MOSFET or IGFET|What is MOSFET ? | Types

MOSFET : It could be further subdivided as follows:

Static Characteristics of a JFET

We will consider the following two characteristics:

  1. Drain characteristic : It gives relation between ID and VDS for different values of VGS (which is called running variable).
  2. Transfer characteristic : It gives relation between ID and VGS for different values of VDS .

We will analyse these characteristics for an N-channel JFET connected in the common-source mode as shown in Fig. 4. We will first consider the drain characteristic when VGS= 0 and then when VGS has any negative value upto VGS(off) .

Depletion-enhancement MOSFET or DE MOSFET

This MOSFET is so called because it can be op- erated in both depletion mode and enhancement mode by changing the polarity of V When negative gate-to-source voltage is applied, the N-channel DE MOSFET operates in the depletion mode. How- ever, with positive gate voltage, it operates in the enhancement mode. Since a channel exists between drain and source, ID flows even when VGS=0. That is why DE MOSFET is known as normally-ON MOSFET.

MOSFET e1615568938125
MOSFET

Enhancement-only MOSFET

As its name indicates, this MOSFET operates only in the enhancement mode and has no depletion mode. It works with large positive gate voltages only. It dif- fers in construction from the DE MOSFET in that structurally there exists no channel between the drain and source. Hence, it does not conduct when VGS = 0. That is why it is called normally-OFF MOSFET. In a DE MOSFET, ID flows even when VGS = 0. It operates in depletion mode with negative values of VGS . As VGS made more negative, ID decreases till it ceases when VGS= VGS(off) in enhancement mode when VGS positive as shown in Fig. 1 (b).

DE MOSFET 1 e1615609396153
Fig. 1 DE-MOSFET

In the case of E-only MOSFET, ID flows only when VGS exceeds VGS(th) as shown in Fig. 2 (c).

E only MOSFET e1615609564354
Fig. 2 E-Only MOSFET

MOSFET Handling

MOSFETS require very careful handling particularly when out of circuit. In circuit, a MOSFET is as rugged as any other solid-state device of similar construction and size.

It is essential not to permit any stray or static voltage on the gate otherwise the ultra-thin SiO2, layer between the channel and the gate will get ruptured. Since gate-channel junction looks like a capacitor with extremely high resistance, it requires only a few electrons to produce a high voltage across it. Even picking up a MOSFET by its leads can destroy it. Generally, grounding rings are used to short all leads of a MOSFET for avoiding any voltage build up between them.

DE MOSFET e1615570660765
DE-MOSFET

These grounding or shorting rings are removed only after MOSFET is securely wired into the circuit. Sometimes, conducting foam is applied between the leads instead of using shorting rings. As shown in Fig. 3, some MOSFETS have back-to-back Zener diodes internally formed to protect them against stray voltages.

MOSFET Handling
Fig. 3 MOSFET Handling

What is Depletion-enhancement MOSFET or DE MOSFET ?

This MOSFET is so called because it can be op- erated in both depletion mode and enhancement mode by changing the polarity of V When negative gate-to-source voltage is applied, the N-channel DE MOSFET operates in the depletion mode.

Read more

What is Enhancement-only MOSFET ?

As its name indicates, this MOSFET operates only in the enhancement mode and has no depletion mode. It works with large positive gate voltages only. It dif- fers in construction from the DE MOSFET in that structurally there exists no channel between the drain and source.

Read more

What is MOSFET Handling ?

MOSFETS require very careful handling particularly when out of circuit. In circuit, a MOSFET is as rugged as any other solid-state device of similar construction and size.

Read more

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