VLSI Final Exam Review UPDATED
ACTUAL Exam Questions and CORRECT
Answers
What does the mobility in MOS mean and what is the general relation between NMOS and
PMOS mobility? - CORRECT ANSWER - Mobility means how fast the electrons/holes
move in a semiconductor
What is the relation between MOS gate's
i) length & resistance?
ii) width and resistance? - CORRECT ANSWER - i) when length increases, resistance of
the channel increases.
ii) when width increases, resistance of the channel decreases.
Which materials do you to mix with intrinsic Silicon (Si) to create NMOS and PMOS? -
CORRECT ANSWER - NMOS = Arsenic or Phosphorus
PMOS = Boron
In general, what gate is better to use to build a circuit if you have a choice of NAND or NOR
gates. Write down at least 3 reasons. - CORRECT ANSWER - 1) smaller overall
2) less leakage
3) add less cap to previous driver
What is HOT electrons? - CORRECT ANSWER - Due to high electric field at drain,
electrons are imparted with high energy, called hot electrons.
What are 3 effects of HOT electrons in CMOS circuits? - CORRECT ANSWER - 1) can
dislodge holes, push towards body and create body current.
2) can penetrate gate oxide and create gate current.
3) leads to slow or failure of circuits.
, How are P-type semiconductors created? - CORRECT ANSWER - by doping an intrinsic
semiconductor with an electron acceptor element during manufacture. Silicon (Si) + Boron (Br)
= P-type
How are N-type semiconductors created? - CORRECT ANSWER - by doping an intrinsic
semiconductor with an electron donor element during manufacture. Silicon (Si) + arsenic (A) or
phosphorous (P) = N-type
What is the relationship between the mobility of N-type vs P-type materials? - CORRECT
ANSWER - Mobility of N-type material (μn) = 2 x Mobility of P-type (μp). N = 2P
What does the junction between p-type and n-type semiconductors form? - CORRECT
ANSWER - a diode
What are the nFET Pass Characteristics? What is the voltage range in which nFET passes? -
CORRECT ANSWER - nFETs pass strong logic 0 voltages, but weak logic 1 values
nFET can only pass a weak logic 1
nFET is said to pass a strong logic 0
can pass a voltage in the range [0, V1]
What is threshold voltage loss? - CORRECT ANSWER - When the output voltage Vy is
reduced to a value less than the input voltage VDD
why is nFET weak? - CORRECT ANSWER - loose p-type
What are the nFET Pass Characteristics? What is the voltage range in which nFET passes? -
CORRECT ANSWER - pFETs pass strong logic 1 voltages, but weak logic 0 values
When should you use nFET? - CORRECT ANSWER - Use nFETs to pass logic 0 voltages
of VSS = 0 V
ACTUAL Exam Questions and CORRECT
Answers
What does the mobility in MOS mean and what is the general relation between NMOS and
PMOS mobility? - CORRECT ANSWER - Mobility means how fast the electrons/holes
move in a semiconductor
What is the relation between MOS gate's
i) length & resistance?
ii) width and resistance? - CORRECT ANSWER - i) when length increases, resistance of
the channel increases.
ii) when width increases, resistance of the channel decreases.
Which materials do you to mix with intrinsic Silicon (Si) to create NMOS and PMOS? -
CORRECT ANSWER - NMOS = Arsenic or Phosphorus
PMOS = Boron
In general, what gate is better to use to build a circuit if you have a choice of NAND or NOR
gates. Write down at least 3 reasons. - CORRECT ANSWER - 1) smaller overall
2) less leakage
3) add less cap to previous driver
What is HOT electrons? - CORRECT ANSWER - Due to high electric field at drain,
electrons are imparted with high energy, called hot electrons.
What are 3 effects of HOT electrons in CMOS circuits? - CORRECT ANSWER - 1) can
dislodge holes, push towards body and create body current.
2) can penetrate gate oxide and create gate current.
3) leads to slow or failure of circuits.
, How are P-type semiconductors created? - CORRECT ANSWER - by doping an intrinsic
semiconductor with an electron acceptor element during manufacture. Silicon (Si) + Boron (Br)
= P-type
How are N-type semiconductors created? - CORRECT ANSWER - by doping an intrinsic
semiconductor with an electron donor element during manufacture. Silicon (Si) + arsenic (A) or
phosphorous (P) = N-type
What is the relationship between the mobility of N-type vs P-type materials? - CORRECT
ANSWER - Mobility of N-type material (μn) = 2 x Mobility of P-type (μp). N = 2P
What does the junction between p-type and n-type semiconductors form? - CORRECT
ANSWER - a diode
What are the nFET Pass Characteristics? What is the voltage range in which nFET passes? -
CORRECT ANSWER - nFETs pass strong logic 0 voltages, but weak logic 1 values
nFET can only pass a weak logic 1
nFET is said to pass a strong logic 0
can pass a voltage in the range [0, V1]
What is threshold voltage loss? - CORRECT ANSWER - When the output voltage Vy is
reduced to a value less than the input voltage VDD
why is nFET weak? - CORRECT ANSWER - loose p-type
What are the nFET Pass Characteristics? What is the voltage range in which nFET passes? -
CORRECT ANSWER - pFETs pass strong logic 1 voltages, but weak logic 0 values
When should you use nFET? - CORRECT ANSWER - Use nFETs to pass logic 0 voltages
of VSS = 0 V
