Size comparison of transistor packages. Two surface-mount packages, SOT23 and SOT223, are shown next to through-hole packages
A small outline transistor (SOT) is a family of small footprint, discrete surface mounttransistor commonly used in consumer electronics. The most common SOT are SOT23 variations, also manufacturers offer the nearly identical thin small outline transistor (TSOT) package, where lower height is important.
The SOT89-3 electrically only has three leads (contact/pin). The wide lead (tab) is physically part of the middle lead on the other side of the package. Some call this package a SOT89-4, since it visually appears to have four leads when looking down at the part.
The SOT89-5 electrically only has five leads (contact/pin). The middle lead is physically part of the middle lead on the other side of the package. Some call this package a SOT89-6, since it visually appears to have six leads when looking down at the part.
^Hollander, Dave (1995). "Packaging Trends and Mounting Techniques for Power Surface Mount Components". Proceedings of 1995 International Conference on Power Electronics and Drive Systems. PEDS 95. 1: 264–270. doi:10.1109/PEDS.1995.404911. ISBN0-7803-2423-4.