SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

434 CHAPTER 9. FIELD EFFECT TRANSISTORS: MOSFET

30 nm30 nm

Figure 9.1: SEM cross-sectional image of a state-of-the-art MOSFET with a physical gate length
of 30nm. Figure courtesy of R. Chau, Intel.

9.2 MOSFET:DEVICESANDIMPACT .......................

MOSFETs can be made so that the current from the source to drain is carried by electrons
(NMOS), by holes (PMOS), or in the case of complementary MOSFET (CMOS), by electrons
and by holes in two devices. In figure 9.2 we show a schematic of an NMOS device. The struc-
ture starts with ap-type substrate. We will see later that a voltage applied to the gate “inverts”
the polarity of the carriers and produces electrons near the oxide-semiconductor interface.In fig-
ure 9.3 we show the well known “Moore’s Law” and its impact on technology. It is well known
that the advances shown in figure 9.3 have been possible because of the Si MOSFET devices.

CMOS Technology

CMOS technology has become the most widely used technology, finding use in wireless,
microprocessors, memories, and a host of other applications. The chief attraction is low power
dissipation. Since both NMOS and PMOS transistors are to be fabricated on the same substrate,
additional steps are needed compared to the NMOS case discussed earlier. The cross-section
of a typical CMOS structure is shown in figure 9.4a. As can be seen, the NMOS transistor is
fabricated within ap-type well that is implanted or diffused into then-substrate. Thep-well acts
as the body or substrate for the NMOS. In addition to creating thep-well, one needs to do ann+
implant for the source and drain of the transistor. In figure 9.4b the symbolic representation of
the CMOS transistor is shown.
It is critical in MOSFETs to follow a voltage convention to make sure that errors are avoided
in calculating critical parameters such as threshold voltage. Consider two materials 1 and 2,
shown in figure 9.5 with work functionsφ 1 andφ 2 which form a junction. We always reference
voltages with respect to the material 2. The electrochemical potential of material 1 with respect
to material 2 isφ 1 −φ 2. Hence the built-in voltage of this structure, which by definition is the