MOSFET: pinch off and punch through

[gilberthsiao]

Hi all,

In MOSFET operation, punch through occurs when drain's depletion region touches source's depletion region.
In saturation region, the only connection between drain and source is drain's depletion region touching depletion region of source and channel. (Channel is pinched off near drain.)
Why that is not called punch through in saturation region?

 

[erikl]

In MOSFET operation, punch through occurs when drain's depletion region touches source's depletion region.

More exactly, it's a continuous depletion region operated in off-state (Vgs<Vth). The electric_field_strength=Vds/L, however, is high enough to accelerate straying charge carriers to a velocity where they may provoke avalanche break-through, which can cause immense and destructive current flow if not limited externally.

In saturation region, the only connection between drain and source is drain's depletion region touching [depletion region of source and] the channel. (Channel is pinched off near drain.)

This is a normal on-state (Vgs>Vth) operation of a FET.

Why that is not called punch through in saturation region?

Because these are totally different mechanisms.

 

[gilberthsiao]

Thank you erikl.
When Vgs is slightly lower than Vth, body is depleted in the surface under gate oxide. So here is a depletion region between drain and source.
Drain/body and source/body junctions are reversed biased, so they have their own depletion regions.
In this condition, drain's depletion region, source's depletion region, and depletion region under gate are merged.
What is the difference between this condition and punch through?

 

[erikl]

When Vgs is slightly lower than Vth, body is depleted in the surface under gate oxide. So here is a depletion region between drain and source.
Drain/body and source/body junctions are reversed biased, so they have their own depletion regions.
In this condition, drain's depletion region, source's depletion region, and depletion region under gate are merged.

This is just the standard off-condition of the (MOS)FET.

What is the difference between this condition and punch through?

AFAIK, punch through occurs in the same condition as above, but with too high Vds: The lateral electric field is high enough to provoke avalanche breakdown between drain & source/bulk, which means current flow even through depletion regions (s. also my posting above).