What is PNP transistor

PNP transistor structure :

PNP transistor consisting of a layer of N-doped semiconductor between two layers of p-doped material. In the collector output, a small current leaving the base is amplified. If its base is pulled low relative to the emitter, the PNP transistor is ON. The emitter-base region is biased forward in this type of transistor so that holes are injected into the base as a minority carrier. The base is very thin and most holes cross the collector's reverse biased base-collector junction. 

PNP potential barrier : 

In PNP transistor as more electrons and holes flow in the region of depletion, the number of positive and negative ions is reduced and the region of depletion becomes narrow. The energy loss in overcoming the barrier potential results in a voltage drop across the PN junction equal to the barrier potential in the range of 0.7 V for Si and 0.3 V for Ge. 

PNP transistor works : 

When you have a small current running from emitter to base of the transistor a PNP transistor will turn ON. When you say turn ON, means that the transistor will open up a channel between emitter and collector and this channel can carry a much larger current. As discussed, we need two depletion layers with specific barrier potential required to diffuse the depletion layer. According to the barrier potential most common type of transistor used is silicon because it is the most abundant element on the earth after oxygen.