4.1 Semiconductions - Diode Characteristics

Question 1

how are schottky diodes constructed?

Answer 1

metal-to-n junction

Question 2

what are schottky diodes known as?

Answer 2

hot-carrier diodes

Question 3

what are schottky diodes characterised by? (3)

Answer 3

-fast switching times
-low forward voltage drop
-low junction capacitance

Question 4

what range are schottky diodes use for?

Answer 4

high frequency
200 Khtz to 2 Mhtz

Question 5

where are schottky diodes used? (4)

Answer 5

-logic gates,
-switch mode power converters
-RF detectors and mixers
-clamping

Question 6

what are the disadvantages of schottky diodes? (2)

Answer 6

-low reverse voltage rating (50v and below)
-high reverse leaking current

Question 7

in rectifier circuits, which part of the waveform affects load resistance in a half-wave rectifier?

Answer 7

on the respective half waves

Question 8

in rectifier circuits, in a half-wave rectifier, if you were to use a charging capacitor, what conditions change?

Answer 8

the diode is not conductive for the entire positive half wave but only opens when the transformer voltage is greater than the voltage

Question 9

in rectifier circuits, what is the opening time expressed as? what is its value?

Answer 9

as an angle
between 30 and 60 degrees

Question 10

in rectifier circuits, what is often placed in front of a diode to protect it? why?

Answer 10

protective resistor

to limit the incoming current and to stop from peaking in the diode

Question 11

in rectifier circuits, in a full-wave recitfier, which part of the waveform has an effect on work load?

Answer 11

both half waves in the same direction

Question 12

in rectifier circuits, in a full-wave rectifier with a charging capacitor, what happens to the hum voltage?

Answer 12

only half as large but twice the frequency of the power grid

Question 13

what is the disadvantage of a full wave rectifier?

Answer 13

a transformer with a double coil or tapped coil is required

Question 14

what is a bridge circuit?

Answer 14

a full-wave rectifier without the need for a tapped transformer

called a WYE rectifier

Question 15

what is the advantage of a bridge circuit when subjected to capacitive load?

Answer 15

each diode only has to handle a maximum sticking voltage of Vtr because at any given time two diodes are in sequence

Question 16

what type of circuit is used to rectify type 3-phase AC?

Answer 16

six diode circuit

Question 17

how does an 3-phase rectifier work?

Answer 17

3 stators are individually in a negative state where current flows through a diode, the load resistor and another diode, then on to the next phase

Question 18

what is produced as a result of 3-phase rectification?

Answer 18

a pulsating DC voltage

Question 19

in a 3 phase rectifier circuit, what is used in order to produce a DC voltage?

Answer 19

a charging capacitor laid in parallel

Question 20

what is used to level out the direct current voltage at the charging capacitor?

Answer 20

filter circuit

Question 21

in the filtering of rectifier circuits, what is used to remove the ripple remaining after the reservoir capacitor?
which is more efficient?

Answer 21

LC or RC

LC

Question 22

what are the advantages and disadvantages of LC Filtering?

Answer 22

Ad:
only a small loss of DC

Dis:
expensive inductance

Question 23

how can Zener diodes be used to stabilise voltage output?

Answer 23

by passing a small voltage from a voltage source, via a current limiting resistor

Question 24

what are cascades often used for?

Answer 24

creation of high voltage necessary for cathode ray tubes, Geiger-muller counters or igniter boxes

Question 25

why are cascades more advantageous than high voltage transformers

Answer 25

low current consumption

Question 26

how does a delon circuit work? (voltage doubler)

Answer 26

half cycles aren’t sent to the consumer together

capacitors are charged, two half cycles are added together and the two capacitor voltages are the new output voltage

Question 27

how does a villard circuit work? (voltage doubler)

Answer 27

based on a one-way rectifier

during the negative half cycle, the capacitor is charged.
during the positive half cycle, the transformer voltage and capacitor voltage are added
diode now in conducting direction and the second capacitor is charge of the transformer and capacitor

Question 28

what is the principle of a cascade circuit?

Answer 28

any voltage can be created but the current capacity is very low

Question 29

where are clamping circuit used? and why?

Answer 29

used in radar and comms equipment

to change the reference level of a waveform without altering its amplitude

Question 30

what are clipping circuit’s purpose?

Answer 30

they remove undesirable characteristics from a waveform but cutting off part of the voltage waveform

Question 31

in clipping circuits, what types of limiters are there? and their purposes (5)

Answer 31

positive limiter - removes all parts above the waveform
negative limiter - removes all parts below the waveform
series limiter - the diode is in series between the input and low
parallel limiter - diode is parallel between the input and load
combined limiter - uses positive and negative limiter in same circuit

Question 32

what does a zener diode have to protect and how is it done?

Answer 32

steep breakdown in reverse biased direction

minimal changes in voltages leads to large current changes

Question 33

in a zener diode, how can the amount of breakdown be influenced?

Answer 33

by the amount of dopant introduced to the material during manufacture

Question 34

in a zener diode, what is zener breakdown?

Answer 34

where valance electrons are torn out of their lattice structure, increasing free electrons, increasing conductivity of the material

Question 35

what effects diodes that use the zener effect?

Answer 35

temperature (negative), leading to decrease in voltage

Question 36

zener diodes which the avalanche effect is the main cause, are affected by temperature how?

Answer 36

positively therefore higher voltage

Question 37

what does the avalanche effect allow charge to transport through?

Answer 37

the barrier layer

Question 38

which bias and condition does a zener diode operate in?

Answer 38

reverse bias
breakdown

Question 39

what must be taken into consideration regarding to a zener diodes maximum power rating?

Answer 39

careful selection of value of resistor

Question 40

why is the load connected in parallel to the zener diode?

Answer 40

so that the voltage across the resistor is always the same as the zener diode

Question 41

what must be higher than the zener diode?

Answer 41

the voltage of the supply

Question 42

what can be generated from a zener diode stabiliser circuit, and what could be used to overcome this?

Answer 42

electrical noise

addition of a large value decoupling capacitor

Question 43

when connecting several zener diodes in series with regular diodes, what must the supply voltage be?

Answer 43

higher than the larger output reference voltage

Question 44

in a stabilisation circuit with zener diodes, how does the stabilisation improve?

Answer 44

stabilisation increases as input voltage increases, but power dissipation increases at a higher rate

Question 45

in a stabilisation circuit with zener diodes, what compromise should be taken to gain good stabilisation and acceptable power?

Answer 45

suggested input voltage of 2-4 times the value of the output voltage