Transistor operation and characteristics

[Urmi Roy]

I've been doing transistor operation in college and I've got pretty confused in some places...please see if you can help me out...

1. The collector-emitter leakage current flows only in the common emitter configuration,when the base current is zero.

Could anyone please tell me why the collector-emitter leakage current flows in the first place and why the base current has to be zero.

Also,why does it flow only in the common emitter configuration?

2. The collector-base leakage current seems to be analogous to collector-emitter leakage current for CB configuration...but it says in my book that only the collector-base leakage current is temperature dependent,not the collector-emitter leakage current ...why?

3.Due to the early-effect,the base width decreases...why does the emitter current increase due to this early effect?(I understad how the early effect increases the collector current,but it is not clear why Emitter current increases.)

(Also,I suppose the increase of emitter current with the early effect is valid for CB and CE configurations.)

 

[Urmi Roy]

I'm waiting eagerly for someone to help me...please realize how desperate I am to clear away these lingering doubts...please please please help!

 

[Urmi Roy]

Okay,could someone tell me atleast this...

In my book, beta= (Ic-Iceo)/Ib...also,Beta=alpha/(1-alpha)...does this expression of beta in terms of alpha include the relation of beta with Iceo(the first expression)?

Also,why are the hybrid parameters defined in terms of changes in values of voltages,currents etc.?
e.g input impedance (in CE config.)=(change in Vbe)/(change in Ib)...it could have been only Vbe/ Ib.

 

[sophiecentaur]

"3.Due to the early-effect,the base width decreases...why does the emitter current increase due to this early effect?(I understad how the early effect increases the collector current,but it is not clear why Emitter current increases.)"
Where else could the current flow to? It can't be flowing out of the base.

 

[jsgruszynski]

Iceo simply comes into play as the largest collector current you can have before breakdown occurs. In other words, always set Ic in your bias circuit to be less than Iceo. That's the only use for Iceo other than for comparing two transistors.

The breakdown occurs because you are breaking down the reverse-biased base-collector junction. This is usually due to avalanche breakdown.

http://en.wikipedia.org/wiki/Avalanche_breakdown

The open base is simply because it eliminates any base-emitter effects like minority carrier injection. This let's you see just the base-collector breakdown itself when measuring for Ice at breakdown.

The same breakdown would occur in common-base or common collector. The important part is that the transistor is in linear bias. That sets the polarity of junction bias.

The temperature dependence is due to doping. The base collector is more light doped than the base emitter. This impacts the depletion layer size which is also temperature dependent. The size impacts the field created by a given reverse bias. Ditto Early effect.

Iceo is a nonlinear parameter. Alpha and Beta are linear model parameters. Linear behaviors are always a subset of the nonlinear behaviors - see Taylor Expansion.