4.1.1 Semiconductors Flashcards
what is a diode
a two-terminal electronic component, which conducts current in a single direction by applying low (theoretically zero) resistance in one direction, and high (theoretically infinite) resistance in the other
when talking about diodes what end is what
the arrow end is the anode and the bar end is the cathode
The most important thing about a diode is its ability to function as a one-directional conductor
The key characteristics of diodes are their small size and weight, low operating voltages, lower power dissipation, high reliability, and long service life. They are also vulnerable to Electro-static Discharge (ESD) during handling and operation.
Semiconductor materials, such as silicon and germanium, exhibit properties whereby their conductivity can be varied over wide ranges by subtle changes in temperature, light intensity, and purity content.
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Semiconductors are manufactured both as single discrete devices, such as diodes and transistors, and as fully integrated circuits, where multiple discrete devices are manufactured onto a single semiconductor substrate.
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The electrical behaviour of semiconductors means they can function as either a conductor or insulator
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A fundamental semiconductor device is a Positive-negative (PN) junction diode
It is fabricated by doping together two sides of a single crystal semiconductor with opposite types of impurities.
this forms a junction at the interface between positive (p-type) and negative (n-type) sides
N-type materials have excess electrons, whereas p-type materials are lacking electrons – they have excess ‘holes’ into which electrons can move.
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A semiconductor diode functions as a kind of check valve for electrical current that can block it or let it through; it achieves this by using the depletion layer.
This depletion layer is formed when p-type and n-type conductive materials are bonded together; electrons diffuse from the n-material into the p-material to fill the holes there. This forms a depletion region (sometimes known as a “barrier region”) along the junction between the materials.
conductor (forward bias).
insulator (reverse bias)
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if the p-type is connected to the ‘+’ of the battery
causing the depletion layer to narrow and the diode to function as a conductor; this is known as forward bias
if the p-type is connected to the ‘–‘ of the battery
causing the depletion layer to widen and the diode to function as an insulator; this is known as reverse bias.
When diodes are connected in series, the forward current flowing through each diode is the same.
Therefore, the maximum forward current must not exceed any individual diode current rating.
Each diode has a small voltage drop across it, which reduces the voltage supplied to the load.
This voltage drop is 0.3 V in germanium diodes and 0.7 V in silicon diodes. This is known as the forward voltage.
When diodes are connected in parallel, the current is divided between them (as with any parallel circuit)
It is possible to use diodes in parallel when a single diode cannot withstand the maximum forward current without posing a risk of damage to components in the circuit
using diodes in parallel increases the forward current rating of the system. The limitation is that if any diode fails the current is shared by the remaining diodes and may cause them to fail too.
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Diodes connected in parallel do not conduct current equally if they have different forward bias characteristics. If this is required (e.g., multiple light sources need to have the same brightness), identical diodes should be selected.
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A Silicon Controlled Rectifier (SCR) – also known as
a thyristor
Silicon Controlled Rectifier (Thyristor) - is a multi-layered semiconductor device with at least four layers of alternating n- and p-type material.
SCRs are fast electronic switches (up to 400 Hz)
They are constructed in four alternating layers, PNPN so that three p-n junctions are formed between the layers.
(Thyristor)
they usually have a gate to control conduction and are constructed
Although there are two-wire versions that operate without a gate.
Thyristors have three operating states which are :
- Reverse blocking mode
- forward blocking mode
- forward conducting mode
reverse blocking mode is
voltage is applied that would reverse bias an equivalent diode.
forward blocking mode is
voltage is applied that would forward bias an equivalent diode but the thyristor has not been triggered into conduction.
forward conducting mode
voltage is applied that would forward bias an equivalent diode and the SCR has been triggered into conduction.
If a voltage is applied such that the anode is positive with respect to the cathode
no conduction will take place until this voltage is greater than the ‘breakdown voltage’ of the thyristor. Once this breakdown voltage occurs, the thyristor will conduct.
If a voltage is applied to the gate, with respect to the cathode, this breakdown voltage will occur at a lower anode cathode potential difference
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The thyristor will continue to conduct until the anode to cathode voltage is removed or reverse biased. Or the current through the anode to the cathode falls below the manufacturer’s specified holding current.
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Every thyristor can only be subjected to a specific maximum current. If this limit is exceeded, the thyristor is destroyed. Similarly, certain maximum voltage levels must not be exceeded in the reverse direction.
In order to keep the thyristor from overheating, it must be cooled.
Thyristors can be used in high-voltage applications, such as power switching, phase controls, battery chargers, and inverter circuits
they can be used produce variable DC voltages for motors. In lighting dimmer systems, in they are used to reduce the average voltage supplied to the lights, by controlling the gate trigger signal position.
Electronic components that emit light are known as
optoelectronic components
optoelectronic components are important in all areas of electronics and are mainly used as sensors or signalling devices. LEDs fall under this category
they are semiconductor diodes that when an electron recombines with a hole, they emit energy in the form of photons.
(LED’s) The semiconductor material that is used determines what
the overall wavelength of light emission, and hence the colour of light emitted.
LEDs usually have a forward-biased current of somewhere between 5 mA and 40 mA, and operate at forward-biased voltages of between 1.4 and 4.5 V (depending on the material and hence colour).
the switching times range from 50 nanoseconds for red light-emitting diodes, to 400 nanoseconds for green light-emitting diodes. A diode has a normal life span of many years.
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LEDs in the visible range are manufactured from what
Gallium Arsenide Phosphide (GaAsP) and Gallium Phosphide (GaP), sometimes with the addition of other dopants.
for diodes that emit infrared light what is used
Gallium Arsenide (GaAS) combinations with minimal amounts of zinc and silicon are used.
LEDs in the visible range are used for a variety of signalling and indicating purposes. what are they
By adding reflectors or lenses and arranging them specially, they are useful for a wide range of alphanumeric displays, with red LEDs being the industry standard
infrared LEDs are used for what
control devices in industry, such as photoelectric beams, monitoring manufacturing facilities, and a wide range of uses across communication technology.
photo conductive diode.
This also an optoelectronic device, but this time reacts to light
The construction of the photodiode light sensor is similar to that of a conventional p-n junction diode, except that the diode’s outer casing is either transparent or has a clear lens to focus the light onto the p-n junction for increased sensitivity
LEDs, as all PN junctions, can be used as photodiodes, as they can both emit and detect light at their junction. All p-n junctions are light sensitive and can be used in a photo-conductive, with the p-n junction of the photodiode always reverse-biased; this way, only the diode’s leakage current can flow.
with the p-n junction of the photodiode always reverse-biased; .
Because of their characteristics, photodiodes can be used in light-detecting circuits, such as proximity detectors and fibre optic data bus receivers.
Varistors are also called VDRs (Voltage Dependent Resistors). The effective semiconductor layer of a VDR consists of many small, highly conductive crystal particles, each of which is covered with an extremely thin barrier layer and sintered together.
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The resistance level of varistors is inversely proportional to the voltage applied, irrespective of polarity
At low voltages, a varistor has a relatively high resistance level due to its barrier layer. However, higher voltages break through the barrier layers, thereby reducing the resistance of the component.
Varistors are chiefly used to protect systems against excess voltage
they can also be used to regulate voltages and suppress interference in small direct current motors, for lightning protection and for protecting contacts against cut-off currents in the case of inductances.
rectifier diodes is usually used in applications that require high current, such as power supplies. They are used to convert Alternating Current (AC) to Direct Current (DC)
The current range that these diodes can handle is from 1 A to 100s of amps.
A recognisable feature of the larger rectifier diodes is what
that they are encased in metal, which acts as a heat sink.
a diode shows a very high resistance (or open circuit) if wired in the reverse bias direction and very low resistance when wired in the forward bias direction
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If the diode is faulty in the open circuit condition, then the reading is high resistance (or open circuit) in both directions
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if it has failed in the short circuit condition (which is less likely), it shows low resistance in both directions.
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When a multimeter is set to the diode test position, it supplies a voltage sufficient to forward and reverse bias the diode. This voltage is approximately 2.5-3.5 V
It is possible to determine the condition of a diode based on results of multimeter readings from both the forward and reverse biased directions.
A serviceable diode shows around 0.6 to 0.7 V on the multimeter when tested in the forward bias direction. The equipment may also beep once.
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When tested in the reverse bias direction it shows the internal voltage of the meter or indicate high resistance as overload (OL)
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If a diode has failed open circuit, it shows the internal voltage of the meter in both forward and reverse bias or indicates overload.
If it has failed in the short circuit condition, it shows zero (or nearly zero) in both directions. An audible ‘BEEP’ may also be heard, depending on meter type.
Current flows through a diode in only one direction
Bi-polar junction transistors are mainly used for switching and amplification
How is a transistor biased correctly to conduct
base-emitter must be forward biased
inductors are not normally integrated into an IC
amplifier stages are isolated from each other with the use of capacitors
U/S integrated circuits must be replaced
