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MULUNGUSHI UNIVERSITY

Pursing the frontiers of knowledge

CENTRE FOR ICT EDUCATION

ICT 241 Digital Design

Assignment type: Lab 2

Fundamentals of CMOS Circuits: CMOS Logic

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Transistors are electrically controlled switches that turn ON or OFF when a

voltage or current is applied to a control terminal. The two main types of

transistors are bipolar transistors and metal-oxide-semiconductor field effect

transistors (MOSFETs or MOS transistors, pronounced “moss-fets” or “M-O-S”,

respectively). MOSFETs are now the building blocks of almost all digital systems.

A capacitor consists of two conductors separated by an insulator. When a

voltage V is applied to one of the conductors, the conductor accumulates electric

charge Q and the other conductor accumulates the opposite charge -Q. The

capacitance C of the capacitor is the ratio of charge to voltage: C = Q/V. The

capacitance is proportional to the size of the conductors and inversely

proportional the distance between them.

nMOS and pMOS Transistors

A MOSFET is a sandwich of several layers of conducting and insulating

materials. The MOSFET sandwich consists of a conducting layer called the gate on

top of an insulating layer of silicon dioxide (SiO2) on top of the silicon wafer, called

the substrate. There are two flavors of MOSFETs: nMOS and pMOS (pronounced

“n-moss” and “p-moss”). The n-type transistors, called nMOS, have regions of n-

type dopants adjacent to the gate called the source and the drain and are built on

a p-type semiconductor substrate. The pMOS transistors are just the opposite,

consisting of p-type source and drain regions in an n-type substrate.

The diagram below shows the two types of MOSFETS: the nMOS and the

pMOS, and below them are their basic schematic representations.

Fig.1. nMOS and pMOS transistors

TASK 1: CMOS NOT gate

Objective: To understand the internals of digital inverters based on CMOS.

i. Using Logisim, draw the schematic of a NOT gate based on CMOS

logic as shown in the diagram below.

Fig.2. CMOS based NOT gate.

ii. Take a screenshot of the schematic drawn in Logisim and compute a

truth table for your circuit, giving an explanation on how every

output is achieved.

TASK 2: CMOS NAND gate

Objective: To understand the internals of digital NAND based on CMOS.

i. Using Logisim, draw the schematic of a NAND gate based on CMOS

logic as shown in the diagram below.

Fig.3. CMOS based NAND

iii. Take a screenshot of the schematic drawn in Logisim and compute a

truth table for your circuit, giving an explanation on how every

output is achieved.

TASK 3: CMOS NOR gate

Objective: To understand the internals of digital NOR based on CMOS.

i. Using Logisim, draw the schematic of a NOR gate based on CMOS

logic as shown in the diagram below.

Fig.4. CMOS based NOR

ii. Take a screenshot of the schematic drawn in Logisim and compute a

truth table for your circuit, giving an explanation on how every

output is achieved.

TASK 4: Compound Logic

Objective: To understand the internals of compound Logic.

i. Using Logisim, draw the schematic of the compound logic circuit

shown below.

Fig.5. Compound Logic

ii. Take a screenshot of the schematic drawn in Logisim and compute a

truth table for your circuit, giving an explanation on how every

output is achieved.

Give a detailed explanation of what compound logic is and its applications. Give

references/citation where needed.

TASK 5: XOR

Objective: To understand the internals CMOS based XOR.

i. Using Logisim, draw the schematic of the XOR circuit shown below.

Fig.6. XOR

ii. Take a screenshot of the schematic drawn in Logisim and compute a

truth table for your circuit, giving an explanation on how a couple of

outputs are achieved.

Present a standard report and give a final conclusion in relation to the work done

in this lab work.