Junctions between semiconductors

It seems possible that the same approach will suffice for Junctions between semiconductors and insulators, since the latter will be described by the same LCAO parameters. However, the treatment of semiconductor metal junctions is better handled with pscudopotentials, so will be discussed in Chapter 18. 

Exclusion of minority carriers is a nonequilibrium transport effect occurring as a consequence of the application of electric field to an isotype contact, i.e., to a junction between semiconductors of the same type, but with different dopant concentrations and/or bandgaps. In n-type semiconductors exclusion is provided by... 

A semiconductor laser takes advantage of the properties of a junction between a p-type and an n-type semiconductor made from the same host material. Such an n-p combination is called a semiconductor diode. Doping concentrations are quite high and, as a result, the conduction and valence band energies of the host are shifted in the two semiconductors, as shown in Figure 9.10(a). Bands are filled up to the Fermi level with energy E. ... 

The first semiconductor lasers, fabricated from gallium arsenide material, were formed from a simple junction (called a homojunction because the composition of the material was the same on each side of the junction) between the type and n-ty e materials. Those devices required high electrical current density, which produced damage ia the region of the junction so that the lasers were short-Hved. To reduce this problem, a heterojunction stmcture was developed. This junction is formed by growing a number of layers of different composition epitaxially. This is shown ia Figure 12. There are a number of layers of material having different composition is this ternary alloy system, which may be denoted Al Ga his notation, x is a composition... 

Contacts are the elementary building blocks for all electronic devices. These include interfaces between semiconductors of different doping type (homojunctions) or of different composition (heterojunctions), and junctions between a metal and a semiconductor, which can be either rectifying (Schotlky junction) or ohmic. Because of their primary importance, the physics of semiconductor junctions is largely dealt with in numerous textbooks [11, 12]. We shall concentrate here on basic aspects of the metal-semiconductor (MS) and, above all, metal-insulator-semiconductor (MIS) junctions, which arc involved in the oiganic field-effect transistors. 

The heart of a light-emitting diode is a junction between a p-type semiconductor and an n-tyqje semiconductor. The different semiconductor types have different electron populations in their bands. The lower-energy band of a p semiconductor is deficient in electrons, while the upper-energy band of an n semiconductor has a small population of electrons. The band structure in the junction region is shown schematically in the figure below. 

More complex phenomena occur when current crosses interfaces between semiconductors. The most typical example is the rectification produced at interfaces between p- and n-type semiconductors. Electric current freely flows from the former into the latter semiconductor, but an electric field repelling the free carriers from the junction arises when the attempt is made to pass current in the opposite direction Holes are sent back into the p-phase, and electrons are sent back into the n-phase. As a result, the layers adjoining the interface are depleted of free charges, their conductivities drop drastically, and current flow ceases ( blocking the interface). 

Macroscopic n-type materials in contact with metals normally develop a Schottky barrier (depletion layer) at the junction of the two materials, which reduces the kinetics of electron injection from semiconductor conduction band to the metal. However, when nanoparticles are significantly smaller than the depletion layer, there is no significant barrier layer within the semiconductor nanoparticle to obstruct electron transfer [62]. An accumulation layer may in fact be created, with a consequent increase in the electron transfer from the nanoparticle to the metal island [63], It is not clear if and what type of electronic barrier exists between semiconductor nanoparticles and metal islands, as well as the role played by the properties of the metal. A direct correlation between the work function of the metal and the photocatalytic activity for the generation of NH3 from azide ions has been made for metallized Ti02 systems [64]. 

Finally, an interesting concept, recently advanced, is the implementation of active materials as nanotube arrays. These systems have high surface area to optimize contact between semiconductor and electrolyte, and good light trapping properties. Their inner space could also be filled with catalysts or sensitizers and/ or pn junctions to obtain charge separation and facilitate electron transport [136]. 

Fig. 7.21. When a junction between a p-type and an n-type of semiconductor is established (a), a diffusion of holes and electrons in the opposite direction takes place (b). This results in a separation of charge (c) and the formation of an electrical potential difference across the interface (d).

So-called wet solar cells show promise, particularly because of their relative ease of fabrication. In this type of photovoltaic cell, the junction is formed, between a semiconductor and a liquid electrolyte. No doping is required because a junction forms spontaneously when a suitable semiconductor, such as GaAs, is contacted with a suitable electrolyte, Three knotty problems (accelerated oxidation of surface of semiconductor exchange of ions between semiconductor and electrolyte forming a blocking layer and deposition of ions of impurities on the surface of the semiconductor) all have been solved and thus the concept now appears technically viable,... 

In solid-state devices semiconductors are interfaced with other materials and junctions are formed. A junction between two semiconductors of opposite polarity is called a pn-junction. If the concentration of one type of dopant is much higher than that of the other (e.g., N > > Afo) it is called an abrupt junction (Fig. C.3). For the approximately equal doping levels we talk about, we use the term two-sided junction. A gradient of dopants is found in graded junctions. The concentrations of carriers on the two sides of the junction (subscripted) are... 

Fig. 2.23 Junction between an n-type and a p-type semiconductor (a) before contact and (b) after contact.

Controlled introduction of impurities forms the basis of much of semiconductor technology indeed p-type (acceptor-doped) and n-type (donor-doped) layers and the junctions between them control carrier confinement, carrier flow and ultimately the device characteristics. Achieving both n-type and p-type conductivity has traditionally proved to be a challenge in wide-bandgap semiconductors. 

In the dark, the junction between an extrinsic (doped) semiconductor and a redox electrolyte behaves as a diode because only one type of charge carrier (electrons for n-type and holes for p-type) is available to take part in electron transfer reactions. The potential distribution across the semiconductor/electrolyte interface differs substantially from that across... 

Thermocouples, or thermal junctions, or thermoelectric thermometers have two intermetallic junctions between two different metals (or semimetals, or semiconductors) A, B in a loop (Fig. 10.21). When these two junctions are held at different temperatures (T i, and T2), then a potential difference A Vis set up this is the Seebeck98 effect. For instance, for a Cu-constantan thermocouple, with T2 = 300 K and T, 273.15 K, AV = 1.0715 mV. Its converse is the Peltier99 effect If a current at a fixed voltage is applied in a loop like in Fig. 10.21, then a temperature difference AT can be maintained (thermoelectric heaters and coolers). The Seebeck effect arises because, before the junctions are made, the two metals have different Fermi levels after the junctions are made, electrons will flow from the higher-level metal to the lower-level metal, until a single Fermi level results across the junction. 

When a Schottky junction between a metal and an n-type semiconductor is illuminated and photons having enough energy for the creation of an electron-hole pair are absorbed in the depletion region, the electric field at the junction is able to separate the photogenerated charges, preventing recombination. Electron movement toward the semiconductor and hole... 

Of perhaps even more interest arc the discontinuities in hands at a hetero-junction, the junction between two semiconductors, which is u.sed in electronic devices. The predicted discontinuity in the valence-band maximum (Harrison, 1977 b) is obtained by simply subtracting the corresponding values from Table 10-1. The discontinuity in the conduction bands is obtained by making a correction on each side of the hcterojunclion for the band gap. In alloys, the values from Table... 

The pseudopotential formulation also provides a simple way to estimate the relative band positions at interfaces between two semiconductors. The estimate is in reasonable accord with the results of the LCAO calculations for heterojunctions moreover, it provides an approach for analysis of junctions between metals and semiconductors. 

Figure 3.72. Possible junction between a semiconductor and an aqueous solution double layer. The former typically extends a few orders of magnitude further than the latter. The way in which the potential regimes match Is highly uncertain. The slope (dv /dx ) depends on the sum of all charges starting from the seat of the surface states to the right.

Photovoltaic devices are based on the concept of charge separation at an interface of two materials having different conduction mechanisms, normally between solid-state materials, either n- and p-type regions with electron and hole majority carriers in a single semiconductor material, heterojunctions between different semiconductors, or semiconductor-metal (Schottky) junctions. In photoelectrochemical cells, the junctions are semiconductor-electrolyte interfaces. In recent years, despite prolonged effort, disillusion has grown about the prospects of electrochemical photo-... 

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