4.1 Semiconductions - Diode Characteristics (2)

Question 1

what does a varactor diode consist of?

Answer 1

standard PN junction

Question 2

which bias does a varactor diode operate in?

Answer 2

reverse bias

Question 3

how is it possible to increase or decrease the width of the depletion layer?

Answer 3

by changing the level of reverse bias

Question 4

what is the construction of a varactor diode?

Answer 4

exactly the same as a capacitor

conductive plates separated by an insulating dielectric

Question 5

using a capacitor and varactor diode together might forward bias the varactor during half periods, how is this overcome?

Answer 5

by using two varactor diodes

Question 6

in LEDs, what does the colour of light emitted depend on?

Answer 6

the material used

Question 7

in LEDs, what is the forward biasing current?

Answer 7

5 to 40 mA

Question 8

in LEDs, what is the forward voltages?

Answer 8

1.4 to 4.5 volts

Question 9

in LEDs, what is the breakdown voltages in the reverse direction?

Answer 9

between 20 and 50v

Question 10

in LEDs, what are the switching times for:
red
green

Answer 10

red = 50 nanoseconds
green = 400 nanoseconds

Question 11

in LEDs, what is the life span?

Answer 11

6 to 11 years

Question 12

how do light dependant resistors work?

Answer 12

they have no depletion layer and change their effective resistance IAW an internal photoelectric effect

more light = less resistance
less light = more resistance

Question 13

how are photo conductive diodes constructed?

Answer 13

similar to a conventional PN junction diode,

outer casing is either transparent or clear to focus light on the PN Junction for increased sensitivity

Question 14

which bias are photo conductive diodes always put in?

Answer 14

reverse biased

Question 15

in photo diodes, how does leakage current increase?

Answer 15

increases as illumination of the junction increases

Question 16

in photo diodes, how are they proportional?

Answer 16

directly proportional to light intensity falling onto the PN junction

Question 17

what are the advantages and disadvantages of photo diodes?

Answer 17

ad:
fast responses to changes in light levels

dis:
relatively small current flow even when fully illuminated

Question 18

what does an optical coupling device do?

Answer 18

separate the electrical potential of the input port from that of the output port

Question 19

how is resistance effected with voltage in VDR?

Answer 19

resistance decreases as voltage increase
resistance increases as voltage decreases

higher voltages break through the barrier layer

Question 20

how are VDR’s used? (3)

Answer 20

-to protect against excess voltage in the system,
-regulate voltages and suppress interference in DC motors
-lightning protections

Question 21

mutlilayed semiconductors are characterised by 2 different load configs, what are they?

Answer 21

highly resistant and non conducting
low resistance and conducting

Question 22

how many layers can a diac be constructed into?

Answer 22

3, 4 and 5 layers

Question 23

what is zero sweep voltage?

Answer 23

when zero voltage is applied to the gate, the transition from blocking mode to conducting mode takes place at a certain voltage

Question 24

in a thyristor, what must be large enough?

Answer 24

the control voltage and must occur within a specific time frame

Question 25

when a thyristor is in its low resistance state, how is it possible to return it to high resistance state?

what is it called?

Answer 25

when the electrical current falls below a specific value

called holding current

Question 26

how are thyristors constructed?

Answer 26

4 alternating layers PNPN, so 3 PN junctions form between layers

Question 27

how can thyristors be used as rectifiers?

Answer 27

when they are orientated with a negative voltage at the anode.

one half of AC cannot pass through

Question 28

how does a thyristor work as a rectifier?

Answer 28

negative half-wave current is blocked

positive half-wave, initially current blocked, electrical impulse is applied to the control electrode allowing to become a transmission gate and current can flow

Question 29

what is the delay called when the control voltage are constantly releaed after a specific point?

Answer 29

ignition delay angle

Question 30

what is a triac?

Answer 30

essentially the wiring of two thyristors in an anti-parallel circuit.
united through one crystal and a common junction

Question 31

when is a triac switched into its low resistance state?

Answer 31

by applying an electrical current in either direction to the control electrode

Question 32

how do dimmer switches work?

Answer 32

-a capacitor is charged,
-when a diac reaches its switching voltage, it switches to low resistance state
-applies the current to the control electrode of the triac switching to its low resistance state.

occurs on both half waves