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Extrinsic Semiconductors – Doped Semiconductors

In general, semiconductors are inorganic or organic materials that can control their conduction depending on chemical structure, temperature, illumination, and the presence of dopants. The name semiconductor comes from the fact that these materials have electrical conductivity between a metal, like copper, gold, etc., and an insulator, like glass. They have an energy gap of less than 4eV (about 1eV). In solid-state physics, this energy gap or band gap is an energy range between the valence band and conduction band where electron states are forbidden. In contrast to conductors, semiconductors’ electrons must obtain energy (e.g., from ionizing radiation) to cross the band gap and reach the conduction band. Properties of semiconductors are determined by the energy gap between valence and conduction bands.

Extrinsic Semiconductors – Doped Semiconductors

An extrinsic semiconductor, or doped semiconductor, is a semiconductor that was intentionally doped to modulate its electrical, optical, and structural properties. In the case of semiconductor detectors of ionizing radiation, doping is the intentional introduction of impurities into an intrinsic semiconductor for changes in their electrical properties. Therefore, intrinsic semiconductors are also known as pure semiconductors or i-type semiconductors.

Adding a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties since these foreign atoms incorporated into the crystal structure of the semiconductor provide free charge carriers (electrons or electron holes) in the semiconductor. In an extrinsic semiconductor, these foreign dopant atoms in the crystal lattice mainly provide the charge carriers that carry electric current through the crystal. Two types of dopant atoms generally result in two types of extrinsic semiconductors. These dopants that produce the desired controlled changes are classified as either electron acceptors or donors, and the corresponding doped semiconductors are known as:

  • n-type Semiconductors.
  • p-type Semiconductors.

Extrinsic semiconductors are components of many common electrical devices, as well as many detectors of ionizing radiation. For these purposes, a semiconductor diode (devices that allow current in only one direction) usually consists of p-type and n-type semiconductors placed in a junction with one another.

n-type Semiconductors

extrinsic - doped semiconductor - n-type - donorAn extrinsic semiconductor doped with electron donor atoms is called an n-type semiconductor because most charge carriers in the crystal are negative electrons. Since silicon is a tetravalent element, the normal crystal structure contains 4 covalent bonds from four valence electrons. The most common dopants in silicon are group III and V elements. Group V elements (pentavalent) have five valence electrons, allowing them to act as donors. That means adding these pentavalent impurities such as arsenic, antimony, or phosphorus contributes to free electrons, greatly increasing the conductivity of the intrinsic semiconductor. For example, a silicon crystal doped with boron (group III) creates a p-type semiconductor, whereas a crystal doped with phosphorus (group V) results in an n-type semiconductor.

The conduction electrons are completely dominated by the number of donor electrons. Therefore:

The total number of conduction electrons is approximately equal to the number of donor sites, n≈ND.

The charge neutrality of semiconductor material is maintained because excited donor sites balance the conduction electrons. The net result is that the number of conduction electrons increases while the number of holes is reduced. The imbalance of the carrier concentration in the respective bands is expressed by the different absolute number of electrons and holes. Electrons are majority carriers, while holes are minority carriers in n-type material.

Donor Level

Donor Level

From the energy gap viewpoint, such impurities “create” energy levels in the band gap close to the conduction band so that electrons can be easily excited from these levels into the conduction band. The electrons are said to be the “majority carriers” for current flow in an n-type semiconductor. This shifts the effective Fermi level to halfway between the donor levels and the conduction band. Fermi level is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. The Fermi level is the surface of the Fermi sea at absolute zero, where no electrons will have enough energy to rise above the surface. In pure semiconductors, the position of the Fermi level is within the band gap, approximately in the middle of the band gap.

p-type Semiconductors

extrinsic - doped semiconductor - p-type - acceptorAn extrinsic semiconductor doped with electron acceptor atoms is called a p-type semiconductor because most charge carriers in the crystal are electron holes (positive charge carriers). The pure semiconductor silicon is a tetravalent element, and the normal crystal structure contains 4 covalent bonds from four valence electrons. In silicon, the most common dopants are group III and group V elements. Group III elements (trivalent) all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces a tetravalent silicon atom in the crystal, a vacant state (an electron-hole) is created. An electron-hole (often simply called a hole) is the lack of an electron at a position where one could exist in an atom or atomic lattice. It is one of the two charge carriers responsible for creating an electric current in semiconducting materials. These positively charged holes can move from atom to atom in semiconducting materials as electrons leave their positions. Adding trivalent impurities such as boron, aluminum, or gallium to an intrinsic semiconductor creates these positive electron holes in the structure. For example, a silicon crystal doped with boron (group III) creates a p-type semiconductor, whereas a crystal doped with phosphorus (group V) results in an n-type semiconductor.

The number of electron holes is completely dominated by the number of acceptor sites. Therefore:

The total number of holes is approximately equal to the number of donor sites, p ≈ NA.

The charge neutrality of this semiconductor material is also maintained. The net result is that the number of electron holes is increased while the number of conduction electrons is reduced. The imbalance of the carrier concentration in the respective bands is expressed by the different absolute number of electrons and holes. Electron holes are majority carriers, while electrons are minority carriers in p-type material.

Acceptor Level

Acceptor Level

From the energy gap viewpoint, such impurities “create” energy levels within the band gap close to the valence band so that electrons can be easily excited from the valence band into these levels, leaving mobile holes in the valence band. They create “shallow” levels, levels that are very close to the valence band, so the energy required to ionize the atom (accept the electron that fills the hole and creates another hole further from the substituted atom) is small. This shifts the effective Fermi level halfway between the acceptor and valence bands. Fermi level is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. The Fermi level is the surface of the Fermi sea at absolute zero, where no electrons will have enough energy to rise above the surface. In pure semiconductors, the position of the Fermi level is within the band gap, approximately in the middle of the band gap.

The P-N Junction – Reverse Biased Junction

The semiconductor detector operates much better as a radiation detector if an external voltage is applied across the junction in the reverse-biased direction. The depletion region will function as a radiation detector. Improvement can be achieved by using a reverse-bias voltage to the P-N junction to deplete the detector of free carriers, which is the principle of most semiconductor detectors. Reverse biasing a junction increases the thickness of the depletion region because the potential difference across the junction is enhanced. Germanium detectors have a p-i-n structure in which the intrinsic (i) region is sensitive to ionizing radiation, particularly X and gamma rays. Under reverse bias, an electric field extends across the intrinsic or depleted region. In this case, a negative voltage is applied to the p-side and positive to the second one. Holes in the p-region are attracted from the junction towards the p contact and similarly for electrons and the n contact. In proportion to the energy deposited in the detector by the incoming photon, this charge is converted into a voltage pulse by an integral charge-sensitive preamplifier.

See also: Germanium Detectors, MIRION Technologies. <available from: https://www.mirion.com/products/germanium-detectors>.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection, and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
  3. Martin, James E., Physics for Radiation Protection 3rd Edition, Wiley-VCH, 4/2013. ISBN-13: 978-3527411764.
  4. U.S.NRC, NUCLEAR REACTOR CONCEPTS
  5. U.S. Department of Energy, Instrumentation, and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.

Nuclear and Reactor Physics:

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  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See above:

Types of Semiconductors