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Effective Mosfet and bjt resistances

In simulating some mofsets I have to bring the gate ~13 volts below the source for it to have a resistance of about 1M. With a bjt it is seems to be much easier to do so. This is in simulation and I'm not sure if it's a modeling issue or what.

vcc | R1 | Q | R2 | gnd

Now bias Q's gate or base using a fixed voltage V. With a bjt the voltage across R2 is simply V - Vbe regardless of what value R2 has.

If Q is a mosfet the source follower configuration will allow the source to rise above the gate if R2 is large. The larger R2 gets the higher the voltage across R2 becomes. The bjt does not have this issue. Surely because of the diode action of the base.

The way I understand this issue is that for a mosfet, it has to turn off more, it's effective resistance has to increase, to compete with R2. To do this the gate must be pulled further and further below the source as R2 rises.

The question I have is what specification on a datasheet gives information about how effective the mosfet will cut off with Vgs = 0?

Do I simply have to compute V_DS/IDSS? For the 2N7002 IDSS = 1uA @ 48V ==> 48/1uA or 48M?

What I'm looking for is the the off resistance of a mosfet. We are all familiar with the on resistance but I would like some way to know the off resistance. Also how it functions for different I_DS, V_DS, and V_GS.

Reply to
James Brown
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Tilt.

BJTs and MOSFETs are three terminal devices. You're only showing two.

I don't think you're going to find that on a data sheet. That I_DSS parameter is probably a combination of 'normal' current through the channel, which (if the device behavior holds) is going to be constant, the leakage through some parallel conductive path (which is going to be proportional to voltage), and possibly some reverse diode leakage.

I think the answer to your question then is "poorly quantized, variable from device to device, not terribly linear, and undependable".

--
www.wescottdesign.com
Reply to
Tim Wescott

See my post of July 25. I measured a 2N7000 as having Idss around a pA.

But it's nonlinear, not really resistive. You can always divide a voltage by a current and compute a resistance, but it doesn't mean the part behaves like a resistor.

John

Reply to
John Larkin

The datasheet

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give IDSS as the current you seek. Maximum 1uA at 25 degrees and 1mA at 125 degrees MAXIMUM. The datasheet normally hides the typical and minimum IDSS by practicing safe specs. The channel length affects the off current. 1um length channels in n-channel enhancement mode MOSFETs have less off current than 0.045um channel length n-channel enhancement mode metal oxide semiconductor field effect transistors. Temperature affects the IDSS. What is the channel length of your device?

High voltages give more IDSS. Use VDS at 0.1 volt to find a minimal IDSS at -5 degrees C. Then pico amps are seen.

Reply to
Globemaker

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