All on-chip temperature sensors operate on the common principle of generating an output voltage, current, or pulse that is correlated with the thermal state in the physical vicinity of the sensor. These devices are generally comprised of two parts, which we shall refer to as the back-end and the front-end. The back-end constitutes the actual sensing device in charge of producing a measurable voltage/current signal, while the front-end performs the conversion of this output signal into a digital representation, which system components utilize in downstream thermal management modules. Temperature sensors, which encapsulate both the backand front-end components together on the actual spot of measurement, are also referred to as local sensors (or their measurements are called local sensing). In another realization, the back-end component might be placed at or near the spot of thermal event for monitoring, while the front-end component is located elsewhere on the chip or even off-chip. One front-end component might also be shared among multiple sensing devices in these realizations. A sensor implementation, where the two components are physically separated in this manner is referred to as a remote sensor. Finally, temperature sensors are classified as analog versus digital. We denote sensors that utilize doped p-and n-regions, a bipolar junction transistor (BJT) planted into the IC, resistive lines, or thermoelectric materials in their back-end structures, as analog sensors. Digital sensors utilize MOSFETs as the fundamental building block in various configurations (MOSFET as a diode junction, inverter delay lines, ring oscillators, leakage sensors, etc.) to construct the back-end.

In this chapter, we will review major active cooling mechanisms and the types of support they receive from sensors. The discussion will start with the most common and perhaps least sophisticated of all active cooling techniques, which is air cooling. Liquid cooling has found commercial use in select processor chips used in HPC systems, for example, supercomputers. Finally, the emerging field of thermoelectric cooling will be discussed.

The impact of thermal effects have first been noted due to their direct relationship with the power consumption, performance, reliability, and cooling needs of the integrated circuits deployed in high-performance systems. With a growing awareness of this phenomenon, various other aspects are now emerging as important design considerations. A striking example is how users physically perceive heat dissipated from electronic devices, particularly, users of mobile and handheld devices. Another dimension to the issue has been emerging as the amount of heat generated by some of these systems becomes so substantial that we now speak of harvesting this energy to convert back to electric energy. Thermal harvesting is a vast domain with a plethora of applications in systems at various macro and micro scales. It is beyond the scope of this book to provide a thorough discussion of this topic. In this chapter a brief introduction to recent development specifically focusing on on-chip thermal harvesting will be presented. Another avenue of innovation is the pursuit of novel materials and design methods for building onchip temperature monitoring devices. Finally, the presence of heat and the mechanisms that generate heat within integrated circuits can be viewed from an entirely different perspective to infer valuable knowledge on the structure and operation of an integrated circuit. Securing integrated circuits and understanding potential threats that may exploit thermal properties is one such interesting domain worth mentioning in the conclusion of this book.