Control functions include, on the one hand, devices that modify the amplitude or phase of a signal and, on the other hand, those that route the signal through different transmission paths in a system. These devices are controlled by signals that can be voltages or currents. We therefore distinguish between three types of device:
These devices require the use of semiconductor components with variable resistor or variable capacitor behavior.
A metal semiconductor inverse-polarized junction exhibits a capacitor for which the value can be changed by adjusting the polarization voltage.
The equivalent circuit of the diode is shown in Figure 5.1.
Diode capacitance can be written as:
The doping profile influences the possible value of the ratio between the maximum and minimum capacitor. For a steep profile, the value is between 5 and 10, while a hyper-steep profile may reach a value above 10.
Figure 5.2 shows an example of variation of the capacitor as a function of voltage supply.