4.1 Semiconductors - Diodes Flashcards

1
Q

What are semiconductors?

A

element or compound that contain relatively few free moving electrons

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2
Q

Which elements are the most important semiconductors? (4)

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

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

A

Gallium arsenide

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4
Q

What is doping?

A

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

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5
Q

what is an n-type semi conductor?

A

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

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6
Q

what is an p-type semiconductor?

A

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

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7
Q

what is the temperature limit of an impure material?

A

+75c germanium
+150c silicon

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8
Q

What is the diffusion voltage of germanium and silicon?

A

germanium = 0.3V
silicon = 0.7V

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9
Q

where is the diffusion voltage only available?

A

in the depletion layer

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10
Q

what can a semiconductor do?

A

block or let current through

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11
Q

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

A

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

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

what is the threshold voltage?

A

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

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15
Q

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

A

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

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16
Q

why does a reverse-biased current occur?

A

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

17
Q

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

A

the type of semiconductor diode type

18
Q

what is maximum forward current?

A

the maximum forward current allowed to flow through the device

19
Q

what will happen if the maxiumum forward current is exceed?

A

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

20
Q

what is peak inverse voltage/maximum reverse voltage?

A

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

21
Q

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

A

avalanche breakdown level

22
Q

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

A

a few volts to thousands of volts

23
Q

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

A

rectifying diodes in AC Rectifier circuits

24
Q

what are the three types of single crystal semiconductor diodes?

A
  1. microdiodes
  2. rectifier diodes
  3. heavy current diodes
25
Q

what are the characteristics of a microdiode?

A
  • 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)
26
Q

what are the characteristics of a rectifier diode?

A
  • primariliy used for electrical power supply devices
  • used to construct rectifier circuits for small outputs
  • max forward-biased current up to 10 amps
27
Q

what are the characteristics of a heavy current diode?

A
  • max forward-biased current of 10-100 amps
  • used in rectifier circuits for large power outputs
28
Q

what is a freewheeling diode?

A

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

29
Q

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

A

reverse biased, (blocking state)

30
Q

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

A

forward bias (allow current flow)

31
Q

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

A

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

32
Q

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

A

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

33
Q

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

A

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

34
Q

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

A

high resistance (open circuit)

35
Q

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

A

very low resistance

36
Q

how can you tell a diode is faulty when tested?

A

high resistance in both directions

37
Q

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

A

low resistance in both directions