Customising BJT Parameters

[viswa]

hi all,
In cmos design we used to customise nmos/pmos by changing width and keeping length constant for a given technology.similarly in bjt which parameter is customised for acheiving the intended purpose.is beta changed or base width or wat else is changed?another one query :"is there any technology scaling in bjt similar to cmos length scaling?"

 

[Vamsi Mocherla]

Dear Viswa,

The most dominating parameter in BJTs is the area of the emitter. You can change that for greater saturation current. Also the width of the base can be changed. However in CMOS processes, the width of the base is not really critical. It is only the area of the emitter. In technologies, the area of the emitter is like the W and L of MOSFETs.

 

[pit1000]

Vamsi Mocherla wrote

" ...the width of the base is not really critical."

Whats about lateral BJTs?

 

[Vamsi Mocherla]

But we do not layout lateral BJTs in CMOS processes. So, mostly it is vertical BJTs that are critical here.

 

[viswa]

hi Vamsi,
wat is meant by lateral bjts?and y do u relate bjts with cmos processes?are they not entirely different?

 

[Vamsi Mocherla]

Well Lateral BJTs are BJTs which are synthesized in the same well, with the current direction moving laterally. Vertical BJTs are used with substrate as collector, Well as base and and diffusion as emitter. In this case, the area of the emitter is the most critical parameter. Mostly, in CMOS processes, since we do not have proper BJTs we prepare Veritcal BJTs. If you are not clear with the explanation, kindly refer to Paul Gray and Meyer, 2nd chapter. It will throw some light into it.