There's a couple of clarifications I need to make before I can address your questions.
- Although textbooks far too often show charts having \$V_{_\text{CE}}\$ as the x-axis for certain illustrations about collector current, they contribute to confusion when doing so as the models don't operate that way. Instead, it would probably be better to show reverse-biasing of \$V_{_\text{CB}}\$ as the x-axis. So I take exception to your phrasing, using \$V_{_\text{CE}}\$, and will instead address myself to reverse-biasing of \$V_{_\text{CB}}\$.
- When the reverse-biasing of \$V_{_\text{CB}}\$ is small, less than 4 times \$V_T\$, what I'll write is no longer accurate because the classical p-n diode reverse conductance can no longer be neglected in the analysis. But as this conductance magnitude decreases by an order of magnitude for every \$60\:\text{mV}\$ increase in reverse-biasing, its importance quickly disappears and so can be neglected once the reverse-biased \$V_{_\text{CB}}\$ voltage difference exceeds 4 times \$V_T\$.
Why collector current in NPN transistor in CE mode is almostindependent of the Collector-Emitter voltage?
Here, you need to go to W. Shockley's 1949 paper, "The Theory of p-n Junctions in Semiconductors and p-n Junction Transistors".
Under his assumption, that changes in collector and emitter barrier thicknesses did not affect the base-layer thickness, he was able to "prove" that the collector current was independent of the reverse-biasing of the collector with respect to the base. (Note that he explicitly mentions #2 above as a requirement for his proof.)
Doesn't increasing reverse bias voltage (Vce) decreases the width ofthe base (due to increase in width of depletion region) and as aresult more electrons are swept from base to collector region and inturn increases the collector current (Ic)?
That would now be J.M. Early's 1952 paper, "Effects of Space-Charge Layer Wideningin Junction Transistors".
(Here, Early assumes that the emitter-side of the barrier doesn't move -- which would await Gummel & Poon, 1970, "An Integral Charge Control Model of Bipolar Transistors" for a correction to that assumption.)
Early shows that as reverse biasing of \$V_{_\text{CB}}\$ increases, what he describes as a "barrier thickness"\$x_m\$ also increases and eats into the base thickness, thinning the base region.
A thinner base layer, he writes, has two effects:
- It "decreases recombination of injected minority carriers in the base layer since the average carrier diffuses across the narrower base in a shorter time" and therefore "increases the transport factor \$\beta\$, which is the fraction of emitted minority carriers which reach the collector."
- It leads to a "decrease in the impedance presented to minority-carrier current injected by the emitter." This is because the "impedance seen by this injected current depends on base-layer resistivity and base-layer thickness".