Category: Digital Circuits Objective Questions

Digital Electronics/Circuits Multiple Choice Questions on “MOS Digital Integrated Circuits”.

1. The full form of MOS is ___________
A. Metal oxide semiconductor
B. Metal oxygen semiconductor
C. Metallic oxide semiconductor
D. Metallic oxygen semiconductor
Answer: A
Clarification: The full form of MOS is “Metal Oxide Semiconductor”. It is a type of transistor having 3 layers: a metal conductor, an insulating silicon layer and a semiconductor silicon layer.

2. What are the types of MOSFET devices available?
A. P-type enhancement type MOSFET
B. N-type enhancement type MOSFET
C. Depletion type MOSFET
D. All of the mentioned
Answer: D
Clarification: MOSFET are of two types: enhancement and depletion type. Further, these are classified into n-type and p-type device. The depletion type is switched on without the application of gate bias voltage and the enhancement type is switched on with the application of gate voltage.

3. Which insulating layer used in the fabrication of MOSFET?
A. Aluminium oxide
B. Silicon Nitride
C. Silicon dioxide
D. Aluminium Nitrate
Answer: C
Clarification: Silicon dioxide is used as an insulating layer in the fabrication of MOSFET. It gives an extremely high input resistance in the order of 10¹⁰ to 10¹⁵ Ω for MOSFET.

4. Which of the following plays an important role in improving the device performance of MOSFET?
A. Dielectric constant
B. Threshold voltage
C. Power supply voltage
D. Gate to drain voltage
Answer: B
Clarification: In MOSFET, the threshold voltage is typically 3 to 6V. This large voltage is not compatible with the supply of 5V which is used in digital ICs. So, for the improvement of the device’s performance the magnitude of threshold voltage should be reduced.

5. A technique used to reduce the magnitude of threshold voltage of MOSFET is the ___________
A. Use of complementary MOSFET
B. Use of Silicon nitride
C. Using thin film technology
D. Increasing potential of the channel
Answer: B
Clarification: Silicon nitride is sandwiched between two SiO2 layer and provide necessary barrier. The dielectric constant of Si3N4 is 7.5, whereas that of SiO2 is 4. This increase in overall dielectric constant reduces threshold voltage.

6. What is used to higher the speed of operation in MOSFET fabrication?
A. Ceramic gate
B. Silicon dioxide
C. Silicon nitride
D. Poly silicon gate
Answer: D
Clarification: In conventional metal gate small overlap capacitance is present, which lowers the speed of operation. With the presence of self aligning property of the poly silicon gate it eliminates this capacitance. Using a process called ion-implantation, polysilicon, the drain and the source get doped. However, the thin oxide under silicon gate acting as a mask for the process and thus develops the gate aligning property.

7. Find the sequence of steps involved in fabrication of poly silicon gate MOSFET?

Step 1: Entire wafer surface of a Si3N4 is coated and is etched away with the help of mask to include 
source, gate and drain.
Step 2: The contact areas are defined using photolithographic process.
Step 3: Selective etching of Si3N4 and growth of thin oxide.
Step 4: The deposition of poly silicon gate.
Step 5: The growth of thick oxide is called field oxide and P implantation.
Step 6: The metallization and interconnection between substrate and source.

A. 1->5->3->4->2->6
B. 1->3->4->2->5->6
C. 1->5->4->3->2->6
D. 1->4->2->5->3->6
Answer: A
Clarification: These steps are the sequence of steps involved in fabrication of poly silicon gate MOSFET. With the help of poly silicon gate doping, it highers the speed of operation of the MOSFET.

8. Why MOSFET is preferred over BJT in IC components?
A. MOSFET has low packing density
B. MOSFET has medium packing density
C. MOSFET has high packing density
D. MOSFET has no packing density
Answer: A
Clarification: MOSFET is preferred over BJT because of its low packaging density. Thus, more number of MOSFET memory cells can be accommodated in a particular area as compared to BJT.

9. Critical defects per unit chip area are ________ for a MOS transistor.
A. High
B. Low
C. Neutral
D. Very High
Answer: B
Clarification: Critical defects per unit chip area is low for a MOS transistor because it involves fewer steps in the fabrication of a MOS transistor. Also, MOSFET has low packaging density.

10. MOS is being used in ___________
A. LSI
B. VLSI
C. MSI
D. Both LSI and VLSI
Answer: D
Clarification: Since more transistor and circuitry functions can be achieved on a single chip with MOS technology that is why MOS is being used in LSI and VLSI. LSI stands for Large Scale Integration and VLSI stands for Very Large Scale Integration.

Digital Electronics/Circuits Multiple Choice Questions on “Multiplexers (Data Selectors) – 1”.

1. What is a multiplexer?
A. It is a type of decoder which decodes several inputs and gives one output
B. A multiplexer is a device which converts many signals into one
C. It takes one input and results into many output
D. It is a type of encoder which decodes several inputs and gives one output
Answer: B
Clarification: A multiplexer (or MUX) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line, depending on the active select lines.

2. Which combinational circuit is renowned for selecting a single input from multiple inputs & directing the binary information to output line?
A. Data Selector
B. Data distributor
C. Both data selector and data distributor
D. DeMultiplexer
Answer: A
Clarification: Data Selector is another name of Multiplexer. A multiplexer (or MUX) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line, depending on the active select lines.

3. It is possible for an enable or strobe input to undergo an expansion of two or more MUX ICs to the digital multiplexer with the proficiency of large number of ___________
A. Inputs
B. Outputs
C. Selection lines
D. Enable lines
Answer: A
Clarification: It is possible for an enable or strobe input to undergo an expansion of two or more MUX ICs to the digital multiplexer with the proficiency of large number of inputs.

4. Which is the major functioning responsibility of the multiplexing combinational circuit?
A. Decoding the binary information
B. Generation of all minterms in an output function with OR-gate
C. Generation of selected path between multiple sources and a single destination
D. Encoding of binary information
Answer: C
Clarification: The major functioning responsibility of the multiplexing combinational circuit is generation of selected path between multiple sources and a single destination because it makes the circuit too flexible. A multiplexer (or MUX) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line, depending on the active select lines.

5. What is the function of an enable input on a multiplexer chip?
A. To apply Vcc
B. To connect ground
C. To active the entire chip
D. To active one half of the chip
Answer: C
Clarification: Enable input is used to active the chip, when enable is high the chip works (ACTIVE), when enable is low the chip does not work (MEMORY). However, Enable can be Active-High or Active-Low, indicating it is active either when it is connected to VCC or GND respectively.

6. One multiplexer can take the place of ___________
A. Several SSI logic gates
B. Combinational logic circuits
C. Several Ex-NOR gates
D. Several SSI logic gates or combinational logic circuits
Answer: D
Clarification: A multiplexer (or MUX) is a device that selects one of several analog or digital input signals and forwards the selected input into a single line, depending on the active select lines. Since many operational behaviour can be performed by using a multiplexer. Whereas, a combinational circuit is a combination of many logic gates which makes the circuit more complex.

7. A digital multiplexer is a combinational circuit that selects ___________
A. One digital information from several sources and transmits the selected one
B. Many digital information and convert them into one
C. Many decimal inputs and transmits the selected information
D. Many decimal outputs and accepts the selected information
Answer: A
Clarification: A digital multiplexer is a combinational circuit that selects one digital information from several sources and transmits the selected information on a single output line depending on the status of the select lines. That is why it is also known as a data selector.

8. In a multiplexer, the selection of a particular input line is controlled by ___________
A. Data controller
B. Selected lines
C. Logic gates
D. Both data controller and selected lines
Answer: B
Clarification: The selection of a particular input line is controlled by a set of selected lines in a multiplexer, which helps to select a particular input from several sources.

9. If the number of n selected input lines is equal to 2^m then it requires _____ select lines.
A. 2
B. m
C. n
D. 2ⁿ
Answer: B
Clarification: If the number of n selected input lines is equal to 2^m then it requires m select lines to select one of m select lines.

10. How many select lines would be required for an 8-line-to-1-line multiplexer?
A. 2
B. 4
C. 8
D. 3
Answer: D
Clarification: 2ⁿ input lines, n control lines and 1 output line available for MUX. Here, 8 input lines mean 2³ inputs. So, 3 control lines are possible. Depending on the status of the select lines, the input is selected and fed to the output.

11. A basic multiplexer principle can be demonstrated through the use of a ___________
A. Single-pole relay
B. DPDT switch
C. Rotary switch
D. Linear stepper
Answer: C
Clarification: A basic multiplexer principle can be demonstrated through the use of a rotary switch. Since its behaviour is similar to the multiplexer. There are around 10 digits out of which one is selected one at a time and fed to the output.

12. How many NOT gates are required for the construction of a 4-to-1 multiplexer?
A. 3
B. 4
C. 2
D. 5
Answer: C
Clarification: There are two NOT gates required for the construction of 4-to-1 multiplexer. x0, x1, x2 and x3 are the inputs and C1 and C0 are the select lines and M is the output.
The diagram of a 4-to-1 multiplexer is shown below: