P-FET gate drivers

[David Hess]

The body diode is the little triangle inside the symbol.  It's always there, whether you notice it or not.  Or, if you were always wondering...
Tim

Yeah I am still wondering what you are saying, are you saying that ALL fets have a freewheeling diode?

Not all MOSFETs have body diodes but I doubt you can find a power MOSFET which lacks one.  It is not a freewheeling diode so much as a parasitic element which may often be used as one.  The body diode in early power MOSFETs had a slow recovery time so it was important to prevent it from conducting in high frequency applications.

Here are just three examples of the Eagle fet library to me the last symbol is the most clear, but I know for a fact that the IRF510 also has a freewheeling diode (datasheet).

Your middle example, the 2N4857, is a JFET (junction field effect transistor) and the diode is an inherent part of the structure connecting the gate to the source and drain.  It is unrelated to the body diode found in a MOSFET.

[SeanB]

Only MOSFET with a separate body diode I have used regularly is the CD4007 6 device IC, where you have 4 devices sharing a separate body diode, and 2 devices with them internally connected. otherwise analogue transmission gates have a separate body diode connection. As to power devices I have seen none.

[David Hess]

Only MOSFET with a separate body diode I have used regularly is the CD4007 6 device IC, where you have 4 devices sharing a separate body diode, and 2 devices with them internally connected. otherwise analogue transmission gates have a separate body diode connection. As to power devices I have seen none.

I think RCA and Siliconix made 4 pin small signal MOSFETs like this with the substrate connection brought out on the 4th pin for chopper and electrometer applications.  I certainly remember running across them in high performance integrating analog to digital converters like you would find in a high resolution multimeters.

The initial power MOSFETs lacked a body diode but this has complications because it allows the parasitic bipolar transistor in the structure to turn on under certain conditions causing destructive latchup so you will not find these anymore except maybe as some specialized lateral power MOSFETs.

[SeanB]

3N series jfets and mosfets are pretty rare these days, they also had a separate substrate connection that was the body diode connection. Was not rated for anything other than reverse voltage and leakage, and was also recommended to never forward bias it.

[T3sl4co1l]

Not all MOSFETs have body diodes but I doubt you can find a power MOSFET which lacks one.  It is not a freewheeling diode so much as a parasitic element which may often be used as one.  The body diode in early power MOSFETs had a slow recovery time so it was important to prevent it from conducting in high frequency applications.

Like I said, the eGaN FETs are possibly the only power device on the market that actually has neither a substrate junction nor a connection to it.  Probably, there are RF devices too.

Interestingly enough, the body diode has always had poor recovery, and as it turns out, necessarily so.  The tricks that make good fast-recovery diodes (gold or other "bad" doping, irradiation) are absolute poison to MOSFETs.  So you get two basic classes: slow (optimized for slightly lower Rds(on)), and slightly-less-slow (optimized for slightly lower t_rr).  Example: IXYS PolarHV vs. HiPerFET series.

Your middle example, the 2N4857, is a JFET (junction field effect transistor) and the diode is an inherent part of the structure connecting the gate to the source and drain.  It is unrelated to the body diode found in a MOSFET.

Actually, you can think of the substrate as a JFET gate, and the insulated gate proper as MOS.  You get dual control ("back gate") if you put signal on the substrate.

Those eGaN FETs are ultimately built on a silicon die, I wonder how much transconductance you get from it if you scratch away the passivation and make a connection.  Would be relevant to dV/dt in high voltage applications (they're up to 300V now).  Maybe they thought of that, and shorted it to source or drain?

Tim

[c4757p]

Not all MOSFETs have body diodes but I doubt you can find a power MOSFET which lacks one.  It is not a freewheeling diode so much as a parasitic element which may often be used as one.  The body diode in early power MOSFETs had a slow recovery time so it was important to prevent it from conducting in high frequency applications.

Like I said, the eGaN FETs are possibly the only power device on the market that actually has neither a substrate junction nor a connection to it.  Probably, there are RF devices too.

Those eGaN FETs are ultimately built on a silicon die, I wonder how much transconductance you get from it if you scratch away the passivation and make a connection.  Would be relevant to dV/dt in high voltage applications (they're up to 300V now).  Maybe they thought of that, and shorted it to source or drain?

First one I clicked on has a substrate pad.

[T3sl4co1l]

Oh, OK.  I forgot, or didn't read then 

So it should be more like RF shielding, than the MOSFET meaning of "substrate".

Tim