Power bipolar transistors are semiconductors in which a base n-type or p–type layer is sandwiched between emitter and collector layers of the opposite type. There are two polarities available: PNP and NPN. PNP devices consist of an n-type layer sandwiched between two p-type layers. NPN devices consist of a p-type layer sandwiched between two n-type layers. With both arrangements, the junctions between semiconductor sections amplify weak incoming signals. In addition, the thick and low-doped collector region results in a large blocking voltage. Typically, power bipolar transistors are operated at lower current densities to improve the power dissipation per unit of area. Larger devices are used with larger currents. Silicon is the most commonly used material because of its high thermal conductivity and relatively low cost. Silicon carbide offers performance advantages, but is a more expensive material.
Performance specifications for power bipolar transistors include collector-to-emitter breakdown voltage, collector-to-base breakdown voltage, maximum collector current, and current gain bandwidth. Static forward current transfer ratio, which is also known as common-emitter current gain, is the ratio of the input DC current and the output DC current. Power dissipation, the total power consumption of the device, is usually measured in watts (W) or milliwatts (mW). Other performance specifications for bipolar transistors include power gain, output power, and temperature range. Some devices support a specific temperature range and feature mechanical and electrical specifications that are suitable for commercial, industrial, or automotive applications. Other power bipolar transistors meet screening levels for military specifications (MIL-SPEC).
Basic IC package types for power bipolar transistors are transistor outline (TO), small outline (SO), and small outline transistor (SOT). For each package type, many variants are available. Transistor outline packages include TO-92, a single in-line package often used for low power devices; TO-220, which is suitable for high power, medium current, and fast-switching power devices; and TO-263, the surface-mount version of the TO-220 package. Small outline transistor packages include SOT23, which is often used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment; SOT89, a plastic, surface mounted package with three leads and a collector pad for good heat transfer; and SOT223, an encapsulated package that provides excellent performance in environments with high temperatures and humidity levels. IC package types for IGBTs also include discrete or deca-watt package (DPAK) and flat package (FPAK).
Packing methods for power bipolar transistors consist of tape reel, rail, bulk pack, and tube technologies. The tape reel method packs components in a tape system by reeling specified lengths or quantities for shipping, handling, and configuration in industry-standard automated board-assembly equipment. Rail, another standard packing method, is typically used only in production environments. Bulk pack devices are distributed as individual parts, while tray components are shipped in trays. The tube or stick magazine method is used to feed power bipolar junction transistors into automatic placement machines for through-hole or surface mounting.
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