Identifying GaAs FET Structures
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 :)