BJT can not be scaled like MOSFET,why？

Nearly all CMOS process will contain some bipolar device which can be used for BG or thermal sensor. The PDKs of fab always tell us BJT can not be scaled like MOSFET, instead, only some fixed dimension of emitter area are given for analog designers, such as 5x5, 10x10, 20x20, etc.
My question is why BJT can not be scaled in process? Why the WL of BJT cannot be scaled like MOSFET?Is it hard to accurately model the scaling effect?

我也想知道。
接触的几个digital/mixed signal cmos工艺里的BJT都很简陋，只有两种
PNP是做在P-sub上的vertical PNP
NPN是是做在n-well和deep n-well里的NPN
symbol只有三端，都不知道怎么用，PNP在P-sub上那端是要接地的，NPN呢?PNP的beta只有零点几，rb十几欧姆，NPN不知道咋用还没仿过。

beta只有零点几不对吧！

回复 2# nanke

确实是这么多，smic55 beta 0.7