Chapter 6 physics
What are the types of impurities
trivalent(P-TYPE) and pentavalent(N-TYPE) electrons added to silicon or germanium
What is the total charge of both n and p type
zero because they are both equal in number of holes and valance electrons
What type of atom is p-type
. Therefore, a hole is created in the silicon crystal, which is doped by trivalent impurity called acceptor atom, like trivalent impurity (Boron, aluminum, and Indium)
What are the methods to increase electrical conductivity of semi conductors
1. Thermal 2. Adding impurities also known as doping
What are the the characteristics of energy bands of conduction band
1. Valance bands interfere (overlap) with conduction bands. 2. No forbidden energy gap between valence bands and conduction bands. Consequently, valence electrons are free in conductors, thus, these metals have high electrical conductivity. 3. Electrical conductivity of metal decreases when temperature increases due to increase in electrical resistance (increase in time average of seismic energy of atoms and molecules).
What is the characteristics of energy bands in insulators
1. Valence band is full of valence electrons. 2. Conduction band is empty of electrons. 3. Forbidden energy gap is relatively wide.
What are the characteristics of conductors
A conductor is a material that allows the flow of electric current through it, electric charges move freely in conductors like (copper, silver, gold, and Aluminum). Conductors have one valence electron very loosely bind to the nucleus. These electrons are easily broken free from the nucleus and become (free electrons). Therefore, conductors have plenty of free electrons, an electronic current arises through the conductor by placing appropriate potential differences on its sides due to the movement of these electrons in one direction. The specific resistance of conductors is (10^-8 - 10^-5 Ωm).
What is the result of generating a electron current
Another current is created in valence band, it is called holes current. The direction of positive holes inside the crystal is toward the direction of the electric field while the electrons move in the opposite direction of the electric field, this means that holes move in the opposite direction of the electrons. See figure right.
What is the energy band in semi conductors
At very low temperature (at zero Kelvin 0k), and absence of light, semi-con- ductors behave like insulators, therefore, at these conditions: 1. Valence bands are full of valence electrons. 2. Conduction band is empty of electrons. 3. Forbidden energy gap is relatively narrow.
At very low temperature what do semi conductors work as and at what temperature ?
At very low temperature (at zero Kelvin 0k), and absence of light, semi-conductors behave like insulators,
What is the name of the two bands
Conduction band and valance band
What is the conduction band
Conduction band, it contains high allowable energy levels, higher than allowable energy levels of valence band. Its electrons are called conduction electrons, these conduction electrons can move freely to participate in electric conduction .Valence band and conduction band are separated by a gap called "forbidden energy gap".
What happens to the fermi level in n-type
Donor level is occupied by electrons freed by donor atoms. As a result, the Fermi level rises and becomes close to the conduction band. It is worth noting that electrons freed by pentavalent impurities leave no holes in the valence band when moving to conduction bond,(as in thermal effect). Therefore, the concentration of electrons in the conduction band is larger than the concentration of holes in the valence bond.
What is better thermal increase or doping to increase semiconductor conductivity
Doping
What can doping control ?
Doping can control electrical conductivity of semi-conductors and increase it significantly due to increase in Charge Carriers (electrons and holes) in the crystal compared to thermal effect.
What current are generated in valence band when imposing an electric field onto the sides of an intrinsic semi conductor ?
Electron current and hole current
What are energy bands
Electrons of any atom are affected by electrons of neighboring atom. This interaction between neighboring atoms divides allowable energy levels in external secondary shells into bands. Each band has a specific convergent secondary energy called Energy Bands.
Why Electrons spread through the junction PN stops when there is a state of balance.
Electrons spread through junction PN generate more positive ions and nega- tive ions on the sides of the junction PN at depletion zone. This creates an electric field (represented by red arrows) in fig.19, the potential difference resulting from this filed prevents electrons from further passing the junction, so, electrons stop spreading, this is called potential barrier. The potential barrier in the PN-diode depends on the type of semi-conductor, percentage of doped impurities, and temperature of the material. Potential barrier in PN-diode at room temperature (300K) is (0.7V) for silicon, and (0.3V) for germanium.
What happens to the free electron left from N-type doping
Free electrons participate in the electric conduction of the doped semi-conductor, this type of pentavalent impurity is called Donor atom.
What type of ion is created in p-type
In doping silicon with trivalent impurities (like Boron), the impurity becomes a negative ion, after accepting one electron from the silicon atom in the crystal structure. The negative ion is not a charge carrier because it binds with the crystal structure strongly by (covalent bonds). It does not participate in electrical conduction of the doped semi-conductor.
What type of ion is N-type
It becomes a positive ion that binds strongly with the crystalline structure. It is not a charge carrier because it does not participate in the electrical conduction of the doped semi-conductor.
What do trivalent impurities cause in
It is worth noting that the trivalent impurity causes a hole in the valence band when accepts one electron of valence electrons, (no transfer of extra electrons into conduction bond as in thermal effect). As a result, holes will be concentrated in the valence band more than the concentration of electrons in the conduction band, so, the holes in the valence band are called majority carriers while electrons in the conduction band are called Minority carriers. Hence, we have an N-type semi-conductor.
What determines availability of electrons at a given level of allowable energy?
Occupying electrons at an allowable energy level is compared to a particular level called Fermi Level. Fermi level is the highest allowable energy level an electron can occupy at absolute zero (0K).In conductors, at zero Kelvin, Fermi level is above the region which full of electrons in conduction band. The energy level occupied by these electrons is below Fermi level. As for intrinsic semi-conductors, Fermi level lies in the middle of the forbid- den energy gap between conduction band and valence band.When the semi-conductor is mixed with impurities, Fermi level shifts either above or below, and this shift depends on the type of extrinsic.
What do we need to control the direction of current or improve signals
PN diode
What is PN DIODE
PN- diode is obtained by doping a semi-conductor crystal (silicon or Germanium) with two types of impurities, one is trivalent (like boron), so we get a semi-conductor zone of the p-type, the other is pentavalent (antimony) . So we get a semi-conductor zone of N-type, the connection zone is dyed with a metal material so that wires can be connected to the crystal diode (PN) and the external circuit, see fig 18. The surface that separates the two zones is called junction.
What are the most common types of semi conductors
Silicon and Germanium
What happens to the energy gap in instrinic silicon at room temperature and what happens also to Germanium?
The forbidden energy gap decreases in intrinsic silicon when temperature increases above zero Kelvin to room temperature (300K), the gap will be (1.1eV intrinsic silicon) and (0.72 eV for intrinsic Germanium).
What is the total current flowing through semi conductors
The total current flowing through the semi-conductor is the sum of electron currents and holes current. Both electrons and holes are called Charge carriers.
How can we illisutarte elecgtron breaking free and escaping attraction power
The vertical axis y represents energy E measured in (eV) on negative scale compared to zero level (E=0) which is the highest level of atom energy, because the electron is bind to the nucleus by attraction. The lowest energy Hydrogen electron can have is (-13.6 eV), this means that when this electron gains (+13.6 eV) energy , it breaks free from the hydrogen atom ( at the ground level). This applies only to single atom.
What happens when increasing the temperature of semi conductors in the resistance
There is decrease in specific resistance of semi-conductor when its temperature increases.
What type of energy do acceptor atom create?
These acceptor atoms add a new energy level called Acceptor level, which lies within the forbidden energy gap and above the valence band, therefore, the Fermi level drops and becomes close to the valence bond.
What do the ions do to the concentration of free electrons and positive holes? n-type
These donor atoms increase concentration of free electrons in conduction bands, and decrease concentration of positive holes in valence bonds (originally generated by thermal effect). Therefore, donor atoms add a new energy level called donor level, which lies within forbidden energy gap and exactly below conduction band.
What do all electronic devices depend on
They all depend on crystal diode, transistors, integrated circuits
But, why a semi-conductor crystal after doped by pentavalent impurities, is called N-type? Or negative crystal? Is the crystal charge negative?
They are called N-type because Majority Carriers of the charge are the electrons, while the minority carriers are the positive holes.
But, why a semi-conductor crystal after doped by trivalent impurities (like Boron), is called P-type? Or the Positive Crystal? Is the crystal charge positive?
They are called P-type because Majority Carriers of the charge are the positive holes in valence band, while the minority carriers are the electrons in conduction band.
How many valance electrons do silicon atoms have
They have 4 valance elelctrons
What is the forbidden gap
This gap neither has allowable energy levels nor occupied by electrons. Each electron must gain energy form outer source in order to move from valence band to conduction band through forbidden energy gap. This outer power source (thermal, light or electric field) must be no less than energy of forbidden energy gap.
When P and N are put together they create a thin zone called
This process causes a thin zone on the two sides of the junction containing positive ions in zone N and negative ions in zone P, and without charge carriers, this zone is called Depletion region.
What is the majority carries and minority carries in n-type
Thus, electrons are called Majority Carriers, because they are genera- ted from doping and thermal and positive holes are minority carriers
Time average of generating (electron-hole) pairs in intrinsic semi-conductor depends on
Time average of generating (electron-hole) pairs in intrinsic semi-conductor depends on: 1- Temperature of semi-conductor. 2- Type of semi-conductor material.
How do we get P-type
To get a crystal semi-conductor P-type, it requires doping (carefully and controllably) of an intrinsic semi-conductor (silicon or Germanium) with a trivalent atom (like Boron) at room temperature. Therefore, each impurity atom dislocates a silicon atom of the crystalline structure and bind with three neighboring silicon atoms. But, the trivalent impurity leaves a covalent bond lacking one electron
What is N-type Semi conductors
To get a semi-conductor crystal N-type, an intrinsic semi-conductor crystal (silicon or Germanium) should be doped by a pentavalent atom (Antimony Sb for example). It should be carefully and controllably doped at room temperature. Consequently, each impurity shifts (dislocates) a silicon atom of the crystal structure and binds with four neighboring atoms. This process is done by four valence electrons out of the five electrons of the impurity, as for the fifth valence electron, it is left free in the crystal structure
What is the valence band
Valence Band It contains low allowable energy levels, partially or completely full of electrons and cannot be empty. Its electrons are called valence electrons, they cannot move among neighboring atoms because they are so close to the nucleus and closely attached to it.
What shells whose elements participate in chemical reactions and determines electrical properties of matter?
Valence shell, which have valence electrons which have greater energy and minimum attraction to nucleus is valence electrons and they are the shells which determine electrical properties and participate in chemical reactions
What happens when an electric field is imposed on the sides of silicon crystal at room temperature ?
When an electric field is imposed on the sides of intrinsic silicon crystal at room temperature, free electrons are attracted easily to the positive side. This free electron movement in the intrinsic semiconductor material is called electron current.
How can intrinsic semi-conductor (like silicon) have electric conductivity by heat?
When the temperature of the semiconductor increases to room temperature (300K), valence electrons gain enough energy to break covalent bonds (thermal energy) and move from valence band to conduction band through the forbidden energy gap. Then these electrons are free in conduction band.
Why do insulators have no electrical conductivity ?
because the forbidden energy gap in the insulator is relatively wide (around 5eV) or more, electrons in valence band cannot pass forbidden energy gap and move to conduction band when the supplied energy is less than forbidden energy gap. As a result, valence band remains full of valence electrons while conduction band is empty. It is worth noting that placing a huge electric field or heat onto the insulator might lead to collapse of the insulator and flow of a very little current through it.
If the thermal effect on semi-conductors increases electric conductivity, why resort to doping with trivalent or pentavalent?
controlling conductivity of the intrinsic semi-con ductor is not possible by thermal effect; therefore, it requires a better way to control electrical conductivity by adding atoms of pentavalent or trivalent elements called "impurities". Impurities are added carefully and accurately (average of 1 to108) at room temperature and at very low rates in an intrinsic semiconductor crystal. This process is called "doping".
What are the characteristics of insulators
do not allow flow of electric current in normal conditions. Valence electrons of insulators are strongly bound to the nucleus. Specific resistance of insulators is (101^0 - 101^6Ωm)
What happens when electrons move from valance to conduction band ?
each moving electron leaves behind an empty spot in valence band, this spot is called (hole) which acts as a positive charge. At this point, free electrons gather in conduction band and equal numbers of holes in valence band, this process generates the so-called "electron-hole" pair.
What are the characteristics of Semi-conductors
electric charges move less freely within them. The specific resistance of semi-conductors ranges between that of the conductors and that of the insulators in electric conductivity, which is around (10^-5 - 10^8Ωm).
