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  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    VLSI DESIGN

    UNIT I INTRODUCTION: Introduction to IC

    Technology MOS, PMOS, NMOS,

    CMOS & BiCMOS technologies.

    BASIC ELECTRICAL PROPERTIES :

    Basic Electrical Properties of MOS and

    BiCMOS Circuits : Ids-Vds

    relationships, MOS transistor

    threshold Voltage, gm, gds, figure of

    merit o ; Pass transistor, NMOS

    Inverter, Various pull ups, CMOS

    Inverter analysis and design, Bi-

    CMOS Inverters.

    VIDYA SAGAR P

  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    UNIT I

    INTRODUCTION: Introduction to IC Technology MOS, PMOS, NMOS, CMOS &

    BiCMOS technologies.

    BASIC ELECTRICAL PROPERTIES : Basic Electrical Properties of MOS and BiCMOS

    Circuits: Ids-Vds relationships, MOS transistor threshold Voltage, gm, gds, figure of

    merit o ; Pass transistor, NMOS Inverter, Various pull ups, CMOS Inverter analysis

    and design, Bi-CMOS Inverters.

    INTRODUCTION:

    The invention of the transistor by William B. Shockley, Walter H. Brattain and John

    Bardeen of Bell Telephone Laboratories drastically changed the electronics industry and

    paved the way for the development of the Integrated Circuit (IC) technology. Over the past

    several years, Silicon CMOS technology has become the dominant fabrication process for

    relatively high performance and cost effective VLSI circuits. The revolutionary nature of this

    development is understood by the rapid growth in which the number of transistors

    integrated in circuits on a single chip.

    The expansion of VLSI is Very-Large-Scale-Integration. Here, the term Integration refers

    to the complexity of the Integrated circuitry (IC). An IC is a well-packaged electronic circuit

    on a small piece of single crystal silicon measuring few mms by few mms, comprising active

    devices, passive devices and their interconnections. The technology of making ICs is known

    as MICROELECTRONICS. This is because the size of the devices will be in the range of

    micro, sub micrometers. The examples include basic gates to microprocessors, op-amps to

    consumer electronic ICs. There is so much evolution taken place in the field of

    Microelectronics, that the IC industry has the expertise of fabricating an IC successfully

    with more than 100 million MOS transistors as of today. ICs are classified keeping many

    parameters in mind. Based on the transistors count on the IC, ICs are classified as SSI,

    MSI, LSI and VLSI. The minimum number of transistors on a VLSI IC is in excess of

    40,000.

    The concept of IC was conceived and demonstrated by JACK KILBY of TEXAS

    INSTRUMENTS at Dallas of USA in the year 1958.The silicon IC industry has not looked

    back since then. A lot of evolution has taken place in the industry and VLSI is the result of

    this. This technology has become the backbone of all the other industries. We will see every

    other field of science and technology getting benefit out of this.

  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    What is VLSI?

    VLSI is Very Large Scale Integration. It is the process of designing, verifying, fabricating

    and testing of a VLSI IC or CHIP.A VLSI chip is an IC, which has transistors in excess of

    40,000. MOS and MOS technology alone is used. The active devices used are CMOSFETs.

    The small piece of single crystal silicon that is used to build this IC is called a DIE. The

    size of this die could be 1.5cmsx1.5cms. This die is a part of a bigger circular silicon disc of

    diameter 30cms.This is called a WAFER.

    Semiconductor technology:

    Semiconductors can be made from crystalline silicon into which impurities have been

    introduced: A high valency implant such as phosphorous gives free electrons, creating an

    n-type region. A low valency implant such as boron gives free holes, creating a p-type

    region. The junction of an n-type and a p-type region in a single crystalline lattice creates a

    diode which only conducts if it is forward biased with the p-type region (the anode) more

    positive than the n-type region (the cathode).

    A light emitting diode has the additional property that it glows when current is flowing

    through it. It is prudent to limit this current to a few milli-Amps by means of a kilohm

    series resistor, or it glows very brightly, but only for a short time.

    Digital switching:

    Most digital logic is based on the idea of switching signals between a high voltage (which we

    will usually treat as being 5V, although modern systems more commonly use 3.3V or less)

    and a low voltage (0V, or ground). The sense may be determined by current flowing or not

    (as in bipolar circuits) or by the presence or absence of charge (as in MOS circuits). A logic

    function takes some input signals and computes an output function using pull-up and

    pull-down circuits which may be passive (always switched on) or active (selectively

    switched).

  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    The diagram shows a circuit with an active pull-down and a passive pull-up. The pull-down

    can be thought of as a remote-control switch, usually made from transistors but possibly

    relays or valves. A further complexity with MOS circuits is that the charge on wires persists

    after they have ceased to be driven; this means that the wires have a memory (typically

    lasting a thousandth of a second or so) of the last value driven on them.

    Bipolar circuits:

    A bipolar transistor is formed by a sandwich of n-type, p-type and n-type regions in a single

    crystalline lattice. It can be thought of two diodes connected anode-to-anode such that a

    current through the forward biased diode overwhelms the reverse biased diode.

    A small current flowing from the base to the emitter of an npn transistor induces a

    largecurrent from the collector to the emitter. A pnp transistor has the opposite polarity.

    Thesecan be used to construct a AND gate using transistor-transistor logic (TTL).

  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    The two transistors on the left calculate the logical function and that on the right is simply

    an inverter. Sometimes this is followed by an additional buffer for a totem pole output. With

    a 1k7 Ohm pull-up, about 1 mA flows through the gate whenever an input is high and the

    gate then dissipates 5 mW. There are also difficulties in finding the right sizes for the

    transistors and resistor.

    MOS Circuits: An enhancement mode, n-channel, metal-oxide-silicon field-effect transistor

    (nMOS FET) is formed on a crystal of p-type silicon. Two n-type regions (known as

    diffusion) lie on either side of a region of the p-type substrate which is covered by a thick

    layer of insulating silicon dioxide (or oxide) and a metal plate.

    When the gate is positive with respect to the source, an n-type channel is formed under the

    gate and current is conducted from drain to source. Even when turned on, a MOS

    transistor has a resistance of about 10 k. The construction of the transistor is symmetric

    with respect to the source and drain - the labels merely indicate the relative voltages. This

    contrasts with the different processing used to make the collector and emitter of a bipolar

    transistor. A p-channel MOS FET has the opposite polarity and conducts when its gate is

    low. However, the resistance of a p-type channel is about 2 times that of an n-type

    channel of the same size.

  • VBIT COURSE MATERIAL VLSI DESIGN

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    In integrated circuits, the metal gate is replaced by one made from polycrystalline silicon (or

    polysilicon) for ease of fabrication. The nMOS transistor operate in three modes:

    cutoff when Vgs Vt and Vds >Vgs Vt

    linear when Vgs >Vt and Vds t = (C/I) V

    Capacitance and current determine speed

  • VBIT COURSE MATERIAL VLSI DESIGN

    potharajuvidyasagar.wordpress.com BY VIDYA SAGAR.P

    METAL-OXIDE-SEMICONDUCTOR (MOS) AND RELATED VLSI TECHNOLOGY:

    The MOS technology is considered as one of the very important and promising technologie