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.