why we use CMOS in VLSI

The idea of metal oxide silicon field effect transistors was patented by j.e.lilienfeld in early 1930 before the invention of BJT. Owing to fabrication limitations, however, MOS technologies became practical much later,in the early 1960s,with the first several generations producing only n-type transistors. It was in the mid 1969s that complementary MOS devices were introduced, initiating a revolution in the semiconductor industry.

CMOS technologies rapidly captured the digital market: CMOS gates dissipated power only during switching and required few devices, two attributes in sharp contrast to their bipolar or GaAs counterparts. It was also soon discovered that the dimensions of mos devices could be scaled down more easily than those of other types of transistors. furthermore CMOS circuits proved to have a lower fabrication cost.

The next obvious step is to apply CMOS technology to analog design. The low cost of fabrication and the possibility of placing both analog and digital circuits on the same chip so as to improve the overall performance and/or reduce the cost of packaging made of CMOS technology attractive. However MOSFET were quite slower and noiser than BJT, finding limited applications.

How did MOS technology come to dominate the analog market as well? The principal force was device scaling because it continued to improve the speed of MOSFETs. The intrinsic speed of MOS transistors has increased by more than three orders of magnitude in the past 30 years,becoming comparable with that of bipolar even though the latter.