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?

 

[Eeyore]

Tomás Ó hÉilidhe wrote:

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

 

[David Tweed]

Tomás Ó hÉilidhe wrote:

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 other responders told you what that current was doing, but didn't
answer the question.

Unfortunately, the answer you want is not simple. It really depends
heavily on the surrounding circuit. Usually, the driver that is charging
and discharging the gate is tied in some way to the source pin, so
the circuit loop that carries this current passes through that pin.

However, if the drain of the MOSFET has a low impedance path to the
same supply as the gate driver circuit, some of the current will flow
there, too.

-- Dave Tweed

 

[Jim Adney]

On Sat, 31 May 2008 22:24:19 +0100 "ian field" <dai.ode@ntlworld.com>
wrote:

"Tomás Ó hÉilidhe" <toe@lavabit.com> wrote in message
news:456b168a-4f54-414d-8cf3-e4289864ddfe@s33g2000pri.googlegroups.com...

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.

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.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[legg]

On Sat, 31 May 2008 11:34:01 -0700 (PDT), Tomás Ó hÉilidhe
<toe@lavabit.com> wrote:

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