Cathode diode9/27/2023 ![]() The current that flows in a pure semiconductor material is very small, and can be thought of as streams of free electrons and holes going in opposite directions as illustrated below. The free electrons from broken covalent bonds move through the semiconductor by 'hopping' from one hole to the next towards the positive terminal, making it appear as if the positively charged holes are moving towards the negative terminal. When a battery is connected across a pure semiconductor material, it attracts free electrons within the crystal structure to the positive terminal and supplies more free electrons at the negative terminal. ![]() Silicon crystal lattice with free electron and hole The structure of a silicon atom is illustrated below. ![]() Semiconductor materials such as silicon or Germanium have four valence electrons. In solids, the atoms usually combine together in a regularly repeating three-dimensional structure known as a crystal lattice. The valence electrons form covalent bonds with the valence electrons of other atoms. It is these valence electrons that give the atom its electrical properties, which will in turn determine how the atom can combine with other atoms. Each shell requires a specific number of electrons in order to be considered complete, and the electrons in the outermost shell of an atom are known as valence electrons. The number of electrons orbiting the atom will be different for each element, but in all cases the atoms will be arranged in one or more orbits known as shells. ![]() The first thing to understand is that for each element in the periodic table, there will be a number of electrons in orbit around the nucleus of the atom. To help you to understand how a diode works, we will attempt an explanation of the properties of semiconductor materials, starting with a look at the nature of the bonds formed between the atoms that make up various materials. Note that the direction of the arrow in the circuit symbol indicates the direction of conventional current flow through the diode. A typical diode together with its circuit diagram symbol is shown below. For this reason, most discrete diode components are marked in such a way as to identify the cathode (usually with a black or white painted band). The very nature of the diode means that it must be connected the right way round in a circuit. Conventional current flow is of course in the opposite direction, so conventional current flows out of the diode via the cathode. Electrons flow from the cathode to the anode. The other terminal, known as the anode, is connected to the p-type region. One terminal of the diode, known as the cathode, is connected to the n-type region. These holes act as positive charge carriers, and the region is referred to as a p-type semiconductor material. The other region is characterised by an absence of electrons (often referred to as "holes") in many of the chemical bonds between the atoms within the region. One of the regions contains a high number of negative charge carriers (free electrons) and is referred to as an n-type semiconductor material. The semiconductor material in a diode consists of two adjoining regions, each of which has been "doped" with chemical impurities to give it specific electrical characteristics. In the other direction (known as the diode's reverse direction), the diode prevents current from flowing. The direction in which the diode allows current to flow is known as the diode's forward direction. Indeed, diodes were the first electronic components to be constructed using semiconductor materials (nowadays the material used is primarily silicon, although germanium is also used for some applications). The term is normally used to refer to a semiconductor diode. A diode is an electronic component with two terminals that conducts electricity in one direction only.
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