Before transistors existed, vacuum tubes were used to amplify signals and logic gates in the first simple computers. While they provide high-quality amplification, vacuum tubes are bulky and require large amounts of power which is why they were phased out soon after transistors became commercially available.
One of the easiest ways to identify a transistor is to look up the part number that is often printed on the component itself. For example, the 2N3904 is a BJT NPN transistor that is housed in a TO-92 package and has three pins. The back of the package has “2N3904” which, when Googled, returns links to the datasheet.
However, some transistors may be in packages too small to have a part number printed or only have a portion of the part number printed. In these instances, the PCB component legend (silk screen) has to be relied upon and a transistor can be identified with the component identification TR or Q.
There is a growing trend with manufacturers to not print component identifications as SMD components are now placed using a pick and place machine (the computer does not need to look at diagrams on the board as it has a copy internally).
Transistors come in many different packages and since these packages are also used by other parts (such as with diodes and voltage regulators), they can be tricky to identify at times.
Transistors come in many different types but, unlike diodes, different transistor types can be housed in the same package. This means that the package that houses a transistor will not tell you the specific type of transistor.
Through-hole transistors have component legs that are inserted through the holes on a PCB and then soldered in place. Through-hole transistors were very common in older designs (before 2000), but are now only found in power applications.
TO-92 Package Transistors
The TO-92 package is one of the most common packages for through-hole transistors in the past and is still a very popular package in prototyping and hobby circuits. TO-92 packages have a curved face and a flat face where the part number is printed. TO-92 packages have their leads very close together which make them more difficult to insert into a PCB if a standard 0.1” pitch is used.
TO-220 Package Transistors
The TO-220 is another common transistor package that is often associated with medium power amplification and switching. The TO-220 package has a tab on the top of the part that is used for mounting to a heat-sink and can be mounted horizontally or vertically directly on to the PCB.
Be warned—other parts also use the TO-220 packaging including voltage regulators such as the 7805 and the LM317 so always double-check the printed part number!
TO-3 Package Transistors
TO-3 packages are used in high-power/high-current situations and can be cheap or very expensive. This package has three connections but only two pins as the third connection is the body itself.
The package has two holes for mounting directly on to a heat sink and the part number is printed on the lid. The 2N3055 is a common example of a TO-3 package and is often used in DC motor control.
TO-18 / TO-39 Package Transistors
The TO-18/39 packages are another transistor package that was commonly used in earlier electronics. Unlike the TO-92, the TO-18 is a metal case that is hollow internally.
Since the case is circular a tab is connected to the bottom of the case to indicate pin 1 and the part number is printed on the side of the case. While not commonly used in transistors anymore, laser diodes in CD and DVD players are commonly housed in a TO-18 package with a lens attached to the top.
TO-126 Package Transistors
The TO-126 is a transistor package that is smaller than a TO-220 but larger than a TO-92.
The width of the legs on a TO-126 is the same as a TO-220 as well as the leg pitch but the package itself is physically smaller.
The component part number is printed on the front while the back may be metalized for mounting to a heat sink. This package is not very common and almost never used in modern electronics.
Surface Mount Transistors
SMD transistors are now the most common type of transistor on the market being available in many different shapes and sizes.
In general, the larger an SMD transistor is, the more power it can handle. One of the problems with SMD transistors is that very small parts may not have their part number printed on them, making identification very difficult.
SOT-23 Package Transistors
The SOT-23 package is one of the smallest transistor packages that have three pins with one of the pins being on the other side of the package (this prevents the package from being placed incorrectly).
SOT-23 packages may not have their part numbers printed as they are so small and if they do have something printed, it may be a portion of the code that only allows the designer to know what the part actually is.
TO-263 / D2PAK
The D2PAK transistor case is very similar to the TO-220 package but lacks the tab and is surface mounted. These parts have their part number printed on the front face, a very small tab, and two legs at the front. D2PAK packages are used in high power situations like DC motors.
SOIC / SOP
Sometimes multiple transistors of identical characteristics are needed in a circuit (such as push/pull amplifiers). In these situations, transistors in SOIC parts are used such as the WSP4884 which houses two NMOSFET devices.
These devices are very easily mistaken for semiconductor devices such as amplifiers, memory ICs, digital logic, and microcontrollers.