Thermal characteristics of high speed devices

An investigation of the ultimate limit in transistor speed has revealed that thermal effects may place a limit on device speed. A mathematical model is presented and used to study the thermal characteristics of very small transistors. The model gives the three-dimensional time dependent temperature response at any point in the region of an active device dissipating power. The physical structure and actual region of power dissipation are taken into account in the model. In addition to predicting high values of thermal spreading resistance for small devices, the model has predicted several other interesting results. The heating effect caused by the power dissipation of small devices was found to be very localized and adjacent devices in an integrated circuit are essentially thermally isolated. For example, no longer can one consider the temperature to be the same in adjacent transistors of a differential pair. Furthermore, it was found that the thermal response of both silicon and germanium devices is much slower than the electrical response. All of these results have been substantiated with laboratory measurements on small high speed devices.