Mosfet measurament question

[sharas]

Hi, Is forcing a specific current between the source and the drain and measuring the voltage at the gate terminal - yield the same result as forcing Vds and sweeping the Gate terminal untill the same current is measured at the drain terminal?

 

[rtarbell]

Theoretically, yes: The two should provide you with the same result.

There is one thing that I can think of:

In your first example, if you are forcing the current through the drain-source of the mosfet, you have to make sure that the Vds is the same as what you force it to in the second example.

In a sense, you have three variables (in terms of DC conditions): Vgs (gate-source voltage), Vds (drain-source voltage), and Id (drain current). If you define any two of them, you should be able to determine the third.

 

[jayc]

Hi, Is forcing a specific current between the source and the drain and measuring the voltage at the gate terminal - yield the same result as forcing Vds and sweeping the Gate terminal untill the same current is measured at the drain terminal?

No, this is not the case.

If you force a current through the source and drain terminal while leaving the gate floating, there is no way to accumulate charge on the gate terminal and it will remain fixed at its initial value. This is surely not what you want.

Remember that when you hold Vds constant and sweep Vgs, you are sweeping the transistor through its three regions of operation. In a simplified ideal transistor model, if Vgs < Vt, you have cutoff. If Vt < Vgs < Vds + Vt, you are in the saturation region. Finally, if Vgs > Vds + Vt, you are in the triode, or linear, region.

Most of the time, we want our transistors to operate in the saturation region. To determine the gate voltage required for a specific current to pass through, we can diode-connect the transitor. That is, connect the gate and drain terminals while passing the current through. This will force the transistor to operate in saturation mode and the gate voltage will be a well-defined function of the drain current.


jayc

 

[sharas]

Hi, Is forcing a specific current between the source and the drain and measuring the voltage at the gate terminal - yield the same result as forcing Vds and sweeping the Gate terminal untill the same current is measured at the drain terminal?

No, this is not the case.

If you force a current through the source and drain terminal while leaving the gate floating, there is no way to accumulate charge on the gate terminal and it will remain fixed at its initial value. This is surely not what you want.

Remember that when you hold Vds constant and sweep Vgs, you are sweeping the transistor through its three regions of operation. In a simplified ideal transistor model, if Vgs < Vt, you have cutoff. If Vt < Vgs < Vds + Vt, you are in the saturation region. Finally, if Vgs > Vds + Vt, you are in the triode, or linear, region.

Most of the time, we want our transistors to operate in the saturation region. To determine the gate voltage required for a specific current to pass through, we can diode-connect the transitor. That is, connect the gate and drain terminals while passing the current through. This will force the transistor to operate in saturation mode and the gate voltage will be a well-defined function of the drain current.


jayc

Thanks, jayc.

Can you explain why charge won't be accumulated on the gate according to Gaus's theorem, if current is flowing between the source and the gate?

 

[jayc]

There is no current flowing between the source and gate. I think you meant to say source and drain.

I am assuming you are talking about running a DC current and measuring the gate voltage. The impedance between the drain/source nodes and the gate is purely imaginary. In particular, it is capacitive, which decouples DC signals. Therefore, if you put a DC signal at the source/drain, the gate cannot see that signal and is electrically isolated.

If you were to actually try running a current through the drain/source terminals while leaving the gate floating, what would likely happen is that as you increase current, the drain voltage will increase linearly at a very high rate depending on the initial gate voltage until the transistor breaks down and you burn up your chip. Meanwhile, since the gate is isolated, it will remain at its initial value.

If this is not clear, please review basic mosfet operation.

Here are a few links to help you out if you need it.
http://en.wikipedia.org/wiki/MOSFET
**broken link removed**

Hope this helps.


jayc