mosfet

[ashish saini]

how a mosfet will behave if we ground its source and gate.

 

[MrAl]

You mean internally or externally?

 

[alec_t]

Welcome to ETO!
What are you doing with the drain while the gate and source are grounded?
Is the MOSFET an enhancement mode type or a depletion mode type?

 

[MrAl]

Hi again,

Ha ha, i took the word 'ground' to mean 'grind' as in changing the physical size of the internal structure (MOSFET characteristics are based a lot on the physical size internally) or else grind the leads off (ha ha).

To take the word 'ground' as in connecting the gate to ground, if it is an N type it will turn off, if it is a P type it will turn on, if the voltages are within spec. If it is a regular FET it might act as a current source.

 

[ronsimpson]

Connect Gate and Source to ground:
The MOSFET will act like a diode!
Dose not matter if (P) or (N).
That is the short answer. There is a long answer.

 

[SPDCHK]

Have a look at Julian Ilett's Youtube tutorial.

 

[crutschow]

................
To take the word 'ground' as in connecting the gate to ground, if it is an N type it will turn off, if it is a P type it will turn on, if the voltages are within spec. If it is a regular FET it might act as a current source.

The OP stated both the gate and source were grounded.
For an enhancement mode MOSFET (by far the most common) the transistor will be off under those conditions for either the N-type or the P-type.

 

[MrAl]

The OP stated both the gate and source were grounded.
For an enhancement mode MOSFET (by far the most common) the transistor will be off under those conditions for either the N-type or the P-type.

Hello,

Strictly speaking you are correct, however i had said only the gate was grounded in my examples
I was thinking that if the transistors were connected as usually connected in a circuit with one supply, then my conclusion would be true.
But yes, we could also have a negative supply for the P channel and that would mean grounding both the gate and source would be reasonable and would keep it OFF indefinitely.

 

[MikeMl]

how a mosfet will behave if we ground its source and gate.

Power MosFets have a body diode: Drain currents [y-axis] vs sweep of V(x) [x-axis].

 

[simonbramble]

unless... over temperature... there will be a small leakage current between drain and source if the gate is connected to the source (ie the FET is supposedly OFF). This leakage current will get worse over temperature, so the open circuit that you think is between the drain and source starts looking like a high resistance (that gets lower as temperature increases). 2N7002's are particularly bad for this.

The above description is for an enhancement type N and P channel device

 

[ronsimpson]

2N7002

I looked at the data sheet from NXP.
At 60V @ 25C the max leakage current is 1uA,
At 60V @ 150C the max current is 10uA, The typical is much much smaller.

There is yet another effect. At about 20% over voltage the D-S looks like a Zenner diode. It will conduct.

The question was; what does the D-S look like when G-S is a short. Answer; diode.

 

[simonbramble]

Definitely a diode in one direction and theoretically an open circuit in the other direction (like a diode), but dont forget the leakage current which was the point I was making.

The NXP device is OK, but now have a look at the Fairchild device. 1mA leakage current at hot. Diodes.com and ON Semi are similar - 500uA at hot.

The OP was asking what a MOSFET looks like when we ground the source and gate. It should be an open circuit. However, if he has a 1MEG pullup resistor on the drain (which is not unreasonable), he might not find the drain up at Vcc when the FET is off.

I agree about the avalanche (Zener) effect