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The Body Effect

The document discusses the body effect in MOSFET transistors. If all the source terminals of the thousands or millions of MOSFETs in an integrated circuit were connected together and to the single silicon substrate body, it would result in a useless circuit. However, the source terminals are not connected to the substrate body in integrated circuits. This means the voltage between the source and body (vSB) is not necessarily equal to zero and the MOSFET behaves as a four-terminal device rather than a three-terminal device. The body effect changes the threshold voltage expression to include a term dependent on vSB. While often insignificant, the body effect can sometimes have a large impact on circuit performance.

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1K views3 pages

The Body Effect

The document discusses the body effect in MOSFET transistors. If all the source terminals of the thousands or millions of MOSFETs in an integrated circuit were connected together and to the single silicon substrate body, it would result in a useless circuit. However, the source terminals are not connected to the substrate body in integrated circuits. This means the voltage between the source and body (vSB) is not necessarily equal to zero and the MOSFET behaves as a four-terminal device rather than a three-terminal device. The body effect changes the threshold voltage expression to include a term dependent on vSB. While often insignificant, the body effect can sometimes have a large impact on circuit performance.

Uploaded by

bobby1990s
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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10/19/2004 The Body Effect.

doc 1/3

The Body Effect


In an integrated circuit using MOSFET devices, there can be
thousands or millions of transistors.

As a result, there are thousands or


millions of MOSFET source
terminals!

But, there is only one Body (B)—


the Silicon substrate.

Thus, if we were to tie (connect) all the MOSFET source


terminals to the single body terminal, we would be connecting
all the MOSFET source terminals to each other!

Jim Stiles The Univ. of Kansas Dept. of EECS


10/19/2004 The Body Effect.doc 2/3

This would almost certainly result in a useless circuit!

Thus, for integrated circuits, the MOSFET source terminals


are not connected to the substrate body.

Q: Yikes! What happens to


MOSFET behavior if the source
is not attached to the body ??

A: We must consider the MOSFET


Body Effect!

We note that the voltage vSB (voltage source-to-body) is not


necessarily equal to zero (i.e., vSB ≠ 0 )! Thus, were back to a
four-terminal MOSFET device.

There are many ramifications of this body effect; perhaps


the most significant is with regard to the threshold voltage
Vt.

We find that when vSB ≠ 0 , a more accurate expression of the


threshold voltage is:

Vt =Vt 0 + γ 2φf + vSB − γ 2φf

where γ and φf are MOSFET device parameters.

Jim Stiles The Univ. of Kansas Dept. of EECS


10/19/2004 The Body Effect.doc 3/3

Note the value Vt0 is the value of the threshold voltage when
vSB = 0, i.e.:

Vt =Vt 0 when vSB = 0.0

Thus, the value Vt0 is simply the value of the device parameter
Vt that we have been calling the threshold voltage up till now!

In other words, Vt0 is the value of the threshold voltage when


we ignored the Body Effect, or when vSB = 0.

It is thus evident that the term:

γ 2φf + vSB − γ 2φf

simply expresses an extra value added to the “ideal”


threshold voltage Vt0 when vSB ≠ 0 .

For many cases, we find that this Body Effect is relatively


insignificant, so we will (unless otherwise stated) ignore the
Body Effect.

However, do not conclude that the Body Effect is always


insignificant—it can in some cases have a tremendous impact
on MOSFET circuit performance!

Jim Stiles The Univ. of Kansas Dept. of EECS

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