The trench walls are oxidized and the remaining volume is filled with polysilicon. The trench is typically 2µm wide and 5µm deep.To further reduce the area between adjacent mesa, trench isolation can be used, making use of trench etching.The base and emitter are formed on the large mesa and the collector on the small mesa.Oxide is either deposited or grown to fill the V-grooves. They utilize wet anisotropic etch (KOH) of the Si wafer with Si3N4 as mask. Oxide isolation processes were intorduced in the late 70’s.Modern isolation techniques: oxide isolation, and trench isolation.The area (and hence junction capacitance) is determined by alignment tolerance, area for side diffusion, and allowance for the spread of the depletion region.The traditional junction isolation technique requires the p+ deep diffusion to be aligned to the n+ buried layer that is covered by a thick epitaxial layer.The most significant advances in reducing overall device size and packing density have come from improved isolation methods.As the advances in MOS development appears, some of the fabrication technology are also applied to the BJT.The original BJT structure survived, practically unchanged, since the mid 60’s.Controlling depth of the emitter’s n doping sets the base width.In the planar process, all steps are performed from the surface of the wafer.The structure contains two p-n diodes, one between the base and the emitter, and one between the base and the collector.īJT Structure - Planar The “Planar Structure” developed by Fairchild in the late 50s shaped the basic structure of the BJT, even up to the present day.Early BJTs were fabricated using alloying - an complicated and unreliable process.The E-B diode is closer to the surface than the B-C junction because it is easier make the havier doping at the top.In integrated version, all 3 contacts appear on the top surface.In discrete, the substrate can be used for one connection, typically the collector.BJT can be made either as discrete devices or in planar integrated form.The First BJT Transistor Size (3/8”L X 5/32”W X 7/32”H) No Date Codes. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.There are therefore two kinds of BJT, the NPN and PNPvarieties.A bipolar transistor essentially consists of a pair of PN Junction diodes that are joined back-to-back.Field Effect Transistors (FETs) are “unipolar” transistors since their operation depends primarily on a single carrier type.The term “bipolar” was tagged onto the name to distinguish the fact that both carrier types play important roles in the operation.It did not become practical until the early 1950s.Initially known simply as the junction transistor.First BJT was invented early in 1948, only weeks after the point contact transistor.The point-contact transistor was commercialized and sold by Western Electric and others but was rather quickly superseded by the junction transistor. It was made by researchers John Bardeen & Walter Houser Brattain at Bell Laboratories in December 1947. The First Transistor: Point-contact transistor A point-contact transistor was the first type of solid state electronic transistor ever constructed. In digital circuits they function as electrical switches, including logic gates, random access memory (RAM), and microprocessors.In analog circuits, transistors are used in amplifiers and linear regulated power supplies. The physics of "transistor action" is quite different for the BJT and FET.Transistors have 3 terminals where the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region.
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