You might edit in a little note saying what version of Mathematica you are using. The way units were handled was completely changed starting with version 9 and examples for a version you don't have won't be helpful.
If you are using version 9 or later then you might compare this with what you showed. And watch for the missing _ in your Ic definition.
In[1]:= boltz = UnitConvert[Quantity["BoltzmannConstant"], "Joules"/"Kelvins"]
Out[1]= Quantity[1.38065*10^-23, ("Joules")/("Kelvins")]
In[2]:= echarge = UnitConvert[Quantity["ElectronCharge"], "Coulombs"]
Out[2]= Quantity[1.6021766*10^-19, "Coulombs"]
In[3]:= Vt[T_] := boltz*T/echarge
In[4]:= Vt[1]
Out[4]= Quantity[0.000086173, ("Joules")/("Coulombs" "Kelvins")]
In[5]:= Ies = 10^-9 (*Unita A Transistor reverse current*)
Out[5]= 1/1000000000
In[6]:= Ic[Vbe_, T_] := Ies*(Exp[Vbe/QuantityMagnitude[Vt[T]]] - 1)
In[7]:= Ic[Vbe, 273]
Out[7]= (-1 + E^(42.5074 Vbe))/1000000000
In[8]:= Plot[Ic[Vbe, 273], {Vbe, 0, 0.7}]
If you insert PlotRange->All just before the final closing ] of that Plot then you will be able to see the current out to 0.7 volts.
Please check this carefully and point out any mistakes that you see.
Thank you
