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To: ForYourEyesOnly who wrote (880)	9/16/1997 2:05:00 AM
From: Rex Dwyer	of 4710

Here's my take on the differences in HBT and MESFET processes. The HBT is a bipolar device. So, it has a base current and a Vbe where MESFETS are like MOSFETS that are essentially driven by a voltage input. HBT processes tend to have big operating voltages. So, it is easier to make linear amplifiers on the HBT process. CDMA system designers use linear output stages and therefore like to design in HBT PAs. The input amps are also nice on HBT because of their linearity. I think RFMD has some good high headroom LNA parts made on the TRW process. HBT is a bit esoteric, and hence come from the old "spook" houses, TRW and Rockwell. Marie's article mentions higher efficiency in the HBT process, but I think that only pertains to the Linear amps. High Betas on the HBT process allow an efficiency advantage over the MESFET linear amps. The Class C (non-linear) amps are very efficient and served up very nicely on a good MESFET process. You have Apples and you have Oranges. All of the wireless systems that I can think of that use GaAs use Class C output stages except for the Qualcomm CDMA systems. Another sidenote. The HBT process, since it is a bipolar transistor, is more like a standard Silicon Bipolar process that many analog IC designers are comfortable with. It may make a difference if design talent is scarce. Yet another sidenote. One mustn't count out the importance of the designer. PAs and LNAs are very tricky devices. The same process could yield two drastically different chips from different designers. SUMMARY: - HBT wins in: úlinear input amps, úlinear power amps (Qualcomm CDMA) - MESFETS win in: úDigital circuitry, úClass C Power amps, úvery low noise input amps that don't have to be super linear. I hope this helps. Just one man's opinion. I could be wrong. Rex

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