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Semiconductors :- Most of the solids can be placed in one of the two classes: Metals and insulators. Metals are those through which electric charge can easily flow, while insulators are those through which electric charge is difficult to flow. This distinction between the metals and the insulators can be explained on the basis of the number of free electrons in them. Metals have a large number of free electrons which act as charge carriers, while insulators have practically no free electrons.
There are however, certain solids whose electrical conductivity is intermediate between metals and insulators. They are called 'Semiconductors'. Carbon, silicon and germanium are examples of semi-conductors. In semiconductors the outer most electrons are neither so rigidly bound with the atom as in an insulator, nor so loosely bound as in metal. At absolute zero a semiconductor becomes an ideal insulator.
semiconductors - Theory and Definition
Semiconductors are the materials whose electrical conductivity lies in between metals and insulator. The energy band structure of the semiconductors is similar to the insulators but in their case, the size of the forbidden energy gap is much smaller than that of the insulator. In this class of crystals, the forbidden gap is of the order of about 1ev, and the two energy bands are distinctly separate with no overlapping. At absolute o0, no electron has any energy even to jump the forbidden gap and reach the conduction band. Therefore the substance is an insulator. But when we heat the crystal and thus provide some energy to the atoms and their electrons, it becomes an easy matter for some electrons to jump the small (» 1 ev) energy gap and go to conduction band. Thus at higher temperatures, the crystal becomes a conductors. This is the specific property of the crystal which is known as a semiconductor.
Effect of temperature on conductivity of Semiconductor
At 0K, all semiconductors are insulators. The valence band at absolute zero is completely filled and there are no free electrons in conduction band. At room temperature the electrons jump to the conduction band due to the thermal energy. When the temperature increases, a large number of electrons cross over the forbidden gap and jump from valence to conduction band. Hence conductivity of semiconductor increases with temperature.
Pure semiconductors are called intrinsic semi-conductors. In a pure semiconductor, each atom behaves as if there are 8 electrons in its valence shell and therefore the entire material behaves as an insulator at low temperatures.
A semiconductor atom needs energy of the order of 1.1ev to shake off the valence electron. This energy becomes available to it even at room temperature. Due to thermal agitation of crystal structure, electrons from a few covalent bonds come out. The bond from which electron is freed, a vacancy is created there. The vacancy in the covalent bond is called a hole.
This hole can be filled by some other electron in a covalent bond. As an electron from covalent bond moves to fill the hole, the hole is created in the covalent bond from which the electron has moved. Since the direction of movement of the hole is opposite to that of the negative electron, a hole behaves as a positive charge carrier. Thus, at room temperature, a pure semiconductor will have electrons and holes wandering in random directions. These electrons and holes are called intrinsic carriers.
As the crystal is neutral, the number of free electrons will be equal to the number of holes. In an intrinsic semiconductor, if ne denotes the electron number density in conduction band, nh the hole number density in valence band and ni the number density or concentration of charge carriers, then
ne = nh = ni
As the conductivity of intrinsic semi-conductors is poor, so intrinsic semi-conductors are of little practical importance. The conductivity of pure semi-conductor can, however be enormously increased by addition of some pentavalent or a trivalent impurity in a very small amount (about 1 to 106 parts of the semi-conductor). The process of adding an impurity to a pure semiconductor so as to improve its conductivity is called doping. Such semi-conductors are called extrinsic semi-conductors. Extrinsic semiconductors are of two types :
i) n-type semiconductor
ii) p-type semiconductor
When an impurity atom belonging to group V of the periodic table like Arsenic is added to the pure semi-conductor, then four of the five impurity electrons form covalent bonds by sharing one electron with each of the four nearest silicon atoms, and fifth electron from each impurity atom is almost free to conduct electricity. As the pentavalent impurity increases the number of free electrons, it is called donor impurity. The electrons so set free in the silicon crystal are called extrinsic carriers and the n-type Si-crystal is called n-type extrinsic semiconductor. Therefore n-type Si-crystal will have a large number of free electrons (majority carriers) and have a small number of holes (minority carriers).
In terms of valence and conduction band one can think that all such electrons create a donor energy level just below the conduction band as shown in figure. As the energy gap between donor energy level and the conduction band is very small, the electrons can easily raise themselves to conduction band even at room temperature. Hence, the conductivity of n-type extrinsic semiconductor is markedly increased.
In a doped or extrinsic semiconductor, the number density of the conduction band (ne) and the number density of holes in the valence band (nh) differ from that in a pure semiconductor. If ni is the number density of electrons is conduction band, then it is proved that
ne nh = ni2
If a trivalent impurity like indium is added in pure semi-conductor, the impurity atom can provide only three valence electrons for covalent bond formation. Thus a gap is left in one of the covalent bonds. The gap acts as a hole that tends to accept electrons. As the trivalent impurity atoms accept electrons from the silicon crystal, it is called acceptor impurity. The holes so created are extrinsic carriers and the p-type Si-crystal so obtained is called p-type extrinsic semiconductor. Again, as the pure Si-crystal also possesses a few electrons and holes, therefore, the p-type si-crystal will have a large number of holes (majority carriers) and a small number of electrons (minority carriers).
It terms of valence and conduction band one can think that all such holes create an accepter energy level just above the top of the valance band as shown in figure. The electrons from valence band can raise themselves to the accepter energy level by absorbing thermal energy at room temperature and in turn create holes in the valence band.
Number density of valence band holes (nh) in p-type semiconductor is approximately equal to that of the acceptor atoms (Na) and is very large as compared to the number density of conduction band electrons (ne). Thus,
nh» Na > > ne
The semiconductor diode is a device that will conduct current in one direction only. It is the electrical equivalent
of a hydraulic check valve. The semiconductor diode has the following characteristics:
· A diode is a two-layer semiconductor consisting of an Anode comprised of P-Type semiconductor material
and a Cathode which is made of N-Type semiconductor material.
· The P-Type material contains charge carriers which are of a positive polarity and are known as holes. In the
N-Type material the charge carriers are electrons which are negative in polarity.
· When a semiconductor diode is manufactured, the P-Type and N-Type materials are adjacent to one another
creating a P-N Junction.
A bias refers to the application of an external voltage to a semiconductor. There are two ways a P-N junction can
· A forward bias results in current flow through the diode (diode conducts). To forward bias a diode, a positive
voltage is applied to the Anode lead ( which connects to P-Type material) and the negative voltage is applied
to the Cathode lead ( which connects to N-Type material).
· A reverse bias results in no current flow through the diode (diode blocks). A diode is reverse biased when the
Anode lead is made negative and the Cathode lead is made positive.
P-N Junction Characteristics
The P-N Junction region has three important characteristics:
1) The junction is region itself has no charge carriers and is known as a depletion region.
2) The junction (depletion) region has a physical thickness that varies with the applied voltage. A forward
bias decreases the thickness of the depletion region; a reverse bias increases the thickness of the depletion
3) There is a voltage, or potential hill, associated with the junction. Approximately 0.3 of a volt is required to
forward bias a germanium diode; 0.5 to 0.7 of a volt is required to forward bias a silicon diode.
Three characteristics must be defined for proper application or replacement of a semiconductor diode:
Voltage Rating is the maximum voltage which the diode will block in the reverse-biased mode.
· This is expressed as the Peak-Reverse-Voltage (PRV) or Peak-Inverse-Voltage (PIV).
· It is important to remember that this is a peak value of voltage not the root-mean-square (RMS) value. As a
"Rule -of-Thumb, to provide a margin of safety, the PIV rating of a diode should be at least 3 times the RMS
voltage of the circuit.
Current Rating is the maximum current the device can carry in the forward biased direction.
· Small, low current diodes are available in an axial lead configuration. The band end is the cathode.
· High current diodes come in a press-fit, stud- mounted, or hockey puck package.
Stud mounted diodes are available in Standard Polarity (stud cathode) and Reverse Polarity (stud anode).
· It is essential that semiconductors operate within the device temperature ratings.
· Semiconductor charge carriers are released thermally as well as electrically. Heat-sinking may be required
during soldering and when the device is in operation to prevent thermal damage.
· The forward resistance of a diode decreases with temperature; this results in an increase in current, which in
turn produces more heat. As a result, thermal run-away can occur and destroy the semiconductor.
If we join a section of N-type semiconductor material with a similar section of P-type semiconductor material, we obtain a device known as a PN JUNCTION. (The area where the N and P regions meet is appropriately called the junction.) The usual characteristics of this device make it extremely useful in electronics as a diode rectifier. The diode rectifier or PN junction diode performs the same function as its counterpart in electron tubes but in a different way. The diode is nothing more than a two-element semiconductor device that makes use of the rectifying properties of a PN junction to convert alternating current into direct current by permitting current flow in only one direction. The vertical bar represents the cathode (N-type material) since it is the source of electrons and the arrow represents the anode. (P-type material) since it is the destination of the electrons. The label "CR1" is an alphanumerical code used to identify the diode. In this figure, we have only one diode so it is labeled CR1 (crystal rectifier number one). If there were four diodes shown in the diagram, the last diode would be labeled CR4. The heavy dark line shows electron flow. Notice it is against the arrow. For further clarification, a pictorial diagram of a PN junction and an actual semiconductor (one of many types) are also illustrated.