6.12 Chapter 4_section 4.4._design rules for fabricating njfet.

Chapter 4_Section 4.4_Design Rules for nJFET.

We have already discussed the structure and physics of nJFET in m38382.

We saw that in Triode Region (Ohmic Region):

V _P = Pinch – Off voltage of the channel given by the following expression:

For small drain-source voltages (v _DS <0.1V):

Therefore N-type Channel of nJFET offer linear resistance given by the following expression:

Thus at small voltages Gate Controlled linear resistances can be realized using a JFET.

In Pentode Region, the drain current saturates at V _DS ^* = (V _GS - V _P ) :

At V _GS = 0, i _DS = I _DSS and from Equation 4.4.5:

Here d _n = depletion penetration in N-substrate at zero gate voltage and is given by the following expression:

The customer will specify the pinch-off voltage and I _DSS . In practice these can range from

0 to -25V for Pinch-Off voltage and I _DSS = 10μA to 10A.

From these specifications the fabrication parameters and geometrical dimensions can be determined.

Section 4.4.1.Design nJFET for pinch-off Voltage -1V and -2V.

Keep the Gate P+ doping at 10 ¹⁹ /cc. Keep L=10μm and W=10μm.

Calculate the total thickness ‘t’ for pinch-off voltage at -1V and -2V. Also calculate the penetration of depletion width in the channel at Gate Voltage = 0V. This Data will be required while calculating I _DSS .

Design procedure for V _P = -2V.

Using Eq.4.4.2. for N _D = 10 ¹³ /cc we get t = 18.4μm and using Equation 4.4.7 we get d ₀ =9.47μm.

At N _D = 10 ¹³ /cc the resistivity of the channel is 430.45Ω-cm. Using Equation 4.4.6. we get :

I _DSS = 12.45μA.

If instead of N _D = 10 ¹³ /cc , N _D = 10 ¹⁴ /cc is used then we get t = 5.9μm and using Equation 4.4.7 we get d ₀ =3 μm.

At N _D = 10 ¹⁴ /cc the resistivity of the channel is 43.45Ω-cm. Using Equation 4.4.6. we get :

I _DSS = 40μA.

Design procedure for V _P = -1V.

Using Eq.4.4.2. for N _D = 10 ¹³ /cc we get t = 14.6μm and using Equation 4.4.7 we get d ₀ =9.47μm.

At N _D = 10 ¹³ /cc the resistivity of the channel is 430.45Ω-cm. Using Equation 4.4.6. we get :

I _DSS = 3.47μA.

If instead of N _D = 10 ¹³ /cc , N _D = 10 ¹⁴ /cc is used then we get t = 4.7μm and using Equation 4.4.7 we get d ₀ =3 μm.

At N _D = 10 ¹⁴ /cc the resistivity of the channel is 43.45Ω-cm. Using Equation 4.4.6. we get :

I _DSS = 12μA.