rf transistors

A BJT or bipolar junction transistor is a current amplifier. They typically have three connections, emitter, base, and collector. This transistor is connected in such a fashion that the current going through the collector is amplified by the current interring into the base. The amplification factor is the collector current divided by the base current and this is called BETA.

A FET or field-effect transistor is a voltage amplifier. It has three connections, source, gate, and drain. This transistor is connected in such a fashion that the current going through the drain is proportional to the gate

voltage. Its gain is referred to as mu and it units are umhos. This is the change in drain current divided by the gate voltage.

BJT's have a low input impedance which may be a disadvantage and are more prone to thermal run-away than FET's. That is they can start conducting prematurely simply do to getting hot.
BJT's can go into an avalanche mode, that is suddenly start conducting and thereby destroying itself.

FET's affect less to temperature than BJT's and are more amune to avalanche affects.

RF transistors simply means "radio frequency transistors". That mean they are operating at frequencies typically used to send radio signals, usually the the megahertz or gigahertz.

BJT's and FET's are normal transistors. These along with MOSFET's comprise almost all the transistors there are.

 

The amplification factor is the collector current divided by the base current and this is called BETA.