4.1 Semiconductors - Diodes

Question 1

What are semiconductors?

Answer 1

element or compound that contain relatively few free moving electrons

Question 2

Which elements are the most important semiconductors? (4)

Answer 2

1. Silicon
2. Germanium
3. Selenium
4. Copper oxide

Question 3

Which intermetallic compound can be used in the same way of germanium or silicon?

Answer 3

Gallium arsenide

Question 4

What is doping?

Answer 4

the shifting of balance of electrons by adding (doping) other atoms to silicon.

Question 5

what is an n-type semi conductor?

Answer 5

atoms with one more valence electron than silicon (n-type = negative)

Question 6

what is an p-type semiconductor?

Answer 6

atoms with one less valence electrons (p-type = positive)

Question 7

what is the temperature limit of an impure material?

Answer 7

+75c germanium
+150c silicon

Question 8

What is the diffusion voltage of germanium and silicon?

Answer 8

germanium = 0.3V
silicon = 0.7V

Question 9

where is the diffusion voltage only available?

Answer 9

in the depletion layer

Question 10

what can a semiconductor do?

Answer 10

block or let current through

Question 11

if the positive pole of a current is in the n-zone and negative pole in the p-zone, what happens?

Answer 11

a depletion zone forms at the junction (no free electrons or empty spaces) and the semiconductors blocks the charge

Question 12

if the negative pole is in the n-zone and positive pole in the p-zone, what happens?

Answer 12

the depletion layer breaks down and the semi conductor will let the current through

Question 13

if no voltage is applied to a semi conductor, what happens?

Answer 13

a depletion zone is formed that has to be overcome before current can flow in the forward-biased direction

0.7V silicon
0.3V germanium

Question 14

what is the threshold voltage?

Answer 14

the voltage required to breakdown the depletion layer at junctions
0.7V silicon
0.3 Germanium

Question 15

when a semiconductor diode doesn’t fully block, what happens?

Answer 15

a residual reverse-biased current still flows (Leakage current)

Question 16

why does a reverse-biased current occur?

Answer 16

caused by residual impurities in the crystal which are broken down by heat waves

Question 17

what does the amount of residual reverse-biased current depend on?

Answer 17

the type of semiconductor diode type

Question 18

what is maximum forward current?

Answer 18

the maximum forward current allowed to flow through the device

Question 19

what will happen if the maxiumum forward current is exceed?

Answer 19

cause more heat to be generated across the junction and the diode will fail due to thermal overload

Question 20

what is peak inverse voltage/maximum reverse voltage?

Answer 20

the maximum allowable reverse operating voltage that can be applied across the diode without reverse breakdown or damage

Question 21

what is peak inverse voltage/maximum reverse voltage usually less than?

Answer 21

avalanche breakdown level

Question 22

what is a typical value for peak inverse voltage/maximum reverse voltage?

Answer 22

a few volts to thousands of volts

Question 23

when might a peak inverse voltage/maximum reverse voltage parameter be used?

Answer 23

rectifying diodes in AC Rectifier circuits

Question 24

what are the three types of single crystal semiconductor diodes?

Answer 24

1. microdiodes
2. rectifier diodes
3. heavy current diodes

Question 25

what are the characteristics of a microdiode?

Answer 25

• mainly used in communication technology
• max forward-biased current below 1 amp
• have different names according to construction (junction diodes, point-contact diodes, planar diodes)

Question 26

what are the characteristics of a rectifier diode?

Answer 26

• primariliy used for electrical power supply devices
• used to construct rectifier circuits for small outputs
• max forward-biased current up to 10 amps

Question 27

what are the characteristics of a heavy current diode?

Answer 27

• max forward-biased current of 10-100 amps
• used in rectifier circuits for large power outputs

Question 28

what is a freewheeling diode?

Answer 28

provides protection against back-EMF that occured when switching inductive loads

Question 29

which bias is a freewheeling diode when the switching device is switched on?

Answer 29

reverse biased, (blocking state)

Question 30

which bias is a freewheeling diode when the switching device is switched to off?

Answer 30

forward bias (allow current flow)

Question 31

without the protection of a freewheeling diode, what would occur?

Answer 31

damage to the switching device due to high di/dt currents causing high voltage spike or transient currents to flow

Question 32

when diodes are connected in series, what happens to the forward current?
why must care be taken when selecting your diode values?

Answer 32

the forward currents remains the same.
forward current must not exceed the lowest diode current rating
small voltage drop across each diode, smaller voltage to load

Question 33

when diodes are connected in parallel, what happens to the forward current?

Answer 33

current divided between the diodes, possible for a single diode to exceed max forward current in a rectifer

Question 34

what resistance will a diode show when wired in reverse bias direction?

Answer 34

high resistance (open circuit)

Question 35

what resistance will a diode show when wired in the forward bias direction?

Answer 35

very low resistance

Question 36

how can you tell a diode is faulty when tested?

Answer 36

high resistance in both directions

Question 37

if a diode fails in a short circuit condition, what will be shown when tested?

Answer 37

low resistance in both directions