SEMICONDUCTOR MATERIALSCreated and Presented by Doren Nedrick

The Basic Principles Of Heat Sensitive DevicesA change in temperature results in a change in its resistance depending on the temperature coefficient.

ThermistorThermistors also called thermal resistors are semiconductor devices whose use as transducers is due to the fact that their resistance changes when their temperature increases.

Thermistors are used for the measurement and control of temperature, being heated either externally or internally by the current they carry.Used as a means of sensing temperature change. Commonly found embedded in motor windings to detect overheating.

Thermionic EmissionThe thermionic emission of electrons is also known as thermal electron emission.Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the forces restraining it.The charge carriers can be electrons or ions, and in older literature are sometimes referred to as "thermions".

Closeup of the filament on a low pressure mercury gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil.

ThermocoupleThermocouple is a thermoelectric device for measuring or sensing temperature difference, consisting of two wires of dissimilar metals connected at two points, between which a voltage is developed in proportion to any temperature difference. Its application therefore is to measure temperatures accurately

SummaryThe Bohr atom is a tool for visualizing atomic structure. The nucleus is positively charged and has the protons and neutrons.The atomic number is the number of protons and determines the particular element.In the neutral atom, the number of electrons is equal to the number of protons. Electrons are negatively charged and in discrete shells.

SummaryThe outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and in metals they account for electrical and thermal conductivity.A neutral Si atom is shown. There are 4 electrons in the valence shell.Is Si a conductor, insulator, or semiconductor? Semiconductor

SemiconductorsAn insulator is any material that opposes the flow of electrons (current).A conductor is any material which allows for free flow of electrons (current). Semi means half. A semiconductor therefore is a material that has a conductivity level somewhere between the extremes of an insulator and a conductor.

An atom is made up tiny particles. Two of these particles, the electron and the proton are important to our studies. Electrons move around the center, or nucleus, of an atom in paths. These paths are usually called shells

An atom can have several shells around the nucleus. Each of these shells can have only up to a certain number of electrons. The number is called the quota of the shell. The first shell can hold two electrons while the second, third, fourth and so on can hold eight. When every shell of an atom contains its quota of electrons, the atom is said to be in stable condition.

The proton carries a positive charge while the electron carries negative. Recall a material with a lot of electrons free to move are called conductors and a material that have a shortage is called an insulator. For semiconductor it is somewhere between that (having just about half its quota) eg silicon and germanium - having four (4) valence electrons,

Crystal Lattice of SiliconThe resistance of a semiconductor decreases with temperature rise

Valence ElectronsElectrons in the outermost shell of an atom are called valence electrons. The combining power of the atom depends on these electrons. The valence electrons form bonds with the valence electrons of neighboring atoms to produce, in the case of most solids, a regular repeating pattern of atoms called a crystal lattice.

An atom can lose one or more electrons. If it does it becomes positively charged, because it then has more protons than electrons, and is called a positive ion. If it gains one or more electrons it becomes a negative ion

Electric charges exert forces on one another.Like charges repel, opposite charges attract.

DopingThe addition of certain impurity atom into the relatively pure semiconductor material is called doping. N and P-Type materials are formed by adding a predetermined number of impurity atoms into a semiconductor base usually silicon or germanium.

N-Type MaterialsThese are extrinsic materials (subjected to doping) created by doping the silicon base with impurity elements that have five (5) valence electrons (pentavalent), eg. Phosphorous and arsenic fig 1 shows that.

Four of its valence electrons form a bond with four neighboring silicon atoms but the fifth is spare and loosely held and takes part in conduction. The impurity (phosphorous) atom is called a donor. It is so called because it has the extra electron which has a negative charge, thus N for negative. In N-Type materials there are a few positive holes are present and are called minority carrier.

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P-Type Materialsthese are extrinsic materials created by doping the silicon base with impurity elements that have three (3) valence electrons, eg. Boron. Fig 2 shows that. The three (3) valence electrons each share an electron with three (3) of the four silicon atoms surrounding it.

The PN junction The operation of many semiconductor devices depends on effects which occur at the boundary (junction) between P- and N-Type materials formed in the same continuous crystal lattice.

A PN junction is represented in fig 3a. As soon as the junction is produced, free electrons near it in the N Type material are attracted across into the P-Type material where they fill holes. At the same time holes pass across the junction from P-Type to N-Type, capturing electrons there. As a result the N-Type material becomes positively charge and the P-Type material becomes negatively charged. The exchange of charge soon stops because the negative charge on the P-Type material opposes the further flow of electrons and the positive charge on the N-Type opposes the further flow of holes.

Biasing Bias is a current or a voltage that is applied to a device (diode) to obtain a desired mode of operation. The potential is applied to control the width of the depletion layer. Two types of bias are forward and reverse bias.

Reverse Bias on a PN junctionIf a battery is connected across a PN junction with its negative terminal to the P-Type side, it helps the junction voltage. Electrons and holes are repelled farther from the junction and the depletion layer widens, fig 6. Only a few minority carriers cross the junction and a tiny current, called the leakage or reverse current flows. The resistance of the junction is very high in reverse bias.

Forward Bias On A PN JunctionIf a battery is connected so as to oppose the junction voltage, the depletion layer narrows. When the battery voltage exceeds the junction voltage, appreciable current flows because majority carriers are able to cross the junction. Electrons travel from the N to the P side and holes in the opposite direction, fig 4. The junction is then forward biased, i.e. the P Type side is connected to the positive terminal of the battery and the N-Type side to the negative terminal.

The resistance of the junction is very low in forward bias.

The Junction diodeConstruction A junction diode consists of a p-n junction with one connection to the p-side, the anode A, and another to the n-side, the cathode K. Its symbol is shown in Fig. 26.3a. In actual diodes the cathode end is often marked by a band, Fig. 26.3b; it is the end from which conventional current leaves the diode when forward biased.

Characteristics of the Semiconductor DiodeTypical Characteristic curves for silicon and germanium diodes at 25 C are shown in fig 6. Conduction does not start until the forward voltage vf is about 0.7V for silicon and 0.3 for germanium thereafter, a very small change in vf causes a sudden, large increase in forward current if. When if is limited to a value within the power rating of the diode (by a resistor in the circuit). The reverse currents ir are negligible (note the change of scales on the negative axes of the graph and remain so until the reverse voltage vr is large enough (from 5V up to 1000V depending on the level of doping) to break down the insulation of the depletion layer. ir then increases suddenly and rapidly and permanent damage to the diode occurs. ir is similar and more constant for silicon and germanium. Fig 7

The average forward current if (av) and the maximum reverse Voltage Vrrm

Application Of The Semiconductor Diode Diodes operate as a rectifier to change AC to DC in power supplies.To prevent damage to a circuit by a reversed power supply.As a clamp diode or DC restorer to prevent dc level shift problems in capacitor coupling circuitThe average forward current if (av) and the maximum reverse Voltage vrrm or PIV rating are usually quoted for a diode.