Basics of CMOS

MOSFET:

The MOS transistor, also called MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or IGFET (Insulated-Gate Field-Effect Transistor) is the most widely used semiconductor device and is at the heart of every digital circuit. Without the MOSFET there would be no computer industry, no digital telecommunication systems, no video games, no pocket calculators and no digital wristwatches. MOS transistors are also increasingly used in analog applications such as switched capacitor circuits, analog-to-digital converters, and filters.

The exponential progress of MOS technology is best illustrated by the evolution of the number of MOS transistors integrated in a single memory chip or single microprocessor, as a function of calendar year. Each memory cell of a dynamic random-access memory (DRAM) contains a MOS transistor and a capacitor. This exponential growth of integration density with time is known as Moore's law.

The integration density of memory circuits is about 5 to 10 times higher than that of logic circuits such as microprocessors because of the more repetitive layout of transistors in memory chips. The increase in integration density is essentially due to the reduction of transistor size.

There are two types of MOS transistors: n-channel MOSFET, p- channel MOSFET.
N-channel MOSFET: The n-channel MOSFET in which current flow is due to electron transport, and the p-channel MOSFET in which holes are responsible for current flow. A circuit containing only n-channel devices is produced by an nMOS process.

Similarly, a pMOS process fabricates circuits that contain only p-channel transistors. Today the most commonly used technology is CMOS (Complementary MOS) in which both n-channel and p-channel transistors are fabricated. Here we will limit our analysis to n-channel devices. The current-voltage expressions describing a p-channel device can readily be derived from the n-channel equations, provided the appropriate changes of sign are made. An n-channel MOS transistor is fabricated in a P-type semiconductor substrate, usually silicon. Two N-type diffusions are made in the substrate and the current flow will take place between these two diffusions. The diffusion with the lowest applied potential is called the "source" and the diffusion with the highest applied potential is called the "drain". Above the substrate, and between the source and the drain lies a thin insulating layer, usually silicon dioxide, and a metal electrode called "gate". An electron-rich layer referred to as the "channel" can be created between the source and the drain underneath the gate insulator when a positive bias is applied to the gate. With appropriate voltages applied at the source and drain electrons can then flow from the source into the drain, through the channel.

P-channel MOSFET:
In a p-channel transistor an N-type substrate is used. The P-type drain is at a lower potential than the P-type source and the application of a negative bias to the gate enables the formation of a hole-enriched channel between source and drain. The metal-insulator semiconductor structure is often referred to as a "MIS" structure, where the "I" stands for the insulator. When the insulator is an oxide, it is called a "MOS" structure.

OPERATION OF N-CHANNEL MOSFET:

The basic operation of the n-channel MOSFET is the following. We will first consider the case where the gate voltage is equal to zero while the P type substrate and the source are grounded The drain is connected to a positive voltage source for instance). Since the source and the substrate are at the same potential there is no current flow in the source-substrate junction. The drain-substrate junction is reverse biased and except for a small negligible reverse leakage current no current flows in that junction either. Under these conditions there is no channel formation, and therefore, no current flow from source to drain. In the second case a constant positive bias is applied to the gate. There is no gate current since the metal electrode is dielectrically insulated from the silicon. Because it is positively biased the gate electrode does, however, attract electrons from the semiconductor, and a thin, electron rich layer forms under the gate insulator. These electrons are supplied by the source and the drain which, being N-type, are large reservoirs of electrons. The electron-rich layer underneath the gate is called "channel". The N-type source and the N-type drain are connected by the electron rich channel, and current is now free to flow between source and drain. The effect of the gate voltage controlling the concentration of electrons in the semiconductor through the gate oxide is called "field effect". The bias on the gate creates an electric field which can either induce or inhibit the formation of an electron-rich region at the surface of the semiconductor. The terms "source", "drain", "channel" and "gate" come to mind quite naturally since the electrons originate at the source, flow through the channel and are finally collected by the drain, the whole process being controlled by the bias on the gate.

In the above equations "lamda" is Channel length modulation.

 

Body effect :
Body effect is major drawback of CMOS technology. This will effect when substrate or body not biased with source. There is PN-juction diode present in between substrate and source so this will effect the change of threshold voltage. in order to avoid this effect we have to give same potential to both substrate and source.
In Body effect when we give the supply voltage to the gate (+ ve) and source to (- ve) at this condition there will be formation of a diode in the reverse bias this will increase the depletion region this makes the decreasing the channel and the threshold voltage will also increases, so in order to maintain the channel we need to increase the gate voltage.
Due to this effect we are biasing the source and substrate to the same potential which will not allow the moment of depletion towards the gate by this the Threshold voltage will not be increased.

Channel Length Modulation :

To understand the Channel length modulation, first we need to know about pinch-off of the channel is introduced. The channel is formed by attraction of carriers to the gate and the current  drawn through the channel is nearly a constant independent of drain voltage in saturation mode.

As the drain voltage increases, its control over the current extends further towards source, so the uninverted region expands towards the source, shortening the length of the channel region. The effect is called Channel Length Modulation. Because resistance is proportional to length, shortening the channel decreases its resistance, causing an increase in current with increase in drain bias for a MOSFET operating in saturation region.