What happens if I use a high frequence LDMOS in a Low frequence application?

[coulson]

recently, I got a NXP LDMOS(AFT05MP075NR1), datasheet says it's for 136 to 520 MHz application. What really confuse me is that why there is even a low frequency limit, what happens if I use this mos in low frequency such as 2MHz,what could goes wrong?

[Damianos]

I think that in these devices the construction (all parasitics: R, L, C) is optimised for the given frequency band. Outside of it, the performance will be degraded significantly, so they are not characterised for that use. For very low frequencies, than "normal", they may perform well, but are unnecessary more expensive than a proper selection. This is just my guess!

[RoV]

In the specific case of that NXP LDMOS device, the datasheet says it is unmatched and simply characterized from 136 to 520 MHz. I think nothing prevents using it at a lower frequency, but certainly there are more economic parts to do the job, with the proper characterization.
More frequently, devices are characterized in a band because they are internally matched in the input and/or output sides to simplify the external matching networks.

There are, however, a few warnings from Polyfet. You can find them somewhere in their site http://www.polyfet.com/. Here an excerpt from a file rftopics.pdf that I saved some time ago from their site:

• Load VSWR - Although MOSFETS are not as sensitive to VSWR as bipolar transistors,
  some caution should be applied. In general, the higher frequency of operation, the higher
  VSWR tolerance of the device. Most manufacturers specify a maximum load VSWR for
  safe operation. The maximum permissible VSWR is less at lower frequencies. Example:
  a transistor which is safe with and infinite VSWR load at 1 Ghz maybe capable of withstanding
  only a 10:1 VSWR at 100 Mhz. Also the maximum VSWR specifications on a data sheet is
  usually for a transistor-circuit combination. Some circuits present higher VSWR’s to the
  transistor than others.
  
• Frequency Range - A transistor should be used within its intended frequency range if
  possible. If a transistor is used at a lower frequency it will be more fragile and more
  susceptible to oscillations. If operated at higher frequencies, lower gain and poorer
  efficiencies can be expected.
  

[virtualparticles]

It would be a good idea to check the stability, Linville or whatever, at lower frequencies. I used an MRF5045 LDMOS PA at UHF frequencies where the data sheet only has information for 700 MHz to 1 GHz. The device is only conditionally stable at UHF so I had to manage the input and output matching very carefully and used a resistor/Inductor series pair to ground on the input to drag it to a known low impedance at VHF frequencies. I also popped about a dozen devices during characterization at $50 each, the price of doing business.