Transistor Packages

SOT-23 Package

SOT-23 Body style Transistor or FET SMD package

Package Out-line and Dimensions for a SOT-23 Transistor
SOT-23 Package Drawing and Package Dimensions

The SOT-23 package is a 3-terminal plastic surface mount component.
A similar package style is the TO-236AA SMD Transistor.

SMD Transistor, TO-236 Package
SOT-23 Package Drawing

Case temperature. Because the SOT-23 is a Surface Mount Device [SMD] heat transfer will also be conducted via the component leads. There are no leads to shorten to increase heat conduction as in a thru-hole component. The TO-236 has to be mounted to a Printed Circuit Board [PCB] so the lands or PWB pads are used to transfer heat. For best results use the largest pads possible while keeping the required spacing between pads.

The weigh of the copper [copper thickness] is of marginal help with 3oz copper providing the same heat transfer as 1oz copper. However the surface area of the PCB pads makes all the difference in conducting heat. The larger the pad surface area, the lower the thermal resistance. Heat conduction increases as thermal resistance decreases.
Refer to Thermal Resistance by Pad Area by Copper Weight graph.

TO-236 Package
SOT-23 Package Drawing
SMD Transistor soldered to a printed circuit board

The text above indicates that this package is used by either transistors or FETs but almost any two or three terminal semiconductor could use this package: Thyristors, IGBTs and so on. Because of the package size, the SOT-23 is a low current device. Additional SOT23 Package Variations.


An additional version of the SOT23 package is the SOT23-MO package style. This variation is a 6-pin package, with three pins on either side of the plastic body. No physical dimensions are provided because there are a number of variations. For example the physical width of the plastic body is 1.75mm for a SOT23-MO-178 package, or 1.45mm for a SOT23-MO-203 package [max dimension]. The length could also vary between package versions; for example the MO-178 package has a length of 3.05mm, while the MO-203 package has a length of 2.15mm.
SOT23-MO-178 Package Out-line for a 6-pin SOT-23 Transistor
SOT-23-MO-178 Package Drawing

When holding one transistor per package it's common for 4 of the pins to be dedicated to the collector, allowing 4 pads on the printed wiring board to dissipate heat. Of course the remaining two terminals are used for the Base and Emitter of the transistor.

The SOT package used with transistors is similar to the SOP package used with digital ICs. Although the dimensions and number of pins changes with the semiconductor function used within the device and number of terminals required.


Related BJT Topics:
Transistor Derating Guidelines
Including Thermal definitions

Types of Transistor cases and FET cases
Manufacturers of FETs and BJTs
Thermal Compounds

SOT-23 Plastic SMD Transistors;
NPN Switching Transistor:
2N2222AUE1, 2N3700UE1
NPN Low Noise Switching Transistor:
PNP Switching Transistor:
2N2907AUE1, 2N4033UE1

SOT-23-6 Plastic SMD Transistors;
NPN Low Saturation Switching Transistor
NPN Low Saturation Switching Transistor
NPN Low Saturation Switching Transistor
PNP Low Saturation Medium Power Transistor
PNP Low Saturation Switching Transistor
PNP Low Saturation Switching Transistor

PC motherboard

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Modified 2/21/12
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