Mumbai University > Electronics ana telecommunication engineering > Sem 3 > Analog electronics 1

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Construction of n- channel Enhancement MOSFET:

The fig shows the basic construction of n-channel enhancement type MOSFET.

Two highly doped n regions are diffused into a lightly doped p type substrate. The source and drain are taken out through metallic contacts to n- doped regions as shown in fig2.5. But the channel between two n-regions is absent in the enhancement type MOSFET.

The $SiO_2$ layer is still present to isolate the gate metallic platform from the region between the drain and source, but now it is simply separated from a section of the p-type material.

For characteristics, transfer characteristics and drain characteristics Operation and characteristics of n-channel enhancement MOSFET:

On application of drain to source voltage V_DS and keeping gate to source voltage zero by directly connecting gate terminal to the source terminal, practically zero current flows-quite different from the depletion type MOSFET and JFET.

If we increase magnitude of $V_GS$ in the positive direction, the concentration of electrons near the $SiO_2$ surface increases. At a particular value of $V_GS$ there is a measurable current flow between drain and source. This value of $V_GS$ is called threshold voltage denoted by $V_T$.

Thus we can say that in an enhancement type n-channel MOSFET, a positive gate voltage above a threshold value induces a channel and hence the drain current by creating a thin layer of negative charges in the substrate of negative charges in the substrate region adjacent to the SiO2 layer, as shown in the fig 2.8.

The conductivity of the channel is enhanced by increasing the gate to source voltage and thus pulling more electrons into the channel. For any voltage below the threshold value, there is no channel. Since the channel does not exist with $V_GS$ = 0V and "enhanced" by the application of a positive gate to source voltage, this type of MOSFET is called an enhancement type MOSFET.

Fig. shows the drain characteristics of an n-channel enhancement type MOSFET. Looking at Fig. 2.9 we can say that as $V_GS$ increases beyond the threshold level, the density of free carriers (electrons) in the induced channel increases, increasing the drain current.

However, at some point of $V_DS$, for constant $V_GS$, the drain current reaches a saturation level. The levelling off of $I_D$ is due to a pinch-off process.

Fig below shows the transfer characteristic for n-channel enhancement type MOSFET.

For $V_GS\gtV_T$ the relationship between drain current and $V_GS$ is nonlinear and it is given as

$I_D=K(V_GS-V_T)^2 $

The K term is a constant that is a function of the construction of the device. The value of K can be determined from,

K=$\frac{I_D(ON)}{(V_GS(ON)-V_T)^2}$