Transistor in saturation mode

[fzn10]

Iv been told that when operating transistor as constant switching devices, it is more efficient to operate them in saturation mode. it that right. Can any one tell me why that is?

Thanks

 

[dknguyen]

Which kind of transistor? BJT or MOSFET? Saturation means completely different things between the two.

For MOSFETs, yes, because it's the mode where the transistor is turned "on" the most with the lowest resistance (passes the most current).

Refer to the picture here:
https://en.wikipedia.org/wiki/Image:IvsV_mosfet.png

 

[fzn10]

its niether its called BD140, jsut a PNP silicon transistor. Does that theory for the MOSFET you just said still apply for that. THANKS

 

[dknguyen]

No, it does not apply. THat is a BJT transistor. The two types of BJTs are NPN and PNP. The two types of MOSFETs are NMOS and PMOS. Saturation is unfortunately used for both devices but they are referring to completely different things.

[EDIT:] I think I may be mistaken. Although saturation means different operating modes for both types of transistors, they both happen to be the most efficient when being used as a digital switch.
**broken link removed**
**broken link removed**

I kind of forget how BJTs work so...yeah...read what I write critically.

 

[fzn10]

Yeah thanks, the second link acutally exaplains it all in the third paragraph

 

[hjames]

dknguyen - you had it right the first time. Saturation in mosfets refers to constant current mode - where Id is independent of Vds. Triode region is the one you want when using a mosfet as a power switch - if you run through the equation and assume that Vds is very close to 0, you end up with Id ~= k*Vds which looks a lot like Ohm's law.


reference:
https://en.wikipedia.org/wiki/MOSFET

 

[dknguyen]

Oh, so it is triode for MOSFETs and saturation for BJTs? I tend to get confused with the region where you switch between it's two extremes when using it as a switch and when you push the transistor way beyond the "on" threshold so it can't switch off as fast. Bleh, gotta review.