What is a diode connected MOS transistor

[lallaby]

diode with mos

Hello guys,

I can not find the pn junction between the Drain and Source of the MOSFET when the Gate and Drain are connected. Why people say it behaves like a diode? Is there anyone who can give me a cross section of the MOSFET to illustrate it? Thanks a lot!

Best regards,
lallaby

 

[LordAli]

transistor with diode drain source

It depends on type of channel. If you have transistor that can create drain-source channel conductive with 0V on gate-source then drain-source will behave like resistor.

If you measure gate-source pins with diode test, you will notice, that there is pn junction like in bipolar transistors in BF245. But voltage applied to the control electrode is usually negative with respect to source. And channel is conductive even with 0V G-S so there's "resistor behaviour between D and S".

 

[kamalifard]

diode connected mos transistor

hi
in most of commertioal mosfet body or substrate is conected to source and in drain to source there is a p-n junction it is not very fast and have about 1000ns trr
for most switching application a parralel diode is added to it inherently that have fast characteristic

 

[usernam]

nmos diode connected fet

To qoute wikipedia.

h*tp://en.wikipedia.org/wiki/Power_MOSFET

It can be seen in figure 1 that the source metallization connects both the N+ and P implantations, although the operating principle of the MOSFET only requires the source to be connected to the N+ zone. However, if it were, this would result in a floating P zone between the N-doped source and drain, which is equivalent to a NPN transistor with a non-connected base. Under certain conditions (under high drain current, when the on-state drain to source voltage is in the order of some volts), this parasitic NPN transistor would be triggered, making the MOSFET uncontrollable. The connection of the P implantation to the source metallization shorts the base of the parasitic transistor to its emitter (the source of the MOSFET) and thus prevents spurious latching.

This solution, however, creates a diode between the drain (cathode) and the source (anode) of the MOSFET, making it able to block current in only one direction.

More details about Power MOSFETS are here.
h*tp://www.irf.com/technical-info/appnotes/mosfet.pdf

 

[LordAli]

mos diode applied gate and source

Aaah, now I'm in a picture.

You mean body diode between drain and source of power mosfet. It's pn junction of parasitic transistor. This parasitic transistor is created during the process when you made mosfet. Nice paper for this is: Design And Application Guide for High Speed MOSFET Gate Drive Circuits. Everything explained there. Can be found on TI site or www.smps.us