A pn junction is where p-type and an n-type semiconductor material meet or join. P-Type material stands for positive doped semiconductor material and N-Type is negative doped. Diodes, Transistors, and many other semiconductor devices rely on pn junctions. Here is a picture of a pn junction.
"image source":http://bedtimestory-ijamdbest.blogspot.com/2008/09/bipolar-junction-transistor-bjt-is-type.html