Semiconductor Formula: Explanation, Examples

Semiconductors are materials having conductivity properties lying between good conductors and insulators of electricity. Some notable examples of semiconductors are germanium and arsenic. The two types of semiconductors are intrinsic or pure semiconductors and extrinsic or impure semiconductors. Charge carriers responsible for current flow in semiconductors are holes and electrons. Both are equal in magnitude but opposite in polarity. Holes are positively charged carriers while electrons are negatively charged carriers.

Key Terms: Intrinsic Semiconductors, Extrinsic, Semiconductors, Mobility, Rectifier Efficiency, Amplification Factor, Transistors, Electrons, Holes

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Intrinsic and Extrinsic Semiconductors

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• Intrinsic semiconductors are pure semiconductors having charge carriers (electrons and holes) produced due to thermal excitation that represent the intrinsic property of the material. No impurity of atoms is added to increase the conductivity during intrinsic conduction that occurs by electron-hole pairs.
• In intrinsic semiconductors, the number of electrons in conduction band is equal to the number of holes in the valence band,nh. Therefore, the intrinsic charge carrier concentration, ni=ne=nh.
• Extrinsic semiconductors are impure semiconductors that are made by doping pure semiconductors with impure atoms to increase conductivity. The number of electrons and holes can be changed by this method of doping.
• There are two types of extrinsic semiconductors, n-type and p-type semiconductors.
• In n-type semiconductors, electrons are the majority carriers while holes are the minority charge carriers, that is, nenh.
• In p-type semiconductors, holes are the majority charge carriers while electrons are the minority charge carriers, that is, nhne.

The video below explains this:

Semiconductors and Insulators Detailed Video Explanation:

Semiconductor Electronics Class 12 Important Notes PDF

Semiconductor Electronics Class 12 Important Notes

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Mobility

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• The mobility of charge carriers can be defined as the magnitude of drift velocity per unit electric field applied.
• The term mobility is represented by the symbol, . If the drift velocity and electric field are represented as Vd and E respectively, then the mobility is given by, =VdE
• The SI unit of mobility is m²V^-1s^-1.
• The mobility of electrons( e) is greater than the mobility of holes (h), that is, e>h

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Transistors

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• Transistors are three-section semiconductors, used as amplifiers and oscillators that help in the conduction and insulation of electric current or voltage. They are of two types, bipolar junction transistor and field-effect transistor. The three sections of the transistors are emitter, collector, and base.
• The emitter current (Ie) is always slightly greater than the collector current, and is given as the sum of collector current (Ic) and base current (Ib), that is, Ie=Ic+Ib
• Low current gains of a transistor are defined into two types, common base current amplification factor and common emitter current amplification factor.
• Common base current amplification factor or ac current gain () is defined as the ratio of small change in the collector current to the small change in the emitter current when the collector-base voltage is kept constant, that is, =IEIC, where VCB=constant.
• Common emitter current amplification factor or ac current gain () is defined as the ratio of small change in the collector current to the small change in the base current when the collector-emitter voltage is kept constant, that is, =IEIB, where VCE=constant.
• The relation between the current gains, and are given by, =1+ and =1-

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Power Gain

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• The ac power gain is defined as the ratio of change in output power to the change in input power.
• The formula for ac power gain is given by, Power gain= (Ic)Rout(Ib)Rin, where RoutRin is the resistance gain.

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Logic Gates

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• Logic gates are Boolean function based digital devices used to carry out logical operations.
• If A and B are the given inputs, then the output, C is given as, C=A+B.
• There are different types of logic gates namely, AND, OR, NAND, NOR, and NOT gates, and the output in these can be calculated as follows:

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Things to Remember

• The electron density in the conduction band is equal to the hole density in the valence band for the case of intrinsic semiconductors.
• Extrinsic semiconductors are made by the process of doping through which the impurities are added to a pure semiconductor and hence the charge carrier can be changed.
• The mobility of charge carriers can be defined as the magnitude of drift velocity per unit electric field applied, that is, =VdE
• With the exponential increase in temperature, the electrical conductivity of intrinsic semiconductor increases. The formula for electrical conductivity is given by, =0exp(-Eg2kBT)
• The relation between the current gains, and are given by, =1+ and =1-
• The formula for ac power gain is given by, Power gain= (Ic)Rout(Ib)Rin, where RoutRin is the resistance gain.
• The emitter current (Ie) in a transistor is the sum of collector current (Ic) and base current (Ib), that is, Ie=Ic+Ib

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