Differences between Lateral and Vertical BJTs

[aryajur]

What are the differences in performance of lateral and vertical Bipolar transistors?

 

[TiwstedNeurons]

Width of the base region should be different. Normally, the vertical BJT has narrower base region (results in bigger β if the doping concentrations keep same on both sides of base and emitter). Please correct me if I'm wrong.

 

[sunking]

structre and process is diff.
beta, Is, breakdown voltage...

 

[Hughes]

Besides the above-mentioned differences, lateral bjt is usually inferior to vertical bjt since the former has a parasitic verital bjt with it.

 

[asic_ant]

As far as Iknow:lateral BJTs are more easy to fabricate,but poor in high frequency performance due to the wider base region.In addition,there is usually a sink(buried layer)in vertical BJTs.

Added after 23 minutes:

I've draw a cross section for you : )

 

[aryajur]

Thanks for the replies. Want to organise the facts and get more information:

So far I have come to know from above:

1. Vertical Bipolars have a smaller Base region than lateral.
Q. Just a little confused, shouldn't we be able to accurately control lateral dimensions in a fabrication process than vertical ones? So does that mean the smallest base width we can accurately control in a fabrication process is still more than the base width we can have in a vertical BJT?

2. Lateral BJTs are inferior since they have a parasitic vertical BJT
Q. What degrading effects does that vertical BJT have on our main lateral BJT perfomance? One thing I am guessing is substrate currents.

3. Because of wider base in lateral BJT they are slower because more stored charge in the base region and hence lower ft.

MORE Questions:
1. How does the early voltage of the 2 devices compare?
2. How does the size in layout of the 2 devices compare?
3. I heard that the β in one of them falls more rapidly for increasing Ic..., can somebody elaborate?

 

[yaxazaa]

vertical is normally better than lateral. Lateral is a by-product of cmos process

 

[krashkealoha]

1. Vertical Bipolars have a smaller Base region than lateral.
Q. Just a little confused, shouldn't we be able to accurately control lateral dimensions in a fabrication process than vertical ones? So does that mean the smallest base width we can accurately control in a fabrication process is still more than the base width we can have in a vertical BJT?

Yes, the control of implants and diffusions are still better than lateral dimensions, which is dependent on your photolithography capability. For example, a transistor with a ft of about 50 GHz has a base width about 45 nm, which is beyond current production photolithography capability.

2. Lateral BJTs are inferior since they have a parasitic vertical BJT
Q. What degrading effects does that vertical BJT have on our main lateral BJT perfomance? One thing I am guessing is substrate currents.

Yes, the substrate current from the parasitic verticle component increases the base current of your lateral BJT.

3. Because of wider base in lateral BJT they are slower because more stored charge in the base region and hence lower ft.

Wider base translates to higher base transit time.

MORE Questions:
1. How does the early voltage of the 2 devices compare?

You will get a better Early voltage * beta product with a verticle BJT.

2. How does the size in layout of the 2 devices compare?

The current carrying capability of the lateral device is worst therefore, the transistor size is usually much larger than the verticle device to carry the same amount of current.

3. I heard that the β in one of them falls more rapidly for increasing Ic..., can somebody elaborate

This is due to the high base resistance of the lateral device. The base connection of a lateral device is usually made through the epi/well, which is high resistance.

 

[aryajur]

MORE Questions:
1. How does the early voltage of the 2 devices compare?

You will get a better Early voltage * beta product with a verticle BJT.

Is it because since the base width is larger so it will have a higher Early voltage. Beta is not better than vertical but Early voltage is much better so Early Voltage and Beta product will be better than the vertical BJT?

 

[krashkealoha]

The base width for the verticle is much smaller than the lateral, therefore the higher Ft. The verticle has more base charge, Q, than the lateral. This allows for a higher Early voltage, and prevents high beta to fall off at high current.