How Do Semiconductor Materials Conduct Current?

TL;DR

Semiconductor materials conduct current when electrons receive enough energy to cross the forbidden gap from the valence band into the conduction band. Their conductivity is higher than that of insulators but lower than that of conductors, while silicon and germanium have stated band gaps of 1.16 and 0.75 electron volts. Read on to compare their resistivity, band structure, and current flow with conductors and insulators.

Transcript

from this presentation we will start analog electronics and in the first lecture we will complete semiconductor materials they are the backbone of electronic devices and circuits that's why it is very important to know what are the different semiconductor materials available to us and also their properties so in this course we will complete some to... Read More

Key Insights

• Electrical conductivity is the basis for classifying materials as conductors, insulators, or semiconductors. A conductor supports a generous flow of charge when voltage is applied, an insulator permits nearly negligible flow, and a semiconductor operates between those two limits.
• Copper and aluminium are commonly used conductors in electrical equipment. Their ability to support a high flow of charge through a cross-sectional area makes them suitable examples of materials with high conductivity and low opposition to current.
• Mica and wood are examples of insulating materials with very low conductivity. Mica is particularly important in electrical industries because it provides both thermal and electrical insulation in electrical equipment, while its flow of charge is described as negligible.
• Semiconductor conductivity is greater than insulator conductivity but lower than conductor conductivity. Silicon, represented as Si, and germanium are identified as the two most important semiconductor materials for understanding later analog-electronics topics such as diodes and transistors.
• Resistivity is the reciprocal of conductivity and a fixed property associated with a particular material. It is represented by rho and can be calculated as resistance multiplied by cross-sectional area and divided by the conductor's length.
• Semiconductor resistivity lies below insulator resistivity and above conductor resistivity. The stated resistivities are 10^-6 ohm-centimeters for copper, 10^12 ohm-centimeters for mica, 50 times 10^3 ohm-centimeters for silicon, and 50 ohm-centimeters for germanium.
• The forbidden band gap is the energy separating the valence band from the conduction band. An electron must receive energy equal to this gap to enter the conduction band, where it can participate in electrical conduction, because electrons cannot remain within the forbidden region.
• Energy-band structure distinguishes the three material classes. Insulators have a large gap near 6 electron volts, semiconductors have a narrower gap near 1 electron volt, and conductors have overlapping valence and conduction bands, allowing conduction without extra transition energy.

Questions & Answers

Q: How do semiconductor materials conduct current?

Electrons must receive enough energy to cross the forbidden gap from the valence band into the conduction band, where they can participate in electrical conduction. Because a semiconductor has a narrower gap than an insulator, it conducts more current than an insulator but less than a conductor.

Q: What is the difference between conductors, insulators, and semiconductors?

Conductors support a generous flow of charge when voltage is applied, while insulators permit nearly negligible charge flow. Semiconductors operate between them, conducting more than insulators but less than conductors. Copper and aluminium are conductor examples, mica and wood are insulators, and silicon and germanium are semiconductors.

Q: What is electrical resistivity, and how is it calculated?

Electrical resistivity is a fixed property of a material that represents opposition to charge flow, and it is the reciprocal of conductivity. It is represented by rho and calculated as resistance multiplied by cross-sectional area and divided by length: rho equals RA/L.

Q: How does resistivity affect current flow?

A material with high resistivity offers more opposition to charge flow, so its current is lower. A material with low resistivity offers less opposition, allowing greater current flow. Semiconductor resistivity is lower than insulator resistivity but higher than conductor resistivity.

Q: What are the stated resistivities of copper, mica, silicon, and germanium?

Copper has a stated resistivity of 10^-6 ohm-centimeters, while mica has 10^12 ohm-centimeters. Silicon is listed at 50 × 10^3 ohm-centimeters, and germanium is listed at 50 ohm-centimeters. These values place the semiconductors between the conductor and insulator examples.

Q: What is the forbidden energy gap in a semiconductor?

The forbidden energy gap is the interval between the valence band and conduction band where an electron cannot remain. An electron must receive energy equal to this gap to move into the conduction band and participate in conduction.

Q: How do energy-band diagrams distinguish conductors, insulators, and semiconductors?

Insulators have a large forbidden gap near 6 electron volts, while semiconductors have a narrower gap near 1 electron volt. In conductors, the valence and conduction bands overlap, so there is no forbidden gap between them.

Q: Why does germanium have a smaller band gap than silicon?

Germanium has a stated forbidden band gap of 0.75 electron volts, compared with 1.16 electron volts for silicon. Germanium has more electron orbits, placing its valence electrons farther from the nucleus and reducing the attractive force. Silicon has fewer orbits and stronger attraction, so more energy is required to move an electron into its conduction band.

Summary & Key Takeaways

• Electrical materials are categorized as conductors, insulators, or semiconductors according to how much charge flows when voltage is applied. Copper and aluminium are common conductors, while mica and wood are insulators. Semiconductors, especially silicon and germanium, occupy the middle range because their conductivity exceeds that of insulators but remains below that of conductors.

• Resistivity is a material property represented by rho, defined as the reciprocal of conductivity and expressible as rho equals resistance times cross-sectional area divided by length. The stated values are 10^-6 ohm-centimeters for copper, 10^12 ohm-centimeters for mica, 50 times 10^3 ohm-centimeters for silicon, and 50 ohm-centimeters for germanium.

• Energy-band diagrams explain differences in conduction through the valence band, conduction band, and forbidden band gap. Electrons must receive enough energy to cross this gap. Insulators have a gap near 6 electron volts, semiconductors have a gap near 1 electron volt, and conductors have overlapping valence and conduction bands with no forbidden gap.