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Brief of MOSCAP


By
Sarah Alsaggaf

December 27, 2016 - Posted in Discussion
​​​There are four essential devices in integrated circuits (IC) which are resistors, diodes, capacitors, and transistor. In general, a capacitor is a device that has two closely spaced conductors, which are called plates, separated by an insulator or a dielectric material such as air, foil or any other insulation. The function of the capacitor is to store the energy in the electric field between these plates; so it works as energy-storage in any device. Also, the capacitor is extremely helpful as a filter since it is utilized to differentiate between signals of high frequency and low frequency. It also can be used for passing Alternating current (AC) and blocking Direct current (DC). There is also a different type of capacitors like capacitors that are made of only dielectric material. MIM -which refers to metal insulator metal device- is a capacitor with a dielectric material that contacted by two metals on the two sides. Additionally, there is another type of capacitor that is MOS capacitor which stands for Metal Oxide Semiconductor. Although the name implies that a metal (M) is being used and this terminology has widely used in the literature, the devices today are fabricated with different gate materials. Starting from 1970, metallic electrodes such as Al have been replaced by heavily doped poly-Si, and the later became the standard in most of the today’s devices, with the main reason being that poly-Si withstands high temperatures without reacting with SiO2 [1]. So, even if the identity of M and O is changed, the term MOS capacitor is the one that is mostly used, but that does not mean the metal has to be normally a metal.

The MOSCAP is a 2-terminal device that is not widely used in itself, but it is required to understand transistors such as MOSFETs (metal-oxide-semiconductor field effect transistors), the most widely used semiconductor device. We can simply say that the MOS capacitor is the heart of MOSFET.

figure.png
Fig. 1 Schematic of a MOSCAP

The structure of MOSCAP, which is depicted in Figure 1, includes:

a) A metal electrode which is the gate,

b) An insulator film (such as SiO2),

c) The semiconductor (such as Si) body.


References

[1]         Hu, Chenming. "Chapter 5: MOS Capacitor." Modern Semiconductor Devices for Integrated Circuits. 2010. Print.

[2]         "Chapter 6: MOS Capacitors." Colorado University. Web. 22 Oct. 2014.

[3]         Donald Neaman, Semiconductor physics and devices, vol. 9, no. 5. 2006.




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