Phosphorus-doped semiconductors

FIGURE 21.11 MO energy levels for doped semiconductors, (a) An M-type semiconductor, such as silicon doped with phosphorus, has more electrons than needed for bonding and thus has negative electrons in the partially filled conduction band. 

A semiconductor such as phosphorus-doped silicon is called an n-type semiconductor because extra electrons (negatively charged) are present in the crystal structure. 

Knights el fl/., 1977), are shown in Fig. 20 (Street eta/., 1981) for both boron and phosphorus doping. These are not commonly interpreted as signals from un-ionized dopants as one would expect in a crystalline semiconductor. at low temperatures, but rather are attributed to singly occupied states in the band tails that are presumably formed from weak or strained bonds (Knights et ai, 1977). Similar results are observed in the a-Ge H system (Stutzmann et al, 1983). 

Scientists add small amounts of phosphorus to the silicon for N-type doping. Figure 15-4 shows the phosphorus atom. Phosphorus has an atomic number of 15 and has five valence electrons. Silicon has four electrons in its outer orbit. So when a phosphorus atom is placed in the silicon lattice, one of its five valence electrons is out of place. This fifth electron has nothing to bond to, so it is free to move. This free electron is what makes the phosphorus-doped silicon an N-type semiconductor. 

In the extrinsic or doped semiconductor, impurities are purposely added to modify the electronic characteristics. In the case of silicon, every silicon atom shares its four valence electrons with each of its four nearest neighbors in covalent bonds. If an impurity or dopant atom with a valency of five, such as phosphorus, is substituted for silicon, four of the five valence electrons of the dopant atom will be held in covalent bonds. The extra, or fifth electron will not be in a covalent bond, and is loosely held. At room temperature, almost aU of these extra electrons will have broken loose from their parent atoms, and become free electrons. These pentavalent dopants thus donate free electrons to the semiconductor and are called donors. These donated electrons upset the balance between the electron and hole populations, so there are now more electrons than holes. This is now called an N-type semiconductor, in which the electrons are the majority carriers, and holes are the minority carriers. In an N-type semiconductor the free electron concentration is generally many orders of magnitude larger than the hole concentration. 

The piasma jet can be cooied rapidiy just priorto coming in contact with the substrate by using a blast of cold inert gas fed into an annular fixture. Gaseous boron or phosphorus compounds can be introduced into the gas feed for the deposition of doped-semiconductor diamond. 

Figure 13.29A). The extra electrons in phosphorus-doped silicon are free to conduct an electric current, so the doped silicon becomes a good conductor. It is called an n-type semiconductor, because the current is carried by negative charges (electrons). 

The conductivity of a semiconductor can be enhanced greatly by doping, the addition of very small quantities of an element with one more or one fewer valence electron than the natural semiconductor. For the purpose of simplicity, we will consider a pure silicon semiconductor. Silicon is a Group 4A element, so it has four valence electrons per atom. A small (parts per million) amount of phosphorus (Group 5A, five valence electrons) can be added, thus doping the silicon with phosphorus. Since each phosphorus atom has an extra electron relative to the pure semiconductor, these extra electrons must reside in the conduction band, where they increase its conductivity. This type of doped semiconductor is called an n-type semiconductor because the semiconductivity has been enhanced by the addition of negative particles, the extra electrons. 

A photovoltaic cell (often called a solar cell) consists of layers of semiconductor materials with different electronic properties. In most of today s solar cells the semiconductor is silicon, an abundant element in the earth s crust. By doping (i.e., chemically introducing impurity elements) most of the silicon with boron to give it a positive or p-type electrical character, and doping a thin layer on the front of the cell with phosphorus to give it a negative or n-type character, a transition region between the two types... 

Silicon can be doped with small amounts of phosphorus to create a semiconductor used in transistors, (a) Is the alloy interstitial or substitutional Justify your answer, (b) How do you expect the properties of the doped material to differ from those of pure silicon ... 

Semiconductor properties are imparted by doping its structure with boron, phosphorus, or arsenic atoms. Silicon is relatively inert chemically but is attacked by halogens and dilute alkalies. It has good optical transmission especially in the infra-red. 

Silicon s atomic structure makes it an extremely important semiconductor. Highly purified silicon, doped with such elements as boron, phosphorus, and arsenic, is the basic material used in computer chips, transistors, sUicon diodes, and various other electronic circuits and electrical-current switching devices. Silicon of lesser purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, and bronze. 

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