Transistor diode model

In a diode model two diodes are connected back-to-back to make a PNP or NPN bipolar junction transistor (BJT) equivalent. This model is theoretical and qualitative.
Examples.
PNP transistor.
To make a PNP transistor, the cathodes of both diodes are back-to-back connected to form a large N type base region.
NPN transistor.
To make an NPN transistor, the anodes of both diodes are back-to-back connected to form a large P type base region.
Base biasing.
As the base region is a combination of two anodes or two cathodes, and is not lightly doped, more base biasing is required for making this model operational.