9.3 Sspd_chapter 7_part 3_basic electrical properties of mos and  (Page 2/3)

All the above cases are Ohmic or Triode Region of operation of (D) n-channel MOS.

7.3.2. Saturation or Pentode Region of operation of (D) n-channel MOS.

For a given V _GS , as V _DS is increased a point comes when (V _DS -V _GS ) = V _P . At this point the channel gets pinched off on the Drain-side and n-channel becomes conical as shown in Figure 7.3.6. At this point let V _DS = V _DS *. Now the current I _DS saturates.

Now if we increase V _DS = V _DS * +ΔV, the apex of the conical channel becomes more pinched off and the incremental voltage ΔV drops across the pinched off region between the apex of the cone and the drain. The voltage drop across the conical channel remains fixed at V _DS * and the resistance of the conical channel is also constant at 1/3 of the full value of the parallaelopied channel resistance. Thus the drain current saturates at :

Where R ₀ = the resistance of the parallaelopied channel.

The saturation region of operation or Pentode Region of operation is shown in Figure 7.3.7.

All these arguments hold good for Enhancement Type MOSFETS also except that (E)nMOS is normally-off device and (D)nMOS is normally – on device. In Enhancement type NMOS, positive voltage has to be applied to the gate and only when Gate Voltage has exceeded the Threshold Voltage(V _Th ) that the n-channel is induced through inversion. In contrast in Depletion type NMOS, n-channel is already present there. Gate Voltage can be negative in which case the n-channel will get depleted and ultimately it will be pinched-off. Gate Voltage can be positive . In this case channel will beome more conductive.

(D)nMOS gives a greater flexibility of design hence it more preferred device.

The Transfer Characteristics of (E)NMOS and (D)NMOS are shown in Figure 7.3.8. As can be seen from the Graph in Figure 7.3.8, (E)NMOS originates at V _Th whereas (D)NMOS originates at pinch-off voltage V _P . Both these transfer characteristics have been plotted in the Pentode region of the Output Characteristics. The saturation current of (D)NMOS at V _GS = 0V is referred to as I _DSS as it is done in JFET.

7.3.3.Theoretical Formulation of Output family of Curves of NMOS.

Just as in BJT,

Where Q _B = charge stored in Base and τ _t = transit time in the Base.

Similarly

Transit time has the following formulation:

Drift Velocity of electron in the channel is as follows:

Where µ _n = 2D mobility of electron in the channel because the channel behaves like a sheet and not like a Bulk.

E _ds = Electric field from the Drain to Source

Substituting Eq.7.3.6 in Eq 7.3.5 which in turn is substituted in Eq.7.3.4. we get:

This is analgous to the expression we get for the transit time of electron in NPN BJT:

Here W = effective Base width, µ _n = Bulk mobility of electron and V _Th = thermal voltage at room temperature = kT/q;

7.3.3.1. Output Curve in Triode Region(non-saturated region).

Figure 7.3.10 gives the 3-D view of NMOS transistor structure and it indicates the symbol used for the width, length of the channel anf for gate oxide thickness. According to the Figure: