Aluminum gate:
– First MOSFET invented in 1959
– Silicon used as channel material
– Aluminum gate common until early 1970s
– Aluminum gate deposition in evaporation vacuum chamber
– Aluminum prone to alloying with silicon
Polysilicon:
– Polysilicon replaced aluminum as gate material in late 1970s
– Polysilicon highly doped to reduce electrical resistance
– Polysilicon deposited via CVD
– Polysilicon tolerant to high temperatures
– Polysilicon used for self-aligned gates
NMOS and CMOS:
– Positive voltages can cause sodium impurities to diffuse
– PMOS less sensitive to sodium impurities
– Polysilicon gates treated with HCl gas to remove sodium
– Polysilicon not ideal for gate capacitance
– PMOS metal gate composed of two layers
Modern processes return to metal:
– Metal gate technology returns from 45nm node onward
– High-dielectric materials used with metal gate
– Candidates for metal gate electrode for NMOS and PMOS
– Metal gate improves strain capacity on the channel
– Metal gate reduces current perturbations in the gate
See also:
– Gate oxide
– Multigate device
References:
– 1960 – Metal Oxide Semiconductor (MOS) Transistor Demonstrated
– Voinigescu, Sorin (2013) High-Frequency Integrated Circuits
– Fujikawa, Shin-ichiro; Hirano, Ken-ichi; Fukushima, Yoshiaki (1978) Diffusion of silicon in aluminum
A metal gate, in the context of a lateral metal–oxide–semiconductor (MOS) stack, is the gate electrode separated by an oxide from the transistor's channel – the gate material is made from a metal. In most MOS transistors since about the mid-1970s, the "M" for metal has been replaced by polysilicon, but the name remained.
