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A surface mount device (SMD) transistor is a type of transistor that is soldered directly onto the surface of a computer component board. Although transistors mounted this way can break off more easily, boards that utilize the SMD transistor are cheaper to produce than others. The alternative to SMD transistors are through-hole transistors, which are attached to the board with metal prongs that are inserted into holes drilled in the board. The drilling of these holes makes through-hole technology more costly and time consuming than SMD technology.
Any type of transistor can be mounted as an SMD transistor, but the most common type of SMD transistor is called a bipolar junction transistor. With this device, an electrical signal is passed through two positive-negative gates. The devices may be designed as positive-negative-positive (PNP) or as negative-positive-negative (NPN). Current is added to the system in the middle section in order to amplify the input as it crosses past the second positive-negative gate.
Another common design for an SMD transistor is as a metal oxide semiconductor field-effect transistors (MOSFET). MOSFETs can be either negative or positive. They are comprised of an input/output of the same charge which connect with a central area of that same charge when the electricity is sent through the center probe. The rest of the body of the transistor is the opposite charge.
Attaching an SMD transistor must be done carefully because the transistor is a delicate piece of equipment that can be easily damaged by the heat of soldering. In order to prevent this damage, the three prongs on the SMD transistor are soldered one at a time, leaving enough time in between solders to allow the transistor to cool back to room temperature.
Semiconductor materials are an important component in an SMD transistor. Semiconductors conduct only a portion of the electrical energy that passes through them, which allows the SMD transistor to be used as a switch or as an amplifier. An SMD transistor is most frequently used to amplify electrical signals. These electrical signals enter the transistor at one point and pass through a barrier into the middle section, where the input is subjected to an electrical charge. The signals then leave, amplified, through the third section. SMD transistors can amplify input by a factor of several hundred.