Electronic – What makes an “rf transistor” different from an ordinary transistor


I have been looking at various types of transistors that can switch at higher speeds than my clunky but powerful IGBT, and I came across these things called "rf transistors" or "rf mosfets" which can switch at speeds anywhere from a few mhz to a few ghz. Here is an example datasheet. I was wondering what makes these things so fast. I tried some googling, but all I could find was products–no details on what exactly makes these devices different.

Also, is there anything I should watch out for when driving one of these devices? I have gotten fairly comfortable driving large IGBTs and other power devices, but I definitely experienced a learning curve. For example, it took some time and a fair amount of burned out transistors to realize I had to isolate the voltage that drives the gate from source-drain voltage. I also learned that although a resistor connected to the gate is not necessary, a small one helps get you a cleaner signal (I suspect it prevents ringing between the secondary of a gate drive transformer and the effective capacitance between the gate and source). Finally, I found that for fast switching, it helps to actively pull the gate voltage below zero rather than just let it drain across a diode. So given that these are some things I learned to watch out for when driving power transistors, are there any new caveats I should watch out for when working with rf transistors?

Best Answer

RF transistors are operated in their linear mode (for analog signals, going into saturation or full cut off will 'slow them down' dramatically), whereas the transistors you refer to are operating as a switch (either fully saturated or fully cut off, they will have an extremely low on-resistance and a high off-resistance). Entirely different modes of operation.

RF transistors can be extremely sensitive to static discharge.