In the IC design, the basic and most essential component is the transistor. These layers allow the transistors to be formed within the semiconductor material. A good insulator like Sio2 has a thin layer with a hundred molecules thickness. The transistors which we use polycrystalline silicon poly instead of metal for their gate sections. Power is only dissipated in case the circuit actually switches. Please refer to the link to know more about the fabrication process of CMOS transistor. NMOS is built on a p-type substrate with n-type source and drain diffused on it.
In NMOS, the majority of carriers are electrons. When a high voltage is applied to the gate, the NMOS will conduct. Similarly, when a low voltage is applied to the gate, NMOS will not conduct. The majority of carriers are holes. When a high voltage is applied to the gate, the PMOS will not conduct. When a low voltage is applied to the gate, the PMOS will conduct. The same signal which turns ON a transistor of one type is used to turn OFF a transistor of the other type.
This characteristic allows the design of logic devices using only simple switches, without the need for a pull-up resistor. The networks are arranged such that one is ON and the other OFF for any input pattern as shown in the figure below.
CMOS offers relatively high speed, low power dissipation, high noise margins in both states, and will operate over a wide range of source and input voltages provided the source voltage is fixed. Furthermore, for a better understanding of the Complementary Metal Oxide Semiconductor working principle, we need to discuss in brief CMOS logic gates as explained below. The inverter circuit as shown in the figure below. A flow of animals through a complex maze of gates would require a farm hand at each gate.
This voltage turns on a transistor by changing its characteristics from that of an open circuit the 'off' position to one that can conduct a small current. This control voltage, in turn, is already available within the chip as a voltage at a point on another circuit. And, being a voltage on a circuit, this control mechanism represents a different bit of information. A computer chip therefore can be designed to make complex decisions about the information flow within itself.
This ability enables sophisticated systems to be created by interconnecting as many as a million gates within a single chip. All of this with no farm hands and no moving parts.
Watson Research Center adds some complementary details: "A logic gate in a microchip is made up of a specific arrangement of transistors. A MOSFET has three components, or regions: a source region, a drain region and a channel region having a gate over it.
The three regions are arranged horizontally adjacent to one another, with the channel region in the middle. The channel region, on the other hand, is designed to be empty of electrons under normal condition, blocking the movement of current.
The larger the gate voltage, the larger is the concentration of electrons in the channel region. The substantial concentration of electrons in the channel provides a path by which the electrons can move easily from the source to the drain. There is movement of charges in the silicon, but there are no mechanical moving parts involved. The connection between pull-up and pull-up networks results with output and power dissipation, as it is a physical process.
Figure 2 shows the results of all the possibilities for the connection of series and parallel transistors. In terms of operation, if any input is high, the result is low, if all the inputs are low — the result is high.
A Compound gate is a structure experiencing more complex logic functions in a single state and formed by combinations of transistors connected in series and parallel. Another important term is the conduction complements , which is the interchanging operations. When transistors appear parallel in the pull-down network, they appear in series in the pull-up network and reverse.
Other important parameters are Pass transistors and Transmission gates. An nMOS transistor is strong 0, and weak 1 it can also be called degraded. And the pMOS transistor is strong 1 and weak 0.
In this case the switch is on when the gate is 1. Figure 4 shows a pass transistor with strong and weak outputs. And Figure 5 shows a Transmission gate input and output. Another important term is Fully restored logic , which is when a nMOS transistor only carries 0, the pMOS transistor only carries 1, and the output is strongly driven and never degrades.
Static CMOS gates are very useful in logic design. They do not dissipate power in contrast to other different technologies. And the idea of a CMOS gates , as mentioned above, is to invert the signal.
This inversion occurs without power dissipation which makes the technology very useful. Different operations can be built using CMOS gates in multiple stages.
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