ICEO, ICBO physical interpretation in BJT

Question

I am stuck with the equation:

I_CEO = (1+β) I_CBO

My questions are:

1. What is the physical interpretation of this equation?
2. Will both the currents exist simultaneously?
3. Are any of the reverse currents getting amplified?

Answer 1

ICBO = Reverse Leakage Current between Collector and Base while Emitter is Open. (IE=0)

ICEO = Reverse Leakage Current between Collector and Emitter while Base is Open. (IB=0)

IC=BIB+(B+1)ICBO
if(IB=0){IC=(B+1)ICBO=ICEO}

These are two different Leakage Currents for two different ways of Active Biasing of a BJT Transistor.

Answer 2

1/ I_CBO is reverse leakage current going from the Collector to the Base. This current is then amplified by β to produce additional Collector current, thus the "1+β" term.

2/ Both currents exist simultaneously, but I_CBO is included in I_CEO.

3/ The reverse current I_CBO is amplified, just like external Base current would be.

Answer 3

ICEO and ICBO exist/or measured when you connect the transistor in two different configuration(common base and common emmiter respectively) so there is no question of asking whether both exist simultaneously(answer is no).Consider you have transistor say T1 to be measured.You measure both Icbo and Iceo when you connect T1 in two different config as mentioned above.So now when you want to know what the value of Icbo wrt measured Iceo Iceo=(1+beta)Icbo will be the expression for solving.In a nutshell to calculate either Icbo or Iceo you need to know to measure just either of Iceo or Icbo for which the expression you mentioned helps.Hope you get it! Cheers

Answer 4

they are leakage currents ,be so small that they are often neglected β =(IC-ICEO)/IB alpha=(IC-ICBO)IE

these two equations are used when leakage is not neglected

*but * β=IC/IB alpha= IC/IE these commonly used

Answer 5

I think the confusion with leakage currents, and other parameters, occurs because many textbooks don't explicitly state that the open terminal (b in the case of Iceo) and (e in the case of Icbo) is left 'open circuit' to allow the measurement of the parameter being measured.

This is also true for h-parameters etc. where terminals are either shorted out or left open to effectively 'freeze' parameters in order to measure another parameter under known and benign (for the measurement) conditions.

The equation relating the relationship between the parameters (Iceo and Icbo) apply when the device is in a correctly biased circuit.

It is feasible for anyone to breadboard a transistor circuit and measure these parameters but one requires accurate meters capable of measuring very small currents.

An easier way is to simulate the circuits using LTspice or similar free simulation software. Of course this last suggestion of simulation is simply to help the student understand how these parameters are measured and used.