MOSFET Structure and Threshold: Voltage Opens a Channel
Raise the gate voltage to open the channel
At the very top is the gate, the thin band beneath it is the insulating oxide, below that is the p-type body, and the source and drain sit on either side. Raise the gate voltage and, below the threshold Vth, nothing gathers at the surface; above it, electrons collect at the surface and a gold channel joining the source and drain appears. Find the voltage at which the channel first shows up.
An insulated gate, field not current
A MOSFET's gate sits on a thin oxide insulating layer. Because the insulator blocks it, practically no DC current flows into the gate, the exact opposite of a BJT, which needed current into its base to work. The gate works with an electric field instead of current. With almost no current going in, its input impedance is very high, so it puts almost no burden on 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 the surface. Apply a positive voltage to the gate and its field pushes holes away and pulls the minority carriers, electrons, toward the surface. At low voltage too few electrons are pulled in, but past a certain threshold enough gather at the surface to form a thin n-type layer, the inversion layer. This layer becomes the channel joining 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 body doping and the materials. The more Vgs exceeds Vth, that is, the larger the overdrive Vgs - Vth, the more electrons are pulled in, the thicker the channel and the larger the current it can carry. This switch, which opens and closes a channel with only a voltage on an insulated gate that draws almost no current, underpins digital chips and power conversion.
Back to the first screen
While the gate voltage was below the threshold, the surface was empty and the source and drain were disconnected. The moment you raised the voltage past the threshold, electrons were drawn to the surface and a gold channel joined the two n-type regions. What opened that path was not current flowing into the gate but the gate's field acting through the insulator. The threshold is the boundary where the channel just forms, and the overdrive raised beyond it thickens the channel. This switch, opening and closing the flow with voltage alone while drawing almost no current, is where the MOSFET begins.
What comes next
Past the threshold, the channel is open. But how much actually flows once the channel exists also depends on the drain voltage. In the next unit you raise the drain voltage and see how the operation divides: the linear region, where the channel behaves like a resistor, and the saturation region, where the channel narrows at its end and the current stops growing.