<p>Surface Mount cases Just like Forest Gump’s chocolates you can’t tell what’s inside by looking at the case of a device. The case is just a case. The typical SOT-23 surface mount package may be a diode in various configurations or it may be a transistor, even an IC. </p><p>The SOT-23 case is so small there is little space for much more than a few characters. The industry is fairly standardized on the coded characters used to mark the devices. Confusion still strikes when you least want it. How big is an SOT-23 package? About 1.5 mm by 3 mm (0.056” by 0.120”) overall. The ever-popular 2N3904 NPN transistor in an SOT-23 case has a “1A” marked on it. The lettering designates an NPN transistor with the same characteristics as a 2N3904 and will cross to many different part numbers from various manufacturers. The 1N4148 would likely be marked with an “A6” code. Again this more describes the characteristics than it crosses to a specific part number. There is no marking on the body to indicate which lead is pin 1. Pin designation may differ from one manufacturer to another but generally referencing the drawing above pin 1 would be in the lower left corner, going anti-clockwise with pin 3 at the top middle unless there are more than three pins. We can have up to eight leads on the SOT-23 body. Package designations may also differ between manufacturers. The above drawing might apply for Panjit. For AUK an SOT-23 may be a package of similar size with the leads going flat out from the body. For Seme Lab the SOT23 is a totally leadless package similar to a ceramic leadless chip carrier with the same lead footprint as the other SOT23 devices. The main standard is… well there ain’t no standard. Be careful of what you buy.</p>