Semiconductors Flashcards
Whats the conductivity of Silicon?
10^-2 to 10^2
What is the relationship between radius and energy level in atoms?
Energy increases with radius
How can energy for orbit change be supplied to electron?
Light or Heat
What is energy state spit?
Something that happens when adjacent atoms interact
What are the gaps between energy bands called?
Band Gaps
What is an energy band?
A range of allowed energy levels in a Solid formed by the overlap of orbitals
Where are all the electrons at T = 0K and what band is empty
Valence Band, Conduction band is empty
How many valence electrons does Silicon have
4
What happens to the electrons of Silicon at 0K?
They are bound to the Nucleus
Is Silicon conductive at 0K
No
How much energy is required to lift an electron into the conduction band for Silicon?
1.1 eV
What density of free electrons called?
Intrinsic Conduction Carrier Density
What is the relation between temperature and free electron number?
Number of free electrons increases with temperature
What is generation?
when an electron leaves the band leaving behind a hole
What is recombination?
When an free electron returns to fill the hole
What is the thermodynamic equilibrium of generation and recombination?
Free electron formation and electrons returning to valence band
What does the conductivity of a material depend on?
Number of electrons in conduction band which depends on the smaller band gap
What is an n-doped semiconductor?
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
What are majority and minority charge carriers in n-doped semiconductors?
Electrons are majority charge carriers, holes are minority charge carriers.
What is a p-doped semiconductor?
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
What are majority and minority charge carriers in p-doped elements?
Holes are majority charge carriers and electrons are minority charge carriers.
When can current flow in a semi-conductor?
When electrons or holes can move freely
What is the formula for current density s?
s = I/A
What is the total current density?
Sum of electron and hole current density?
What are the two physical causes of a current?
Drift and diffusion current
How do holes and electrons respond to an electric field with semiconductor?
Holes move with the Electric Field and Electrons move against it
What causes Drift current?
Application of electric field
What is a property of velocity of carriers in drift current?
Velocity is constant because of lack of arbitrary accelerations
What is the formula for charge carrier velocity?
v_n = -\micro_n E
v_p = \micro_p E
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.
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
What is the formula for total drift current?
s_DRIFT = \sigma E
where sigma is the electrical conductivity
Does diffusion current depend on electric field or charge of particle?
No
What is diffusion current?
Movement of particles from region of higher to lower concentration
What are the formulas for diffusion currents?
s_diff,n = qD_n dn/dx
s_diff,p = qD_p dp/dx
Where the D values are the diffusion coefficients
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
What is the basic use of a p-n junction>
Blocking or conduction of a current by applying different voltages (semiconductor diode)
What is the flow direction and blocking direction in p-n juction?
n-side is flow and p-side is blocking
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
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
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
What happens if the voltage increases too much in blocking direction of diode?
There is a breakthrough and current increases sharply
What are the three types of breakdowns in a p-n junction?
Avalanche
Tunnel/Zener
Thermal
What are Z-diodes?
- Diodes that use tunnel breakthrough
What are the applications of Z-diodes?
- Voltage limitation
- Voltage stabilization
- Design for operation in reverse direction
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
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
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
What is a photodiode?
Produces current when exposed to light
What are the applications of photodiodes?
- Galvanic Isolation
- High Voltage transmission
- Displays
What are passive components?
Resistors, capacitors, inductors
What are transistors used for?
To switch and control electrical voltages
What are the two designs of transistors?
- Bipolar Junction Transistor
- Field Effect Transistor
What are the three ports of BJT?
Base, collector and emitter
- The base port is the control port
What are the two types of BJTs?
- n-p-n
- p-n-p
Where do diodes face in npn and pnp diodes?
Inwards for pnp and outwards for npn
What are the three ways a bipolar transistor can be operated?
Emitter circuit
Collector circuit
Base Circuit
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.
What region should a BJT transistor be operated in?
Saturation region
What three areas can the output characteristic field for BJT be divided in?
Saturation area
Linear Area
Breakthrough Area
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
What is the gain factor?
/beta = (dI_C)/(dI_B)
typical values of beta are 10-250
What are the three terminals of the MOSFET?
Gate, drain and source
what is MOSFET?
Metal oxide field effect transistor
What are the three circuits for mosfet?
Source (=emitter)
Drain(=collector)
Gate(=base)
What are the two types of MOSFETS?
n and p type mosfets
What is the structure of the MOSFET?
three doped regions of different types
a gate connecting the two similarly doped regions via an insulator
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
What kind of power output is required for MOSFET at low frequencies?
Very low power
What are the three ranges for the MOSFET characteristic curve?
- Ohmic/triode 1
- Saturation 2,3
- Breakdown 4
What are IGBTs?
Insulated gate bipolar transistors
What are the three terminals of IGBTs?
Collector, gate, emmiter
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)
What are good applications of IGBTs
High power, high voltage, and high current
What does the structure of an IGBT consist of?
A Darlington circuit of a pnp BJT and an n-channel MOSFET
When is a Darlington circuit used?
- The Darlington circuit is used when very large currents are to be
switched with low powers
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
What are the operational ranges of the characteristic ranges of the IGBT
Reverse blocking range
Linear
Active range
Does the IGBT or MOSFET have greater safe operating range?
IGBT
What are some new wide band gap semiconductor materials?
Silicon carbide and gallium nitride
What is the order of band gap?
Si < SiC < GaN
What is the order of critical field?
Si < SiC < GaN
What is the order of electron mobility?
SiC< Si < GaN
What is the order of electron saturation?
Si < SiC < GaN
What is the order of thermal conductivity?
GaN < Si < SiC
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.
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.
