Non-matched GaAs FET are usually the most common because they are wide band and they can be used on different frequencies. They have a smaller package (compared with internally matched types) due to the fact that they have no internal matching networks. For instance a 1.1W device characterized by the manufacturer up to 14.5 GHz (such as Mitsubishi MGF2430A) can be used from much lower frequencies up to 15 GHz because it has no pre-matching circuits; the devices is therefore meant to be wide band, typically used in X band but also from 2 - 3 GHz to 15 GHz.
From the other side there is a drawback in using these devices in wide band applications: we have to match them to the frequency we need because rarely they are already matched to 50Ω.
Internally matched GaAs FETs
These GaAs FETs are pre-matched with a matching network close to 50Ω inside the device, this is applicable for a moderate bandwidth, it means that into his own bandwidth the 50Ω matching is very easy. For their operation just a connection on a 50Ω track is needed, additional tunings are also possible (with the usual straw slid on the input and output tracks) but they will not lead to substantial improvements. Compared to all not matched types they have a larger case that also serves to enclose the various matching networks.
Unlike the standard not matched types, they work only within their frequency band, with an over-range that depends a lot on power as when the devices have limited power, for example 1 or 2W, the internal matching and combining networks are rather simple and therefore fewer, so they can be used far beyond the optimal bandwidth, the classic example is the RFMA7185-S1 model, optimized for 7.1 - 8.5 GHz, that is usable also from 6 to 10 GHz and above.
For devices that have more power, 4W or more, matching and combining networks are more complicated and numerically more abundant so the frequency band is narrowed.
