This is a good place to start if you are choosing from different transistor package types for your design.
Introduction
Nowadays, you can choose from countless transistor package types for your designs. It would be best to base your decision on your design’s space requirements, thermal profile, cost, and specific application. You don’t want to end up with an over- or under-specified requirement. Over-specifying may lead to unnecessary costs that could be avoided. Much worse, under-specifying can lead to disastrous consequences when your product is left on the field.
Your design's requirements
Space Requirements
A transistor package should fit the space requirements of your design. For power designs, you may need to allot space for a heatsink or a large PCB area to dissipate the heat generated by your transistor. With this, you’ll need power packages such as the TO-220, TO-247, and TO-3 (through hole types), and the DPAK or D2PAK packages (surface mount types).
 You can opt for the popular TO-92 through-hole packages for small general-purpose designs. If you need a lower profile, you can opt for the smaller SMD types such as SOT-23, SOT-223, or SOT-89 variants.
Thermal Profile
If you need a through-hole-mounted power transistor that attaches to a heatsink, you may choose packages such as the TO-220, TO-247, TO-3, and others. These transistors are specially made with a thermally conductive back that can attach to an external heatsink.
Otherwise, if you can’t use through-hole-mounted components and need a lower profile, you can opt for the DPAK or D2PAK packages. These packages can be soldered and mounted to a PCB through surface-mount technology. You may have already seen DPAKs mounted on your PC motherboards or laptops.
Cost
Generally, higher power transistors costs much more. If you want to save on material cost, you may opt to use your PCB as a heat spreader rather than using an external heatsink. The DPAK and D2PAK packages are specially made for this situation.
Small, general-purpose SMD-type transistors usually cost less (in cents) individually, or they may come in a reel. These transistors typically come in a SOT-23 package. If you need more power, you can opt for transistors that come in a SOT-223 or SOT-89 package, which are usually more costly than the SOT-23s.
Specific Application
Note that some transistors may need to be used in a rugged environment, such as in automotive or industrial applications. To withstand these applications, you may require robust package types. It is best to consult your transistor datasheet to find out these details.
Common Transistor Package Types
Through Hole Packages
Listed below are the common through hole packages available for transistors.
TO-92
TO-18
This is the “metal-can” type package that can have slightly more power dissipation than a TO-92. It was usually used on classic transistors such as the 2N2222.
TO-220
A popular power transistor package that incorporates a thermally conductive tab at its rear side. This tab is meant to attach to a heatsink, and a heatsink transfer compound is usually applied to transfer heat efficiently. You’ll see many popular transistors such as TIP31, TIP41, IRF540N, IRFZ44N, and IRF9540N use theTO-220 packaging.
TO-247
Similar to the TO-220, the TO-247 uses its rear side for heat spreading to a heatsink. However, the TO-247 has a wider and thinner body, making it more thermally efficient to transfer heat than the TO-220. With these characteristics, TO-247s are more widely used for industrial and automotive applications.Â
TO-3
Also called a “metal can” package like the TO-18, this transistor package has high power dissipation (due to its size) and is usually used for high-current applications. If used for heavy loads, it usually requires a large heatsink with fins. You may have seen TO-3s used in power amplifiers, power supplies, and motor control circuits. Some popular transistors using the TO-3 packaging are 2N3055, MJ2955, MJ1000, and KD503.
SMD Packages
SMD transistor packages are very useful if you need a low-profile design. Additionally, they can be efficiently assembled with a pick-and-place machine.
SOT-23
A popular package for general purpose transistors of the SMD type. Usually a very inexpensive package with modest specs. Popular transistor housed in a SOT-23 are MMBT3904, MMBT3906, MMBT5551, and MMB2222A.
SOT-223
If you require higher power dissipation than SOT-23 (or SOT-89), SOT-223s are the way to go. They have a wide surface-mount pad that can attach to copper on a PCB to disperse heat efficiently. Popular transistors housed in a TO-223 package are 2N4401, 2N4403, and PZT651T1.
SOIC or SOP-8
You’ve previously learned about the SOIC and SOP IC packages. However, did you know that transistors also use these types? The 8-pin SOIC or SOP is a good choice for transistors since they have short leads and a very low profile, perfect for efficient power dissipation. They come in various pin configurations too. For example, pins 8, 7, 6, and 5 can be used as Drain terminals, 1, 2, and 3 as Source, and 4 as Gate. Alternatively, some packages can have both P-Channel and N-Chanel MOSFETs inside them, making them a very versatile device.Â
SOT-89
Another alternative to SOT-23 and SOT-223 is the SOT-89 package. It is more thermally efficient than SOT-23 but less than SOT-223. You’ll notice that the SOT-89 has smaller pins and heat spreader pads than the SOT-223. This allows you to use SOT-89 in more constrained applications.
DPAK (TO-252) and D2PAK (TO-263) series
Originally from Motorola, DPAK, and D2PAK (short for Decowatt and Double Decowatt), power SMD packages are useful complements to their through-hole TO-220 counterparts. DPAKs have their pins and metal tabs meant to lie flat on the PCB for SMD soldering. DPAK is also known as TO-252, while D2PAK is TO-263.
A D2PAK has a wider body than a DPAK and is more thermally efficient.
Conclusion
You have just learned about different transistor package types and where to use them for various applications. Choose wisely to make your transistor designs more efficient.
