electronic Flashcards
what accounts for the mass of an atom?
neutron and proton
charge on electron
-q = -1.5x10⁻¹⁹
given in exam
Boltzmann’s constant
governs energy gained by electrons as a result of temp above absolute zero,
value given in exams
Pauli’s Exclusion Principle
no two electrons orbiting an atom can have identical quantum numbers, ‘nlms’.
Two orbiting electrons may have the same energy level but have opposite spins in which case s = ±½ and this means their quantum numbers differ in the spin characteristic s.
When several atoms are in close proximity, the energy associated with the individual quanta cannot be identica
continuum or band of energy levels
Band Theory of Solids
- spacing between levels decreases as n umber of neighbouring atoms increases
- in crystalline structure of solid element, there are TONS of neighbouring atoms which exert influence on each other
- each individual quantum which existed in an atom considered in isolation becomes a continuum or band of energy levels around the original quantum
energy bands in insulators
-characterised by large energy gap between valence and conduction bands
- at room temp no electrons gain sufficient energy to make transition between bands
- electrons remain firmly bonded to their atoms in valence band
energy bands in conductors
- conduction and valence bands overlap
- plentiful supply of free energy levels close to those occupied by electrons in upper region of valence band of metals
- at room temp e- can easily move into vacant levels in conduction bands
- outer electrons of metals break free of parent atoms + become free charge carriers
- free neg-charged electrons can readily be made to move, forming an electric current
energy bands in semiconductors
- have energy gap between conduction and valence bands that is much lower than that of insulators
- at room temp, no. of electrons can make the transition from valence band to conduction band but much smaller no. than conductors
controlling extent of conduction in semiconductors
-doping the semiconductor materials w/ impurities in the form of another element from neighbouring group in Periodic Table
most common semiconducting material
Silicon Si
also used:
Germanium Ge
Gallium Ga
Arsenic As
Fermi Level
In the context of electronic materials,
defined as the energy associated with the highest energy level occupied by an orbiting electron at absolute zero temperature, 0K
Fermi Level and absolute zero
at absolute zero temp, all available energy orbitals below Fermi Level are occupied + all of those above are unoccupied
Fermi Level for conductors, semiconducts, and insulators
- Conducting materials: Fermi level located somewhere in conduction band
- Semiconducts: it is not an occupied level + lies between valence and conduction band
intrinsic silicon - total current flwoing through material
- consists of sum of both components of positive and negative charges
- why this semiconductor technology is referred to as -bipolar-
free electrons and holes in intrinsic silicon
-always created in pairs
intrinsic silicon
un-doped silicon
Fermi-Dirac Probability Function
-indicates probability at any temp that an energy level is occupied by an electron
Fermi-Dirac Function notes
- function only applies to energy levels that exist + are available in material
- function has rectangular shape at T=0K
probability function - superimposed at room temp, with one curve being probability of occupancy and the other being of vacancy
- sum of all corresponding points on curves is unity for all energy values
- bc free electrons + holes are generated in pairs
- bc of this symmetry, Fermi Level for intrinsic Si is placed midway between conduction and valence bands
concentrations of electrons and holes in intrinsic Silicon
they are equal
density of atoms in intrinsic silicon material
5 x 10²² cm⁻³
femi energy in intrinsic silicon
-halfway between the valence band edge and conduction band edge
degree/intensity of doping
-classified according to number of impurity atoms implanted into Si per unit volume, relative to atomic density of pure Si
intrinsic silicon and temp
- at absolute zero: an insulator
- at room temp: moderate conductor
n-type doped silicon
produced by doping Si with a group V material
eg. Phosphorous (P)
-extra electron, becomes free negative charge carrier
n-type: majority and minority carriers
-conc of electrons greater than holes
- electrons: majority carriers
- holes: minority carreirs
p-type doped silicon
produced by doping Si with a group III material
eg. Aluminium (Al)
- 3 electrons in outer shell
p-type: majority and minority carriers
conc of holes greater than of electrons
- holes: majority carriers
- electrons: minority carriers
fermi level in p-type semiconductor
lower, towards valence band
potential difference V between two points
the difference between the two potentials or voltages as measured relative to ground.
unit: Volts
potential difference and sign
-positive when Potential V₁ relative to grund is more positive than potential V₂ relative to ground
electromotive force EMF
When an electric field acts along a path where charged particles are free to move, it exerts an influence on the mobile charge carriers called an electromotive force
-measured in Volts
EMF in an electric circuit
voltage between terminals of battery is the EMF which it generates
electric current
When charged particles free to move in a conducting material are placed under influence of an electric field acting in a defined direction, they will tend to move in a direction determined by that of the field
electric current definition
the quantity of charge flowing through a piece of conducting material per unit time
measured im Amperes (A)
electric field strength definiton
force per unit charge acting at a point in the field
carrier mobility
velocity with which a carrier will drift, on average, in a unit field strength of Vm⁻¹
unit: m²V⁻¹s⁻¹
mobility of electrons in conduction band and valence band
Generally, mobility of electrons in conduction band is somewhat greater than that of holes in valence band, i.e.
μₙ > μₚ
charge flux density
-average charge flow considered per unit area normal to direction of flow
electrons and holes and current
Electrons and holes drift in opposite directions in any given electric field.
However, since charge on the electron is negative, both electrons and holes make positive contributions to conventional current which is specified as being positive in the direction of the electric field
diffusion current
rate at which diffusion takes place
-determines charge flux density due to diffusion
diffusion coefficients
measure of relative ease with which the carriers can spread outward on a wave-like surface with decreasing concentration
dimensions of coefficients
cm₂s₋₁ (area/unit time)
thermal voltage
the equivalent potential difference within an electric field through which an electron having a charge of magnitude q must accelerate in order to gain the same amount of energy as it has due to temperature, i.e. kT Joules.
p-n junction
-two pieces of semiconductor are joined together,
-imbalance of free carrier concentrations in both
conduction + valence bands on each side of junction formed between p-type and n-type materials.
-gives rise to carrier concentration gradients across junction.
-electrons diffuse across the junction from n-type to p-type, while holes diffuse from p-type to n-type material, setting up associated diffusion currents.
-Electrons entering p-type material readily recombine with the plentiful holes present here
-holes entering the n-type material readily recombine with the plentiful electrons here
junction region
referred to as the depletion region, having a much reduced conc of mobile or free charge carriers
equilibrium conditions: net transfer of either type of carrier
Under equilibrium conditions, there is no net transfer of either type of carrier from one material to the other i.e. the combined diffusion and drift currents are zero for both holes and electrons.
capacitance depends on
- dependent on the width of the junction region + on the doping concentrations of both p-type and ntype materials
- dependent on any electric field which may be applied externally + is hence a bias voltage dependent property in semiconductor devices
effect of presence of junction capacitance
-has effect of slowing down changes in current which may be induced in response to externally applied conditions
ideal diode equation
characterizes current-voltage relationship of diode as an electrical circuit element
resistivity
measure of how strongly a material opposes flow of electric current through it
what resistivity depends on
- particular atomic strcuture
- measured in Ohm-metres
what resistance depends on
- resistivity of material
- physical dimensions
resistance unit
Ohm Ω
capacitance
ability of a body to store electrical energy in form of charge associated with an electric field
permeability
-property which determines degree to which material becomes magnetised when subjected to influence of a magnetic field
Hall Effect - where it applies
-applies to materials which conduct electricity and are also magnetisable
seebeck coefficient
measure of sensitivity of the thermal emf, Vₜₕ to temp gradient
