The success of CMOS technologies stems from their ability to fabricate on the same substrate both NMOS and PMOS transistors. This is indispensable to design digital gates dissipating very small static power. As we will discuss in the next chapter, having both devices simultaneously available is also the key to building high performance amplifiers. Fig. 2.14 shows the simplified cross section of a standard bulk CMOS process and allows us to grasp some important features of these technologies. The transistors are built over a common substrate, that in the example is p-type. In CMOS fabrication, p-type substrates tend to be preferred because p-doped wafers are easier and cheaper to produce. Furthermore, since the hole mobility ...
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