I'm having a hard time understanding the concept of BJTs in saturation and active mode. So far I have got that the base-emitter junction is forward biased, so electrons go from the emitter to the base, where some of them recombine, but most of them diffuse to the collector as there is a concentration gradient.

But, during saturation the base-collector junction is reverse biased, so my question is how can current flow out of the collector in the first place? And why does it stop increasing (apart from the Early effect which I do understand) when VCE~0.7 i.e. it becomes forward biased? I have read that it's because VBC is decreasing as VCE increases, and after VCE~0.7 VBC cannot get any smaller, which does make sense, so I suppose the main question is how Ic comes about.

I know this is a common question and I have spent a long time reading many posts of forums but nothing I have read fully explains it to me. I would like to know physically what is happening in the BJT, and what is meant by 'saturation' and 'active'.

 

A rule of thumb for saturation is 1/10 the base emitter current than what you are going to have for the collector emitter current. Basically saturation is when the transistor is within 0.2V (usually a lot less) voltage drop between collector emitter. The transistor is considered to be fully conducting.

 

But, during saturation the base-collector junction is reverse biased, so my question is how can current flow out of the collector in the first place?

In the saturation condition both junctions (Base-Emitter & Base-Collector) are forward biased.