Radio Frequency Integrated Circuit Design - Webs

104 Radio Frequency Integrated Circuit Design
Figure 5.5 Diffusion resistors: (a) diffusion resistor without well isolation (n doping); and (b)
diffusion resistor with well isolation (p doping).
lines is deliberate, the oxide between the two lines is thinned to increase the
capacitance per unit area. This type of capacitor is called a metal-insulator-metal
capacitor (MIM cap). More capacitance per unit area saves chip space. The
capacitance between any two parallel-plate capacitors is given by (5.5), as discussed
previously. Since this expression holds for a wide range of applied voltages,
these types of capacitors are extremely linear. However, if there is too much
buildup of charge between the plates, they can actually break down and conduct.
This is of particular concern during the processing of wafers; thus there are
often rules (called antenna rules) governing how much metal can be connected
directly to the capacitors.
Capacitors can also be made from two layers of poly silicon separated by
a layer of dielectric. However, since poly silicon is closer to the substrate, they
will therefore have more bottom-plate capacitance. A simple model for an
integrated capacitor is shown in Figure 5.6.
5.9 Applications of On-Chip Spiral Inductors and
Transformers
The use of the inductor is illustrated in Figure 5.7, in which three inductors
are shown in a circuit that is connected to a supply of value VCC . A similar