Power MOSFET: Diode and quantity direction

Hi,

In datasheet of a P-Channel Power MOSFET Si2333DS I see the symbol of the following. Does anyone have idea why there is a diode drawn inside?



Another question is how I_D is defined. This is an P-Channel enhancement MOSFET, and the majority carrier holes would flow from S to D with V_D<V_S and V_G<V_S. So if we define source-to-drain as the positive current direction, I_D should also be positive.



However, the datasheet page 1 lists negative I_D values. So does it mean the positive current direction is D-to-S?
[h=3]Error[/h]I also found in the top-left figure of page 3, V_DS, V_GS and I_D are marked as positive. But for a P-Channel Power MOSFET to work, at least both V_DS, V_GS should be negative, with I_D still depends on the positive direction definition.
As a comparison, in the datasheet of Fairchild BSS84, the axis are labeled as –V_DS, -I_D, and all V_GS’s are negative.
So if Fairchild BSS84 is correct, then Vishay Si2333DS datasheet must be wrong?


Matt

read: 1. Basic Characteristics of P-Channel HEXFET Power MOSFETs
http://www.irf.com/technical-info/appnotes/an-940.pdf

for example............
http://www.infineon.com/dgdl/Revers...b2614&fileId=db3a304412b407950112b41887722615

Tip - Logic Level FET's (P-Channel) [67] | General Electronics | Electronics

If I understand right, your primary problem is the definition of Id. I presume, that we are talking about technical current notation, not electron flow. Id is in fact understood as the current into the drain terminal, drain-to-source, not source-to-drain current as you assumed. Thus Id is negative for a PMOS transistor in active operation, as well as Vgs and Vds.

If you refer to the said BSS84 and Si2323 datasheets, all respective quantities are shown negative for both in the tabular part. The difference is, that the sign has been omitted for the Si2323 diagrams, which is in fact incorrect.

P.S.: I think, there's also an erroneous assumption involved with your question. Either what MOS and actual Vds polarity you have, Id current direction will follow Vds. There's no internal current source opposite to Vds.

Finally you have been asking about the diode symbol. The built-in diode is a side-effect of usual power MOSFET technology where source and substrate terminal are internally connected, refer to semiconductor text books.

FvM,

Your answer cleared every of my doubts. Thanks very much.

Matt

---------- Post added at 16:59 ---------- Previous post was at 16:58 ----------

kak111,

I read the materials and this question is resolved. Thanks very much.

Matt

FvM and kak111,

There is also a question with breakdown voltage.



Both Si2333 and BSS84 are p-channel power MOSFETs. For the MOSFET to be turned on, V_DS needs to be negative. However, the breakdown V_DS for both are also negative, having the same direction as normal working V_DS.

This seems to be opposite with a similar concept, like “maximum reverse voltage” in diodes (and LEDs, etc.) in which such a voltage is in the reverse direction than the forward voltage, and once exceeded, would damage the device.

1. With p-channel power MOSFET, the breakdown voltage has same direction as ordinary V_DS.
2. With diode, “maximum reverse voltage” has reverse direction than V_FORWARD.

According to Avalanche breakdown - Wikipedia, the free encyclopedia, the mechanism of avalanche breakdown is somewhat similar to chain effect in that fast-moving electrons and holes knocks other electrons and holes free when the voltage is high enough. The principle was discussed there (Wikipedia article) for reverse LED voltage, but does it also apply to p-channel power MOSFET?

But why there could be different direction between “breakdown voltage” of p-channel power MOSFET and “maximum reverse voltage” of LED? Could anyone explain the reason for the difference?

Matt

However, the breakdown VDS for both are also negative, having the same direction as normal working VDS.

Click to expand...

Yes, that's how a MOSFET works.

Although bipolar junctions are involved in MOSFET technlogy, e.g. substrate diodes, the basic MOS transistor operation doesn't utilize bipolar junctions. In so far, there's no reverse voltage breakdown involved. Basic MOSFET breakdown is rather occuring in isolation layers.

Secondly, power MOSFET breakdown can occur in the reverse biased drain-substrate diode. This diode is also the reason, why only higher voltages of one polarity can be applied to the transistor.

The datasheet values are not referring to internal structures but to external terminals. They clarify, that the breakdown voltage has the same polariry as the normal operation voltage of the transistor.

FvM,

Many thanks for the detailed answer. Though I am still not well with the physics, the confusion is however cleared.

Matt