Two questions about CB transisters

[sayurabh]

plz reply my two questions my concepts aren not cleared
Q1.why in a CB transister in output characterstics or u cna say graph between collecter current(i) and collecter base(vcb) voltage, i is zero when vcb is between -1 and 0 why? plz tell me every book has written " this is a saturation region" but do not tell the basic concept behind this every graph has logic on the based of the logic graph is made and then logic is verified
Q2why in a cb transister considering the input characteristics that is graph between Emitter current(i) and emitter base voltage (vbe) ,on increasing collecter base voltage(vcb) the graph tends to more incline to the i that is y axis that is slope increases why tell me the reason

 

[jinal patel]

transisters

what a question... great depth.
Answer to your first question:
You dought is like this. Whsn Vcb is between -1 to zero means the polarity of Vcb is reversed fine.
Now initially the polarity of Vcb was in between 0 to some positive.. and in this specific case you are considering the opposite polarity MEANS NOW YOURCOLLECTOR TO BASE JUNCTION IS IN FORWARD BIASED though initially it was in reversed biased. Now what happen that the flow of electrons (Not current)are in reversed i.e <---- in this direction. And flow of emitter to base electrons are in ----> this direction. so when Vcb is greater than some specific amount.. then iit will make zero resultant effect means both electrons flow are in equal and in opposite direction so final current will reduced to zero.

But remember this thing that in CB transister, We can not make Ic=0 by using normal transistor action i.e emitter base junction forward and collector base junction reverse biased.

TO make Ic=0 you must have to make both junction in forward biased. hence this operation is called saturation region(F-F)

 

[yjkwon57]

Re: transisters

For your first question: This can be well explained with the Ebers-Moll BJT model. Both junctions are forward biased such that emitter current is equal to collector current.

For your second question: This can be well explained by the Early effect. The collector-base junction is reversely biased, so that the thickness of the collector-base depletion layer is increased, thus, alpha (Ic / Ie) is increased.

 

[jinal patel]

transisters

About your 2nd question: its again related to the potential of reversed biased bettery. now think about NPN transistor. for reversed biased collector juinction, the positive terminal of battery is connected to the last N of NPN transistor fine.
Now due to this positive terminal more number of electron will attract from emitter side. If you increased the potential of reverse voltage Vcb than again due to higher positive potential, no number of electron attracted from emitter terminal will increased and hence it will surely chang you slop of curve.

 

[sayurabh]

Re: transisters

great answers to my first question dude
but sori for 2nd question i am unable to understand ur concept but i am trying make my own concept plz check it out it is right or wrong my concept are very low i am pursuing ece i have read ae in kollege but it is totally based on cramming i want to make my concepts strong in electronics back to the question
my concept is that if i aplly reverse biased condition in collecter base juction so on increasing the vcb the collecter current reamin to be conctant according to reverse biase graph that on incresing reverse voltage the reverse current remains constant on very small value u cna say in microamp but suddenly it started to increase adruptly on increasing more reverse voltage u cna say cut off limit of diode if i apply reverse biased concept the collecter current should be constant how can Ic/Ie can be increased so how can be Ie can be increased becoz in input characterstics of cb transisters the on increasing vcb the current is increasing that is Ie(taking Ie on y axis and vbe on x-axis),curve tends to become straight how can this happen?

 

[jinal patel]

transisters

Hi sayurabh.
First of all let me clear you about your last line that the curve tends to become straight.
In CB transister(or also CE), the input characteristics curve is not straighted. For ideal diode it will become linear.
Now first of all you just clear this thing that in any transister, the output current (Ic) is always function of input current ( Ie (CB) Ib(CE))
But since we apply reverse voltage in collector side, we always get Straight Curve of Ic. BUT THE IMPORTANT THING IS THE VALUE OF Ic-COLLECTOR CURRENT WILL BE DECIDED BY INPUT CURRENT Ie FOR CB. FOR EXAMLE IF EMITTER WILL EMMIT 100 ELECTRONS (JUST FOR UNDERSTANDING), THAN 98% OR 95% ELECTRONS WILL TRAVEL TO WORDS COLLECTOR, the remaining 2 to 5% electrons will travel through base since base is lightly doped and size of base region is thin. Now for any specific case you will get the value if Ic/ Ie generally reffer as Alpha constant. so this constant is always less than 1 because never all electrons reach to words the collectlr.
Now again if you will increase the value of input forward bias voltage Veb, then emitter will emmit more electron so you will get different value of Ic with specific value of Ie. So curve of Collector reverse voltage to Collector Current Ic remain straight BUT WITH DIFFERENT VALUES OF Ie.

 

[neils_arm_strong]

transisters

for the second question
see if the base width decreases,then more and more carriers aare injected across the junction easily and injection ratio increases.As alpha is related to injection efficiency,it will also increase.

 

[masri]

Re: transisters

hi
i havent understanded your question well
but this topic is clearly demonstrated
in twoo usefull books
1_electronic devices( author bolysted)
2_electronic devices and circuits (neamen)
8O

 

[jinal patel]

transisters

No masri, the question asked by Sayurabh is not clearly mention in any book. Just bottom line is given in all books. but why saturation region occures and why we get collectior current even if collection to base voltage zero, is not given in any book. also the fundamental difference between output characteristics of CE and CB is not clearly mention in any standard books.
Mark this thing that in CE configuration, the output current(Ic) is NOT zero when output collector voltage(Vcb) is zero, while in CE configuration the output current(Ic) is zero when output collector voltage(Vce) is zero. Why this thing happen? is not cleary mention in any standard books.

Added after 1 minutes:

(please read CB instead of CE in last 3rd line. Means "Mark this thing that in CB configuration"