The metal-oxide-semiconductor (sometimes called metal-oxide-silicon) field-effect transistor is more commonly known as the MOSFET. It is also simply called MOST for metal-oxide-semiconductor transistor. It belongs to a general group of devices, called IGFET (insulated-gate field-effect transistor) or MISFET (metal-insulator-semiconductor field-effect transistor), that have a gate isolated electrically from the semiconductor by a dielectric layer. A MOSFET has oxide as the insulator, and thus it is commonly referred to silicon devices because of the near ideal oxide-semiconductor interface that is grown thermally. A MOSFET, on the other hand, implies an IGFET made on a compound semiconductor.
The idea of field effect can be traced back to Lilienfeld in 1926 and to Heil in 1935. During the late 1940s, experiments were focused on making field-effect devices. In fact, it was in the course of such activities that the bipolar transistor was discovered experimentally in 1947. The first experimental FET was shown by Shockley and Pearson in 1948. Up to that point, field effect was only used to modulate majority-carrier conduction near the surface in bulk material. Ross in 1955 proposed using the field effect on minority carriers in the surface inversion layer, and the Si-Si02 system was first proposed in 1960 by Atalla. After the invention of the bipolar transistor, most activities were shifted toward that area. Another road block was the unavailability of a good oxide- semiconductor system without excessive leakage, oxide charges, and traps. The pioneering work by Ligenza and Spitzer on oxides grown in steam at high pressure was instrumental in the development of the first MOSFET reported by Kahng and Atalla in 1960. Since then, the popularity of the MOSFET has been rising. Early MOSFETs were dominated by p-channel devices because of the inability to realize enhancement mode for n-channel devices due to positive oxide charges. When oxide quality improved, n-channel devices provided improved performance due to higher electron mobility. Most circuits now utilize complementary MOS devices (CMOS) that include both channel types. The MOSFET is the most common transistor found in commercial ICs because of its simplicity, low cost, small size, and low power. Device modeling on MOSFET characteristics was developed around 1963 by Ihantola and Moll, Sha, Hofstein and Heiman.
For an n-channel device, the current is conducted by electrons and the source and drain are formed by /j+-regions (in the order of 1020 cm-3) for good contacts to the channel. The source and drain are formed by ion implantation after the gate structure is defined so that they are self-aligned to the gate. The gate-to-source and -drain overlap is critical for the formation of a continuous channel. The device is symmetrical so that the source and drain can be interchanged. The distance between the metallurgical junctions is the effective channel length Lc. The substrate is of an opposite type to ensure source and drain isolation. The oxide is grown thermally for good interfacial properties. The most common gate material is polysilicon, but metals and silicides can also be used. In the latter cases, refractory metals and silicides are necessary for compatibility with high-temperature processing. For a channel length of 0.3 nm, typical parameters are: oxide thickness 10 nm, substrate doping 3xl017 cm'3, source and drain junction depth 0.2 nm. An optimized MOSFET usually has nonuniform substrate doping in the p-direction and has a bell-shape profile which peaks at about 0.2 nm in depth. The higher bulk concentration serves to prevent punch-through between the source and drain, and the low doping at the surface maintains a low threshold voltage, and minimizes mobility degradation due to a high surface field.
For each type of channel, the threshold gate voltage to turn on the channel can be adjusted. If the channel is off at zero gate voltage (normally off), it is called an enhancement device because gate voltage is required to “enhance” a channel. If the channel is already on at zero gate voltage (normally on), it is called a depletion device because gate voltage is required to “deplete” the channel. A common way to achieve a depletion-mode device is to incorporate a buried channel. This buried channel is formed by ion implantation.
