MOSFET gate current... where does it go?

[Tomás Ó hÉilidhe]

Consider an NMOS transistor which is turned off when there's 0 V on
the gate, and turned on when there's 5 V on the gate.

I've been told that a tiny bit of current flows into the gate of a
MOSFET transistor... well where does this current go? Does it come out
the drain, or the source, or does it split between the two?

 

[ian field]

Consider an NMOS transistor which is turned off when there's 0 V on
the gate, and turned on when there's 5 V on the gate.

I've been told that a tiny bit of current flows into the gate of a
MOSFET transistor... well where does this current go? Does it come out
the drain, or the source, or does it split between the two?

The gate has a significant capacitance, any current that flows is either
charging or discharging that capacitance.

 

[Eeyore]

Consider an NMOS transistor which is turned off when there's 0 V on
the gate, and turned on when there's 5 V on the gate.

I've been told that a tiny bit of current flows into the gate of a
MOSFET transistor... well where does this current go? Does it come out
the drain, or the source, or does it split between the two?

It comes back out exactly where it went in. Up the gate pin. You have to
both charge and discharge the gate.

Graham

 

[Jim Adney]

The gate has a significant capacitance, any current that flows is either
charging or discharging that capacitance.

To add a bit of clarification:

The gate capacitance is between gate and source, so, while you're
charging that capacitance, the charging current is going in the gate
and coming out the source. Discharge is the reverse.

The actual electrons are going in the gate while charging, and back
out the gate on discharge.

-

 

[legg]

Consider an NMOS transistor which is turned off when there's 0 V on
the gate, and turned on when there's 5 V on the gate.

I've been told that a tiny bit of current flows into the gate of a
MOSFET transistor... well where does this current go? Does it come out
the drain, or the source, or does it split between the two?

The current is needed to form/discharge voltages on the circuit
capacitances.

Current i = dQ/dt Amps, delta-coulombs delta-seconds
Charge q = CV Coulombs Farads Volts

There is also a leakage current, very small, that bleeds through
imperfect insulation in the capacitive structures.

Other losses, in series resistance, will affect energy transfer.

RL