Semiconductor junction rectifier

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. 

Early solid-state devices relied on observing the ionization in intrinsic semiconductors. Early devices were impractical due to the requirement of extremely pure material. Modem devices are based on semiconductor junction diodes. These diodes have a rectifying junction that only allows the flow of current in one direction. Incident radiation creates ionization inside the bulk of the diode and creates a pulse of current in the opposite direction to the normal current flow through a diode that is straightforward to detect. 

Hetero junctions, forming a Schottky barrier like a metal-semiconductor junction, normally change the energy levels of conduction and valence bands. When the Fermi level of the semiconductor equilibrates with the energy level of the redox couple in the solution, the electric energy level at the surface is pinned and a depletion layer is formed. This is postulated since the rectified current can be observed at semiconductor plate electrodes. The bending of the band in the semiconductor at the surface can be described as a solution of the one-dimensional Poisson-Boltzmann equation... 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Rectification the asymmetric current-voltage characteristics of a semiconductor junction, in which charge-carrier flow is impeded in one direction, but not in the other direction Reverse bias the sign of the applied potential opposite to forward bias. In reverse bias, the current through a diode is essentially independent of the applied potential Schottky barrier a rectifying semiconductor/metal junction Surface states energy levels arising from atoms at the semiconductor surface... 

The rectification properties of semiconductor interfaces are the most important electrical characteristic of semiconductor contacts. Certain types of devices, such as transistors, require both ohmic and rectifying contacts on a given semiconductor surface, whereas other devices, such as Schottky barriers, are based on the inherent rectification properties of semiconductor/metal Junctions. Numerous photonic devices, such as photon detectors and photovoltaic cells, require rectification at a semiconductor Junction, and light-emitting diodes require both ohmic contacts and rectifying Junctions in a well-defined geometry. Thus, successful fabrication of a desired device structure depends entirely on the electrical properties of the specific semiconductor contacts that are formed in the process. The principles described above allow the rational fabrication of contacts with the desired properties, and also describe the operation of the resulting devices within a simple, chemically intuitive, kinetic framework. 

Schottky barriers are metal-semiconductor junctions that have the ability to rectify current, because the work fimction of the metal is greater than that of the semiconductor. The junction thus creates a barrier between the semiconductor and the metal that decreases when the junction is forward biased and vice versa. Conduction in Schottky devices is by majority carriers, principally electrons. In conventional p-n devices reverse conduction is predominately via minority carriers. In p- -junction devices, charge is stored in the junction during forward conduction and has to be removed if the jimction is reverse biased before the diode can switch off. The junction capacitance and the capacitive reactance are voltage dependent. 

The electrical properties of a single crystal of germanium (or silicon) can be drastically changed by alloying the element with very small amounts of other elements. As explained in the following paragraphs, these effects are the basis of the operation of the semiconductor junction rectifier, the transistor, and integrated circuits. 

A bilayer-coated electrode has two electroactive films, each having different reduction potentials. The inner layer is in direct contact with the carrier electrode surface and acts as a mediator to the outer layer which is mainly in contact with the solution. Provided that the redox levels in the two layers are appropriate, the interface between the two polymer films acts analogously to a semiconductor junction as a charge rectifying junction. The method of preparing first the inner layer on the electrode and then, in a second step, the outer layer may in principle be deduced from the conventional methods already described. 

A metal-semiconductor junction is an integral part of any semiconductor device, and hence, it is crucial to understand their nature. Properties of a semiconductor-metal junction often closely resembles that of a semiconductor-electrolyte junction, as both can be rectifying in nature. However, the space-charge region in the metal is usually neglected because the high density of states causes very little penetration of the electric field beyond the surface. 

A metal-semiconductor junction will therefore form a barrier for electrons and holes if the Fermi energy of the metal is somewhere between the conduction and valence band edge. Such a contact that is rectifying in nature is called a Schottky contact. 

Eq. (14.1) is known as the Mott-Schotlky equation. We note llial for a given n-lype semiconductor, the harrier height increases as the work function of the metal increases. It is therefore expected that high work function metals will give a rectifying junction, and low work function metals an ohmic contact (it is the reverse for a p-type semiconductor). 

The Aviram-Ratner D-ct-A molecule is analogous to a pn junction rectifier the electron-rich donor region D would be similar to the electron-rich semiconducting n region, while the electron-poor A region would be similar to a semiconductor s p region [79]. However, note that under forward bias the preferred direction of Aviram-Ratner electron flow is from A to D, while in a pn junction rectifier the preferred direction is from n to p. 

Rectifying junction - [SEMCONDUCTORS - SILICON-BASED SEMICONDUCTORS] (Vol 21)... 

Fig. 2.20 Current-voltage characteristic for a metal-semiconductor rectifying junction.

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