fetsemiconductors
I found a photo in my grandfather's effects that I believe to be of an RF GaAs FET. He was working in a GaAs FET lab at the time this photo was dated (1975). It was labeled on the back "300μm gate width on mesa".
I believe it was related to US Patent US4160984A filed in 1977; he is listed as one of the inventors.
The structure is foreign to me; where are the gate, source, and drain? Any additional information on the structure would be appreciated.
Let's start with the electret microphone basics.
The actual microphone is formed by a (very very) thin sheet of mylar film that is charged. This is connected to the metal body of the electret capsule but insulated from the pick-up plate. The mylar film and the pick-up plate form a capacitor. The charge on the mylar film produces a voltage difference between it and the pick-up plate. (V = Q/C)
As the mylar film is moved by the sound waves it changes the distance between it and the pick-up plate and changes the capacitance value as the distance between the 'plates' increases and decreases.
This is effectively what your electret microphones consist of.
Most electrets contain a FET which converts the very high impedance of the 'capacitor' voltage into a more useful form. It consists of a FET with the gate connected to the pick-up plate, the drain connection as a '+ pin' and the source connected to the ground (metal body).
You could add a FET but given that electrets are so inexpensive, by the time and money you would spend adding a FET to your electrets it would be a lot easier and cheaper and more reliable just to buy a new electret microphone with the FET built into it.
At the moment of discharge, there is current flowing from the charged object, through the transistor, and finally to ground or other lower potential.
Ohm's law is in effect here, and the higher the resistance, the higher will be the voltage observed across the two terminals of the "device under destruction".
FET's have a very high resistance on their gate lead. For MOS FET's, that resistance is extremely high. A BJT has a much lower resistance, although, when you are dealing with static discharge, there's no assurance you won't blow that up as well.
As a side effect, BJT circuits usually have relatively small resistors across their terminals to bias the device, and that will limit the IR drop even further.
Best Answer
Here's an annotated version of what is what, from my experience laying out GaAs and GaN HEMTs:
You can see the gate finger peeling off of either side of the two gate contacts if you look closely. I can't tell which side is drain, which side is source, but if it's a symmetric device, they aren't any different physically (e.g. a FET designed for switch applications). My guess is that the dashed-line box is the mesa region.
Very cool photo and background :)