So I was rummaging through my spare and sample parts bin last week, and found an bunch of empty 32 pin quad flat pack packages. These packages might be good way to package a nanodevice and provide high-speed signal inputs and outputs, connections for DC bias, and perhaps room for a semiconductor based temperature sensor to make a good eval board to study device performance. The open package might also allow access to the nanodevice for functionalization or other modifications.
The package datasheet leads me to believe that it can operate to 18 GHz. I decided to build a mockup to show at the Princeton Graphene Workshop
to get some feedback. What I learned surprised me but more about that later. The schematic for the mock-up eval board is shown below for a graphene FET assuming one could be purchased or fabricated. I propose, using a hexagon around a standard MOSFET symbol to indicate a graphene FET.
The top and bottom view of the mock-up is shown in the next two photos. The edge mound SMA connectors used in the mock-up are good to 12.5 GHz using flexible cable and 18 GHz using semi-rigid coax cable connections. The header is really a place holder for a dual row ribbon connector to allow twisted pair DC connection to the FET and source and drain resistors. A heatsink is connected to the bottom of the board to provide a good thermal path to the package and the graphene FET so that the temperature can be controlled using a TEC cooler or similar thermal controller. I used a flat pack prototype board for this quick mock-up but a real board would have multiple layers and use microstrip traces for good RF connections between the edge launch connectors and the package.
So to my surprise, the first person I showed it to was a theorist and did not have any input on the features, design, or practicality of the mock-up. I guess I thought that most of these new nanodevices are hand made so most people in the field would have more familiarity with hardware. Next, I spoke with and experimentalist and found that the design basically blows. To make Quantum Hall Effect (QHE) measurements and similar meV energy measurements, the temperature needs to be in the mK range so kB
T energy levels are less than the energy levels that are to be measured. Thus, the semiconductor temperature will not work because it's only good to -50 °C. The other problem is that electrical connections also conduct heat into the device being measured and this interferes with getting good results. The measurement bandwidth is around 100 GHz so the package is no good either. I'm shocked to find such hard core hardware requirements are needed to measure QHE conductivity and the like.
Well, back to the drawing board. An new package and connector are needed. The table below indicates that a 1.0 mm connector would be a good candidate.
| SMA||7 to 18 GHz
| 3.5 mm||34 Ghz
| 2.92 mm ||40 GHz
| 2.4 mm||50 GHz
| 1.85 mm||65 GHz
| 1.0 mm||110 GHz|