INTEGRATED-CIRCUIT MOSFETs

By fur the greatest number of MOSFETs manufactured today arc in integrate circuit s. The enhancement-type MOSFET has a very simple structure (Figure 7-33) that makes its fabrication in a crystal substrate a straightforward and economics procedure. Furthermore, a very great number of devices can be fabricated in a single chip. Enhancement MOSFETs account for the vast majority of very large
scale integrate I (VLASIC) circuits manufactured. and they arc the primary ingredients of digital ICs such as microprocessors and computer memories. The f imbrication of integrated-circuit MOSFETs is accomplished using the same photo lithographic techniques and batch production methods we discussed in Chapter 6. Because tens of thousands of components may be fabricated in a single chip, the techniques we described for producing very line masks and for direct writing of patterns using electron beams are particularly appropriate to technology. Ion implantation. which allows close control of impurity concentration and  layer depth. is widely used to control the values of threshold voltages and other
MOSFET characteristics. Figure 7-46 shows cross-sectional views of PMOS and NMOS FETs embedded in crystal substrates. Note that a layer of polycrystalline silicon is deposited over the gate of an NMOS device to form the gate terminal. This layer improves device
performance but adds to the complexity of the manufacturing procedure. PMOS devices arc less expensive to produce but do not perform as well as NMOS circuits, primarily because the mobility of the majority carriers (holes) in P material is smaller than that of the majority carriers (electrons) in N material. NMOS circuits are generally preferred and can be produced with the greatest number of components per chip for a given performance capability. Another type of digital integrated circuit using enhancement MOSFETs has
both PMOS and NMOS devices embedded in the same substrate. These circuits arc called complementary MOS. or CMOS, circuits. They are more difficult to PMOS transistor polycrystalline silicon NMOS transistor construct than either PMOS or NMOS circuits, but they have the best performance characteristics, especially in terms of switching speed. We will discuss applications of CMOS circuitry in Chapter 8. Figure 7-47 shows a cross-sectional view of a CMOS circuit containing one PMOS and one NMOS transistor. Note that it is necessary to embed a P-iype layer in the N substrate for the NMOS transistor. This .”equentIy called a ‘tub,” is necessary for the formation of the inducer! N channel of the NMOS transistor. Also note the N” and P+ regions used to isolate the transistors. The CMOS structure can be made with either noncrystalline silicon
or aluminum gate electrodes. The more complex structure of a CMOS IC is evident in the figure.