RF transistors

[chorb]

Just finished up a course in transistors, and I have found the subject to be fascinating, and in looking at different companies that manufacture transistors I came across RF transistors but I havnt been able to figure out what makes them different than say a regular FET. Google didnt provide any answers, so what makes them different?

 

[f95toli]

I guess you are refering to HEMTs: High Electron Mobility Transitors.
The principle of operation is more or less the same as for an "ordinary" FET so there is no "fundamental" difference.
However, HEMTS are made from III-V materials (i.e not silicon) such as GaAs, InP etc and they use a heterojunction instead of an n-doped region.

Look at
http://en.wikipedia.org/wiki/HEMT

 

[djhuber82]

RF transistors are usually junction devices (BJTs or HBTs), not FETs. These transistors are better suited to building high speed amplifiers and other RF blocks due to a number of factors, including: more transconductance for a given bias current, better matching, and higher speed operation. They are generally not used for digital because unlike CMOS they need to be biased at nonzero DC current.

 

[f95toli]

AFAIK all ordinary HEMTs are FETs (made from e.g AlGaAs/GaAs).
You have source, drain and gate and the current is controlled by the field so it is definitly a field effect transistor.
Isn't an old name for HEMT Heterostructure FET (or something similar)?

You can actually use HEMTs in "ordinary" low-frequency electronics as well, just as you wouyld use an ordinary FET. They are very noisy but the advantage is that they work at low temperatures (Si devices usually freeze out at around 20K)

 

[FrustratedUser]

The most important thing that everyone missed here is the physical size of the transistor itself.
I actually used to do photo lithography and some processing of HEMT transistors on III-V materials back in '92-'94. They were trying to make a MMIC amplifier for 119GHz.

Anyway. One really important dimension is the gate length which (among many other parameters) sets the f-max. f-max is the frequency where the gain of the transistor is 1 (one).