Cool Running Semiconductors

Solid-state semiconductors don't handle heat very well. If they're operated at high power, they tend to burn out. Heat poses other problems as well—the hotter the device, the greater the electrical resistance (and the lower the efficiency). Digital semiconductor devices also have capacitive elements—small elements that store electrical charge—and if the devices are to run at their fastest, these capacitive elements must be charged as rapidly as possible. Here again heat poses a dilemma: faster charging requires higher voltages and currents, but the resulting resistive heating raises the device's ambient temperature.

Increasing thermal conductivity can remove this extra heat faster, which in turn allows devices and circuits to be driven with a higher current–speed. Alternatively (without increasing the current), increasing thermal conductivity also allows devices to be more closely packed, which increases the system's overall speed (by reduced interconnect resistances and capacitances) thus improving time constants and circuit speed.

Under an Office of Naval Research basic research effort, Dr. George Brandes' team at Advanced Technology Materials Inc. (ATMI) of Danbury, CT have grown thin layer silicon from isotopically purified silane gas (Si28H4). Using the latest scanning thermal conductivity probe Prof. Fred Pollack of the City University of New York (also funded by ONR) measured the thermal conductivity of these silicon films at room temperature to be ~30% higher than regular silicon. Removing the Si27 and Si29 isotopes allows a purer phonon (improved lattice vibrational frequency) distribution, smoothing the pathway for heat conduction.

"ONR hopes to further this approach in isotopically pure silicon carbide for extremely high power devices," say ONR's Colin Wood, science manager for the research.

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