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UKCAA MODULE 4 ELECTRONIC FUNDAMENTALS > SEMICONDUCTORS > Flashcards

SEMICONDUCTORS Flashcards

1
Q

draw the symbols for all all 12 diodes

A
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2
Q

What are the key characteristics of diodes?

A

•Diodes are small, lightweight devices •low operating voltages,
•low power dissipation,
•high reliability,
•long service life.
They function primarily as one-directional conductors.

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3
Q

What are diodes made of?

A

Diodes are made of semiconductor materials such as silicon and germanium. These materials can vary their conductivity based on temperature, light intensity, and purity content.

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4
Q

What is a PN junction diode?

A

A PN junction diode is formed by doping together two sides of a single crystal semiconductor with opposite types of impurities: n-type (excess electrons) and p-type (excess holes).

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5
Q

How does a semiconductor diode function?

A

A semiconductor diode acts as a check valve for electrical current. It can block current flow in one direction (reverse bias) and allow current flow in the opposite direction (forward bias), controlled by the biasing voltage applied to the junction.

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6
Q

What happens in forward bias of a diode?

A

In forward bias, the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal to the n-type. This reduces the width of the depletion layer, allowing current to flow through the diode.

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7
Q

What is the depletion layer in a PN junction diode?

A

The depletion layer is a region formed at the junction of p-type and n-type semiconductor materials. It is depleted of mobile charge carriers (electrons and holes) and acts as a barrier to current flow in reverse bias.

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8
Q

What happens in reverse bias of a diode?

A

In reverse bias, the positive terminal of the battery is connected to the n-type semiconductor and the negative terminal to the p-type. This widens the depletion layer, preventing significant current flow through the diode.

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9
Q

How does ESD affect diodes?

A

Diodes are vulnerable to Electro-static Discharge (ESD) during handling and operation, which can damage them if proper precautions are not taken.

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10
Q

What happens when diodes are connected in series?

A

When diodes are connected in series, the same forward current flows through each diode. The voltage drop across each diode reduces the voltage supplied to the load. It’s crucial not to exceed the maximum forward current rating of any individual diode.

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10
Q

What is the characteristic voltage drop of diodes in series?

A

Each diode in series typically has a forward voltage drop of 0.3 V (for germanium diodes) or 0.7 V (for silicon diodes).

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11
Q

What happens when diodes are connected in parallel?

A

Diodes connected in parallel divide the current among them, similar to any parallel circuit. This configuration increases the overall forward current rating of the system, but failure of one diode can affect others.

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12
Q

How do diodes in parallel behave with different characteristics?

A

Diodes in parallel do not conduct current equally if they have different forward bias characteristics. For applications requiring consistent performance (e.g., uniform brightness in multiple LEDs), identical diodes should be chosen.

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13
Q

What is a Silicon Controlled Rectifier (SCR)?

A

A Silicon Controlled Rectifier (SCR), also known as a thyristor, is a semiconductor device with four layers of alternating n- and p-type material. It acts as a fast electronic switch capable of handling high currents and voltages, up to 400 Hz.

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14
Q

Why are diodes not typically used for voltage division in forward bias?

A

While diodes do drop voltage like resistors in a potential divider, this practice is not common. Reverse biased Zener diodes are preferred for voltage regulation

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15
Q

How does an SCR operate?

A

SCRs have three main states: reverse blocking mode (no conduction), forward blocking mode (no conduction until triggered), and forward conducting mode (conduction after triggering). They are triggered into conduction by applying a sufficient gate current.

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15
Q

What are common uses of SCRs in the aerospace industry?

A

SCRs are used in high-voltage applications such as power switching, phase control, battery chargers, and inverter circuits. They are essential for controlling variable DC voltages and in lighting dimmer systems.

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16
Q

What are typical characteristics and applications of LEDs?

A

LEDs operate at forward voltages between 1.4 V and 4.5 V and forward currents between 5 mA and 40 mA, depending on their color. They are used extensively for signaling, indicating, and display purposes in aircraft, due to their reliability and efficiency.

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17
Q

What are Light Emitting Diodes (LEDs)?

A

LEDs are semiconductor diodes that emit light when forward biased. They are categorized by the wavelength and color of light they emit, determined by the semiconductor material used

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18
Q

What are Photo-conductive Diodes?

A

Photo-conductive diodes are light-sensitive optoelectronic devices. They function similarly to standard diodes but are designed to respond to light, particularly in the infrared and red wavelengths.

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19
Q

How are Photo-conductive Diodes used in aerospace applications?

A

They are used in light-detecting circuits such as proximity detectors, fiber optic data bus receivers, and various sensing applications where response to specific wavelengths of light is crucial.

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20
Q

What are Varistors (VDRs)?

A

Varistors are voltage-dependent resistors made of semiconductor materials. Their resistance decreases as voltage increases, providing protection against excess voltage surges and interference suppression.

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21
Q

What are Rectifier Diodes used for?

A

Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC). They handle high currents, ranging from 1 A to hundreds of amps, and are crucial for providing stable DC power in aircraft systems.

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21
Q

Where are Varistors commonly employed in aerospace?

A

Varistors are used for lightning protection, voltage regulation, and interference suppression in aerospace systems, ensuring reliability and safety during operation.

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22
Q

What are the basic steps to test a diode using a multimeter?

A

Set the multimeter to the diode test position, which supplies a voltage sufficient to forward and reverse bias the diode. Measure the resistance in both forward and reverse bias directions to determine the condition of the diode.

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23
Q

What should a serviceable diode show when tested in the forward bias direction with a multimeter?

A

A serviceable diode shows a voltage drop of around 0.6 to 0.7 V and may emit a beep.

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24
Q

What should a serviceable diode show when tested in the reverse bias direction with a multimeter?

A

In reverse bias, a serviceable diode shows the internal voltage of the meter or indicates high resistance as overload (OL).

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25
Q

What indicates an open circuit failure in a diode when tested with a multimeter?

A

An open circuit failure shows the internal voltage of the meter or overload (OL) in both forward and reverse bias directions.

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26
Q

What indicates a short circuit failure in a diode when tested with a multimeter?

A

A short circuit failure shows zero or nearly zero voltage in both directions, and the meter may emit an audible beep.

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27
Q

Why is it important to understand how to test diodes in avionics systems?

A

Proper testing ensures avionics systems are fit for purpose, helping to diagnose and prevent circuit issues, thus maintaining safety and reliability in aerospace applications.

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28
Q

What does a multimeter supply when set to the diode test position?

A

The multimeter supplies a voltage of approximately 2.5-3.5 V to forward and reverse bias the diode.

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29
Q

How can you confirm a diode is serviceable using a multimeter?

A

A diode is confirmed serviceable if it shows around 0.6 to 0.7 V in the forward bias direction and high resistance or overload (OL) in the reverse bias direction.

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30
Q

What audible indication might a multimeter provide when testing a serviceable diode in forward bias?

A

The multimeter may emit a beep.

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31
Q

What is the significance of the multimeter reading zero in both directions when testing a diode?

A

This indicates a short circuit failure in the diode.

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32
Q

name the higher order dopants or N-type

A

N nitrogen, P phosphorous, As Arsenic, SB antimony, Bi Bismuth, Uup ununpentium

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33
Q

name the lower order dopants or P-type

A

B boron, Al Aluminium, Ga Galium, TI Thallium, Uut Ununtrium

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34
Q

What are semiconductor materials, and which elements are commonly used?

A

Semiconductors are elements or compounds with electrical conductivity between conductors and insulators. Common materials include silicon (Si), germanium (Ge), selenium (Se), copper oxide (CuO), and gallium arsenide (GaAs).

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35
Q

What is the electron configuration of Germanium and Silicon?

A

Germanium: 32 electrons, with 4 valence electrons.

Silicon: 14 electrons, with 4 valence electrons.

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36
Q

How are the atoms in semiconductors structured?

A

Semiconductors are made up of atoms interconnected through covalent bonds between their valence electrons, forming a lattice structure.

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37
Q

Why can’t a pure silicon crystal conduct current?

A

A pure silicon crystal can’t conduct current because the electrons are not free to move; they are bound in covalent bonds.

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38
Q

What is doping, and how does it affect the electrical properties of semiconductors?

A

Doping is the process of introducing impurities into a semiconductor to change its properties. It increases the number of free charge carriers, improving conductivity.

39
Q

What are higher-order dopants, and what type of semiconductor material do they produce?

A

Higher-order dopants have more valence electrons than the semiconductor material (e.g., phosphorus with 5 valence electrons). They produce n-type semiconductor material by introducing surplus electrons.

40
Q

What are lower-order dopants, and what type of semiconductor material do they produce?

A

Lower-order dopants have fewer valence electrons than the semiconductor material (e.g., boron with 3 valence electrons). They produce p-type semiconductor material by creating holes (missing electrons).

41
Q

What is a PN junction, and why is it important in electronics?

A

A PN junction is formed by joining n-type and p-type semiconductor materials. It is important in electronics for its rectifying properties, allowing current to flow in only one direction, essential for diode rectifiers.

41
Q

What are charge carriers in semiconductors, and how are they classified in n-type and p-type materials?

A

Charge carriers are free electrons (negative) and holes (positive). In n-type materials, majority carriers are free electrons, and minority carriers are holes. In p-type materials, majority carriers are holes, and minority carriers are free electrons.

42
Q

What is the depletion layer in a PN junction?

A

The depletion layer is the region around the PN junction with no charge carriers, high resistance, and forms a barrier preventing current flow under unbiased conditions.

43
Q

What is the diffusion voltage, and how does it differ between germanium and silicon?

A

The diffusion voltage is the voltage across the PN junction due to charge displacement. It is approximately 0.3 V in germanium and 0.7 V in silicon.

44
Q

What happens to a PN junction under reverse bias conditions?

A

Under reverse bias (positive terminal to n-region, negative terminal to p-region), the depletion zone widens, preventing current flow, and the PN junction acts as an insulator.

45
Q

What happens to a PN junction under forward bias conditions?

A

Under forward bias (negative terminal to n-region, positive terminal to p-region), the depletion zone narrows, allowing current to flow through the junction.

45
Q

What is the difference between intrinsic and extrinsic conductivity in semiconductors?

A

Intrinsic Conductivity: Conductivity of pure semiconductor materials without any impurities, dependent on temperature.
Extrinsic Conductivity: Conductivity of doped semiconductors, determined by the added impurities and less dependent on temperature.

46
Q

What are the temperature limits for germanium and silicon in terms of maintaining extrinsic conductivity?

A

Germanium (Ge): +75° C
Silicon (Si): +150° C
Exceeding these limits can cause intrinsic conductivity to surpass extrinsic conductivity.

47
Q

How do majority and minority carriers differ in n-type and p-type semiconductors?

A

N-type:
Majority Carrier: Free electrons
Minority Carrier: Holes
P-type:
Majority Carrier: Holes
Minority Carrier: Free electrons

48
Q

How does a diode convert alternating current (AC) to direct current (DC)?

A

A diode uses the rectifying property of a PN junction to allow current to flow in only one direction, blocking the reverse flow, thus converting AC to DC.

49
Q

Why can’t the diffusion voltage across a PN junction be used as a battery?

A

The diffusion voltage is confined to the depletion zone and cannot be extracted as a usable voltage source; it only influences the junction’s ability to conduct under forward bias.

50
Q

What role does the depletion layer play in the functioning of a semiconductor diode?

A

The depletion layer acts as a barrier that controls the flow of charge carriers, allowing current flow under forward bias and preventing it under reverse bias.

51
Q

What happens when silicon is doped with phosphorus?

A

Doping silicon with phosphorus introduces extra electrons (since phosphorus has 5 valence electrons), creating an n-type semiconductor with enhanced conductivity due to the increased number of free electrons.

52
Q

What happens when silicon is doped with boron?

A

Doping silicon with boron creates a p-type semiconductor by introducing holes (since boron has 3 valence electrons), which act as positive charge carriers, enhancing conductivity.

52
Q

What is the barrier potential of a PN junction, and what are its typical values for silicon and germanium?

A

The barrier potential is the voltage required to overcome the built-in electric field of the depletion layer. Typical values are 0.7 V for silicon and 0.3 V for germanium.

53
Q

How does temperature affect the intrinsic conductivity of a semiconductor?

A

Increasing temperature provides energy to break covalent bonds, generating more free electrons and holes, thus increasing intrinsic conductivity.

54
Q

What happens to the depletion zone in a PN junction when reverse biased?

A

The depletion zone widens as holes and electrons are pulled away from the junction, preventing current flow.

55
Q

What happens to the depletion zone in a PN junction when forward biased?

A

The depletion zone narrows as holes and electrons are pushed towards the junction, allowing current to flow.

56
Q

What is Peak Inverse Voltage (PIV) and why is it important in diodes?

A

PIV, or Maximum Reverse Voltage (VRmax), is the maximum reverse voltage that can be applied across a diode without causing reverse breakdown and damage. It is crucial for rectifying diodes in AC rectifier circuits to prevent diode failure. Typical values range from a few volts to thousands of volts.

57
Q

What is the Maximum Forward Current (IFmax) and what happens if it is exceeded?

A

IFmax is the highest level of forward current a diode can withstand. Exceeding IFmax generates excess heat across the junction, leading to thermal overload and diode failure. Proper cooling is necessary when operating near this limit to prevent damage.

58
Q

What is leakage current in a diode, and when does it become significant?

A

Leakage current is the small amount of current that flows when a diode is in reverse bias. It is generally negligible but becomes significant if the reverse voltage reaches breakdown, causing an avalanche of current flow. Keeping the reverse voltage below breakdown ensures minimal leakage current.

59
Q

Why is junction operating temperature important, and how is it specified?

A

Junction operating temperature affects diode reliability. Exceeding the maximum junction temperature can cause diode failure and fire risk. This temperature is specified in the diode’s datasheet and pertains to the internal junction, not the package temperature.

59
Q

How is power dissipation in a diode calculated, and why must it be managed?

A

Power dissipation in a diode is calculated as the voltage drop across the diode multiplied by the current flowing through it, resulting in heat. Managing this heat within tolerable limits is essential to prevent diode damage. The maximum power dissipation is a function of the current.

60
Q

What is the highest working frequency (Fm) of a diode, and why does it matter?

A

Fm is the upper limit frequency at which a diode can operate. High frequencies affect the diode’s recovery time, making it act like a capacitor and fail to rectify properly. This parameter is critical for applications requiring fast switching.

60
Q

What is the purpose of a clamping circuit?

A

Clamping circuits change the reference level of a waveform without altering its amplitude. They are used in radar and communications equipment. The circuit uses a diode and capacitor to clamp the high level of the output to a specific voltage.

60
Q

How does a positive limiter circuit work?

A

In a positive limiter circuit, the diode is forward biased during the positive half-cycle, limiting the output voltage to 0.7 V (for silicon diodes). It blocks the negative half-cycle, allowing it to pass to the load unaltered. This limits the positive half of the input waveform.

61
Q

How does a negative limiter circuit work?

A

In a negative limiter circuit, the diode is forward biased during the negative half-cycle, limiting the output voltage to -0.7 V (for silicon diodes). It blocks the positive half-cycle, allowing it to pass to the load unaltered. This limits the negative half of the input waveform.

62
Q

How does a half-wave rectifier function?

A

A half-wave rectifier allows current to pass only during the positive half-cycle of AC, blocking the negative half-cycle. With a capacitive load, it charges the capacitor during the positive half-wave, creating pulsating DC output.

63
Q

How does a full-wave rectifier function?

A

A full-wave rectifier allows current to pass during both half-cycles of AC, inverting the negative half. With a capacitive load, it smooths the output to a lower ripple DC, requiring a transformer with a double or tapped coil.

64
Q

What is the advantage of a bridge rectifier circuit?

A

A bridge rectifier provides full-wave rectification without the need for a tapped transformer. It uses four diodes to rectify both half-cycles of AC, ensuring each diode handles a maximum voltage of VTR, improving efficiency with capacitive loads.

65
Q

Why is a three-phase rectifier beneficial in aircraft?

A

A three-phase rectifier produces smooth DC from three-phase AC, preventing voltage from falling to zero. It uses six diodes to rectify the AC, providing a steady, non-pulsing DC output suitable for various aircraft electrical components.

66
Q

What is the principle behind voltage doubler circuits like Villard and Delon?

A

Voltage doubler circuits, like Villard and Delon, use capacitors and diodes to sum the peak AC voltage, doubling the output voltage. Villard charges a capacitor during the negative half-cycle and sums it with the positive half-cycle. Delon adds the two half-cycles together, providing a higher DC output.

67
Q

How does a voltage tripler circuit work?

A

A voltage tripler uses three stages of peak detection to output three times the peak input voltage. It charges capacitors in sequence during each half-cycle of AC, creating a higher DC voltage output without the need for large transformers.

68
Q

What is the construction of a thyristor?

A

A thyristor is constructed of four alternating layers (PNPN), forming three P-N junctions.

69
Q

How does a thyristor transition from blocking mode to conducting mode?

A

The transition occurs at a specific voltage (V) when no current flows through the gate.

70
Q

What is the holding current (IH) in a thyristor?

A

It is the specific value of current below which a thyristor can return to its high resistance state from its low resistance state.

71
Q

What distinguishes a Schottky diode from a conventional P-N junction diode?

A

A Schottky diode has a metal-to-N junction instead of a P-N semiconductor junction.

72
Q

How does the firing angle (φ) affect a thyristor in a controlled power supply unit?

A

The firing angle determines when the thyristor switches to a low resistance state, controlling the power output to the load.

73
Q

What are the typical forward biased current and voltage ranges for LEDs?

A

The forward biased current is between 5 mA and 40 mA, and the forward biased voltage is between 1.4 V and 4.5 V.

74
Q

List two advantages of Schottky diodes.

A

High switching speeds and low forward voltage drop (typically 0.25 V to 0.4 V).

74
Q

What are the common applications of Schottky diodes?

A

Logic gates, switched-mode power converters, RF detectors, mixers, and clamping circuits.

75
Q

What determines the color of light emitted by an LED?

A

The semiconductor material used in the LED.

76
Q

What materials are used to manufacture LEDs that emit infrared light?

A

Gallium Arsenide (GaAs) combinations with minimal amounts of zinc and silicon.

77
Q

How does the resistance of a photoconductive diode vary?

A

The resistance varies with the intensity of light incident on it.

78
Q

What is a common application of photodiodes?

A

Cameras, light meters, CD and DVD-ROM drives, TV remote controls, scanners, fax machines, and copiers.

79
Q

Why are photodiodes operated in reverse bias?

A

Because the reverse leakage current (dark current) is more sensitive to changes in illumination than forward current.

80
Q

What is the primary function of a varactor diode?

A

To function as a variable capacitor.

81
Q

How does changing the reverse bias voltage affect a varactor diode?

A

It changes the width of the depletion region, thereby altering the capacitance.

82
Q

What is a typical application of varactor diodes?

A

Electronically controlled tuning of resonant circuits in oscillators and filters.

83
Q

What is another name for a varistor?

A

Voltage Dependent Resistor (VDR).

84
Q

How does the resistance of a varistor change with voltage?

A

The resistance decreases as the voltage increases.

85
Q

What is the primary use of varistors?

A

To protect systems against excess voltage.

86
Q

What current range can rectifier diodes typically handle?

A

They can handle higher current ranges, from 1 A to hundreds of amps.

86
Q

What is the primary function of a rectifier diode?

A

To convert Alternating Current (AC) to Direct Current (DC).

87
Q

What is a Zener diode specifically designed for?

A

For a steep breakdown in the reverse biased direction to maintain a stable voltage.

88
Q

What is the Zener effect?

A

It is the effect where valence electrons are torn out of their lattice structure at a critical field strength, increasing the material’s conductivity abruptly.

88
Q

How are larger rectifier diodes typically encased?

A

They are encased in metal, which acts as a heat sink.

89
Q

How is a Zener diode used in a voltage regulator circuit?

A

By passing a small current through the diode from a voltage source via a current limiting resistor, the Zener diode maintains a constant voltage drop (Vout).

UKCAA MODULE 4 ELECTRONIC FUNDAMENTALS (5 decks)

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