Basics of semiconductor physics and chemistry Flashcards
What are valence electrons?
The electrons in the outermost shell
How many electrons can a shell contain?
2*n^2 where n stands for the period
What are the three types of bonding?
Atomic bond, Ionic bond, Metallic Bond
How does the atomic/co-valent bond work?
Atoms share electrons in order to achieve the lowest energy state
How does the ionic bond work?
One atom donates an electron whereas another accepts an electron. The atoms stick together due to the charge difference between the,
What are cations and anions?
A charged atom
is known as an ion, while a positive ion is called cation and a negative ion anion,
respectively
How does Metallic bonding work?
The outer energy levels of atoms overlap, and valence electrons are shared between atoms, they can move around
What properties do greater attraction forces and number of free electrons have
greater attraction forces mean higher melting and boiling points, the
number of free electrons affects the conductivity.
What energy state do atoms want to achieve
The “octet” - 8 valence electrons, apart from helium with 2. Nobles gasses exist in this state and are unreactive
Why are insulators non-conductive
They have no free charge carriers
Are atoms in an atomic bond conductors?
No, there are no free electrons due to sharing electrons so atoms in atomic bonds are insulators
Are atoms in ionic bonds conductors?
In the solid state, ions are arranged in a grid network. By electrical forces, the particles are held together. There are no free charge carriers to enable a current flow. Thus
substances composed of ions can be both conductor and insulator.
What are semiconductors?
Semiconductors are solids whose conductivity lies between the conductivity of conductors and insulators. Due to exchange of electrons - to achieve the noble gas configuration - semiconductors arrange as lattice structure. Unlike metals, the conductivity
increases with increasing temperature.
How are holes formed?
An electron detaches from an atom, a hole is left. Another electron jumps to fill this hole, this happens in a chain. Holes and atoms move in opposite directions
What is the band gap?
The valence electrons - which serve as
charge carriers - are located in the valence band, in the ground state the conduction
band is occupied with no electrons. Between the two energy bands there is the band
gap, its width affects the conductivity of materials.
What is the valence band?
with a very
large number of atoms, a single electron can no longer be assigned to one single atom.
As a result, the energy bands of the individual atoms merge to a continuous band, the
valence band.
What occurs in the band model of conductors?
In conductors, the valence band is either not fully occupied with electrons, or the filled
valence band overlaps with the empty conduction band. In general, both states occur
at the same time, the electrons can therefore move inside the partially filled valence
band or inside the two overlapping bands.
In conductors there is no band gap between
the valence band and conduction band.
What occurs in the band model of insulators?
In insulators the valence band is fully occupied with electrons due to the covalent
bonds. The electrons can not move because they are “locked up” between the atoms.
To achieve a conductivity, electrons from the valence band have to move into the conduction band. This prevents the band gap, which lies in-between the valence band and
conduction band.
Only with considerable energy expenditure (if at all possible) the band gap can be
overcome; thus leading to a negligible conductivity.
(Very large band gap)
Is the valence band higher than the conduction band?
No, the conduction band is above the valence band
How large is the band gap in semiconductors?
In semiconductors, there is a band gap, but compared to insulators it is so small
that even at room temperature electrons from the valence band can be lifted into the
conduction band
Why are there positive charge carriers in semiconductors?
The electrons can move freely and act as charge carriers. In addition,
each electron also leaves a hole in the valence band behind, which can be filled by
other electrons in the valence band. Thus one gets wandering holes in the valence
band, which can be viewed as positive charge carriers.
Compare the amount of holes and electrons in a semiconductor
There are always pairs of electrons and holes, so that there are as many negative as positive charges, the semiconductor crystal as a whole is neutral.
What is an intrinsic semiconductor?
A pure undoped semiconductor is known as intrinsic semiconductor.
Why aren’t intrinsic semiconductors used for components?
Conductivity comparable to metals is only possible at very high temperatures
How can we influence the number of free holes and electrons?
Doping
How many electrons in the valence for P doping?
3 valence electrons eg. boron
How many electrons in the valence for n doping?
5 valence electrons eg. phosphorus
Do p dopants accept or donate electrons?
P dopants have 3 electrons so can catch an extra electron, leaving a hole in the valence band. Therefore they accept.
Do n dopants accept or donate electrons?
N dopants have 5 electrons so can donate an electrons. Therefore the dopants become positively charged
Electronic band structure in doped semiconductors
Through the introduction of a dopant with five outer electrons, in n-doped semiconductors there is an electron in the crystal which is not bound and therefore can be
moved with relatively little energy into the conduction band. Thus in n-doped semiconductors one finds a donator energy level near the conduction band edge, the band
gap to overcome is very small.
Analog, through introduction of a 3-valent dopant in a semiconductor, a hole is available, which may be already occupied at low-energy by an electron from the valence
band of the silicon. For p-doped semiconductors one finds an acceptor energy level
near the valence band.
What is the fermi level?
The fermi level is the highest energy state an electron can exist in at absolute 0. It is also the energy required to add or remove an electron from a body
In an N doped extrinsic semi-conductor, is the fermi level closer to the conduction or valence band?
The fermi level is closer to the conduction band
In an N doped extrinsic semi-conductor, is the fermi level closer to the conduction or valence band?
The fermi level is closer to the valence band
