Here is my comment to the "re-model":
There are two important BJT characteristics:
(1)Ic=f(Vbe) . This is Shockleys fundamental exponential relation. For gain calculation, the SLOPE of this function is important - and this is the transconductancegm=d(Ic)/d(Vbe) .
(2)Ib=f(Vbe) . This is the dynamic input characteristic of the BJT. The SLOPE of this curve gives the inverse of the dynamic input resistancehie=d(Vbe)/d(Ib).
(3) The product of both expressions is gm*hie=d(Ic)/d(Ib)=h21=beta.
(4) Hence: hie=beta/gm. This equation relates the most important BJT parameters (a) dynamic input resistance, (b) current gain and (c) transconductance.
(5) As we can see, there is no resistive element called "re" at all.
In some books the inverse transconductance is called re=1/gm - however, this is misleading because re=1/gm is NOT a resistive element (although it has the unit V/A=ohms). It is the inverse transconductance - nothing else and it must not called "intrinsic emitter resistance" (this error can be found in some books or papers).
Therefore, we should use the transconductance gm only (and NOT re) - as the most important and gain determining quantity.
Comment (explanation): Dividing the voltage between two nodes by the current through these two nodes gives the resistance between the two nodes (Ohms law). However, this definition does NOT apply to the quantity re=1/gm=d(Vbe)/d(Ic), because Vbe is the voltage between B and E - and Ic is the current through a third node (C).
