6.9 Ae lecture 8_part a_mosfet & Jfet continued

AE_Lecture 8_MOSFET&JFET_Part A continued.

In Part A of Lecture 8 we have seen the quantitative formulation of Enhancement and Depletion type MOSFET in Triode as well as in Pentode region. In this lecture, the continuation of Lecture 8_Part A, we will study the quantitative formulation of nJFET in ohmic and saturation region also known as triode and pentode region respectively.

Enhancement Type MOSFET was termed as Normally-Off device since with Gate-Voltage zero we have no current.

Similarly Depletion Type MOSFET is termed as Normally-On device since here even under Gate Voltage zero we do have a drain current because in NMOS(D), n-channel has been ion-implanted at fabrication stage hence it conducts at zero Gate Voltage.

In exactly the same fashion, JFET has a conducting channel at Gate Voltage zero. Hence it conducts under zero Gate Voltage therefore it is called Normally – On device. It is a depletion-mode device.

The physical structure of nJFET in a simplified manner is given in Figure 1.

Figure 1. Physical structure of nJFET is given for negligible Drain-to-Source biasing and significant Drain-to-Source biasing respectively.

For negligible Drain-to-Source biasing, we have uniform n-channel as shown in the upper diagram. In this biasing condition, nJFET is operating in ohmic region or in Triode region.

For V _DS >0.1 V, we have trapezoidal n-channelas shown in the lower diagram. The device is still in Triode region but its ohmic behavior is non-linear.

When V _DS = (V _GS – V _P ) then the n-channel gets pinched off near the Drain and current saturates at that point. Now the device has entered saturation region. For V _DS >(V _GS – V _P ), the current remains constant and device is said to be in Pentode region.

In Ohmic Region:

i _D = K _n [ 2(v _GS – V _P ) v _DS – v _DS ² ] for 0<v _DS ≤ ((v _GS – V _P )……………………….1

For small values of v _DS ,

i _D = K _n [ 2(v _GS – V _P ) v _DS ]……………………………………………………………2

At v _DS ^* = (V _GS – V _P ), device enters the saturation region since channel is pinched off near the drain.

In saturation region, irrespective of v _DS current becomes constant at the value decided by v _DS ^* = (V _GS – V _P ). Substituting v _DS ^* = (V _GS – V _P ) in Equation 1, we get:

I _D = K _n (V _GS – V _P ) ² for v _DS ≥ v _DS ^* …………………………………………3

At V _GS = 0, we have I _D = K _n ( -V _P ) ² = this denoted by I _DSS ;

Hence

Typical values of Pinch-off voltage V _P = 0 to -25V and typical values of I _DSS is from 10µA to 10A.

For a given value of I _D , Equation 3 gives two values of V _GS . Only one value is physically tenable. It must lie in the range:

……………………………….V _P <V _GS <0

The line of demarcation or the pinch-off locus is a parabola. By substituting

v _DS ^* = (V _GS – V _P ) in Equation 3 we get:

I _D = K _n (V _GS – V _P ) ² = I _D = K _n (V _DS ^* ) ² which is a parabola and is defined as pinch off parabola.

Figure 2. IC version of JFET.