Question 2 (20 marks) (a) List all four main generations of integrated circuits.

Question

Question 2 (20 marks) (a) List all four main generations of integrated circuits. (4 marks) (b) State the reason why it is necessary to replace SiO2 with (4 marks) material of higher dielectric constant in ultra deep submicron devices. Oxide Polysilicon - well p-substrate Themal oxidsP-dlass Nitide Polysilicon Al p-tub n-tub Tl y-epitay n-substrate Figure 1 The cross sectional area of a single well Complementary Metal Oxide Semiconductor (CMOS) structure is shown in Figure 1(a) and the cross sectional area of a twin well CMOS structure is shown in Figure 1(b). State the reason why twin well design is better than single well design for CMOS devices (4 marks)

Explanation / Answer

Answer (a) - Generation of main IC's are

1- Small scale interation (number of transistors used-1 to 10)

2- Medium scale integration (number of transistors used 10 to 500)

3- Large scale intergration (number of transistors used 500 to 2000)

4- Very large scale integration (number of transistors used 20,000 to 1,00,00,000)

Answer (B)

The thickness of SiO2 layer presently used as the gate dielectric is becoming so thin that the gate leakage current due to direct tunneling of electrons through SiO2 will be so high , exceeding 1A/cm^2 at 1V , the circuit power dissipation will increase upto unaccepted values . In addition it becomes increasing difficult to produce and measure accurately flims of such small thickness . Finally ,the reliability of SiO2 against electrical breakdown declines in thin film . Thus for these three reason ,it is desired to replace SiO2 as a gate oxide.

Answer (C)

Twin well technology is very popular because of the following reason

using twin well technology we can optimize NMOS and PMOS separately. This means that transitor parameter such as threshold volage , body effect and the channel transconductance of both types of transistor can be tuned independently whereas the conventional n-well CMOS suffers from among other effects , the problemof unbalanced drain parasitics since the doping density of well region typically being about one order of magnitude higher than the substrate .This problem is absent in twin -tub process.

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