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jason_class Community Member Offline Posts: 39 singapore	about P+ poly gate Mar 08^th, 2006, 7:27pm Hi Murphy and All I come across text that mentioned p+ poly gate for nmosfet has Vfb as Vfb= +Eg/2q - (Vb). Can you please help to explain how do we come out with the positive sign of +Eg/2q and negative sign of Vb? Then Eg, q and Vb are in the absolute values right? Kindly enlighten Thank you Murphy and All best regards Jason
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Croaker Senior Member Offline Posts: 235	Re: about P+ poly gate Reply #1 - Mar 14^th, 2006, 8:20am I'm not sure what your variables mean. What is Vb? Are these the levels in the poly or in the substrate?
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jason_class

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Posts: 39
singapore

Re: about P+ poly gate
Reply #2 - Mar 14^th, 2006, 4:32pm

Hi Murphy

Thank you for spending time on my enquiry.
Vb is the separation between the Ei and Ef at the substrate side (The potential)
As for Vfb flatband voltage but in the equation, the Qi/Ci is assumed to be zero ,
making Vfb= Vms
Eg is the bandgap energy

I saw in the book for the below 3 cases;
1) n+ poly gate on p silicon, Vms= - (Eg/2q)- Vb

2)p+ poly gate on n silicon , Vms = +(Eg/2q) +Vb

3)p+ poly gate on p silicon , Vms = +(Eg/2q) -Vb

Do you know how to get the equation for the 3) case?

That is my exact question.

Kindly enlighten
Thank you Murphy

best regards
jason

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Croaker Senior Member Offline Posts: 235	Re: about P+ poly gate Reply #3 - Mar 15^th, 2006, 8:02am I'd have to look at the book or see where the equations come from. Good luck, Marc
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jason_class Community Member Offline Posts: 39 singapore	Re: about P+ poly gate Reply #4 - Mar 15^th, 2006, 4:02pm Hi Murphy The book is written by Yuan Taur and Tak H Ning. Title is Fundamental of Modern VLSI Devices. Page 75 and 188. Thank you Murphy best regards jason
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Croaker Senior Member Offline Posts: 235	Re: about P+ poly gate Reply #5 - Mar 15^th, 2006, 9:47pm jason_class wrote on Mar 15^th, 2006, 4:02pm: Hi Murphy The book is written by Yuan Taur and Tak H Ning. Title is Fundamental of Modern VLSI Devices. Page 75 and 188. Thank you Murphy best regards jason The 1998 one? I will check it out from the library and see if I can see what you are asking about.
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jason_class Community Member Offline Posts: 39 singapore	Re: about P+ poly gate Reply #6 - Mar 16^th, 2006, 6:51am Yes Murphy There are reprints done for each year up to 2002 as I can see. Maybe there are even newer ones. The one I am loaning is year 2002 which is exact reprint from year 1998. Thank you for the help best regards Jason
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vivkr

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Posts: 780

Re: about P+ poly gate
Reply #7 - Jun 20^th, 2006, 5:09am

jason_class wrote on Mar 14^th, 2006, 4:32pm:

Hi Murphy

Thank you for spending time on my enquiry.
Vb is the separation between the Ei and Ef at the substrate side (The potential)
As for Vfb flatband voltage but in the equation, the Qi/Ci is assumed to be zero ,
making Vfb= Vms
Eg is the bandgap energy

I saw in the book for the below 3 cases;
1) n+ poly gate on p silicon, Vms= - (Eg/2q)- Vb

2)p+ poly gate on n silicon , Vms = +(Eg/2q) +Vb

3)p+ poly gate on p silicon , Vms = +(Eg/2q) -Vb

Do you know how to get the equation for the 3) case?

That is my exact question.

Kindly enlighten
Thank you Murphy

best regards
jason

Hi Jason,

You can derive all three equations very easily in the following manner:

1. Draw the band diagram of the bulk semiconductor, and of the gate polysilicon separately.
Both have the same Ec (Conduction band), Ev (Valence band) and Ei (Intrinsic level). Only the Ef (Equilibrium Fermi level) is different.

2. The bulk is lightly doped and so (Ei-Ef) is finite. For the gate, assume that Ef is either at Ec (n+ gate) or Ev (p+ gate).

3. Now, the Vfb can be easily found by taking the difference of potentials that would be required to align the 2 Fermi levels on either side. The (Eg/2q) is just half the bandgap.

For your first question, (p+gate on p bulk), Ef(gate) = Ev.
Thus, aligning the 2 sides needs the gate to be raised up by Ef(bulk)-Ev which is equal to
(Ei-Ev)-(Ei-Ef(bulk)) = (Eg/2) - q*Vb

You can derive the other 2 equations in a similar manner.

Regards
Vivek

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