This is a simple small signal model of a npn transistor according to the following equations:
Simplified (linearized) Shockley-equation
Ron*iB = if vBE≤Vth then 0 else (vBE - Vth)
Ebers-Moll-equation
iC = β*iB
For vCE<vBE saturation of iC is taken into account (linearly to 0).
For vCE>vBE the slope of iC is taken into account (Early voltage VY).