Many years ago in the dark and distant past the thermionic valve was king. The number of separate electrical connections were used to identify the type of valve, so a 2 electrode valve would be a diode, 3 would be a triode etc… Now the properties of Germanium crystals were known about but until manufacturing processes were sufficiently developed and the little people at bell labs found out about the fun things that happen at NPN and PNP junctions they were rarely used. The germanium diode was the first to be developed as a logical step from the crystal detector in the early radios. so called crystal diodes as they showed similar properties to the thermionic diodes but made from crystals. The next advance was when the “transistor” effect was discovered. The component seemed to behave like a triode and was therefore named the crystal triode. The name was soon replaced with “transistor” and people got down to finding uses for then and the manufacturing problems began. One fundamental problem problem was “how do we package these things”. Standard packages soon followed and were named Transistor Outlines. The different outlines were given an identifying number. The first was TO1 then TO2 and TO3. The Most common ones now are TO92 and TO220. Diodes were similarly treated
with the DO series of packages. When smaller and smaller transistors were made for surface mount a new series of packages was designed Small Outline Transistor or SOT. The one I am interested in at this moment is SOT23 which is 1.63mm wide x 2.95mm long x 1.10mm high and has 6 legs (originally designed to have 2 on one side a 1 on the other for a single transistor or able to have 2 transistors squeezed into it.)
Now that digression explains that this little black epoxy package should hold 2 transistors but the PIC10F series micrcontrollers are now avalable in SOT23 packae. Thats an entire CPU,programme and ROM Memory and other things like 8 bit ADC, clock generator and power management hardware all crammed into the space of one small or two tiny transistors!!!
I now that is impressive!!!!