what will happen if base area(BJT) is large?

[electronics_kumar]

This may sound like a small question for someone....
In BJT base area was kept small to achieve less transit time..may somemore pluspoints also..but in Phototeansistor base area is large...why it so?
(is there any tradeoff between transit time and factor 'X' in phototransistor.What 'X'?

 

[pourfardm]

I think there is no tough rules that base must be small.
Base in comparision with emitter is smaller and has less doping.

The only thing that are important (i give you an example of pnp transistor ) is that the lengh of emitter must be longer than the lengh that are needed to combine electrons (to increase β) and the lengh of base must be shorter than the lengh that are needed for combination of holes so by this fact more holes can transfer from emitter to base and the holes with suppress electrons in carriers.

But in your mentioned transister we need more surface for recieving ray of light and we do it by changing the density of dopes and their type.

 

[demodb]

because in a BJT you get electrons or holes (dependant on type of tor) traveling from collector to emittor and you don't want them to recombine in the base. So if the base is too long the chance of recombining is too large.

 

[suvendu]

less no. of electrons will go towards the collector region.hence o/p current will be less.we need to constrict the base region so that less no. of electrons combines in the base region and max will pass throgh collector region.

 

[Jack// ani]

Yes suvendu is very true.....purpose of base is just to switch up the transistor with least amount of current, that is why base is lightly doped and small enough to handle only that much current. It helps to reduce the base current passing into the collector.

 

[Samer El-Saadany]

Hi,

In BJT we use a very narrow Base just to decrease the recombination rate of the carriers, so that nearly all the charge carriers emitted from the emitter could be collected at the collector side. i.e we try to have Ie≈Ic.
But in reality we have Ie=[(β+1)/β]Ic

If the Base was wide I think we would have more recombination in the Base region.

Thanks,

Samer El-Saadany

 

[A.Anand Srinivasan]

The larger area means more recombination in the base and hence lesser current gain β because of increase in Ib and decrease in Ic

 

[dineshbabumm]

Base in a BJT is purposefully canstructed small as it leads to recombining effect of holes n electrons if it is large..so it is small..but base is extremely essential as it is used as a switch in some cases n controlling unit in amlifiers..

 

[Indian]

agreeing with all above, you see the main purpose of base is to simulate the signal given to it that is it should modify the collector current as per the base voltage, with its job when it is forward biased with respect to emitter then a large number of electrons will be attracted towards the base plate(in NPN) we dont need the electrons to be combined with the holes and get lost our prime interest is to focus these eletrons to the collector plate this is the prime cause for keeping base with very small size and lightly doped where as collector is made of large size