MOSFET Intro

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) allows us to control current flow using a voltage.

When $V_{GS}>V_t$, the current $I_D$ begins to flow (see diagram with graph below). $I_D$ is then proportional to $V_{OV}^2$

Clarifying:

• $V_{GS}$ is gate voltage with respect to the source voltage. We just assume the source is grounded.
• $I_D$ is the drain current that is being output.
• $V_t$ is a threshold voltage.
• $V_{OV}$ refers to over voltage, such that $V_{OV}=V_{GS}-V_t$
• $V_{DS}$ is the voltage difference between the drain and source terminals of the transistor.

Basically, $V_{DS}$ is a hose pressure being squeezed. Higher $V_{DS}$ means higher $I_D$.

Simplified Model

In MTE 220, we use this simplified model:

• If $V_{GS}<V_F$: OFF switch
• If $V_{GS}>V_F$: ON switch

We may have to model the fact that there is some resistance in the exit pathway, so we add a resistance RDS on.

Nozzle analogy: