic the worse.

Like Ernie said, be absolutely sure that you never apply more than +/-20Vgs.

Any electronics class should have static mats and wrist straps. It is the industry standard and they are not expensive, and most importantly, they work. Actually this experience was extremely valuable, given many people don't believe in ESD protections. The teacher couldn't have come up with a better demonstration of their necessity.

I have to admit I'm guilty of not using them on a regular basis, but I do have the stuff in the garage, just in case. I have a CPU project (old style) that will likely have the mat and strap broke out.

ESD precautions really have to be followed if you want reliable results. How much trouble you actually get if you don't follow the rules will depend on your environment, including the local climate, and whether or not you use air conditioning.

Really low humidity is not good news, paradoxically in our normally humid environment here in UK the danger can be greatest in a warm building during a freezing winter cold spell. Add a nice synthetic carpet and the wrong footwear, and you can find yourself drawing fat (and painful) sparks whenever you touch any grounded metal. You then have a recipe for destroying office equipment, let alone naked semiconductors.

Winter is usually not good for any climate, just for the reasons you stated. Summer is usually the safest, but simple ESD precautions will always work.

When my family moved to Luke AFB, near Phoenix Arizona. They can hit 120°F (49°C) easily, and the climate is bone dry. Add air conditioning to that and 1" sparks are routine.