Semiconductor device physics

Sept. 1, 2006
Successfully designing diodes and other semiconductor circuits depends on an understanding of the physical processes taking place at the atomic level.

Successfully designing diodes and other semiconductor circuits depends on an understanding of the physical processes taking place at the atomic level. Chip designers are especially concerned with electron movements in crystals and the “holes” they leave behind, how each moves, and the effects of one type outnumbering another in different regions of a crystal. These processes contribute to the construction and operation of integrated circuits used in such products as cell phones, microwaves, and motion controllers.

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Rendez-vous

Semiconductor diodes are the simplest electronic devices, consisting of p and n-type silicon. At room temperature, thermal ionization breaks the silicon's covalent bonds, freeing electrons from their parent atom. The positive charges, or holes, left behind are quickly filled by electrons from nearby atoms. This process is called recombination.

Doping

Doping, in the context of integrated circuit fabrication, involves adding impurity atoms to a semiconductor crystal to change its electrical properties. Adding a phosphorous atom to silicon, for example, introduces a free electron (one not shared in a covalent bond), making the silicon more n-type (mostly negatively charged carriers). In this case, phosphorous is a donor atom. Adding boron to silicon, on the other hand, produces p-type (mostly positively charged carriers) material because each boron atom uses one silicon electron to complete its covalent bond. Here, boron is classified as an acceptor atom.

Diffusion and drift

Carriers move through silicon crystals via diffusion and drift. Diffusion is the movement of particles from an area of high concentration to low concentration and is caused by thermal agitation. Drift results when an electric field is applied across a crystal, superimposing a small velocity that accelerates free carriers. Holes drift in the direction of the electric field, while electrons drift opposite the electric field.

In the real world

When fabricating integrated circuits, one of the most common doping methods is ion implantation. Ion implanters produce ions, charged particles, and accelerate them with an electric field. These ions strike the silicon surface and penetrate to varying depths. How deeply they embed in the crystal depends on the ion beam's energy, which is controlled by the accelerating field voltage. Varying the beam current (flow of ions) controls the density, or number, of ions implanted.

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