Semiconductors

Applied Physics GS-122

By Atif M.Khokhar

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Intro Semiconductors Types of Semiconductors Formation of Extrinsic Semiconductors (Doping) Properties of Extrinsic Semiconductors Advantages of Doping Current flow in Extrinsic Semiconductors

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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You will be able to 

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Understand the Lack in Electrical properties of Intrinsic Semiconductors Understand the properties of Extrinsic Semiconductors Material Get the information of Impurity Elements Get the awareness of Advantages of adding impurity

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Germanium

RIPHAH INTERNATIONAL UNIVERSITY,ISLAMABAD, PAKISTAN

2/6/2008

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 Material for which gap between valence band and conduction band is small (gap width in Si is 1.1 eV, in Ge 0.7 eV).  At T = 0, there are no electrons in the conduction band, and the semiconductor does not conduct (lack of free charge carriers)  At T > 0, some fraction of electrons have sufficient thermal kinetic energy to overcome the gap and jump to the conduction band; fraction rises with temperature; e.g. At 20o C (293 K), Si has 0.9x1010 conduction electrons per cubic centimeter; at 50o C (323 K) there are 7.4x1010 . Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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 Electrons moving to conduction band leave “hole” (covalent bond with missing electron) behind under influence of applied electric field, neighboring electrons can jump into the hole, thus creating a new hole.  Holes can move under the influence of an applied electric field, just like electrons both contribute to conduction.  In pure Si and Ge, there are equally many holes (“p-type charge carriers”) as there are conduction electrons (“n-type charge carriers”)pure semiconductors also called “intrinsic Semiconductors”. Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Silicon Energy Bands At finite temperatures, the number of electrons which reach the conduction band and contribute to current can be modeled by the Fermi function. That current is small compared to that indoped semiconductors under the same conditions.

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Germanium Energy Bands At finite temperatures, the number of electrons which reach the conduction band and contribute to current can be modeled by the Fermi function. That current is small compared to that in doped semiconductors under the same conditions.

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Doping •If we purposely include some “impurities” in the crystal, we can add more electrons. • This works if the impurity atoms have one more electron per atom than the host semiconductor. • Since we increase # of electrons, Fermi energy increases • Intrinsic means no doping.

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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 donor (N-type) impurities:  dopant with 5 valence electrons (e.g. P, As, Sb)  4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”;  left over electron is only loosely bound only small amount of energy needed to lift it into conduction band (0.05 eV in Si)  “n-type semiconductor”, has conduction electrons, no holes (apart from the few intrinsic holes)

example: doping fraction of 10-8 Sb in Si yields about 5x1016 conduction electrons per cubic centimeter at room temperature, i.e. gain of 5x106 over intrinsic Si. Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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T= 0 K

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Silicon Doping (Formation of N-type)

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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N-type Semiconductors Current

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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We can do the whole exercise again for HOLES

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RIPHAH INTERNATIONAL UNIVERSITY,ISLAMABAD, PAKISTAN

2/6/2008

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 Acceptor (p-type) impurities:  dopant with 3 valence electrons (e.g. B, Al, Ga, In) only 3 of the 4 covalent bonds filled  vacancy in the fourth covalent bond  Hole  “p-type semiconductor”, has mobile holes, very few mobile electrons (only the intrinsic ones).

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Silicon Doping (Formation of P-type)

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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P-type Semiconductors Current

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Silicon Doping (Summary)

Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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Silicon Doping (Energy States)

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advantages of doped semiconductors: Can “tune” conductivity by choice of doping fraction can choose “majority carrier” (electron or hole) can vary doping fraction and/or majority carrier within piece of semiconductor  can make “p-n junctions” (diodes) and “transistors” Lecture delivered @ HITEC UNIVERSITY, Taxila, by Atif M. Khokhar

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