Why can’t we use BJT instead of MOSFETs in VLSI design?
The main thing is that bjt uses a lot more space than mosfet .That is to say that in a predefined volume, you can add more mosfets than bjt that’s why mosfets are used in VLSI design
Mosfets are much faster than bjt they can be used in technologies with higher switching speeds. Bjt are much slower than mosfets because of its high recovery time.
The mosfets can consume very less energy, even until an energy is saved secondly is a bipolar device, that is, the carrier are both electrons and holes and still has a reverse current of saturation wasting energy
All these reasons led to the conclusion that new technologies must be designed to support a larger number of mosfets, not bjt, in a VLSI
Design shows at the beginning, some differences between bjt and mosfet: –
Bjt is a bipolar device, which means that the current is controlled by majority and minority carriers, while mosfet is a unipolar device, which means that the current is carried by majority carriers, which also makes it usable. l for use in a high frequency application.
Bjt is a current controlled device while MOSFET is a voltage controlled device.
Bjt is a 3-terminal device that is transmitter, collector and base, while MOSFET is a 4-terminal device mainly source, gate, drain and substrate (also called body) and can be useful for maintaining low leakage currents.
The circuit used to build the bjt is much more complex compared to mosfet.
On top of that, as indicated by VLSI, we are more concerned about three aspects that are power, speed and surface.
When using MOSFETs, the advantage is that power dissipation in circuits and leakage currents is minimal. It was used instead of BJT. In addition, as mentioned, MOSFET has supported the high frequency application.