Solid State Control Components
The introduction of the PIN diode more than 45 years ago has led to the development of a large family of RF and microwave control components, including switches, attenuators, modulators, and phase shifters that have become essential elements of most modern microwave systems. Today, the types of PIN diodes available to the component designer is quite extensive and permits a choice of electrical characteristics such as junction capacitance, minority carrier lifetime, reverse voltage breakdown, saturation resistance and resistance vs. current law as well as mechanical format when selecting a diode for a particular application. While a complete treatment of the PIN diode will not be presented here, some of the more important relationships in diode characteristics are described below.1
The unique property of the PIN diode that makes it particularly suitable for control component use is that, in its useful operating frequency range, it behaves as a current variable resistor in its forward biased state. Depending upon the diode construction, this resistance can vary from as low as a few tenths of an ohm when the diode is fully ON to as high as 10,000 ohms with zero bias current applied. The PIN diode displays this behavior because, unlike P-N junction diodes, a thin layer of Intrinsic material is inserted between heavily doped layers of P and N material. When DC current flows through the diode, a stored charge is created in the I layer which establishes the conductance of the diode. The charge is in the form of holes and electrons which have a finite recombination time. As long as the period of any time-varying current is sufficiently short compared to this recombination time, there is effectively no modulation of the diode conductance and, ignoring parasitic reactances, the diode behaves as a pure resistor.
If we define a transition frequency f0 as
then for frequencies significantly below f0 the PIN diode will behave as a P-N junction, rectifying the applied a-c signal. For frequencies well above f0 the diode will behave as a linear resistor. The range of t varies from as low as 10 nsec to as high as 5 µsec, and correspondingly f0 varies from about 16 MHz to 32 kHz.
The degree to which the PIN diode will rectify the a-c signal and thereby generate harmonic power depends not only on the minority carrier lifetime but upon the ratio of the a-c current to the applied d-c current. In general, as the applied signal power rises and the operating frequency decreases, diodes with long minority carrier lifetimes and high bias current are required for satisfactory operation. Unfortunately, such diodes exhibit relatively long switching time and low modulation rates.
When one uses a PIN diode in the microwave frequency range, parasitic reactances will have first order effects. The most important of these is the diode junction capacitance which limits the diode impedance in its back biased state. For low frequency diodes in chip format, employing relatively large junction areas, the junction capacitance is of the order of 0.2 to 1.0 pF. At the other extreme, beam lead diodes exhibit the lowest available junction capacity, ranging from 0.02 to 0.08 pF. For high frequency multi-throw switches, beam lead diodes are frequently employed at the common junction because of their small physical size and low junction capacity. Even with a capacitance as low as 0.02 pF, at a frequency of 18 GHz, the diode will have an impedance of only about 450 ohms in its back biased state due to this reactance. In similar manner, the intrinsic diode inductance as well as that of the connecting ribbons have a significant effect upon the frequency related behavior of the PIN diode.
The diode saturation resistance presents a loss mechanism in the RF and microwave circuit. This resistance can vary from a few tenths of an ohm in a chip diode, to as high as 5 ohms in a low-capacity beam lead diode. In general, there is an inverse relationship between diode junction capacity and saturation resistance. Therefore, in high frequency applications, where low capacity is generally required for best isolation and/or impedance match, higher insertion loss generally arises due to the loss attributed to the diodes.
In the sections that follow more detailed discussions are presented of the circuit topologies, design tradeoffs and performance characteristics of GMC’s families of control components. GMC’s large number of custom designs, which have evolved from these products, have not been included because of space limitations. Consultation with the factory is recommended for such requirements.
