Sometimes you have to Sweat the “Small” Stuff Phonon Transport Modeling by: Michael P. Medlar, RIT PhD Student
We often hear that phrase, “Don’t sweat the small stuff”, in our everyday lives. However, sometimes the small stuff is the most important. I’m referring to phenomena that occur on very small length and very short time scales. Happenings on these scales can have a large effect on the performance of devices we use every day. One of the best examples of this is the computer. Virtually all of the devices we incorporate into work and play are computers of a sort. From phones to laptops and TV’s to gaming. At the heart of these devices is the Central Processing Unit (CPU) which acts like the brain of the computer. In operation, the CPU can generate a lot of heat. So much so that the surface can be hot enough to boil water. The heat is mostly generated by the basic Image by <a href="https://pixabay.com/users/bru-no-1161770/?utm_ component of the CPU, which is the transistor. In source=link-attribution&amp;utm_medium=referral&amp;utm_ fact, billions of them. A transistor is a very small campaign=image&amp;utm_content=4393383">Bruno /Germany</a> from <a href="https://pixabay.com/?utm_source=link-attribution&amp;utm_ electrical switch. The footprint of the transistor is medium=referral&amp;utm_campaign=image&amp;utm_ on the order of 100 nm in width. That is 0.0000001 content=4393383">Pixabay</a> meters. A human hair is about 100,000 nm in diameter. Thus, you could fit about 1000 transistors side to side over this same width (now that’s splitting hairs). Modern transistors are known as FinFET’s (Fin Field Effect Transistor) because they make use of long thin fins within the device. The silicon fins are built upon a lower region called the BOX (buried oxide) and surrounded by the gate. When the transistor is switched on (the switching effect is produced by the gate region), electrical current flows through the fins from the inlet to the exit. When it is off, no current is flowing. The “on” state is represented with a “1” and the “off” state is a “0”. That is why you will often hear people say that computers just deal with a bunch of 1’s and 0’s. And to an extent, they are correct. The switching operation occurs very quickly. On the order of 100’s of picoseconds. A picosecond is 0.000000000001 seconds. During the switching operation, these transistors can get hot because the current (moving electrons or holes) transfers some energy to the material (silicon mostly). The transistors can get so hot, that device designers don’t let them switch on and off quite as fast as they could go and devices don’t last as long as designers would like. To aid in the thermal design of transistors, engineers need to know the local thermodynamic conditions. Methods to predict heat transfer in 28 | The ROCHESTER ENGINEER MARCH 2021
campus feature article
