4.1 Semiconductors - Diodes

Question 1

What are semiconductors?

Answer 1

element or compound that contain relatively few free moving electrons / have a small band gap between valence band and conduction band

Question 2

Which elements are the most important semiconductors? (4)

Answer 2

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

Question 3

What is doping?

Answer 3

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

Question 4

what is an n-type semi conductor?

Answer 4

atoms with one more valence electron than the pure semiconductor are added to it (n-type = negative because the global charge is negative)

Question 5

what is an p-type semiconductor?

Answer 5

atoms with one less valence electrons than the pure semiconductor are added to it (p-type = positive charge)

Question 6

what is the temperature limit of an impure material?

Answer 6

+75c germanium
+150c silicon

Question 7

What is the diffusion voltage of germanium and silicon?

Answer 7

germanium = 0.3V
silicon = 0.7V

Question 8

what can a semiconductor do?

Answer 8

block or let current through

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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 12

what is the threshold voltage?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

why does a reverse-biased current occur?

Answer 14

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

Question 15

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

Answer 15

the type of semiconductor diode type

Question 16

what is maximum forward current?

Answer 16

the maximum forward current allowed to flow through the device

Question 17

what will happen if the maxiumum forward current is exceed?

Answer 17

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

Question 18

what is peak inverse voltage/maximum reverse voltage?

Answer 18

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

Question 19

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

Answer 19

avalanche breakdown level

Question 20

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

Answer 20

a few volts to thousands of volts

Question 21

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

Answer 21

rectifying diodes in AC Rectifier circuits

Question 22

what are the three types of single crystal semiconductor diodes?

Answer 22

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

Question 23

what are the characteristics of a microdiode?

Answer 23

• 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 24

what are the characteristics of a rectifier diode?

Answer 24

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

Question 25

what are the characteristics of a heavy current diode?

Answer 25

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

Question 26

what is a freewheeling diode?

Answer 26

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

Question 27

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

Answer 27

reverse biased, (blocking state)

Question 28

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

Answer 28

forward bias (allow current flow)

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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 31

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

Answer 31

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

Question 32

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

Answer 32

high resistance (open circuit)

Question 33

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

Answer 33

very low resistance

Question 34

how can you tell a diode is faulty when tested?

Answer 34

high resistance in both directions

Question 35

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

Answer 35

low resistance in both directions