Heat in semiconductors is primarily transferred through phonons. Just as sound propagation requires air vibrations, heat transfer requires phonons to carry energy and move. The thermal conductivity of materials depends on the phonon velocity, heat capacity, and mean free path. Doping with impurities (such as germanium in silicon) increases phonon collisions, similar to placing obstacles on a road, which leads to a decrease in thermal conductivity. For example, when 0.5% of silicon atoms are replaced by germanium, the thermal conductivity drops sharply from 220 to 50 W/m-K. However, as chip components shrink to the nanoscale, traditional thermal conductivity theories become less accurate. Recent studies have discovered that doping can anomalously enhance thermal conductivity in the hotspot regions of nanoscale transistors! This is because nanoscale heat sources excite "oblique phonons," which contribute little to vertical heat transfer. Doping atoms, through scattering effects like billiard balls altering direction, can redirect phonons to the effective heat transfer direction, thus improving vertical heat flow. This breakthrough overturns the conventional understanding that "doping always reduces thermal conductivity," providing a new approach for chip cooling.

Speaker

A/Prof. Zhongyong Wang

Global Institute of Future Technology, SJTU

Time

 2025.3.5 12:00-13:30