The relationship between the physical device and its basic 
equivilant circuit is shown for two "four layer devices", 
the SCR (Silicon Controlled Rectifier) and IGBT (Insulated 
Gate Bipolar Transistor).  The SCR can be illustrated by a 
one dimensional model but the IGBT requires the gate electrode 
to extend into three dimensions.

This description does not attempt to explain anything other than 
the equivalent circuit that illustrates the basic operation of 
an IGBT.

-KenKnaell 16Aug2012