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Semiconductors Flashcards

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1
Q

Whats the conductivity of Silicon?

A

10^-2 to 10^2

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2
Q

What is the relationship between radius and energy level in atoms?

A

Energy increases with radius

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3
Q

How can energy for orbit change be supplied to electron?

A

Light or Heat

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4
Q

What is energy state spit?

A

Something that happens when adjacent atoms interact

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5
Q

What are the gaps between energy bands called?

A

Band Gaps

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6
Q

What is an energy band?

A

A range of allowed energy levels in a Solid formed by the overlap of orbitals

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7
Q

Where are all the electrons at T = 0K and what band is empty

A

Valence Band, Conduction band is empty

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8
Q

How many valence electrons does Silicon have

A

4

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9
Q

What happens to the electrons of Silicon at 0K?

A

They are bound to the Nucleus

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10
Q

Is Silicon conductive at 0K

A

No

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11
Q

How much energy is required to lift an electron into the conduction band for Silicon?

A

1.1 eV

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12
Q

What density of free electrons called?

A

Intrinsic Conduction Carrier Density

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13
Q

What is the relation between temperature and free electron number?

A

Number of free electrons increases with temperature

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14
Q

What is generation?

A

when an electron leaves the band leaving behind a hole

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15
Q

What is recombination?

A

When an free electron returns to fill the hole

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16
Q

What is the thermodynamic equilibrium of generation and recombination?

A

Free electron formation and electrons returning to valence band

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17
Q

What does the conductivity of a material depend on?

A

Number of electrons in conduction band which depends on the smaller band gap

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18
Q

What is an n-doped semiconductor?

A

A semiconductor in which some silicon atoms have been replaced with a pentavalent element like phosphorus so that the fifth electron has a small band gap which leads to more free electrons than holes

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19
Q

What are majority and minority charge carriers in n-doped semiconductors?

A

Electrons are majority charge carriers, holes are minority charge carriers.

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20
Q

What is a p-doped semiconductor?

A

Semiconductor in which some silicon atoms are replaced with trivalent element like boron so the band gap to the valence band is very small and electrons can be easily absorbed leading to more holes than free electrons

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21
Q

What are majority and minority charge carriers in p-doped elements?

A

Holes are majority charge carriers and electrons are minority charge carriers.

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22
Q

When can current flow in a semi-conductor?

A

When electrons or holes can move freely

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23
Q

What is the formula for current density s?

A

s = I/A

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24
Q

What is the total current density?

A

Sum of electron and hole current density?

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25
What are the two physical causes of a current?
Drift and diffusion current
26
How do holes and electrons respond to an electric field with semiconductor?
Holes move with the Electric Field and Electrons move against it
27
What causes Drift current?
Application of electric field
28
What is a property of velocity of carriers in drift current?
Velocity is constant because of lack of arbitrary accelerations
29
What is the formula for charge carrier velocity?
v_n = -\micro_n E v_p = \micro_p E
30
How does carrier mobility vary with electric field strength and temperature and doping density?
Is constant with lower electric field strength and decreases with increasing field strength due to scattering. Similarly, it decreases with increasing temperature and doping density.
31
What is the formula for current density of charge carriers?
s_drift,n = q* n * \micro_n * E s_drift,p = q* p * \micro_p * E where n is the electron density, p is the hole density and q is the elementary charge
32
What is the formula for total drift current?
s_DRIFT = \sigma E where sigma is the electrical conductivity
33
Does diffusion current depend on electric field or charge of particle?
No
34
What is diffusion current?
Movement of particles from region of higher to lower concentration
35
What are the formulas for diffusion currents?
s_diff,n = q*D_n* dn/dx s_diff,p = q*D_p* dp/dx Where the D values are the diffusion coefficients
36
What happens when you join a p and n doped region?
Diffusion motion where electrons move to the p-doped region and holes to the n-doped region Formation of depletion region Creation of electric field that counteracts diffusion current with drift current Thermodynamic Equilibrium of diffusion and drift current
37
What is the basic use of a p-n junction>
Blocking or conduction of a current by applying different voltages (semiconductor diode)
38
What is the flow direction and blocking direction in p-n juction?
n-side is flow and p-side is blocking
39
What happens if you apply voltage in the flow direction of p-n junction?
Diffusion dominates, depletion region decreases, Electrons flow from n-region to p-region, Electric field of p-n junction is decreased Current > 0
40
What happens if you apply voltage in blocking direction of p-n junction?
- Electric field of junction is enhanced - Drift motion dominates - Current < 0 but very small - Electron-hole pairs generated - Depletion region increases
41
What is the Diode Equation?
I_D = I_S *( exp( U_pn/U_T ) -1) Where I_D is the diode current, I_S is the saturation current and U_T is the temperature voltage
42
What happens if the voltage increases too much in blocking direction of diode?
There is a breakthrough and current increases sharply
43
What are the three types of breakdowns in a p-n junction?
Avalanche Tunnel/Zener Thermal
44
What are Z-diodes?
- Diodes that use tunnel breakthrough
45
What are the applications of Z-diodes?
* Voltage limitation * Voltage stabilization * Design for operation in reverse direction
46
How do LEDs work?
* Electrons cross the pn-junction and recombine with holes when a positive voltage is applied in the forward direction. * Energy is released during recombination, which is emitted in the form of a photon. * This process is called electroluminescence. * The emitted power is proportional to the forward current during operation. * Color emitted depends on band gap
47
What are the voltages / wavelengths corresponding to the different color LEDs
IR - 1.3 - 930 red - 1.8 - 650 yellow - 2.2 - 590 green - 2.4 - 570 blue - 3.5 - 470 white - 3.5
48
What are the applications of LEDs?
* Lighting of all kinds * Radiation of infrared light in sensor technology * Generation of UV radiation in medical applications * Signal generation in optical fibers
49
What is a photodiode?
Produces current when exposed to light
50
What are the applications of photodiodes?
- Galvanic Isolation - High Voltage transmission - Displays
51
What are passive components?
Resistors, capacitors, inductors
52
What are transistors used for?
To switch and control electrical voltages
53
What are the two designs of transistors?
- Bipolar Junction Transistor - Field Effect Transistor
54
What are the three ports of BJT?
Base, collector and emitter - The base port is the control port
55
What are the two types of BJTs?
- n-p-n - p-n-p
56
Where do diodes face in npn and pnp diodes?
Inwards for pnp and outwards for npn
57
What are the three ways a bipolar transistor can be operated?
Emitter circuit Collector circuit Base Circuit
58
How can a transistor be used in an Emitter circuit?
Connect control voltage to collector and emitter. To make the transistor conductive provide voltage greater than diffusion voltage between the base and the emmiter.
59
What region should a BJT transistor be operated in?
Saturation region
60
What three areas can the output characteristic field for BJT be divided in?
Saturation area Linear Area Breakthrough Area
61
What is the main characteristic of the BJT?
Multiplication of base current with the collector current. In normal operation I_B is much smaller than I_C
62
What is the gain factor?
/beta = (dI_C)/(dI_B) typical values of beta are 10-250
63
What are the three terminals of the MOSFET?
Gate, drain and source
64
what is MOSFET?
Metal oxide field effect transistor
65
What are the three circuits for mosfet?
Source (=emitter) Drain(=collector) Gate(=base)
66
What are the two types of MOSFETS?
n and p type mosfets
67
What is the structure of the MOSFET?
three doped regions of different types a gate connecting the two similarly doped regions via an insulator
68
What happens when a voltage is applied to the MOSFET gate?
the insulated connection affects the electric field between drain and source, allowing or preventing current flow
69
What kind of power output is required for MOSFET at low frequencies?
Very low power
70
What are the three ranges for the MOSFET characteristic curve?
- Ohmic/triode 1 - Saturation 2,3 - Breakdown 4
71
What are IGBTs?
Insulated gate bipolar transistors
72
What are the three terminals of IGBTs?
Collector, gate, emmiter
73
What is the advantage of IGBTS?
Combines the advantages of the BJTs (high reverse voltage and low forward resistance) and MOSFETs (little power required for driving)
74
What are good applications of IGBTs
High power, high voltage, and high current
75
What does the structure of an IGBT consist of?
A Darlington circuit of a pnp BJT and an n-channel MOSFET
76
When is a Darlington circuit used?
* The Darlington circuit is used when very large currents are to be switched with low powers
77
How is the IGBT controlled?
The IGBT is controlled via the gate of the MOSFET: If the gate emitter voltage UGE exceeds the threshold voltage, current flows across the BJT
78
What are the operational ranges of the characteristic ranges of the IGBT
Reverse blocking range Linear Active range
79
Does the IGBT or MOSFET have greater safe operating range?
IGBT
80
What are some new wide band gap semiconductor materials?
Silicon carbide and gallium nitride
81
What is the order of band gap?
Si < SiC < GaN
82
What is the order of critical field?
Si < SiC < GaN
83
What is the order of electron mobility?
SiC< Si < GaN
84
What is the order of electron saturation?
Si < SiC < GaN
85
What is the order of thermal conductivity?
GaN < Si < SiC
86
What are application possibilities of SiC?
* SiC photodiodes: Sensitive to UV light. * SiC MOSFETs: Low on-resistance, smaller cooling surfaces. Lower switching times enable precise results with lower radiation loads, e.g. in medical imaging technology.
87
What are application possibilities for GaN?
* GaN-HEMT (High-Electron Mobility Transistor): For high-frequency applications due to low gate capacitance, thus enabling smaller passive components. * 48 V DC/DC converter: Currently used for 5G infrastructure, AI applications and LIDAR systems due to stable power supply, high power density and high switching frequencies at low cost.

Electrical Engineering (3 decks)

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