Generalized transistor equation for Collector Current including the reverse saturtion current


44I am referring to Millman's Integrated electronics and there I came across and equation
(I_c) = -\alpha(I_E)+I_{Co}(1-\exp^{V_C/V_T}) –>eqn(5.6)
where $$ I_{Co}(1-\exp^{V_C/V_T}) $$ represents shows reverse saturation current . Can any one explain me how this equation came . Also is not reverse saturation current independent of applied voltage then why is there $$V_c$$ there in equation ?

Best Answer

Ico is the reverse saturation current when in CB mode the emitter is assumed open hence the,there is a reverse bias between base and collector,and it is dependent on the value of applied voltage.
Hence the BJT acts like a p-n juction and the equation seriously satisfies with the Shockley diode equation .

So the current will decrease as Vc increase (Vc is positive pnp case and Vc is negative in case of npn), hence increase in Vc will result increase Ico which is also intuitively correct as the reverse current will increase as the reverse voltage will go up.