5.5 Sensors

Integrated circuits are often employed in applications where the signals to be processed are not electrical at their sources of origin. They can be, for example, temperature, light intensity, magnetic field, mechanical pressure, etc. Sensors are used for converting such signals to electrical form, so that they can be subsequently processed by electronic circuits. A variety of technologies for making sensors have been developed, but many are not compatible with VLSI technology. However, some sensors can be made on VLSI chips [ ^{[43] [44] [45] [46] [47] [48]}]. For example, the thermal properties of on-chip devices, notably bipolar transistors, have been used to make thermal sensors for many years. Optical sensors can be made by using the optical properties of pn junctions and transistors to sense light intensity. The Hall effect on transistors can be used to make magnetic field sensors. Mechanical sensors can also be made, by using device mechanical properties (such as piezoresistance) to sense strain, pressure, acceleration, etc.

Since VLSI fabrication technology is normally not optimized for making sensors, on-chip sensors are by-products of the process and their performance is not always satisfactory. Fortunately, this can be often compensated for by using sophisticated electronic means, leading to on-chip smart sensors.