Classification of MOSFET

MOSFET is a very widely used field-effect transistor in both analog and digital circuits. The transistor also became a bipolar transistor, which controlled the flow of current and amplified smaller signals into higher amplitude electrical signals. Both MOSFETMOSFETs and transistors have ON states, so what is the difference between the two when in ON?

    MOSFET and transistors, in the ON state, MOSFETs usually use Rds, and transistors usually use saturated Vce. So is there a situation where the triode is saturated with Rce and the MOSFET with saturated Vds?

    The transistor ON state works in the saturation region, the on-current Ice is mainly determined by Ib and Vce, because the base drive current Ib of the transistor generally cannot be kept constant, so Ice can not be simply determined by Vce alone, that is, it cannot be expressed by saturated Rce (because Rce will change). Since Vce is small in the saturated state, the transistor is generally represented by a saturated Vce.

    MOS transistor works in the linear region (equivalent to the saturation region of the transistor) in the ON state, similar to the transistor, the current Ids is determined by Vgs and Vds, but the driving voltage Vgs of the MOS transistor can generally remain unchanged, so Ids can only be affected by Vds, that is, in the case of Vgs fixed, the on-resistance Rds remains basically unchanged, so the MOS  adopts the Rds method.

    Current can flow bidirectionally through the D and S of the MOSFET, which is the outstanding advantage of the MOSFET, so that there is no concept of DCM in synchronous rectification, and energy can be transferred from input to output, and can also be returned from output to input. It can realize the two-way flow of energy.

    Since the D and S of MOS are interchangeable, why define DS?

    For the MOS transistors inside the IC, they must be completely symmetrical when manufactured, and the purpose of defining D and S is to discuss the current flow direction and facilitate calculations.

    Second, since D and S are defined, what is the difference between them?

    For power MOS, sometimes because of special applications, such as withstand voltage or other purposes, a light doped region withstand voltage is made at the D end of NMOS, and D and S will be different.

    Third, after D and S are swapped, how are the characteristics of MOS different from the original? Examples include Vth, the Maitreya effect, parasitic capacitance, on-resistance, breakdown voltage Vds.

    After DS swapping, when Vgs=0, as long as Vds > 0.7V pipe can also be turned on, but not before replacement. When Vgs > Vth, the reverse layer channel has been formed, and the characteristics of the two are the same after interchange.

    Determination of D and S

    Let's just say that the current can be from D--to--S, and it can also be ---- to---D. But this does not mean that the names of the two terminals D and S are interchangeable.

    The width of the DS channel is controlled by the GS voltage. When G is fixed, who S is is determined.

   If the above is determined to be the S-end, it is considered to be D.

    Thinking that what was originally D is S, and applying a voltage to G and this S, the channel does not change and is still closed.

    When Vgs does not reach Vth, Cgs is charged by driving resistor R, and the model at this stage is a simple RC charging process.