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N-type semiconductors are intrinsic semiconductors doped with materials that have one or more extra valence electrons. This creates an excess of negative (n-type) electron charge carriers. The most common example is the addition of Group V elements (phosphorous, arsenic, antimony) which contain five loosely bound valence electrons, to regular crystal arrays of Group IV elements (silicon, germanium, tin) which contain four valence electrons.

History of Semiconducting Systems

Semiconductors are essential for certain processes. These semiconducting materials were studied in laboratories as early as the 1830's. Ferdinand Braun created the first successfull semiconducting device known as the cat's whisker diode or crystal diode rectifier. This diode was invented during the development of the radio. He used the rectifying properties of galena crystal. Galena crystal is also known as a semiconducting material that is composed of lead sulfide. In 1909, Braun shared the Nobel Prize in physics with Guglielmo Marconi for the development of wireless telegraphy.

Secondly, semiconductors are needed for transistors. A transistor is simply a solid-state electronic device that controls the flow of electric current. Until World War II, most systems used vacuum tubes for the amplification and control of electric current. Many disadvantages arise when referring to vacuum tubes. They are very bulky and fragile, consume a lot of power, and tend to overheat. The demands of radar in particular during World War II encouraged scientists to look for another method of amplifying and controlling electric current in communication devices. The transistor was invented just after World War II in the late 1940's. William Shockley, John Bardeen, and Walter Brattain from the Bell Telephone Laboratories were responsible for the discovery.

Basic Science of N-type Semiconductors

Semiconductors are defined by their unique electric conductive behavior. Metals are good conductors because at their Fermi level, there is a large density of energetically available states that each electron can occupy. Electrons can move quite freely between energy levels without a high energy cost. Metal conductivity is observed to decrease as a function of temperature increases because thermal vibrations in their crystal lattice disrupt the free motion of electrons. Insulators, by contrast, are very poor conductors of electricity because there is a large difference in energies (called a band gap) between electron-occupied energy levels and empty energy levels that allow for electron motion. Insulator conductivity is slightly enhanced with increased temperatures because heat provides energy to promote electrons across the band gap to the higher electron conduction energy levels (called the conduction band). Semiconductors, on the other hand, have an intermediate level of electric conductivity when compared to metals and insulators. Their band gap is small enough that small increases in temperature allow for the promotion of many electrons from the lowest energy levels (in the valence band) to the conduction band. This creates electron holes, or unoccupied levels, in the valence band, and very loosely held electrons in the conduction band. ^[1] ^[2] An intrinsic semiconductor is made up ideally of one pure element, typically silicon. At normal temperatures, the conductivity of intrinsic semiconductors is still relatively low. Conductivity is greatly enhanced by a process called doping, in which other elements containing more or less valence electrons are added to the intrinsic crystal in very small amounts to create what is called an extrinsic semiconductor. When the dopant has more valence electrons than the intrinsic element, the product is called an n-type semiconductor. Conductivity is enhanced in n-type semiconductors because a band of electrons is created, occupied by the extra electrons, that is much higher in energy and closer to the intrinsic semiconductor’s conduction band. Much less energy is needed to promote electrons from the new band to the conduction band to conduct electricity. ^[1]

The Fermi Level

The Fermi level plays an important role in describing the behavior of doped semiconductors. A substance’s Fermi level is defined as the highest occupied energy level found in that substance at absolute zero temperature (0 Kelvin or -273⁰C). At higher temperatures, energy from heat is available to promote electrons into slightly higher energy levels. However, picturing density of states to be filled to the Fermi level helps scientists understand different behaviors between insulators, metals, and intrinsic and extrinsic semiconductors. As seen in figure one, the Fermi level of n-type semiconductors is elevated from that of the corresponding un-doped intrinsic semiconductor. This makes the conduction band much more thermally accessible at temperatures above absolute zero. ^[2]

Figure 1: Representative density of states diagrams of metals, insulators, intrinsic and n-doped semiconductors. Shaded areas represent energy levels filled at absolute zero, below the Fermi level.

Charge Carriers

The concept of free electrons and corresponding holes in a solid semiconductor is a useful tool for understanding conduction through the solid. At temperatures above absolute zero, some amount of electrons in an n-type semiconductor are expected to be excited from the dopant band into the conduction band. This allows creates a state with a few loosely held, very mobile electrons occupying some of the large amount of states available in the conduction band. The vacancies left in the dopant band Group V ions containing a positively charged “hole” in their valence shell. While the amount of holes per filled levels in the dopant band is low, there is a large number of holes or levels to occupy per electron in the conduction band (see figure 2). The electrons can easily move between these available states and conduct a current. In the case of n-type semiconductors, the electrons are considered to be the majority charge carrier. The holes of the dopant band are considered minority carriers. ^[3]

Figure 2: Creation free electrons and holes in n-doped semiconductors temperatures above absolute zero. Shaded areas represent electron-filled levels.

Doping and Synthetic Techniques

The synthesis of n-type semiconductors involves the use of vapor-phase epitaxy. In vapor-phase epitaxy, a gas containing the negative dopant is passed over the gallium arsenide wafer (or whatever substrate is chosen). In the case of n-type GaAs doping, hydrogen sulfide is passed over the gallium arsenide, and sulfur is incorporated into the structure^[4]. This process is characterized by a constant concentration of sulfur on the surface^[5]. In the case of semiconductors in general, only a very thin layer of the wafer needs to be doped in order to obtain the desired electronic properties. The diffusion of sulfur is described by the conservation equation. The resulting equation can be simplified as a 1 dimensional system on a semi-infinite medium to model the diffusion into the thin layer. A similarity transform is suitable to solve this problem^[6]. See the figure 3 below.

The reaction conditions typically range from 600-800C for the n-doping with group VI elements ^[4], and the time typically consists of 6-12 hour residence times depending on the temperature used.

The p-n Junction

Perhaps the most important current use of n-type semiconductors is in p-n junctions. These are p-type and n-type semiconductors brought together in close contact, creating what is called the depletion region. The importance of this contact or junction is the creating of an insulating region between the two relatively good conductors by recombination of p-type holes with n-type free electrons. P-n junctions form the basis of how a lot of current technology works by creating diodes. In a diode, current can flow easily in one direction but not the other, which has been useful in applications such as the light emitting diode (LED) and the field effect transistor (FET). ^[7]

Other applications and continuing research

Rectifying Junction

A rectifying junction can be made when certain metals are in contact with a semiconductor crystal. In the case of an n-type germanium crystal in contact with tungsten, current is allowed to flow when the tungsten is held at a positive potential, however, there is negligible current in the case where tungsten is held at a negative potential. The reverse is true in the case of p-type semiconductors^[5]. These junctions have some advantages over the standard pn junction (diode) in that there is a smaller voltage drop, thus better emulating an ideal diode. Schottky diodes also allow for faster switching times compared to p-n junctions.

The n-type junction (Schottky diode) can also be used in producing a hydrogen fuel cell. Water and hydrogen would combine on a palladium layer and generate a thermionic current sent into an n-type silicon carbide semiconductor with a potential greater than the Schottky barrier. These solid state fuel cells can potentially allow for integration of fuel cells into portable electronics^[8].

N-type semiconductors in organic devices

Organic semiconductors have been of great research interest for use in low cost, ultra thin, and flexible products such as displays and solar energy conversion cells. While many p-type organic semiconductors have been thoroughly characterized, n-type organic semiconductors have proven hard to obtain. Both types are needed for the diodes and transistors that make desirable devices possible. Researchers at Northwestern University have worked to synthesize n-type organic semiconductors of the arylene diimide family that are resistant to thermal and environmental stresses, which is one of the largest challenges in the field. ^[9] Several groups have been searching for n-type organic semiconductors for use in organic field-effect transistors (OFET). Compounds being explored include Buckminster fullerene (C₆₀) and chemically modified oligothiophenes. Semiconductors are made from these compounds by reduction with electron withdrawing groups or, alternatively, modification of solid state surface properties to control electron trapping. ^[10] Organic thin film transistors (OTFTs) are being explored because their low synthasis temperatures allow them to be deposited on thin plastic substrates without damage, making very thin and flexible devices possible. A group at the Georgia Institute of Technology is working to find n-type semiconductors for this purpose to complement the already more available p-type semiconductors. Many of the same compounds being explored as OFETs are being studied for use in OTFTs. ^[11]

References

^ ^a ^b Smart, L.; et al. (2005). State Chemistry: An Introduction. pp. 165–171. ISSN 0-7487-7516-1. {{cite book}}: Check |issn= value (help); Explicit use of et al. in: |author1= (help)
^ ^a ^b Miessler, G.; et al. (1965). Inorganic Chemistry (3rd ed.). pp. 237–240. ISSN 0-7487-7516-1. {{cite book}}: Check |issn= value (help); Explicit use of et al. in: |author1= (help)
^ Halliday, D.; et al. (2005). Fundamentals of Physics (7th ed.). pp. 1151–1153. ISSN 0-471-21643-7. {{cite book}}: Check |issn= value (help); Explicit use of et al. in: |author1= (help)
^ ^a ^b Schubert, E. F. (2005). Doping in III-V Semiconductors. pp. 241–243. ISBN 0-521-01784-X.
^ ^a ^b Middleman, S. (1993). Process Engineering Analysis in Semiconductor Device Fabrication. pp. 29, 330–337. ISBN 0-07-041853-5.
^ Deen, William M. (1998). Analysis of Transport Phenomena. pp. 91–94. ISBN 978-0-19-508494-8.
^ HyperPhysics
^ Karpov E. G.; Nedrygailov I. I. (2009). "Solid-state electric generator based on chemically induced internal electron emission in metal-semiconductor heterojunction nanostructures". Applied Physics Letters. 94. doi:10.1063/1.3147853.
^ Jones, B.; et al. (2007). "Tuning Orbital Energetics in Arylene Diimide Semiconductors". Prog J. Am. Chem. Soc. 129: . 15259-15278. doi:10.1021/ja075242e. {{cite journal}}: Explicit use of et al. in: |author1= (help)
^ Facchetti, A. (2007). "Semiconductors for organic transistors". Materials Today. 10 (3): 29–37. ISSN 1369-7021.
^ Newman, C.; et al. (2004). "Introduction to Organic Thin Film Transistors". Chem. Mater. 16: 4436–4451. doi:10.1021/cm049391x. {{cite journal}}: Explicit use of et al. in: |author1= (help)