How to Identify an SMT Transistor

Written by j.t. barett
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How to Identify an SMT Transistor
Surface mount transistors are flat rectangles with three metal tabs. (Goodshoot/Goodshoot/Getty Images)

A transistor is an electronic component that amplifies and switches electric currents. They come in a variety of shapes and sizes, from microscopic devices on integrated circuits (ICs) to matchbook-sized modules. Some electronic equipment uses transistors packaged with surface-mount technology, or SMT. Traditionally, the transistor has a plastic or metal package with three wires to connect the device to a circuit. An SMT transistor dispenses with wires and uses stubby metal tabs. SMT transistors are smaller, lighter and less expensive than their leaded counterparts. You can identify them as flat, rectangular shapes in a circuit.

Skill level:
Moderately Easy


  1. 1

    Examine the components on a circuit board. Pay particular attention to dark plastic rectangular structures a few millimetres in size.

  2. 2

    Count the shiny metal tabs on the flat components. Some have two tabs, some have three, others have several. Those having three tabs may be SMT transistors.

  3. 3

    Examine the silk-screened lettering on the board next to each component. Those having a "Q" or "T" followed by a numeral are likely to be SMT transistors. For example, if you see the notation "T5" printed next to a component, this means it is the 5th transistor of several in the circuit. Parts designated with a "C," "D" or "R" may be surface-mount components, but are capacitors, diodes or resistors, respectively.

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