Ions in depletion mode

[Manuv16589]

Hi,
MOSFET has 3 modes of operation. Accumulation, depletion and inversion. In depletion mode When gate voltage is greater than threshold voltage ions are attracted to the region beneath the gate. Where do these negative ions(nmos) come from?

 

[vlsiinterviewquestion]

Hi Manu,
These negative ions come from substrate. If you think of a MOSFET with p-type substrate, its majority carriers are holes or positive ions. As soon as you increase the voltage at gate above zero, positive ions near the gate are repelled and a depletion region is formed. It's called depletion region because that region becomes depleted of majority positive ions. When gate voltage is increased above threshold voltage Vth, it attracts minority carries from the ptype substrate which are nothing but electrons.
Let me know if this is still not clear.

 

[Manuv16589]

Hi Manu,
These negative ions come from substrate. If you think of a MOSFET with p-type substrate, its majority carriers are holes or positive ions. As soon as you increase the voltage at gate above zero, positive ions near the gate are repelled and a depletion region is formed. It's called depletion region because that region becomes depleted of majority positive ions. When gate voltage is increased above threshold voltage Vth, it attracts minority carries from the ptype substrate which are nothing but electrons.
Let me know if this is still not clear.

thanks for the reply. I understood what you have explained. But where do these negative ions come from. Are they the dopant atoms?

 

[vlsiinterviewquestion]

Hi,
They're actually electrons as I mentioned and not negative ions. They are the minority carriers from p substrate. As you may recall p type semiconductor material is made by doping it with acceptor atoms which take away one electron from silicon valence band and form holes. Based on the amount of doping concentration in ptype material majority of charge carriers are holes, but there are always electrons from the valence band of the silicon atoms available to carry charge. Let me know if this answers your question.

 

[Manuv16589]

Understood...thanks for the explanation