mobility
/ moh-BIL-it-ee /
Push a crowd of people across a field with a steady wind and ask how fast they drift along: a clear field lets them move briskly, a muddy one slows them to a crawl. Mobility measures the same thing for charge carriers in a material — how much drift speed you get for a given electric push.
Precisely, mobility is the drift velocity an electron picks up per unit of electric field: high mobility means even a gentle field sends the carriers scooting along. It depends on how long the carrier coasts between collisions and how heavy it acts, so cleaner, colder, less-obstructed materials tend to have higher mobility. Multiply mobility by how many carriers there are, and you get the conductivity.
It matters most in semiconductors and transistors, where high mobility means faster, more efficient devices. The honest caveat: mobility isn't the same as the carriers' actual speed — those electrons are already zooming randomly at enormous velocities. Mobility describes only the slight, steady drift the field coaxes out on top of that frantic motion.
Electrons race through pure silicon several times more nimbly than through silicon stuffed with impurities. Chipmakers chase ever-higher mobility — and materials like gallium arsenide or graphene, where carriers drift especially freely, can switch faster than ordinary silicon.
Cleaner crystals give carriers higher mobility, so chips made from them can switch faster.
Mobility describes a single carrier's responsiveness; conductivity is mobility times the number of carriers. A material can have high mobility yet conduct poorly if it has very few carriers — which is exactly the situation in a lightly doped semiconductor.