Semiconductor-based transducers

85.41 - Diodes, transistors and similar semico~lductordevices; photosensitive semiconductor devices, including photovoltaic cells whether or not assembled in modules or made up into panels; light emitting diodes; mounted piezo-electric crystals (+). 8541.10 - Diodes, other than photosens:itive or light emitting diodes - Transistors, other than photoriensitive transistors : 8541.21 - - With a dissipation rate of less than 1 W - - Other 8541.30 - Thyristors, diacs and triacs, other than photosensitive devices 8541.40 - Photosensitive semiconductor devices, including hotovoltaic cells whether 8541.29 P or not assembled in modules or made up into pane s; light emitting diodes - Other semiconductor devices 8541.60 - Mounted piezo-electric cryst:ils 8541.50 (A) DIODES, TRANSISTORS AND SIRlTLAR SEMICONDUCTOR DEVICES These are defined in Note 8 (a) to this Chapter. The operation of the devices of this group is based on the electronic properties of certain " semiconductor " materials. The main characteristics of these materials is that at room temperature their resistivity lies in the range between that of conductors (metals) and that of insulators. They consist, for mstance, of certain ores (e. ., crystal galena), tetravalent chemical elements (germanium, silicon, etc.) or combinations o chemical elements (e.g., trivalent and pentavalent elements, such as gall~um arsenide, indium antimonide). f Semiconductor materials consisting of a ,tetravalent chemical element are generally monocrystalline. They are not used in their ure state but after ve light doping (in a proportion expressed in parts per million) with a speci IC " impurity " (dopanT. ? For a tetravalent element, the " im urity " may he a pentavalent chemical element (phosphorus, arsenic, antimony, etc.) or a triva ent element (boron, aluminium, gallium, indium, etc.). The former produce n-type semiconductors with excess electrons (negatively charged); the latter e semconductors with electron deficiency, that 1s to say that holes (positively ;iZfominate. ZEdiZl Semiconductor materials combining tri- and pentavalent chemical elements are also doped. In the semiconductor materials consisting of or'zs, the impurities contained naturally in the ore act as dopants. The semiconductor devices of this group generally comprise one or more "junctions between p-type and n-type semiconductor materials. ", They include : (I) Diodes which are two-terminal devices with a single p n junction; they allow current to pass in one direction (forward) but offer a very high resistance in the other (reverse). They are used for detection, rectification, switching, etc. The main types of diodes are signal diodes, power rectifier diodes, voltage regulator diodes, voltage reference diodes. (11) Transistors are three- or four-terminal devices capable of amplification, oscillation, frequency conversion, or switching of electrical currents. The operation of a transistor depends on the variation in resistivity between two of the terminals upon the application of an electric field to the third terminal. The applied control signal or field is weaker than the resulting action brought about by the change m resistance and thus amplification results. Transistors include : (1) Bipolar transistors, which .are three-terminal devices consisting of two diode type junctions, and whose transistor action depends on both positive and negative charge carriers (hence, bipolar). (2) Field effect transistors (also known as metal oxide semiconductors (MOS) ,which may or may not have a unction, but which depend on the induced epletion (or enhancement of availa le charge carriers between two of the terminals. The transistor action in a eld effect transistor em loys only one e of charge carrier (hence, unipolar). MOS type transistors which ave four termina s are known as tetrodes. A d b e ? (111) Similar semiconductor devices. The " similar " devices referred to here are semiconductor devices whose operation depends on variations in resistivity on the application of an electric field. They include : (I) Thyristors, consisting of four conductivity regions in semiconducting materials (three or more p n junctions) through which a direct current passes in a predetermined direction when a control pulse initiates conductivity. They are used as controlled rectifiers, as switches or as am lifiers and fimction as two interlocking, complementary transistors with a common col ectorlbase junction. f (2) Triacs (bi-directional triode thyristors), consisting of five conductivity regions in semiconducting materials (four p n junctions) through which an alternating current passes when a control pulse initiates conductivity. (3) Diacs, consisting of three conductivity regions in semiconducting materials (two p n junctions) and used to provide the pulses required to operate a triac. (4) Varactors (or variable capacitance diodes). (5) Field effect devices, such as gridistors. (6) Gunn effect devices. However, this group does not include semiconductor devices which differ fiom those described above in that their operation depends primarily on temperature, ressure, etc., such as non-linear semiconductor resistors (thermistors, varistors, magneto-resiston, etc.) &eatling 85.33). For photosensitive devices the operation of which depends on light rays (photodiodes, etc.), see group (B). The devices described above fall in this heading whether presented mounted, that is to say with their terminals or leads or packaged (components), unmounted (elements) or even in the form of undiced discs (wafers). However, natural semiconductor materials (e.g., galena) are classified in this heading only when mounted. The headin also excludes chemical elements of Chapter 28 (for exam le, silicon and selenium) doped for use in e ectronics, 1n the form of discs, wafers, or similar forms, olis ed or not, whether or not coated with a uniform epitaxial layer, provided they have not been se ectively doped or diffused to create discrete regons. k P k' (B) PHOTOSENSITIVE SEMICONDUCTOR DEVICES This group comprises photosensitive semiconductor devices in which the action of visible rays, infka-red rays or ultra-violet rays causes variations in resistivity or generates an electromotive force, by the internal photoelectric effect. Photoemissive tubes (photoemissive cells) the operation of which is based on the external photoelectric effect (photoemission),belong to heading 85.40. The main types of photosensitive semiconductor devices are : (I) Photoconductive cells (light dependent resistors), usually consistin of two electrodes between which is a semconductor substance (cadmium sulphide, kad sulphide, etc.) whose electrical resistance varies with the intensity of illumination falling on the cell. These cells are used in flame detectors, in exposure meters for automatic cameras, for counting moving objects, for automatic precision measuring devices, in automatic door opening systems, etc. (2) Photovoltaic cells, which convert light directly into electrical energy without the need for an external source of current. Photovoltaic cells based on selenium are used mainly in luxmeters and exposure meters. Those based on. silicon have a higher output and are used, in particular, in control and regulating equipment, for detecting light impulses, in communication systems using fibre optics, etc. Special categories of photovoltaic cells are : (i) Solar cells, silicon photovoltaic cells which convert sunlight directly into electric .They are usually used in groups as sources of electric power, e.g., in rockets or satel ites employed in space research, for mountain rescue transmitters. enerF The heading also covers solar cells, whether or not assembled in modules or made u into panels. However the heading does not cover panels or modules e uipped wxtE elements, however simple, (for example, diodes to control the direction o the current), which su 1 the power directly to, for example, a motor, an electrolyser (heading @.dl). ii) Photodiodes (germanium, silicon, etc.), characterised by a variation in resistivity when t rays strike their n junction. They are used in automatic data processing (reading storage), as pFtotocathodes in certain electronic tubes, in radiation yrometers, etc. Phototransistors and photothyristors belong to this category of p otoelecttic receivers. R The devices of this category differ, when packaged, from the diodes, transistors and thyristors of Part (A) above by their housing, wh~chis partly transparent to permit the passage of light. (jii)Photocou les and photorelays consisting of electroluminescent diodes combined with photodio$s, phototransistors or photofhynstors. Photosensitive semiconductor devices fall in this heading whether presented mounted (i-e., with their terminals or leads), packaged or unmounted. (C) LIGHT EMITTING DIODES Li ht emitting diodes, or electroluminescent diodes, (based, inter alia, on gallium arsenide or ga ium phosphide) are devices which convert electric energy into visible, infi-a-red or ultra-violet rays. They are used, e.g., for displaying or transmitting data in control systems. Laser diodes emit a coherent light beam and are used, e.g., in detecting nuclear particles, in altimetering or in telemetering equipment, in communication systems using fibre optics. (D)MOUNTED PIEZO-ELECTRIC CRYSTALS This heading also excludes unmounted piezo-electric crystals (generalIy heading 38.24,71.03 or 71.04). PARTS k Sub'ect to the general provisions regarding the classification of parts see the General Exp anatory Note to Section XVI), parts of the goods of this heading are classi ed here.