Temperature Dependence of Semiconductor Conductivity
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28-09-2010, 05:18 PM
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After successfully completing this laboratory workshop, including the assigned reading, the lab blue sheets, the lab quizzes, and any required reports, the student will be able to:
• Describe how semiconductor conductivity varies with temperature.
• Draw a band diagram for a semiconductor.
• Describe how doping a semiconductor improves conductivity.
• Define n-type and p-type semiconductors.
• Relate the bonding type to the materials’ electrical properties.
• Determine the band gap energy for a semiconductor from measured data in the intrinsic region.
• Distinguish between intrinsic and extrinsic temperature regimes and identify the applicable temperature range from an examination of measured data.
• Express the mathematical relationships between carrier concentration and temperature in the intrinsic, extrinsic and ionization regimes.
• Express the functional relationship between mobility and temperature in the intrinsic, extrinsic and ionization regimes.
Band Structure of a Semiconductor
The band structure of semiconductors is such that the outermost band of electrons, the valence band, is completely full. If a voltage is applied, there is no conduction of electrons because there are no empty spaces to allow the electrons to move around. In order for conduction to occur, electrons must be excited to the next highest band, known as the conduction band. The conduction band is normally empty but is separated from the valence band by only a small amount of energy. Valence electrons can surmount this barrier by absorbing a small amount of energy from heat or light. This then creates a free electron in the conduction band and a hole (missing electron) in the valence band,
Doping (replacing Si atoms with atoms of another element) is frequently used instead of temperature to control conductivity. Doping can be localized to certain areas whereas the affect of temperature is a less localized influence. If Si is replaced by elements from Column V of the periodic table such as phosphorous, there will be one extra electron in the valence band, This electron is easily broken loose to create a free electron. Silicon doped with Column V elements in known as p-type and the dopants are called donors. Replacing Si with an element from Column III (such as boron) creates a hole in the valence band. The silicon is known as n-type and the dopants are called acceptors.