Interchanging of drain and source of MOS transistor

[shuteng]

Hi all,

1. I learnt in my varsity that the drain and source of a transistor
can be interchanged when drawing the layout (IC) of a transistor. Have
been thinking about it for sometime and I conclude that the drain and
source of a MOS transistor is equally doped hence, they can be
interused unlike that of (collector and emittor of BJT). Is my
hypothesis correct? If so, does it also means that the source and
drain can be interchanged on schematics?

2. In microelectronics, if a nMOSFET is used as a pass gate, does it
matter whether I connect the (driver) signal to the drain or source?
What are the minimum gate voltages needed to pass the signal from the
driver to the other devices(assuming 1V Vt)?


a) _|_ b) _|_
signal=3V ___| |___ signal=0V ___| |___
D S D S

c) _|_ d) _|_
signal=3V ___| |___ signal=0V ___| |___
S D S D

I apologise for my long questions and thanks in advance
shuteng

 

[R. Steve Walz]

Hi all,

1. I learnt in my varsity that the drain and source of a transistor
can be interchanged when drawing the layout (IC) of a transistor. Have
been thinking about it for sometime and I conclude that the drain and
source of a MOS transistor is equally doped hence, they can be
interused unlike that of (collector and emittor of BJT). Is my
hypothesis correct? If so, does it also means that the source and
drain can be interchanged on schematics?

-------------
Only if their construction is symmetrical.

2. In microelectronics, if a nMOSFET is used as a pass gate, does it
matter whether I connect the (driver) signal to the drain or source?

----------------------------
Not if the answer to 1) is true.

What are the minimum gate voltages needed to pass the signal from the
driver to the other devices(assuming 1V Vt)?

a) _|_ b) _|_
signal=3V ___| |___ signal=0V ___| |___
D S D S

c) _|_ d) _|_
signal=3V ___| |___ signal=0V ___| |___
S D S D

I apologise for my long questions and thanks in advance
shuteng

-------------------------------
Go look it up in a device book such as Floyd's "Electronic Devices".
I'm NOT going to do your reading for you. That's what your computer's
for.

-Steve

 

[Beau Schwabe]

Hi all,

1. I learnt in my varsity that the drain and source of a transistor
can be interchanged when drawing the layout (IC) of a transistor. Have
been thinking about it for sometime and I conclude that the drain and
source of a MOS transistor is equally doped hence, they can be
interused unlike that of (collector and emittor of BJT). Is my
hypothesis correct? If so, does it also means that the source and
drain can be interchanged on schematics?

For the most part yes, the Source and Drain can be interchanged.
Where it matters is if you have a single transistor with an even
number of gate fingers. This creates an ODD number Source
and Drain ratio, and in some sensitive circuits this parasitic
difference can affect how the circuit will perform.

2. In microelectronics, if a nMOSFET is used as a pass gate, does it
matter whether I connect the (driver) signal to the drain or source?

In a pass gate, any parasitics as mentioned above will be seen by the
signal no matter which way it is connected, so no in this case
Source/Drain swapping does not matter. ...Unless you have multiple
signals and matching is a concern. In this case you can still swap
Source and Drain, however ALL of the other signals should be connected
to the pass gates in a similar/uniform way.

What are the minimum gate voltages needed to pass the signal from the
driver to the other devices(assuming 1V Vt)?

This is highly dependent on the process in which you are using and how
the transistors are built. i.e. Gate Oxide, Salicide, etc.

 

[Fritz Schlunder]

Hi all,

1. I learnt in my varsity that the drain and source of a transistor
can be interchanged when drawing the layout (IC) of a transistor. Have
been thinking about it for sometime and I conclude that the drain and
source of a MOS transistor is equally doped hence, they can be
interused unlike that of (collector and emittor of BJT). Is my
hypothesis correct?

No, not exactly.

Most people don't emphasize very much where the substrate (bulk) of the
device is connected. The substrate connection is however very important to
the MOSFET's operation. In normal MOSFETs used in normal fashion, the
substrate is normally short circuited to the source contact. This
connection is really what defines the "source" pin (and consequently the
remaining pin as the drain pin).

Although most people talk about a gate-source potential that produces an
electric field that causes channel inversion, I think it is more accurate to
claim the gate-bulk potential difference is what produces the electric field
that causes channel inversion. It just so happens in the marjority of cases
the source is short circuited to the substrate, so a gate-source potential
is the same thing as a gate-bulk potential.

If you look at the MOSFET cross sectional view you notice an NPN structure
(for an N-channel device). When you short circuit the source to the bulk
(by making ohmic contacts to some metal which connects the two parts) you
effectively end up with a PN junction (between source/bulk and drain). This
PN junction becomes forward biased whenever the source is more positive than
the drain potential. Therefore, a standard MOSFET used in the normal
fashion cannot block more than about one diode drop in the reverse
direction.

If you take a look at International Rectifier's MOSFET datasheets they
specifically show a diode symbol as part of the MOSFET symbol to represent
this PN junction.

http://www.irf.com/product-info/datasheets/data/irf2804.pdf

They also characterize the PN junction's performance in the section labelled
"Diode Characteristics" as well as in figure 7.

Note, my comments only apply to MOSFETs, not to other FET devices such as
the JFET. JFETs are very different beasts despite the same pin naming
convention.

If so, does it also means that the source and
drain can be interchanged on schematics?

No, you most definitely cannot interchance the source/drain connections on a
shematic and expect the circuit to function properly. In one direction the
MOSFET will block only one diode drop, but in the other the full amount it
is rated to block...

2. In microelectronics, if a nMOSFET is used as a pass gate, does it
matter whether I connect the (driver) signal to the drain or source?
What are the minimum gate voltages needed to pass the signal from the
driver to the other devices(assuming 1V Vt)?


a) _|_ b) _|_
signal=3V ___| |___ signal=0V ___| |___
D S D S

c) _|_ d) _|_
signal=3V ___| |___ signal=0V ___| |___
S D S D

I'm not sure I understand quite what you are depicting/asking here so I
can't answer this...

 

[Beau Schwabe]

No, not exactly.

Most people don't emphasize very much where the substrate (bulk) of the
device is connected. The substrate connection is however very important to
the MOSFET's operation. In normal MOSFETs used in normal fashion, the
substrate is normally short circuited to the source contact. This
connection is really what defines the "source" pin (and consequently the
remaining pin as the drain pin).

For discrete components I would completely agree with you, but if you
are actually "inside" building the transistor the Source and Drain are
in fact interchangeable with respect to odd fingered transistors.
Even fingered transistors are better for abutment when you have
multiple transistors and you want to share the Sources or Drains, but
exhibit slight parasitic differences because of the odd number of
Source to Drain ratio created. Most of the time this difference will
not matter, but in some circumstances, this will create an unbalanced
loading of the circuit that can propagate against your favor.
(snip)