What makes an RF transistor an RF transistor (as opposed to a regular transistor)
And how does it make RF?

 

RF Transistors are fabricated and characterized for operation at RF frequencies. In the HF range (3-30 MHz) most transistors can provide gain. As the frequency goes up to the VHF range (30-300 MHz.) the caracterization of transistors becomes more important. In the UHF range (300-3000 MHz) charaterization and layout become crtical. Above that you're into microwave voodoo land.

The transistors don't make any RF by themselves, but they can be part of an oscillator circuit that makes RF.

 

The 2 biggest features of any transistor in that area is gain and noise. Gain should be good, noise should be low.

 

RF transistors have high Ft (transition frequency) and low Cob (collector-base capacitance).

 

You will generally find on an RF transistor data sheet the s-parameters plotted as a function of frequency. This information can be helpful in selecting a device. After selecting a device, it is not uncommon to verify the manufacturers data on a VNA or similar equipment.

 

Thanks for the basic explanation guys, it makes sense!