Monday, September 11, 2017

The mystery of the DMOS

dual gate MOSFET

The devices used in the above link is what is being discussed. You might notice they have no protection diodes. I keep hearing they do?
Any way. If you take a standard E-MOS and strap a 100k resistor from gate to drain you can put it in a circuit and build an amp with no further biasing required. It will "self regulated" at a point where the drain voltage "matches" the gate voltage required by the amp. In other words the drain voltage will be at supply as the power is applied. When the MOSFET conducts the drain voltage drops and the gate voltage follows until it reaches a balance point. Knowing this let's look inside the D-MOS.

The D-MOS is made just like any other MOSFET but it gets one extra step in the process. It has a channel implanted to provide forward bias. Remember the 100K resistor?  Neat huh?

The implant shift the bias line and we now have a MOSFET which conducts without external bias.

It's a bit of magic accomplished in the fabrication.

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