[63] Mosfet Body Diodes

Date: December 23rd, 2007 | Comments : [4] | Categories: DIY.

mosfet cross-section Mosfets are 4-terminal devices with a gate that controls the conduction of the drain to source channel. The body (or substrate) of the typical n-channel enhancement mosfet is a P-type material with the drain and source formed from N-type materials.

The oxide layer insulates the metal gate connection from the other layers. There are holes in the oxide layer left for the metallic contacts of the drain and source.

mosfet body diodesN-P junctions act as diodes, and because the layers of the mosfet have P-type and N-type materials in close contact, diodes are formed from the body to the drain or source, as shown in the drawing here of the 4-terminal mosfet. If you were to leave the body connection floating, there could be no voltage flow through the two PN body diodes because they are reverse connected to each other.

However, in the 3-terminal mosfets (like the BS170) that are commonly used in guitar effects pedals, the body and source are connected internally and this shorts out the PN junction of the lower diode and removes it from the circuit. The top body diode can now conduct from drain to source because of the internal connection and therefore has to be taken into consideration during circuit design.

This means that every 3-terminal mosfet has a body diode due the construction of the device. It is not added as a separate component inside the mosfet.

However, there is something even more interesting in the 4-terminal mosfet. The drain-to-body-to-source structure is an N-P-N layered structure… NPN, as in transistor! There is a bipolar transistor in every mosfet!

Can you float the gate and use the drain-body-source as if they were collector-base-emitter? I don’t know but I plan on trying it. I would guess that it will probably be a poor bipolar but that may make it a special sounding unit (or an awful one). HFE will likely be low. It would probably be best to tie off the gate to keep it from acting as an antenna.

One last observation… I believe that the CD4007 has access to the substrate (body) and might be interesting for experimentation.

 

4 Responses to “Mosfet Body Diodes”

[266] epicac Says: 12:08 pm, December 24th, 2007

Great thing you’ve added the planar picture.

It definitely will make a poor bipolar transistor. I think (well, fantasize) the low conductivity of the base layer (P substrate on the image) will make the input impedance of the Body pin really high (as in bi-polars’ common base wiring). Also, the same low conductivity of the channel will limit the source-drain current, and it will probably require a high voltage to open the transistor altogether.

In other terms, I’m waiting for your results anxiously. Especially if you’ll be testing the common emitter/common collector schemes (of the bi-polars).


[267] admin Says: 9:01 am, December 25th, 2007

Update: 4-terminal mosfets are still in production though much less common than the 3-terminal devices. Many of them are listed in the 3Nxxx part number range. Typically they are in TO-72 or TO-206AF packages with the body (substrate) pin connected internally to the metal case.


[272] DDD Says: 8:34 am, January 11th, 2008

Jack, according to you, one can use 4-terminal MOSFET as a solid state device with TWO control terminals (for example, double-gate MOSFET tetrode) or a combination of MOSFET and BJT in “one bottle”.
It seems to be much more interesting than using it as a single BJT.


[295] $uperpuma Says: 1:34 pm, February 14th, 2008

I have what I believe to be a small lot of these 4 terminal MOSFETs… would be nice to have a circuit to use them in 🙂




 

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