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ELEC 212

SECOND SEMESTER EXAMINATIONS 2021-22

CMOS INTEGRATED CIRCUITS

Answer ALL three questions.

The use of a calculator IS allowed.

1.   a)    Describe the meaning of ‘direct ’ and ‘indirect ’ semiconductors. State the typical semiconductors that belong to each group. 6

b)   Derive and illustrate with the aid of a diagram, the kinetic energy versus wavevector for a free electron. 7

c)    State the mass-action law for semiconductors and explain the significance of each term. 5

d)   The conductivity of extrinsic p-type silicon is designed to be 150 (am)- 1.

The mobility of holes is 0.05 m2 / Vs.

i)         Calculate the doping concentration of extrinsic dopant ions to satisfy this requirement. 4

ii)        State one possible dopant for p-type semiconductor. 1

iii)       Calculate the position of the Fermi level (in eV) from the valence band edge for the semiconductor in i). 4

e)    Describe what needs to be added to the p-type silicon semiconductor in d) to fabricate a MOS capacitor. Explain the surface field effect. 5

f)    The MOS capacitor is designed to have a threshold voltage, VT  = 0.8 V.

Calculate the gate oxide thickness for the MOS capacitor to satisfy the required threshold voltage. The doping concentration of a p-type substrate is 2 根 1022 m-3. Assume an ideal MOS capacitor. 8

Total

40

2.   a)    Develop an expression for the current flowing in the channel of an n-MOSFET in terms of the gate (VG), drain (VD) and threshold (VT) voltages, in the linear region: 10

ID = μCo [(VG VT )VD ] ,

where the symbols have their usual meanings.

b)   Define the device constant in the equation in a). What parameters can the designer vary and why? 3

c)    State  the  difference between the  enhancement  and the depletion types  of MOSFET. Sketch the symbols and transfer characteristics of these two types of devices for p- and n-channel transistors. 5

d)   The n-MOSFET  in  a)  is  used to  fabricate  a  complementary  metal  oxide semiconductor (CMOS) inverter. Design its aspect ratio (W/L)n  to obtain a symmetric CMOS inverter. Themobilities of electrons and holes are μn = 545 cm2/Vs and μp  = 130 cm2/Vs respectively, the threshold voltages for p- and n-MOSFETs equal to VTn  = |VTp | = 0.8 V, VDD  = 4 V, and the aspect ratio for p-MOSFET (W/L)p = 8. 10

e)    State the two key parameters, which govern the speed of CMOS inverters. 2

Total

30

3.   a)    Explain briefly the operation of a CMOS inverter. State its main advantages and disadvantages. 6

b)    Sketch the transfer characteristic of aCMOS inverter and illustrate the regimes of operation of referring n- and p-MOSFET transistors. 4

c)    Draw the  circuit diagram  of a two-input NOR gate, fabricated in  CMOS technology. Describe the principles involved in its design. 8

d)   Two-input NOR and two-input NAND gates are to be designed for a process technology with the gate length L = 0.5 μm. The aspect ratio (W/L) of a minimum-size symmetric inverter is W/L = 2.

i)         What is the width/length (W/L) of n- and p-MOSFETs required in the design? 8

ii)        Comment  on the  issues concerning the use of NOR versus NAND gates in CMOS integrated circuit design. 4

Total

30