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let me make my question simple, Lets take an example of band gap reference i can use either simple diode or BJT as a diode for PTAT current generation.
so wht make difference in both the cases
The diode-connected transistor obeys the diode equation better than a pn-junction diode for more current range (about 7 decades)
Even though it is called a diode it is still an active device. The collector current is much higher than the base current.
The effect of base spreading resistance is very low (high beta) in tx case but not so in pn diode case.
For a good transistor, you need heavily doped emitter, moderately doped thin base region and a large lightly doped collector which is readily available in the CMOS process., whereas for a diode you need almost identical n and p regions and expect low series resistances.
You would anyway need a dedicated nwell to construct a working diode (for BG) in CMOS process ., so use the transistor that gets formed and get better results...
These are only as far as I know...Experts can jump in with more valid reasons
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