Switching characteristics of MOS transistor

Switching characteristics of MOS transistor

Static characteristics

As a switching element, MOS transistors also operate in two states: off or on. As MOS transistors are voltage controlled components, their operating state is mainly determined by the gate source voltage uGS.

The working characteristics are as follows:

UGS turn-on voltage UT: The MOS transistor operates in the cutoff region, the leakage source current iDS is basically 0, the output voltage uDS ≈ UDD, and the MOS transistor is in the "off" state

UGS>turn-on voltage UT: The MOS transistor operates in the conduction region, and the leakage source current iDS=UDD/(RD+rDS). Among them, rDS is the drain source resistance when the MOS transistor is conducting. Output voltage UDS=UDD • rDS/(RD+rDS), if rDS<RD, then uDS ≈ 0V, and the MOS transistor is in the "on" state

Dynamic characteristics

When MOS transistors switch between conduction and cutoff states, there is also a transition process, but their dynamic characteristics mainly depend on the time required for charging and discharging stray capacitance related to the circuit, while the time for charge accumulation and dissipation during conduction and cutoff of the transistor itself is very small.

When the input voltage ui changes from high to low and the MOS transistor transitions from a conducting state to a cutoff state, the power supply UDD charges the stray capacitor CL through RD, with a charging time constant τ 1=RDCL. Therefore, the output voltage uo needs to go through a certain delay to change from low level to high level. When the input voltage ui changes from low to high and the MOS transistor transitions from the off state to the on state, the charge on the stray capacitor CL is discharged through rDS, and its discharge time constant τ 2 ≈ rDSCL. It can be seen that the output voltage Uo also needs to go through a certain delay before it can transition to a low level. But because rDS is much smaller than RD, the transition time from cutoff to conduction is shorter than the transition time from conduction to cutoff. Due to the fact that the drain source resistance rDS of the MOS transistor is much larger than the saturation resistance rCES of the transistor when it is turned on, and the external resistance RD of the drain is also larger than the collector resistance RC of the transistor, the charging and discharging time of the MOS transistor is longer, resulting in a lower switching speed of the MOS transistor than that of the transistor. However, in CMOS circuits, due to the low resistance of both the charging and discharging circuits, the charging and discharging processes are relatively fast, resulting in a high switching speed of the CMOS circuit.