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 I_D and V_DS for different values of V_GS (which is called running variable).
2. Transfer characteristic : It gives relation between I_D and V_GS for different values of V_DS .

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 V_GS= 0 and then when V_GS has any negative value upto V_{GS(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, I_D flows even when V_GS=0. That is why DE MOSFET is known as normally-ON 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 V_GS = 0. That is why it is called normally-OFF MOSFET. In a DE MOSFET, I_D flows even when V_GS = 0. It operates in depletion mode with negative values of V_GS . As V_GS made more negative, I_D decreases till it ceases when V_GS= V_GS(off) in enhancement mode when V_GS positive as shown in Fig. 1 (b).

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

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.

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.