4.1 Semiconductions - Diode Characteristics (2) Flashcards
what does a varactor diode consist of?
standard PN junction
which bias does a varactor diode operate in?
reverse bias
how is it possible to increase or decrease the width of the depletion layer?
by changing the level of reverse bias
what is the construction of a varactor diode?
exactly the same as a capacitor
conductive plates separated by an insulating dielectric
using a capacitor and varactor diode together might forward bias the varactor during half periods, how is this overcome?
by using two varactor diodes
in LEDs, what does the colour of light emitted depend on?
the material used
in LEDs, what is the forward biasing current?
5 to 40 mA
in LEDs, what is the forward voltages?
1.4 to 4.5 volts
in LEDs, what is the breakdown voltages in the reverse direction?
between 20 and 50v
in LEDs, what are the switching times for:
red
green
red = 50 nanoseconds
green = 400 nanoseconds
in LEDs, what is the life span?
6 to 11 years
how do light dependant resistors work?
they have no depletion layer and change their effective resistance IAW an internal photoelectric effect
more light = less resistance
less light = more resistance
how are photo conductive diodes constructed?
similar to a conventional PN junction diode,
outer casing is either transparent or clear to focus light on the PN Junction for increased sensitivity
which bias are photo conductive diodes always put in?
reverse biased
in photo diodes, how does leakage current increase?
increases as illumination of the junction increases
in photo diodes, how are they proportional?
directly proportional to light intensity falling onto the PN junction
what are the advantages and disadvantages of photo diodes?
ad:
fast responses to changes in light levels
dis:
relatively small current flow even when fully illuminated
what does an optical coupling device do?
separate the electrical potential of the input port from that of the output port
how is resistance effected with voltage in VDR?
resistance decreases as voltage increase
resistance increases as voltage decreases
higher voltages break through the barrier layer
how are VDR’s used? (3)
-to protect against excess voltage in the system,
-regulate voltages and suppress interference in DC motors
-lightning protections
mutlilayed semiconductors are characterised by 2 different load configs, what are they?
highly resistant and non conducting
low resistance and conducting
how many layers can a diac be constructed into?
3, 4 and 5 layers
what is zero sweep voltage?
when zero voltage is applied to the gate, the transition from blocking mode to conducting mode takes place at a certain voltage
in a thyristor, what must be large enough?
the control voltage and must occur within a specific time frame
when a thyristor is in its low resistance state, how is it possible to return it to high resistance state?
what is it called?
when the electrical current falls below a specific value
called holding current
how are thyristors constructed?
4 alternating layers PNPN, so 3 PN junctions form between layers
how can thyristors be used as rectifiers?
when they are orientated with a negative voltage at the anode.
one half of AC cannot pass through
how does a thyristor work as a rectifier?
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
what is the delay called when the control voltage are constantly releaed after a specific point?
ignition delay angle
what is a triac?
essentially the wiring of two thyristors in an anti-parallel circuit.
united through one crystal and a common junction
when is a triac switched into its low resistance state?
by applying an electrical current in either direction to the control electrode
how do dimmer switches work?
-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
