Lately I discovered the CLY15 GaAsFET. This FET is often used in GSM and WiFi equipment and is manufactured in large quantities.
It is able to produce appr. 2 Watts up till 2.4 GHz. It's very small (SOT223 housing). Power supply voltage is max. 6 Vdc.
I found that it requires a negative Gate Source voltage to operate.
Because this type of FET is so cheap ($2), I ordered 2 of them at eBay for some trials.
If less power is required, the CLY5 or CLY10 can be a better choice.
CLY 15 specs