Electrical – Polarities of high current bypass, and short circuit protection, transistors on LM7912 – NPN or PNP

PNP transistors work the same way as NPNs do but all voltages and currents are reversed. You connect the emitter to the higher potential, source current from the base and the main current flows into the emitter and then exits through the collector.

\$V_\rm{BE}\$ will be \$-0.7\,\rm{V}\$ but it's magnitude should be the same in both PNP and NPN if you use complementary parts.

The main division is between BJTs and FETs, with the big difference being the former are controlled with current and the latter with voltage.

If you're building small quantities of something and aren't very familiar with the various choices and how you can use the characteristics to advantage, it's probably simpler to stick mosly with MOSFETs. They tend to be more expensive than equivalent BJTs, but are conceptually easier to work with for beginners. If you get "logic level" MOSFETS, then it becomes particularly simple to drive them. You can drive a N channel low side switch directly from a microcontroller pin. IRLML2502 is a great little FET for this as long as you aren't exceeding 20V.

Once you get familiar with simple FETs, it's worth it to get used to how bipolars work too. Being different, they have the own advantages and disadvantages. Having to drive them with current may seem like a hassle, but can be a advantage too. They basically look like a diode accross the B-E junction, so this never goes very high in voltage. That means you can switch 100s of Volts or more from low voltage logic circuits. Since the B-E voltage is fixed at first approximation, it allows for topologies like emitter followers. You can use a FET in source follower configuration, but generally the characteristics aren't as good.

Another important difference is in full on switching behaviour. BJTs look like a fixed voltage source, usually 200mV or so at full saturation to as high as a Volt in high current cases. MOSFETs look more like a low resistance. This allows lower voltage accross the switch in most cases, which is one reason you see FETs in power switching applications so much. However, at high currents the fixed voltage of a BJT is lower than the current times the Rdson of the FET. This is especially true when the transistor has to be able to handle high voltages. BJT have generally better characteristics at high voltages, hence the existance of IGBTs. A IGBT is really a FET used to turn on a BJT, which then does the heavy lifting.

There are many many more things that could be said. I've listed only a few to get things started. The real answer would be a whole book, which I don't have time for.