What is Complementary MOSFET(CMOS)?

Complementary Metal Oxide semiconductoris a type of transistor fabrication process that uses complementary & symmetrical pairs of P-type & N-type MOSFET for logic functions.

Generally,the CMOS technology is associated with VLSI (Very Large Scale Integrated) circuit,where a few million or even billions of transistors (MOSFETs to be specific) are integrated into a single chip or die.

The reasons for the dominant use of CMOS technology in the fabrication of VLSI chips are reliability, low power consumption, considerably low cost & most important scalability.

These are four important CMOS process technology

(1) N- well process - In the N-well process,an N-type well or tub is diffused in the P-type substrate. The PMOS transistor M2 is fabricated within this well. The well region acts as the substrate for the PMOS transistor.The additional processing step are the formation on N-region & the ion-imlantation step of P-type source & drain regions.A typical CMOS inverter circuit &  the cross sectional view of the N-well CMOS structure is shown in figure.

                   CMOS inverter circuit

(2) P-well Process - 

 The P-well processes were the commonly used form of CMOS fabrication & the process steps are similar to N-well process except that the P-well diffusion are formed for fabricating NMOS devices. The P-well processes are preferred in situations where the characteristics of PMOS & NMOS devices are to be more balanced.

(3) Twin-tub CMOS- 

The twin-tub CMOS process employs both N-well & P-well diffusion regions for fabricating PMOS & NMOS devices respectively in the wells. 

(4) Silicon-On- Insulator (SOI) process - The emerging process of SOI technology achieves closer packing of PMOS & NMOS transistors & it avoids such problems as latchup & lower parasitic capacitances.A thin layer of single crystal silicon is epitaxially grown over an insulator such as sapphire or magnesium aluminate spinel.