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Author Topic: Multiplying transistor  (Read 815 times)
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I have NO idea where best to put this, but this is an idea I had for a possible new type of transistor:

Basically, an ordinary IGFET, but with two gates perpendicular to each other, with a more heavily doped substrate, or doped with a different-valence element, or made of a different material altogether.  When a channel begins to form from one gate, it either does not conduct or has a high resistance.  When the other gate begins to form a channel, the intersection of the two channels becomes a highly conductive area, thus making the current flow proportional to the product of the gate voltages.

Why does this not work?  I don't claim to be an expert on semiconductor physics, perhaps such a material as the one I described doesn't exist?

If this does work, why are we not using it?  I can't have been the first person to think of it.
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gonna need a diagram to understand it better

is it like a darlington pair but with IGFET?
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no an IGFet is just another name for a Mosfet, Dual gate mosfet's have been around for years 3N140 40673... for a few, the extra gate is frequently used to set the operating point of the fet or as a mixer injection point for a radio detector or RF mixer (typically used to mix 2 signals to provide a third or sum/difference frequency output... and those geometry's were tested and explored well in the 70's and 80's. There is a device that is similar it uses a mos type gate structure to control a bipolar transistor. This was done (Ixys is one) to avoid structural issues that are issues that are unique to a fet and high current and voltage like 600 - 800 V @ 100 A extreme but indicative of why the two technologies were joined at the Hip... There are many articles about it but it does exceed the scope of this forum... IMO
Look at this data sheet http://ixapps.ixys.com/PartDetails.aspx?pid=5189&r=1, surprised?

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I'm not sure I got my point across adequately, so let me try a diagram:

(can't get image to work, here's a link)

In this cross-section, grey is the non-conducting substrate, red and blue are the two separate high-resistivity channels, and purple, where the channels overlap, is the effective (conducting) channel.  Since resistance is proportional to cross-sectional area, the product of the voltages on the two gates is inversely proportional to the resistance of the unit.
« Last Edit: June 14, 2012, 10:50:56 pm by xolroc » Logged

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Quote
set the operating point of the fet
and how is this different? your diagram doesn't properly describe the typical geometry of a dual gate Mosfet and perhaps that is where the confusion exists. It was a common technique to do it that way, bias one of the gates to drive the device into saturation to "Cut off" the device. Operating bias was also done that way in mixers as the bias components aren't in the direct RF signal path and therefore contribute no additional losses or "Side effects". I do understand what you are getting to BTW... This was my answer the first time, is it possible that you don't understand mine?. I have used dual gate Mosfet's for many years as mixers and switches.
"Hint"... I an an auld Phart, been in the electronics business as a tech and engineer for nearly 50 years.

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Oh it's entirely possible I didn't understand what you were talking about.  I make no claims of being anything more than a hobbyist!  I had THOUGHT that a dual gate MOSFET worked in an entirely different way, hence the different geometry of my design.
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no quite the opposite two areas on the drain channel electrically isolated but interacting gate area's either one capable of fully enhancing the device but as either can modify or modulate the drain current useful for many things and yes the gate area's charges can join together internally but the device is now fully on so it makes no difference. Hope this helps to visualize a difficult to explain subject. There were smoe books put out by RCA who were among the leaders in the 60's with that product development. At one time a 40673 was to be found in nearly every design in many places, the second gate was an AGC, ALC control element, a mixer an oscillator... and the 3N140 was used for lower frequency operation < 20 Mhz where the 40673 was used up to about 300 400 Mhz in two way radio also CATV amplifiers and up/down block converters... high gain device too. Never did see any power applications too big for vhf or uhf service.

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Currently we can only make 2-dimensional silicon devices easily - your device has a number of problems though - the intersection of the channels from the two gates is tiny (gate-oxide and the channels they induce are extremely thin and have to be made broad to carry enough current (power MOSFET design is basically an exercise in getting as much channel width into a given area of silicon as possible).

Secondly it don't believe it will work like you think - current is proportional to drain-source voltage in saturation where the channel width is in control, and channel width depends (non-linearly) on the field strength - have two gates and the total field strength will be a linear combination of the two field strengths (Maxwell's equations), so you might as well have one gate, you'll get the same basic response (which is non-linear).

If you want a multiplier from a MOSFET you can use the MOSFET as a voltage-controlled conductance, put one signal voltage across the source/drain, the other on the gate, and take the resultant current as the product term.  Found interesting paper on MOSFET transconductance multipliers here: http://www.cse.psu.edu/~kyusun/class/cse577/11s/lec/lecture/multiplier.pdf
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