Solid-State Switches

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This page offers a comparison between FETs and PIN diodes used as switch elements. By FETs, we include MESFETs, PHEMTs, and HEMTs. Although MEMS switches are built largely of silicon, they aren't considered solid state devices, they are closer to a mechanical switch than an electrical switch.

Experience shows that PIN diode switches are almost always slightly lower in loss, but there are other things you need to consider. By reading this page we think that you'll be well equipped to decide when to use PIN diodes, and when to use FETs.

New for January 2011! Here's an important consideration for designing with shunt switch elements that you need to know about!

MMIC versus hybrid switch

If you only have room for a MMIC switch, chances are it will use FETs. Not many foundries offer PIN diode MMICs, and when they do, they charge you more. A lot more. Even though the basic stack is nearly the same as a cheap HBT. Only M/A-COM and TriQuint currently offer foundry runs with PIN diodes on GaAs.

If you are designing a hybrid microcircuit with discrete parts, chances are you'll want PIN diodes. For discretes, FETs have no cost advantage.

Switch device models

Switch devices (PIN diodes and FETs) are modeled simply as resistors in the on state, and capacitors in the off state. In the case of non-contacting MEMS, the switch element is modeled as two different capacitors in the two states. At frequencies X-band and above, often a more complicated model is required, which takes into account the distributed properties of switch devices. Ground inductance and bond-wire inductance must be accounted for, as well as transmission-line properties of the device due to its physical area.

Need figure showing device models

The off-capacitance of a PIN diode is a function of reverse voltage, the more negative the voltage the less capacitance. For FETs the capacitance is not a strong function of reverse voltage.

Here's how to calculate the on resistance of a device, R_ON, from a "DC" measurement of S21:

By "DC", we mean as close to DC as you can measure. For example, 250 MHz data will do just fine. How did we come up with this formula? It is similar to the problem we show on this page:

Comparison of PIN diodes and FETs for switching

PIN diodes have been used to create switches from 10's of MHz to over 100 GHz. FETs switches can work down to DC, and recently engineers have been able to make them into useful switching elements at millimeterwave frequencies as well.

PIN diodes have a lower frequency limitation due to carrier lifetime, that means you won't be able to use them to switch DC.

A summary of the differences between PIN diodes and FETs used as switching elements is shown below. Compared are "generic" PIN diodes (both Si and GaAs) and FETs (MESFETS and PHEMTs) from no particular foundry. These qualities are discussed further in subtopics below. One further advantage of silicon PIN diodes is that silicon is less susceptible to ESD damage.

Note: this table needs another column, for 0.1 micron HEMT.... and it doesn't even talk about GaN HEMT...

Two terminals versus three terminals

PIN diodes are two-terminal devices, whereas FETs are three-terminal devices. In a practical sense, for FETs, the gate terminal is decoupled from the source and drain, so you don't have to build a bias tee to separate your bias signals from your RF signals. You don't even have to add blocking caps, you lucky guy! The third terminal makes it easy to create a FET switch that works all the way down to DC.

The bias networks for PIN diode switches often involves quarter-wave capacitively terminated stubs, which limit useable bandwidth to perhaps an octave. If this isn't bad enough, you have to have blocking caps to keep all that DC current where you want it.

Switching Figure of Merit for FETs and PIN diodes

PIN diodes have a significant high-frequency advantage over FETs. Their useful upper frequency response limit can be much higher due to lower off-state capacitance (C_OFF) for a given on-resistance (R_ON).

The microwave industry uses a Figure of Merit to rate the switching characteristics of different switch elements. Multiplying R_ON times C_OFF would give a number with units in seconds. But we like to rate things in terms of frequency, so the FOM is stated thus:

Figure of Merit = 1/(2xC_OFF x R_ON)

We have calculated the FOM for the four technologies in the table above. The higher the FOM, the easier it is to achieve bandwidth. For a PIN diode, off-capacitance can be on the order of 50 femto-farads, with on-resistance of 3 ohms at 5 milliamps current to as low as 1.7 ohms at 20 milliamps. In the later case the figure of merit (FOM) for a PIN diode is equal to 1872 GHz. For a MESFET switch, the on-resistance of 1.5 ohms and off-capacitance of 400 femto-farads gives a FOM of "only" 265 GHz. The gain-bandwidth product of the PIN diode switch enjoys a 7X advantage over a MESFET switch.

It's time to convert the switch element FOM into a Rule of Thumb.

If you divide the switching Figure of Merit by 10, you will arrive at the highest frequency that the device can be made to perform as a switch (thanks, Dr. Matt!) Thus MESFET switches will work up to about 26 GHz, PHEMTs will work up to 40 GHz, and PIN diodes will work up to 180 GHz! Yes, there are exceptions to this rule, and yes, we expect to hear from readers that put this Rule of Thumb to the test!

DC dissipation of switch elements

FETs require only a voltage signal for switching, instead of a DC current. This means that they have essentially zero DC power consumption, compared to the 10 mW or so it takes to turn on a PIN diode. This is a huge advantage for phased array systems, where perhaps up to 1,000,000 switch elements are needed to control phase shifters and attenuators on thousands of T/R modules.

Geometry lesson

Here we will discuss the physical features of switching PIN diodes and FETs used for microwave switching. Sorry if this section is incom