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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?

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  • \$\begingroup\$ Can you link the datasheets to the devices you were looking at? \$\endgroup\$ – jippie Nov 10 '12 at 16:45
  • \$\begingroup\$ I added an example datasheet in the first paragraph. \$\endgroup\$ – Alex Eftimiades Nov 10 '12 at 16:50
  • \$\begingroup\$ iirc, one of the differences is that the junction between the elements are thinner due to RF frequencies. \$\endgroup\$ – Rob Nov 10 '12 at 22:05
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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.

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  • \$\begingroup\$ Thanks for the response. So how are they so much faster than normal mosfets? \$\endgroup\$ – Alex Eftimiades Nov 10 '12 at 19:44
  • \$\begingroup\$ @Feynman - They are faster when operated in the linear mode without saturation. A saturated type mode of operation when used in a switching application tends to require full charging and discharging of the gate and junction capacitances on the part. This takes time and slows down operation. RF circuits also tend to have resonant circuits as part of the drivers to the FET which can generate AC voltages are carefully tuned between the input and output which can increase the performance in a signicant way. \$\endgroup\$ – Michael Karas Nov 10 '12 at 21:24
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    \$\begingroup\$ Component package is different too. The regular leads in a TO-92 or TO-220 have a high inductance. You might want to compare the capacities mentioned in the datasheet for a switching transistor vs an RF transistor. \$\endgroup\$ – jippie Nov 11 '12 at 4:32

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