Power MOSFET switching; threshold gate voltage

A power MOSFET is a voltage-controlled, majority-carrier switch. Its switching characteristics show how the drain current responds when the gate-source voltage is switched ON and OFF; the delays come from charging and discharging the gate capacitances $C_{gs}$ and $C_{gd}$.

          D
          |
     | |--+
 G --| |     n-channel
     | |--+  power MOSFET
          |  (body diode D->S
          S   not shown)

Switching characteristics

 vGS ^      _______________
     | VGS |               |
     |  VT +-.             | .
     |----'   `            |  `-------
 iD  ^          ____________
     |         /|           |\
     |--------' |           |  `------
     |<tdon>|tr|           |<tdoff>|tf|

• Turn-on delay $t_{d(on)}$: time to charge the input capacitance ($C_{gs}$) from zero to the threshold voltage $V_{GS(th)}$; no drain current flows yet.
• Rise time $t_r$: gate voltage rises from $V_{GS(th)}$ to the value $V_{GSP}$ needed to carry full drain current; $i_D$ rises to its full value and $v_{DS}$ falls (Miller plateau while $C_{gd}$ charges).
• Turn-off delay $t_{d(off)}$: when the gate drive is removed, the input capacitance discharges from $V_{GS}$ down to $V_{GSP}$; $i_D$ is still full.
• Fall time $t_f$: gate voltage falls from $V_{GSP}$ to $V_{GS(th)}$; drain current falls to zero.

$t_{on} = t_{d(on)} + t_r$ and $t_{off} = t_{d(off)} + t_f$. Because the MOSFET is a majority carrier device there is no storage time, so it switches in tens of nanoseconds; the switching speed depends only on how fast the driver can charge and discharge the gate capacitances.

Threshold gate voltage

The threshold gate voltage $V_{GS(th)}$ (or $V_T$) is the minimum gate-source voltage at which an inversion layer (channel) forms under the gate, so drain current starts to flow.

• For $V_{GS} < V_{GS(th)}$ the MOSFET is OFF (only leakage current).
• For $V_{GS} > V_{GS(th)}$ the drain current in saturation is $i_D = K(V_{GS} - V_{GS(th)})^2$.
• Typical power MOSFETs have $V_{GS(th)} = 2$ to 4 V and are driven fully ON with $V_{GS} = 10$ to 15 V to get low $R_{DS(on)}$.
• $V_{GS(th)}$ falls with temperature, so noise immunity is lower when hot; a negative off-bias is sometimes used.