Sunday, August 03, 2008

Why the field effect is important.

I'm probably overdue for making a post introducing some physics topic for a nonspecialist audience, so here you go....

What is the field effect? Well, it's the physical process that is used in field effect transistors (FETs), the hundreds of millions of little three-terminal switches that are the basis of the computer you're using right now. In a FET the electrical conduction between two electrodes (a source and a drain) is modulated by applying a voltage to a third electrode (the gate) that is
capacitively coupled to (though separated by a thin insulating dielectric layer from) the semiconductor material between the source and drain (the channel). In an ideal (flat band, for the experts) FET, if the gate voltage is adjusted to be positive relative to the source and drain electrodes, a layer of electrons will be capacitively attracted from the source and drain into the channel region. Once there are carriers accumulating in the channel, conduction can take place from the source to the drain. Alternately, one could imagine cranking up the gate voltage to be negative relative to the source and drain, in which case one would expect to accumulate a layer of holes rather than electrons. A device that can really be modulated to have either electrons or holes as the dominant carriers is said to be ambipolar. Of course, real devices are more complicated than this. For example, there can be surface states that live at the semiconductor/dielectric interface close to the gate; the source and drain electrodes may have preferential alignment to either the conduction or valence bands depending on the materials involved; etc. Still, the FET is the basis for modern electronics, and techniques exist for fabricating these little gadgets by the billions in Si with incredible reliability.

That's why the FET is important for industry. Why is the FET important for physics? The FET idea can be rephrased this way: the FET is a way of changing the chemical potential of charge carriers (or, in a related sense, the density of charge carriers) without altering the composition of the material. Think about that for a sec. Chemists understand doping very well. If you're doing chemistry and want to remove some electrons, you add a halogen atom somewhere since those are hugely electronegative and grab an electron. Alternately, you want an extra electron in there? Throw in an alkali metal atom - they love to give up electrons. This kind of doping is commonly done in, e.g., the high temperature superconductors, where the parent copper oxide compound is actually an (antiferromagnetic) insulator that only becomes conducting (and superconducting) if doped chemically. The problem with chemical doping is that adding new atoms to a solid changes its structure and usually increases disorder, since we generally can't control precisely how the extra atoms are distributed. In FET structures, conversely, one can try to tune the carrier density just by turning up a voltage, no chemistry required. In graphene, for example, people have gated readily from having lots of electrons to having lots of holes and back, all in one device. Of course, FET structures do have some serious limitations: they only really put charge onto a surface or interface (rather than the bulk), and it is extremely challenging to get field effect charge densities to be comparable to those achieved easily with chemical doping. Still, the field effect is a tremendous tool for tuning electronic properties without mucking up the disorder or structure. If you're interested in this in more detail, please check out this Rev Mod Phys paper.

1 comment:

Will said...

Already understood the chemistry aspect of it pretty well, but not the industry as much.