Generally, transistors of semiconductor devices are classified as NMOS, PMOS, or CMOS according type of channel employed in the transistors. An NMOS type transistor is formed with an N-channel, and a PMOS transistor with a P-channel. In addition, a CMOS (complementary metal oxide silicon) has both NMOS and PMOS, and, thus, both an N-channel and a P-channel are formed therein.
To form a CMOS type transistor, an n-well and a p-well are first formed in a horizontal direction on a semiconductor substrate by an ion implantation process, and then shallow trench isolation (STI) is formed. An STI structure prevents a malfunction between neighboring devices by electrically isolating the devices on a semiconductor substrate.
A well in a semiconductor substrate is classified as a p-well or an n-well according to the type of ions implanted in the well. A p-well is formed on the semiconductor substrate to form an NMOS structure, and an n-well is formed to form a PMOS structure. Subsequently, a gate oxide layer is formed on the semiconductor substrate, and then a polysilicon layer is formed thereon to form a gate stack. The gate stack forms a gate electrode of the NMOS and the PMOS using a photolithography process and an etching process.
Then, n-type dopants and p-type dopants are respectively implanted into the semiconductor substrate using the gate electrode of the NMOS or the PMOS as an implantation mask. Thus, a source/drain region is formed outward of the gate electrode on an active region of the semiconductor substrate.
As described above, the NMOS and the PMOS structures are formed in a horizontal direction in a typical CMOS process and, thus, a CMOS circuit occupies a larger area than does an NMOS circuit or a PMOS circuit. As a result, the CMOS circuit has a drawback in terms of integration.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.