Field effect transistors

In a field effect transistor, the flow of current through the conducting region is controlled by an electric field. FET is a unipolar device since carrier conduction is only by majority carriers. There are two types of field effect transistors-junction field effect transistor (JFET) and metal-oxide-semiconductor field effect transistor (MOSFET). MOSFET is also known as insulated gate FET (IGFET) and metal oxide silicon transistor (MOST). JFET has been classified into two: N-channel JFET where electrons are majority carriers and P-channel JFET where holes are majority carriers. The fabrication technology of JFET is similar to that of npn and pnp transistors. The epitaxial layer of monolithic IC forms the N-channel of the JFET. Diffusion method or ion-implantation method is used for the introduction of p+ gate. The drain and source connections are made through the ohmic contact regions. In the MOSFET fabrication, a metallic gate is grown over an insulating layer by oxidation process. The threshold voltage of MOSFET depends on the material used for making gate, insulating material thickness, doping concentration etc.  Silicon nitride due to its superlative masking properties is used as an effective dielectric in the MOSFET fabrication process. Aluminium can be used as the gate electrode material. If NMOS and PMOS devices are fabricated on the same silicon chip, they are called complementary MOSFETs (CMOS). It can be produced by various techniques like M-well process, P-well process, twin-tub process or silicon-on-insulator process.