Design of Microwave Active Devices

Chapter 5

Control Functions

Chapter written by Jean-Luc GAUTIER.

5.1. Semiconductor components for control functions

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:

– variable attenuator;

– variable phase shift;

– switch.

These devices require the use of semiconductor components with variable resistor or variable capacitor behavior.

5.1.1. Varactor diode

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.

Figure 5.1 Equivalent circuit of a varactor diode

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.

Figure 5.2 Capacitor variation versus voltage

5.1.2. PIN diode ...