term 'bipolar' means electrons and holes responsible for current flow.Does it mean one at a time or both at same time as in gases current is due to motion of both+ and -tive ions at same time but in solid conductors it is either +(conventional) or -tive(electronic but not both at same time)?

 

THE MOVMENT OF ELECTRONS IN REAL PRACTICE aND ALSO THE MOVMENT OF VOID SPACE I.E ELECTRON DEFICIENT AREA THAT IS CALLED HOLE
THERE IS A PROCESS OF RECOMBINATION OF ELECTRON AND HOLE

 

a good small statement from wiki

Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-carrier' devices.

 

let me add a little more;

we must first remember that many of the principles of diodes & their behavior would greatly help us in understanding other semiconductors like the BJT. by having as very good background about diodes, understanding bjts is easy.

N-type semicon material conduct electricity by means of its supply of free electrons and its counter part the P-type by its supply of positively charged holes. remeber that we are talking here of electron current. for an NPN the P-region is much narrower than the N-region. thus the P-region is much less heavily doped than the the N-region, the holes are fewer and farther apart in comparison to the free electrons in the N-region.

now lets introduce free electrons from the emitter lead to the collector lead. assuming also that the base lead is open. now thinking in terms of diodes, the free electrons continue their way from the N-region of the emitter to the P-region of the base into the N-region of the collector. but this process will be in a brief instance of time.

now let's remember a rule in diodes, electrons can flow from N to P but not from P to N. now looking back of what i cited, it seems there is a conflict.
but let us examine the structure bcoz it is expected that the free electrons would be captured in the base region by falling into holes so no electrons can pass from the base to the collector. as i have said lets examine the structure. the transistor is made of a very lightly doped and narrow base region and the holes are sparsely spread. becoz of that most electrons are able to cross without falling into a hole. and the ones that fell in the hole are stuck and accumulates until creating a negative charge in the base region. becoz of this the transistor is able to throttle the emitter-collector working current. remember that like charges repel.

in this scenario the excess bound electrons in the base region repel the free electrons trying to cross from emitter to collector hence making it harder for the current to pass. it takes only in nanoseconds for the current to be shut off.

the only way to get the working current going again is to withdraw some of the excess bound electrons from the base region. we do this by applying a positive voltage to the base lead. this way it sucks out bound electrons into the base-emitter control circuit. now that we have lowered the barrier electron current is resumed from emitter to collector.

moz

 

The term "bipolar" means "two poles." In this context, it means two junctions between doped material. Contrast Bipolar Junction Transistor (BJT) with Unipolar Junction Transistor (UJT).

All current is current. We cannot have hole flow without electron flow, or electron flow wihtout hole flow. Which point of reference we choose can our ease of understanding, but makes no difference to the reality we are attempting to model.