Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Whilst perusing through Ampeg's product page, I ran across a page for an amp with MODFET outs (http://ampeg.com/products/b/b500dr/index.html).

MODFET?  Somebody must've made a typo.

But a quick Google search showed that Harvard (http://adsabs.harvard.edu/abs/1988STIN...8922029.) even the IEEE (http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=144683) has written up papers about these devices.

According to wikipedia's MODFET article (http://en.wikipedia.org/wiki/MODFET), MODFETs can be manufactured by epitaxial growth of strained SiGe layer, with the germanium content linearly increasing to about 40-50%. The high content of germanium allows formation of a quantum well structure with high conduction band offset and high density and mobility of charge carriers, resulting in parts with ultra-high speed and low noise.

Now my chemistry/quantum physics knowledge is only pedestrian, so anyone is welcome to correct me, but doesn't a high charge carrier density and mobility decrease gate-source and gate-drain capacitance, which is a major design detriment for MOSFET amplifier designers?

And if it's able to perform well at high frequencies, I would guess that the Rds-on would be lower than conventional MOSFETs.

The benefits of a low noise device go without saying.

Now, I don't have the time to delve into this, and in my brief searching, I found nothing that explored the linear operation of these devices.  They may very well be better used in switch-mode amplifiers.

Can anyone give me the skinny on these devices?  Mouser and Digikey don't have any MODFETs available for order, so I imagine that this technology is still in its infancy.

edit: forgot to include the ampeg amp product page

Looks like theres not much information about them in the internet... I have to admit that this is the first time I hear about such devices. Google brought up this article:

http://nsr.mij.mrs.org/1/4/complete.mac.html

It seems to focus on fabrication methods and math is not my strenght. There are some charts at the bottom of the page and it looks like the Current-Voltage Characteristics (Fig. 2a) are very similar to MOSFETS.