The PIN Diode is the heart of the high-speed microwave switch, consisting of a P type (acceptor) region and an N type (donor) region separated by either a P or N type Intrinsic (I) region.  Diode characteristics are controlled by the levels of doping of both P & N regions and the width and area of the I region.
The three vital and inter-related parameters specifying pin diodes are:

• Breakdown voltage, 
• Junction capacitance, and
• Series resistance.

For high power switching applications, breakdown voltage must be increased while reducing series resistance.
For high speed broadband switches, breakdown voltage must be reduced and junction capacity minimized.

Pin diodes are current-controlled devices that, for forward bias introduce low impedance close to RS (series resistance). For reverse voltage they introduce high impedance - a function of junction CJ junction capacitance. 

Fig. 1 represents an 'all-shunt' arrangement - the simplest switch configuration.  The advantages of using this type over PIN diode switches are

• More effective handling of high power levels depending on available reverse bias, and
• Faster switching speed (assuming equally fast switching drive waveforms).

The switch's operating bandwidth is defined by the selection of blocking capacitors and bias circuitry, together with diode reverse bias capacitance.  A reduction in diode shunt resistance results in the isolation of the 'all-shunt' switch, and can be achieved either by increasing the current or decreasing the overall diode resistance.

It is possible to add a fourth shunt diode for increased isolation.  This results in:

• Increased insertion loss, with power handling and speed virtually unaffected
• Typically fast switching speeds of SPST reflective switches

Fig. 1: Basic All-Shunt Reflective

A/m concepts can be applied to multi-throw reflective switches (see Fig. 3).  This is achieved through the use of a series PIN diode, which isolates the low insertion loss ON port from the high insertion loss OFF port. 
OFF port isolation improves as frequency and diode capacitance are lowered.  However, the series diode increases switch insertion loss and, due to its minimal power dissipation ability, it also slows switching speed and limits operating power levels.  While this design allows the flexibility of an increased number of throws, the quantity is limited by the capacitance of the switch junction, which is, in turn, limited by the diode junction capacity.

Fig. 3: SPDT Reflective Switch

The two basic topologies for SPST absorptive switch designs:

1. The All-Series Configuration – the preferred configuration for broadband design up to 18 GHz.
2. The Series Shunt, or Series Termination, Configuration.  Limited to a maximum 16 GHz, the shunt series configuration is required for fast switching speed and high isolation.  This configuration is frequency limited up to 16 GHz, above which (between 16 and 18 GHz) certain degradation in VSWR at OFF position is noticeable.

Fig. 4: Series Termination Absorptive Switch

The series shunt, or series termination, approach is also used in multi-throw designs. However, an all-series scheme is more suitable in cases where broadband insertion loss [or?]VSWR are more important than switching speed. (see Fig. 5). The 50.Q terminating impedance is developed by the series combination of the diode and the terminating resistance to ground.  Speed is limited by the capacity of multiple series diodes. Power handling is limited by the diodes' and resistors' ability to dissipate RF power.

Fig. 5: Shunt Termination Absorptive Switch

Driver design and selection are vitally important to overall pin switch performance. The driver must be capable of supplying the necessary back bias voltage for the required diode capacity, and it must source or sink the required bias currents to drive the diodes to their rated forward bias resistance. In addition, for fast switching performance, transition time between output levels must be minimized as far as possible.   (See Fig. 6 and Glossary)

Fig. 6: Switching Characteristics