Electronic Devices Lectures
The ideal component:
Take no space – to the nano meter
Weigh nothing – 0.1 lb
Use no power – small in size so small in power (micro or nano amps)
Costs nothing – $2 divided by 5x10-9
Require no maintenance -
Require no training to operate.
Last forever
SWPCMT and F
“Smart weightless power costs more training but lasts forever.”
Study the manual for buying semiconductors.
Semiconductor fundamentals:
Crystals
Some background
Bell Laboratories (1947) - the 1st transistor was made
John Bardeen
Walter Brattain
William Shockley
2012 – 20nm 128Gb NAND chip with 4.5x1012 transistors
The first integrated circuit
Robert Noyce
The mayor of Silicon Valley
Fairchild (1957)
Intel (1968)
Jack Kilby
Texas instruments
Nobel prize
First IC at Fairchild semiconductor (1958)
First IC at Texas Instruments (1958)
Silicon Valley history
Left graph - # of transistor in a chip
Right graph – Size of chip
Units
Device dimensions
Microns (micrometers) – 10-6
Oxide thickness
Angdstroms (A) or nanometers (nm) – 10-9
Wafer diameters
Millimeters(mm) or inches
Sometimes mils are used for wafer thickness or chip size
Crystal growth, crystal structure
What is a semiconductor – so – so conductivity
Insulator (0 - glass) – semiconductor – conductor (∞-gold)
Conductivity equation
Inverse of the resistance : 1/R
Proportional to current
Q = 1.602 * 10-19 (c)
Electron charge
N – number of free electrons (e-)
P – number of free holes (h+)
Mn - how fast the e- is moving
Mp – how fast the h+ is moving
To increase the conductivity, change the number of e- or h+
Dopant is the technique used change e- and h+ numbers
Resistivity
Inverse of conductivity
Easy to vary resistivity of a semiconductor
Semiconductor resititivty depends on carrier density and mobility
Types of semiconductors
Elemental – using one material
Compound – uses more than one material
Focus on Si, Ge, Ga and As
Group IV
Silicon is the most common
In most integrated cirucits and power devices
SIC for high temperature devices
Group III and V
GeAs is the most common
Used for light emitting de
High speed devices/circuits
Light emitting
GaN, GaAsP
Optical communication
GaInAsP
Group II and VI
Lightning arresters
ZnO
Infrared detectors
HgCdTe
Review Questions
What material parameters determine the semiconductor resistivity?
Which one material parameter influences the semiconductor resistivity the most?
What does “group IV” etc. mean?
Which groups form semiconductors?
Crystal growth, Crytal Structure
How to make a semiconductor
Bring single crystal seed into the melt
Dip seed into melt and withdraw slowly
Pull seed with proper pull rate and roation
The ingot is pure silicone
Czochraliski crystal growth
Slicing process for making wafers
Wire coated in diamond splinters.
Cost for wafers
How many cpu can you make with a single wafer
1 cpu is 506 mm2
Crystals structure
Amorphous
Randomly distributed, working like an insulator
Glass
Polycrystalline
Metal, some organization, working like an conductor
Single crystal
Semiconductor, pure silicone
Simple cubic lattice
Only 1 atom in total actually in the box
Atoms on every corner
Body centered cubic (bcc) lattice
Only 2 atoms in totally actually in the box
Atoms on every corner AND 1 in the middle
Face centered cubic (fcc) lattice
Only 4 atoms actually in the box
Atoms on every corner AND 1 atom on the surface of each face of the cube
Diamond latticue – fcc with 4 additional atoms
Only 8 atoms actually in the box
Example
Example
Si = diamond lattice
8 atoms in a diamond lattice divided by the area will give the number of atoms per cubic centimeter
Unit cell
A small portion of any crystal that could be used to reproduce the crystal
The original lattice can be readily reproduced by merely duplication the unit cell
The unit cell need not be primitive (the smallest unit cell possible)
Wafter orientation, miller indices
Wafter orientation
Millier indices
the surface of semiconductors wafers is specified by a set of number. These numbers are called
the numbers that specify a wafter surface
Surface orientation is critical in device processing steps and directly affects the characteristics.
example
the atoms that make up the three principal planes in the diamond lattice
Zincblende structure
Same as diamond structure EXCEPT there are two differ types of atoms in the lattice
Ex, GaAs, InP, GaP, GaN etc…
Crytal structure
Top view of zincblende structure showing the atoms in the three principle planes
Direction
Set up a vector of arbitrary length in the direction of interest.
Decompose the vector into its components by projecting along the coordinate’s axes.
Normalize to lowest integers.
Miller indices cannot be established for a plane passing through the origin of coordinates
The orgin of coordinates must be moved to a lattice point not on the plane to be indexed
The procedure is acceptable because of the equivalent nature of parallel planes
Review questions
What are miller indices
How are they determined
How many atoms are there in the (100) plane
How many atoms are there in the three dimensional diamond lattice unit cell?
What is the distance between (111) planes in a simple cubic lattice?
What are the angles between (110) and (100) planes in a simple cubic lattice?
Energy levels, bonds, and bands
Energy levels
Electronic atom structure
Isolated atom – an atomic core or nucleus surrounded by electrons.
Various quantum numbers
Principle quantum number (n) – determines the energy of an election.
Azimuthal quantum number (l) - determines the angular momentum magnitude.
For each value of n there are several l values that govern the spatial distribution of the electron
Each l value constitutes a subshell.
Energy needed to use the electron at that specific level
Energy band diagram
When Si atoms are brought close to eachother, discrete energy levels broaden into bands.
Conduction band (Ec) – higher band, can hold 8 electrons
Band gap (Eg) – electrons cannot exist in this gap, determines the characteristics of the device (ie metal, semiconductor, insulator)
1 eV = 1.602 x 10-19 J
The amount of energy gained/lost by the charge of a single election moved across an electric potential difference of 1 volt.
For silicone = 1.12 eV
For GaAs = 1.42 eV
Valence band (Ev) – lower band, can hold 8 electrons
Lower band gap energy = lower resistance = higher conducivity
Bonds and bands
Covalent bonds – stable
A crystal containing on Si atoms with no impurities gives rise to the conduction and valence bands with no levels in the band gap (forbidden zone)
At low temperatures, there are no electrons in the conduction band and no hoes in the valence band. Aka valence band is full. AND CURRENT = 0;
No bands are broken
Band electros in crystalline silcon are not tied to any one particular atom.
As the temp of the SI crytal increases, some bonds break (electrons break loose),
Free elections – free electrons only in the conduction band
Free holes - only in the valence band
Holes = where a electiron is missing
Current (i) = dQ/Dt
Q = charge = free elctrons and free holes
Election hole pari (EHP) – for every free hole/electron there is a free hole/electron
Dopant atoms – in Si have more or less than 4 electrons in the outermost shell
Donor
Group five atoms (P, As, Sb).
Have 5 electrons but Si only needs 4 for a colvant bond. Makes for a unstable condition making it easy to donate the extra electron
At low temps, insufficient energy to excite electro to conduction band
Higher temp, donor donates electron
N -type – more electrons than holes
Acceptor
Group 3 atoms (B, Ga, In)
Has three electron but si needs 4 for a covalent bond. Makes for an unstable condition meaning it need to accept an electron
At low temps, insufficient energy to excite hold to valence band
At higher temps, acceptor accepts electron
P – type – more holes than electrons
Intrinsic semiconductor – an extremely pure semiconductor sample containing any significant amount of impurity atom
N = number of electrons/cm^3
P = number of holes/cm^3
N = p = ni
Ni
Carriers
Review questions
A si atom has how many electrons?
How many of these electrons are in the outermost shell?
What is the band gap?
Why is a semiconductor an insulator at low temperatures
What is a donor and how does it function
What is an acceptor and how does it function?
Density of states = # of chairs available for electrons
Effective mass(Mn*) – of electrons within a crystal is a function of the semiconductor material and is different from the mass of electrons within a vacuum. Different from mass of electrons in the free space
Electrons moving in a vacuum
Electrons moving in a semiconductor crystal: collide with semiconductor atoms making it hard to calculate the speed of the atom
Fermi Function f(E), gives the probability of a state being occupied by an electron
Example, if the state density is 6 and the fermi function is 0.5, that means that half of the density will be filled aka we have 3 electrons
For E >= Ef + 3kT, the fermi function becomes the Boltzmann approximation (kT = 0.026 eV at T = 300k)
Increasing temperature = increase in probability of seeing e- in Ec = decrease the probability of seeing e in Ev
Density state vs fermi function
Gc(E) – density of states - probability of finding an electron on a state
F(E) – fermi function - probability of those states being occupied
Gc(E) * f(E) = # of electron present
Gv(e) * (1 – f(e) ) = # of free holes
Review questions
What is the density of states g(e)?
How does g(e) depend on energy E?
What is an effective mass?
What is the fermi function f(e)
How does f(e) depend on temperature
What is the fermi energy or fermi level Ef
What is the occupancy of a state at e = Ef
Electron density – deponds on the density of states and on fermi function
N I the area under the (E – Ec) vs gc(e) * f(e) curve
