Electric Contacts in Semiconductors - Essay Example

Assignment Number Electric Contacts First Last Dr. TeacherFirst TeacherLast Number 30 June 2010 Electric Contacts in Semiconductors Electrical contacts history and background The study of electrical properties of semiconductors and their use has been going on for at least a century. Braun is credited for the earliest analysis of metal-semiconductor contacts when he observed the dependence of resistance in a contact to the polarity of applied voltage and surface conditions (Sze & Ng, 2007: p. 134). Later, transport in semiconductor contacts was linked to the band theory of solids based on Wilson’s research in 1931 (Sze & Ng, 2007: p. 134). Schottky’s work in 1938 laid the foundation for modern research followed by that of Mott and Bethe (Sze & Ng, 2007: p. 134). Metal-semiconductor contacts provide a path for charge to flow from one point to another in semiconductor circuits. Presence of appropriate contacts to semiconductors allows several semiconductor uses that depend on rectification and amplification of current. Applications of semiconductors have included diodes, organic light emitting diodes, transistors and solar cells. Electrical contacts are usually created by deposition of metal on a doped semiconductor substrate. Doping is done by applying impurities to the semiconductor to make it a better conductor and can be of N-type or P-type depending on the charge. This paper will consider two types of contacts, Ohmic and Schottky contacts. Most metal semiconductor contacts are Schottky contacts, and give non-linear rectifying current-voltage characteristics. A common application is their use in diodes. They, however, are not good for allowing outside contact for semiconductor or integrated circuit applications. For such applications, a contact which displays properties of conduction for both polarities is required. Such contacts are called Ohmic contacts, named for obeying Ohm’s law. An Ohmic contact displays a linear non-rectifying behaviour as opposed to non-linear rectifying behaviour of Schottky contacts. The characteristics of both contacts are depicted in Figure 1. Figure 1: Ohmic and Schottky Contacts Ohmic Contacts Ohmic contacts display linear non-rectifying current-voltage behaviour, must follow the Ohm’s law (V = IR) and have as low resistance at the contact as possible (Colinge & Colinge, 2006: p. 149). This would mean that conduction is provided both from the metal to semiconductor and vice versa. As mentioned earlier, such contacts are required for several applications such as integrated circuits. Ideal Ohmic contacts can only be made for selected materials. However, it is difficult to make Ohmic contacts for most semiconductor materials, particularly P-type with larger band gap, as there are no appropriate metals with “low enough work function to yield a low barrier” (Sze & Ng, 2007: p. 189). To overcome this problem, tunnelling is used followed by annealing to create Ohmic contacts out of materials which would “a priori form a Schottky diode” (Colinge & Colinge, 2006: p. 149). Tunneling is provided by doping the semiconductor heavily so that there is a thin barrier between the semiconductor and the metal which allows carriers to tunnel through. For example, Aluminium can be used on p-type GaAs and gold can be used on n-type GaAs (Grundmann, 2006: p. 423). There are several ways for deposition including sputtering, evaporation, and others. Subsequent to deposition the contact is usually annealed at high temperature or the metal alloyed with the semiconductor to improve its resistivity. Schottky Contacts Schottky contacts display non-linear rectifying current-voltage behaviour. These are metal-semiconductor contacts with large barrier height and low doping concentration. In metal semiconductor contacts, the resistance or conduction properties are markedly different from a contact between two metal surfaces. The significant difference in Fermi levels in the metal and semiconductor make it difficult for current to flow between them in both directions. This results in a rectifying contact with high resistance level. Electrons try to flow from the solid with lower work function, which is the difference between Fermi level and vacuum level of the semiconductor in relation to the metal, to the other side until the Fermi levels are in equilibrium (Wan, 2008: p. 50). The lower work function solid assumes a slightly positive charge while the one with higher work function assumes a slightly negative charge resulting in a potential to be created. This potential becomes the reason for band bending to take place at the semiconductor surface. The metal side does not display any band bending properties. Very few carriers can get across this barrier to the other material. If a bias is applied to this contact it can result in either making the barrier appear lower or higher. This results in a Schottky Barrier where conduction takes place for one polarity and not for the other. Diodes based on Schottky contacts are majority carriers, meaning that the current flow depends on electric field rather than diffusion, and this allows the device speed to be very fast (Mishra & Singh, 2007: pp. 118, 227). Roughly all metal semiconductor contacts exhibit the same rectifying behaviour. These contacts, called Schottky contacts, make good diode applications but are not good for allowing outside contact for semiconductor or integrated circuit applications. References Colinge, J.P., & Colinge, C.A. (2006) Physics of semiconductor devices. New York, NY: Springer. Grundmann, M. (2006) The physics of semiconductors: an introduction including devices and nanophysics. New York, NY: Springer. Mishra, U.K., & Singh, J. (2007) Semiconductor Device Physics and Design. Dordrecht, Netherlands: Springer. Sze, S.M., & Ng, K.K. (2007) Physics of semiconductor devices. Hoboken, NJ: Wiley. Wan, M. (2008) Conducting polymers with micro or nanometer structure. Berlin, Germany: Springer. Read More