Memory Switches

Mixing micromachines with microelectronics is getting a fair amount of attention these days. There is IBM's millipede project, for example (see link). And there is at least one company using carbon nanotubes to make memory cells.

Well, here is yet another notion. Start with any "electrical circuit schematic" that has a mechanical switch in it. It is symbolized as a little line attached to a hinge-point, with a contact-point near the other end (sometimes two contact points). You visualize the short line swinging on that hinge to make or break contact.

OK, they can and do carve such things out of silicon these days. Texas Instruments, for example, carves millions of little hinged mirrors for its "DLP" (digital light processor) chips. Carving a simple on-off switch should be almost trivial. Carving billions of them for modern Random Access Memory may be a little more difficult, but not impossible.

So, in use each memory cell consists of an input wire, a carved switch-wire, an output wire, and a capacitor. When the switch is carved, it is "doped" such that it can be electrostatically attracted or repelled (by a fleeting charge on the capacitor), thus moving it to make or break the circuit between input and output wires (and the state of the connection is interpreted as the standard binary Zero or One).

In a way, this notion is similar and opposite to an ordinary DRAM memory cell, which consists of one transistor (the switch) and one capacitor. But in a DRAM cell the capacitor holds the data as a quantity of electric charge, and because it quickly leaks away, it must constantly be "refreshed" -- the "D" in DRAM stands for "Dynamic".

Here, the position of the switch indicates the data, and we don't care if the capacitor loses its charge, because after being moved into position, the switch-wire stays there until the capacitor is momentarily charged up to move it again. That makes this a "static" kind of memory cell, which does not need to be periodically refreshed. Also, this memory cell can remember its data when the power is off, which is only to be expected if we don't care about the capacitor losing its charge (although being physically wacked might be expected to cause a lot of switches to move to new positions).

Experiments with nanotubes indicate that micro-mechanical switches ARE capable of keeping up with the extreme speeds of modern electronics. Meanwhile, a typical static RAM cell (the fastest type of standard memory available) is usually made from a group of six transistors, is therefore expensive compared to DRAM, and it must constantly be powered lest it forget.