# Magnitude of $I_{CBO}$ in a typical transistor

I read in a book that $$\I_{CBO}\$$ doubles for every 10 degree rise in temperature and hence contributes to thermal runaway. However I am confused since essentially it is the reverse saturation current through Base-collector junction while Emitter-Base junction is open circuited, so it should be something in μA and even if it rises 10 folds , it will still be negligible compared to other curremt changing factors so I tried to search for magnitude of this current to resolve my confusion however i was unable to find it on internet. So kindly provide the data.

• Use $ for inline MathJAX on EE.SE (and use the preview before posting!). – Transistor Apr 5 '19 at 19:37 • Whta is wrong with this expressions : \mu and I_CBO ? – Kutsit Apr 5 '19 at 19:39 • They should be $ \mu $ and $ I_{CBO} $. – Transistor Apr 5 '19 at 19:41 • You can also use HTML &Omega;, &mu;, &deg;, etc. as well as <sup>...</sup> and <sub>...</sub> in the posts but they don't work in the comments. Again, use the preview before posting. I can see you posting several times a minute at the moment. – Transistor Apr 5 '19 at 19:44 ## 2 Answers In a real application there is typically voltage between emitter and collector, so the $$\I_{CBO}\$$ current can effectively be base current and thus be multiplied by the gain and cause a lot of dissipation. "Is" in the SPICE model for a given part will give you a sort of "typical" value for that particular part at room temperature. It is usually in the nA range at room temperature. • But do we really care about$I_{CBO}$when we are using BJT's in modern applications? – G36 Apr 5 '19 at 19:42 • Got it but still can you please tell me the magnitude at room temperature ? Because I want to have a idea of the magnitude. – Kutsit Apr 5 '19 at 19:42 • Yes, you care about it, but a resistor from emitter to base can deal with it in may cases. – Spehro Pefhany Apr 5 '19 at 19:44 • The maximum possible values for$I_{CBO}\\$ can be found here onsemi.com/pub/Collateral/P2N2222A-D.PDF for a small signal BJT's. The typical value will be one order of magnitude smaller. – G36 Apr 5 '19 at 19:53
• It's typically in the nA range. This is more of a concern for Darlingtons, Sziklai pairs and similar DC coupled transistors where the junction temperature is very high and the gain is very high. Is in the SPICE model will give you a guess for Icbo. – Spehro Pefhany Apr 5 '19 at 19:55

This is leakage current, from the CB junction being stressed near avalanche breakdown.