Diodes and their applications

Question 1

What is a semiconductor?

Answer 1

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator

Question 2

What are the most common semiconductors?

Answer 2

Silicon and Germanium

Question 3

to do with conductivity

What do semiconductors do in their pure state?

Answer 3

Semiconductors will not conduct electrical current easily due to a strong covalent bond
This strong covalent bond is responsible for silicons properties as a semiconductor

Question 4

What is Electron-hole current?

Answer 4

• Electron current = when voltage applied to a crystal silicon, the free electrons will be attracted to the positive end
• Hole current = electrons move within the valence

Question 5

What is doping?

Answer 5

Doping involves adding a small amount of foreign atoms (dopants) to an intrinsic (pure) semiconductor to change its electrical properties.
Phosphorus and boron are the most common used doping material
Phosphorus has 5 electrons in its valence chell and boron has 3.

Question 6

What is N-type Silicon?

Answer 6

• silicon doped with phosporus
• results in an excess of electrons

Question 7

What is P-type Silicon?

Answer 7

• doped silicon with boron
• creates holes (positive charge carriers) where electrons can jump

Question 8

What is a PN junction?

Answer 8

• formed when a p-type semiconductor is brought into contact with an n-type semiconductor
• PN junction is the basis for diodes, certain transistors, solar cells, and other semiconductor devices

Question 8

Describe majoriity and minority carriers.

Answer 8

• N-type semiconductor material harbours an abundance of negative charge carriers
• holes in an n-type material are call the minority carriers
• P-type silicon - majority current carriers are holes
• electrons in a P-type material are minority carriers

Question 9

What happens when a PN junction forms?

Answer 9

• Free electrons from N region diffuse into P region, N region loss of electrons and creates a layer of +ve charge
• P region loss of holes creates a layer of -ve charge
• Forms a depletion zone and electric field builds up
• P.d. of the two layers is called the barrier potential

Question 10

Biasing the PN junction

Answer 10

• The diode is an electronic one-way valve
• When a positive voltage is applied to the n-side and negative to the p-side, the depletion zone widens, preventing current flow
• the diode is reversed, no current flows
• when a +ve charge is applied to the p-side and negative to the n-side, the depletion zone shrinks, allowung current to flow through the junction
• the diode is forward biased, current flows from P-type to N-type

Question 11

V-I characteristiics of diodes in the forward bias region

Answer 11

• threshold voltage (Vth): to conduct, a minimum voltage, typically 0.7V for silicon diodes and 0.3V for germanium diodes, must be appled.
• Once this voltage is surpassed, the diodes resistance drops, allowingg significant current to flow
• The forward current increases exponentially with the applied forward voltage beyond the threshold

Question 12

V-I characteristics of diodes in the reverse bias region

Answer 12

• in this region a small leakage current flows due to minority charge carriers
• Breakdown voltage: if the reverse voltage is increase beyond a certain point, the diode will conduct a large currnet in the reverse direction. Special types of diodes like Zener diodes are designed to operate in this breakdown region.

Question 13

Ideal diode V-I characteristics for both the forward and reverse bias regions.

Answer 13

Forward Bias:
- No threshold voltage
- zero resistance
- instantaneous current flow

Reverse bias region:
- zero current
- infinite resistance

Question 14

Half-wave rectifiers

Answer 14

• One way to derive DC from an AC power supply is using AC/DC rectifiers
• A single diode connected in series with an AC circuit
• Only one-half of the alternating current can pass through the rectifier

Question 15

Full-wave rectifier

Answer 15

• converts the entire AC waveform into a pulsating DC signal
• allows unidirectional current through the load during the entire of the input cycle

Question 16

Smoothing

Answer 16

• the rectifier circuits used to produce a direct current of voltage
• the circuit introduced so far still has a large alternatiing components
• uses capacitior

Question 17

Diodes for signal control

Answer 17

Light emitting diodes LED
- indicator lights
Photodiodes
- optical communication
Schottky diodes
- used for high-speed circuitry and RF devices
Zener diodes
- can be used in reverse-biased mode
- used as a precision voltage reference
Thermal diodes
Laser diodes

Question 18

Zener diode

Answer 18

• a special diode and also known as voltage regulator diodes
• widely used as voltage reference solutions in electronics circuitry
• in the range of about 4V to 75V, depending on the type of diode