Can MOSFET in sub-threshold work as a switch?

Hello All,

As the title stated, Can MOSFET in sub-threshold work as a switch?

I looked at EKV model and I found only one model for the MOSFET where the drain current is exponential of Vgs with of course other parameters. So, I guess we can't bias the mosfet working in sub-threshold to work as a switch, on-off, i.e there is no triod region in sub-threshold?

Yes. That region is right before it starts to act as a wire (with very small resistance, depending on the model, obviously).

The answer to your question vary in order to what you really want. If you want a real switch, then you will saturate the transistor (working with voltages above threshold voltage). But actually it is possible to make a sort of switch by not saturating the transistor... it will have a low output resistance (depends of the model and the voltage you are working at... there's a formula for RDSon) that can work or not with your circuit... whenever the "sort of switch" doesn't have such a high load that could affect the output signal (voltage divider).

What do you want to switch? can you post any circuit? Are you sure you can only use MOSFET instead BJT(lower threshold voltage)?

Hope being useful, feel free to ask again.

Subthresold lies between "on" (well above threshold)
and "off" (well below). The same FET type used for
subthreshold circuit design could be used as a switch
if you drove it right. But that wants (for "on") more than
what you're running subthreshold logic from, supply, if
you want a very low on resistance.

If the other impedances are very high then maybe the
FET will be sufficiently "switch-like", but this comes down
to cases.

BrunoARG said:

Yes. That region is right before it starts to act as a wire (with very small resistance, depending on the model, obviously).

The answer to your question vary in order to what you really want. If you want a real switch, then you will saturate the transistor (working with voltages above threshold voltage). But actually it is possible to make a sort of switch by not saturating the transistor... it will have a low output resistance (depends of the model and the voltage you are working at... there's a formula for RDSon) that can work or not with your circuit... whenever the "sort of switch" doesn't have such a high load that could affect the output signal (voltage divider).

What do you want to switch? can you post any circuit? Are you sure you can only use MOSFET instead BJT(lower threshold voltage)?

Hope being useful, feel free to ask again.

Click to expand...

I guess you mean very high resistance, right? because it seems that Ids in subthreshold is independent of Vds and that is make g_ds=0, i.e r_ds=infinity. This is was my concern, it seems that MOS in subthreshold looks like the worst switch, it has very high channel resistance.

I attached here the circuit I want to use. Voltage doubler, doubling the voltage from 300mV to 600mV through MOS Cap, the problem, I want to have multiple douplers to get 1.8V ( 3 cascaded voltage douplers )
so in each stage, I need high gate voltage, higher than the drain+Vthreshold, to have my transistor switch on perfectly. This makes the last transistor for example needs Vg=2V which is something I don't have. I have only 1V clock voltage signal. So, I though of biasing the transistor in subthreshold. It seems also impractical solution because of high resistance as I mentioned earlier. What do you think guys?

Question #1 - is your 300mV made from a higher (pin)
supply resource? Or it instead, the only power source
that the chip sees? If the former then maybe you only
need a crude low power on-chip regulator to make an
"above the rail" (as the subthreshold logic sees it)
supply for switch gate drive.

0.3V is low for anything but dimly lit solar, single cell
photo-powered, or "energy harvesting" sources. Any
chemical battery would provide you at least 1V and
usually more.

You might also, if using low voltage FETs (or zero-VT)
consider a resonant gate drive capable of "ringing up"
the gate voltage usefully. But this will eat some serious
chip area at any frequency where RF drive losses don't
claim more than they offer.

Yes. It is actually coming from very small pv cell and it is the only source. I want to boost it to 1.5v.

I checked the zero-Vt transistors and it seems not useful in my design and also it costs more money to fabricate.

I like the idea of resonance gate drive, will definitely look at it. I hope the inductor area requirement is not as big as the required inductor in regular boost.

I saw people using boost converters instead of the voltage doubler, that is ok fr discrete solution but for my case it is practical.

Interesting.

I recently ran across, on LinkedIn, an outfit making
"photo-pile" cells for delivering power across fiber optic
cable from a laser diode source. They demonstrated
pretty high efficiency (at least, from optical power
in to voltaic power out - laser diode efficiency is
Somebody Else's Problem) and a range of voltages
by series-connecting cells. They used a GaAs photo-
diode which gives them a higher output voltage per
cell than silicon.

Now two things occur to me here. One is, stacking
of photodiodes. Not, perhaps, practical in a junction
isolated (cheap) technology but not, depending on
details of well depth vs absorption length, out of the
question. Entirely practical in a SOI technology (like
TowerJazz CA18HB, a 1.8V/5V PDSOI with a ~1.5um
film thickness, - and this is only one example). You
could stack (electrically) any number of diodes you
like, provided you can optically couple to them all.
OnSemi has a neat backside-illuminated imager flow
that they run at Gresham, a 110-nm (if I recall)
CMOS technology that they thin post-fab, used for
visible-range star tracker applications on satellites
(and perhaps other platforms). Again it's an SOI
but now without a "handle" , very thin, and the
ability to throw the light in from the back gives you
a free hand with frontside interconnect, zero concern
for shadowing and so on.

The other branch of my thought is, using (as I mentioned
in the beginning) a GaAs photodiode would give you from
a single cell, a higher voltage and likely a higher efficiency
(hence higher available power) from the cell(s). If the goal
of this project includes that the power receiver be
homogeneously (silicon) integrated, perhaps that precludes
such options. But if heterogeneous (e.g. 2.5D chip stacking)
is acceptable maybe you can improve the power picture
(and even do it with zero chip area impact - receiver sits
atop the silicon chip, maybe within the I/O pad ring, as
opposed to consuming mucho silicon area for the optical
input region). There exist commercial GaAs PIN diodes that
are optimized for fiber optic receiver use - and some with
"multi-lane" (like 4x1) fiber cables, which if the connections
were not common-anode might then be "stackable" too -
point being this is commercial, cost-squeezed stuff that may
be already flip-chip compatible, cheap, multi-vendor-sourced
and available to you.

dick_freebird said:

Interesting.

I recently ran across, on LinkedIn, an outfit making
"photo-pile" cells for delivering power across fiber optic
cable from a laser diode source. They demonstrated
pretty high efficiency (at least, from optical power
in to voltaic power out - laser diode efficiency is
Somebody Else's Problem) and a range of voltages
by series-connecting cells. They used a GaAs photo-
diode which gives them a higher output voltage per
cell than silicon.
.

Click to expand...

Interesting. would you please share the link to their page?

regarding the photo-diodes I want to use, they are not stackable because most likely I'll be just using regular 180nm CMOS technology. so I'm kind of stuck to the 300mV-700mV output voltage from photo-diode, I can connect more in parallel though to provide higher current.