VLSI DesignVLSI Design

MOS Transistors

Levels Of Integration

SSI (Small Scale Integration) – Up to 100 components per chip.

For Example: 7404 Inverter

MSI (Medium Scale Integration) – Up to 1000 components per chip.

For Example: 74161 Counter

LSI (Large Scale Integration) – Up to 100,000 components per chip.

For Example: Simple 8-bit microprocessors

VLSI (Very Large Scale Integration) – For more than 1 million components per chip.

For Example: Pentium 4

ULSI (Ultra Large Scale Integration) – For more than half a billion components per chip.

For Example: DRAM Chip

Basic MOS Transistor Basic MOS Transistor

• An MOS (Metal-Oxide-Semiconductor) structure is created by superimposing several layers of conducting and insulating materials to form a sandwich-like structure.

• These structures are manufactured using a series of chemical processing steps involving oxidation of the silicon, the diffusion of impurities into the silicon, and the deposition and etching of aluminum to provide interconnection.

CMOS technology provides two types of transistors: an n-type transistor (nMOS) and a p-type transistor (pMOS).

Transistor operation is based on electric fields so the devices are also called Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).

Each transistor consist of a stack of the conducting gate, an insulating layer of silicon dioxide and the silicon wafer.

nMOS TransistornMOS Transistor

• An nMOS transistor is built with a p-type body and has regions of n-type semiconductors adjacent to the gate called the source and drain.

• In an nMOS transistor, the body is grounded so the p-n junctions of the source and drain to body are reverse-biased. If the gate is also grounded, no current flows through the reverse-biased junctions.

Hence the transistor is OFF.

Transistor symbol & switch-level models:

• If the gate voltage is raised, the electrons outnumber the holes and a thin region under the gate called the channel is inverted to act as an n-type semiconductor. Therefore, a conducting path of electron carriers is formed from source to drain and current can flow.

Hence, the transistor in ON.

pMOS TransistorpMOS Transistor

• A pMOS transistor consist of p-type source and drain regions with an n-type body.

• In a pMOS transistor, when the gate is at a high potential, the source and drain junctions are reverse-biased and no current flows.

So the transistor is OFF.

• When the gate voltage is lowered sufficiently, it inverts the channel and a conducting path of positive carriers is formed from source to drain. So the transistor is ON.

Modes Of OperationModes Of Operation

In order to make a useful device, there must be the capability for establishing and controlling a current between source and drain, and this is commonly achieved in one of two ways, giving rise to the Enhancement Mode and Depletion Mode transistor.

Enhancement ModeEnhancement Mode

WORKING : In a basic enhancement mode device, the

channel is not established and the device is in a non-conducting condition.

If the gate is connected to a suitable positive voltage with respect to the source ,then the electric field established between the gate and the substrate gives rise to a charge inversion region in the substrate under the gate insulation and a conducting path or channel is formed between source and drain.

Depletion ModeDepletion Mode

WORKING :

The channel may also be established so that it is present under the condition Vgs = 0 by implanting suitable impurities in the region between source and drain manufacture and prior to depositing the insulation and the gate.

Under these circumstances, source and drain are connected by a conducting channel, but the channel may now be closed by applying a suitable negative voltage to the gate.

Bibliography Bibliography CMOS VLSI Design

- Neil H. E. Weste, David Harris, Ayan Banerjee

(Perason Education)

Basic VLSI Design

- Douglas A. Pucknell, Kamran Eshraghian

(PHI)