Logic Gates

Question 1

What symbol is generally used to represent power supply? (1)

Answer 1

V_DD

Question 2

Why has the power supply to transistors dropped as chips have progressed? (3)

Answer 2

1. Smaller transistors are used
2. Prevents overloading transistors
3. Saves power

Question 3

Describe in words how logic levels are defined using two gates. (6)

Answer 3

1. The first gate is the driver
2. The second gate is the receiver
3. The output of the driver is the input to the receiver
4. Outputs in the range V_DD - V_OH give logic high; V_OL - GND logic low
5. Inputs in the range V_DD - V_IH are interpreted as logic high; V_IL - GND low. All other inputs to the receiver are forbidden
6. V_OH - V_IH and V_IL - V_OL give the noise margins

Question 4

Draw a diagram showing the input/output characteristics for a driver/receiver circuit. (1)

Answer 4

Question 5

What are the DC transfer characteristics of a gate? (1)

Answer 5

A description of the output voltage as a function of the input voltage (usually shown as a graph)

Question 6

What are the ideal transfer characteristics for an inverter? (1)

Answer 6

Question 7

What might the realistic transfer characteristics look like for an inverter? (1)

Answer 7

Question 8

What are unity gain points? (1)

Answer 8

Points on a transfer characteristic graph (input against output voltage) where the gradient is -1

Question 9

Why do we choose logic levels at unity gain points? (2)

Answer 9

1. Usually maximise noise margins
2. e.g. reduce V_IL -> small increase in V_OH; increase V_IL -> large drop in V_OH

Question 10

What is static discipline? (1)

Answer 10

Given logically valid inputs, every circuit element should produce logically valid outputs

Question 11

What is a transistor? (2)

Answer 11

1. An electrically controlled switch
2. Turns ON/OFF when a current is applied to a control terminal

Question 12

What are the two main types of transistor? (2)

Answer 12

1. Bipolar transistors
2. MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors)

Question 13

What does MOSFET stand for? (1)

Answer 13

Metal-Oxide-Semiconductor Field Effect Transistor

Question 14

How many MOSFETs can fit on a 1cm² silicon chip? (1)

Answer 14

Approx. 1 billion

Question 15

What is a semiconductor? (1)

Answer 15

A material that can conduct electricity under some conditions but not others (depends on the concentration of dopants)

Question 16

What are MOSFETs built from? (1)

Answer 16

Silicon

Question 17

What structure does silicon form and why? (2)

Answer 17

1. Forms a crystalline lattice structure
2. Group IV atom so forms four covalent bonds

Question 18

Why is silicon by itself a poor conductor? (1)

Answer 18

All its electrons are tied up in covalent bonds

Question 19

What are dopant atoms? (1)

Answer 19

Impurities added to a substance to produce semiconductors

Question 20

What happens when a group V atom is added to silicon? (2)

Answer 20

1. n-type silicon is produced
2. There is an extra electron not involved in the bonds which is free to move

Question 21

What happens when a group III dopant is added to silicon? (2)

Answer 21

1. p-type silicon is produced
2. There is a missing electron called a hole (a silicon electron moves to create an ionized dopant atom and the hole can continue to “move” in this way)

Question 22

What is a diode in the context of silicon? (1)

Answer 22

The junction between the p-type and n-type diode

Question 23

What is the p-type region in a silicon semiconductor called? (1)

Answer 23

Anode

Question 24

What is the n-type region in a silicon semiconductor called? (1)

Answer 24

Cathode

Question 25

When is a semiconductor forward biased? (1)

Answer 25

Voltage on the anode \> cathode so a current flows

Question 26

What is the anode in a semiconductor? (1)

Answer 26

p-type region

Question 27

What is the cathode in a semiconductor? (1)

Answer 27

n-type region

Question 28

When is a diode reverse biased? (1)

Answer 28

Voltage on cathode \> anode so no current flows

Question 29

What happens at a p-n junction when no voltage is applied? (3)

Answer 29

1. Electrons from the n-type move to fill the holes in the p-type and vice versa (diffusion current) 2. This leaves a depletion region in the centre with no free charges (i.e. no electrons/holes) but is charged due to the dopant atoms (p 27) 3. The depletion region creates a field that opposes the diffusion of electrons between the two types so eventually equilibrium is reached when diffusion and drift currents are balanced

Question 30

What happens during forward bias? (3)

Answer 30

1. On the p side holes are "pushed" towards the junction where they neutralise some of the -ve charge in the depletion region 2. On the n side electrons are "pushed" towards the junction where they neutralise some of the +ve charge in the depletion region 3. This reduces the field of the depletion region allowing the net diffusion current to increase significantly (drift current unchanged)

Question 31

What happens during reverse bias? (5)

Answer 31

1. In the p region holes are removed giving more -ve atoms 2. In the n region electrons are removed giving more +ve atoms 3. Overall the depletion region grows larger and its field increases 4. Diffusion current is reduced; drift current is unchanged 5. This leads to a small net drift current

Question 32

What is drift current? (1)

Answer 32

The current produced by the field in the depletion region

Question 33

What is diffusion current? (1)

Answer 33

The movement of the electrons/holes due to the difference in concentration between the two regions

Question 34

Draw a diagram of an nMOS. (1)

Answer 34

Question 35

Draw a diagram of a pMOS. (1)

Answer 35

Question 36

What happens when the gate in an nMOS transistor is 0V? (2)

Answer 36

1. The diodes between the source/drain and p substrate are reverse biased (because the source/drain are nonnegative) 2. Therefore there is no current flow and the transistor is OFF

Question 37

What happens when the gate in an nMOS transistor is at V_DD? (3)

Answer 37

1. An electric field is established that attracts +ve charge on the top plate and -ve charge on the bottom plate 2. If the voltage is large enough a channel is established which is effectively n-type 3. There is now a continuous path from source to drain so electrons can flow and the transistor is ON

Question 38

What happens in a pMOS when the gate is at 0V (source and substrate at V_S)? (3)

Answer 38

1. An electric field is created from the substrate to the gate 2. This causes a +ve p-type layer below the gate between the two p-type wells 3. Electrons can now move throught this p-type channel so the transistor is ON

Question 39

What happens in a pMOS when the gate is at V_S (source and substrate at V_S)? (2)

Answer 39

1. Reverse biased p-n junctions are created 2. Therefore no current can flow and the transistor is OFF

Question 40

What is a CMOS? (2)

Answer 40

1. A complementary MOSFET 2. Contains both nMOS and pMOS

Question 41

Why are CMOS used? (2)

Answer 41

1. Solves stray capacitance problems of nMOS 2. Power consumption is reduced (power only dissipated when switching)