Can two separate p-n junction diodes placed back to back be used to form a p-n-p transistor ?

Explanation

No, When two p-n junction diodes are placed back to back the n-region will form th base. This region will be of considerable thickness and doping will also be very high. For a transistor the thickness of the base shoud be very small and also the doping. So two p-n junction diodes placed back to back cannot form a p-n-p transistor.