D1 · MOSFET

MOSFET Structure and Threshold: Voltage Opens a Channel

Where the BJT was governed by current, the MOSFET opens a channel with the gate voltage alone. Raise the gate voltage yourself and see at which threshold a path linking source and drain appears.

Raise the gate voltage to wake the channel

On top is the gate, and the thin band beneath it is the insulating oxide. Raise the gate voltage Vgs. Below a threshold Vth there is nothing at the surface, but above it electrons gather at the surface to form a golden channel linking source and drain.

Gate-source voltage VgsVgs = 0.3 V

Channel state

Vgs = 0.3 V   (Vth = 1.0 V)

Vov = Vgs - Vth = 0.0 V

Vgs is below the threshold. The gate field is too weak to gather electrons at the surface, so source and drain stay disconnected.

No channel · off

An insulated gate, field not current

The gate of a MOSFET rests on a thin oxide insulating layer. Because the insulator blocks it, essentially no current flows into the gate. This is the very opposite of the BJT, which needed a base current to work. The gate works by its electric field instead of current. Since almost no current enters, the input impedance is very large and it barely burdens the signal source.

Cross the threshold and a channel forms

The body under the gate is p-type, so there are almost no electrons at its surface. Put a positive voltage on the gate and its field pushes holes away and pulls electrons, the minority carriers, toward the surface. At low voltage too few are drawn, but past a threshold enough electrons gather at the surface to form a thin n-type layer, an inversion layer. This layer becomes the channel linking the n-type source and drain on either side, and only then does a path for current open.

Threshold voltage Vth and overdrive

The gate voltage at which the channel just begins to form is the threshold voltage Vth. Its value is set by the oxide thickness, the doping, and the materials. The more Vgs exceeds Vth, that is, the larger the overdrive Vgs - Vth, the more electrons are drawn in, the thicker the channel, and the more current it can carry. This switch, opening and closing a channel with voltage alone through an insulated gate that draws almost no current, is why MOSFETs underpin digital chips and power conversion.

ObserveVov = Vgs - Vth

The overdrive is the gate voltage in excess of the threshold.

ChooseQch ∝ Cox (Vgs - ?)

The channel charge is proportional only to the excess over threshold.

Fill inIg ≈ ?

With an insulated gate, the gate current is almost zero.

On your ownVgs > ?

The channel conducts only past this condition.

Back to the first screen

When the gate voltage was below threshold, the surface was empty and source and drain were disconnected. The moment you raised the voltage past the threshold, electrons were drawn to the surface and a golden channel linked the two n-type regions. What opened that path was not a current into the gate but the gate’s electric field acting across the insulator. The threshold is the boundary where the channel just appears, and the overdrive raised above it thickens the channel. This switch, opening and closing a flow with voltage alone without drinking a drop of current, is the starting point of the MOSFET.

A MOSFET is a transistor controlled by voltage, not current. Its gate rests on a thin insulating oxide, so almost no current flows in and the input impedance is very large. When the gate-source voltage Vgs exceeds the threshold voltage Vth, the gate field pulls minority carriers to the surface to form a channel (inversion layer) linking source and drain. Below Vth there is no channel and it is off; above, it conducts. The excess overdrive Vgs − Vth sets the strength of the channel.

What comes next

You crossed the threshold and opened the channel. But how much actually flows once the channel exists also depends on the drain voltage. The next unit raises the drain voltage and sees how the behavior splits: a linear region where the channel acts like a resistor, and a saturation region where the channel end pinches and the current stops rising.