Fundamentals of Microelectronics - B. Razavi (Wiley, 2006) WW(1).pdf - Najnowsze ksiazki(18) - walbo48

Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006]

June 30, 2007 at 13:42

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IntroductiontoMicroelectronics

Over the past ﬁve decades, microelectronics has revolutionized our lives. While beyond the realm

of possibility a few decades ago, cellphones, digital cameras, laptop computers, and many other

electronic products have now become an integral part of our daily affairs.

Learning microelectronics can be fun. As we learn how each device operates, how devices

comprise circuits that perform interesting and useful functions, and how circuits form sophisti-

cated systems, we begin to see the beauty of microelectronics and appreciate the reasons for its

explosive growth.

This chapter gives an overview of microelectronics so as to provide a context for the material

presented in this book. We introduce examples of microelectronic systems and identify important

circuit “functions” that they employ. We also provide a review of basic circuit theory to refresh

the reader’s memory.

1.1 Electronics versus Microelectronics

The general area of electronics began about a century ago and proved instrumental in the radio

and radar communications used during the two world wars. Early systems incorporated “vacuum

tubes,” amplifying devices that operated with the ﬂow of electrons between plates in a vacuum

chamber. However, the ﬁnite lifetime and the large size of vacuum tubes motivated researchers

to seek an electronic device with better properties.

The ﬁrst transistor was invented in the 1940s and rapidly displaced vacuum tubes. It exhibited

a very long (in principle, inﬁnite) lifetime and occupied a much smaller volume (e.g., less than 1

in packaged form) than vacuum tubes did.

But it was not until 1960s that the ﬁeld of microelectronics, i.e., the science of integrating

many transistors on one chip, began. Early “integrated circuits” (ICs) contained only a handful

of devices, but advances in the technology soon made it possible to dramatically increase the

complexity of “microchips.”

Example 1.1

Today’s microprocessors contain about 100 million transistors in a chip area of approximately

3 cm 3 cm. (The chip is a few hundred microns thick.) Suppose integrated circuits were not

invented and we attempted to build a processor using 100 million “discrete” transistors. If each

device occupies a volume of 3 mm 3 mm 3 mm, determine the minimum volume for the

processor. What other issues would arise in such an implementation?

Solution

The minimum volume is given by 27 mm 3 10 8 , i.e., a cube 1.4 m on each side! Of course, the

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Chap. 1

Introduction to Microelectronics

wires connecting the transistors would increase the volume substantially.

In addition to occupying a large volume, this discrete processor would be extremely slow ;the

signals would need to travel on wires as long as 1.4 m! Furthermore, if each discrete transistor

costs 1 cent and weighs 1 g, each processor unit would be priced at one million dollars and weigh

100 tons!

Exercise

How much power would such a system consume if each transistor dissipates 10 W?

This book deals with mostly microelectronics while providing sufﬁcient foundation for gen-

eral (perhaps discrete) electronic systems as well.

1.2 Examples of Electronic Systems

At this point, we introduce two examples of microelectronic systems and identify some of the

important building blocks that we should study in basic electronics.

1.2.1 Cellular Telephone

Cellular telephones were developed in the 1980s and rapidly became popular in the 1990s. To-

day’s cellphones contain a great deal of sophisticated analog and digital electronics that lie well

beyond the scope of this book. But our objective here is to see how the concepts described in this

book prove relevant to the operation of a cellphone.

Suppose you are speaking with a friend on your cellphone. Your voice is converted to an elec-

tric signal by a microphone and, after some processing, transmitted by the antenna. The signal

produced by your antenna is picked up by the your friend’s receiver and, after some processing,

applied to the speaker [Fig. 1.1(a)]. What goes on in these black boxes? Why are they needed?

Transmitter (TX)

Receiver (RX)

Microphone

Speaker

(a)

(b)

Figure 1.1 (a) Simpliﬁed view of a cellphone, (b) further simpliﬁcation of transmit and receive paths.

Let us attempt to omit the black boxes and construct the simple system shown in Fig. 1.1(b).

How well does this system work? We make two observations. First, our voice contains frequen-

cies from 20 Hz to 20 kHz (called the “voice band”). Second, for an antenna to operate efﬁciently,

i.e., to convert most of the electrical signal to electromagnetic radiation, its dimension must be a

signiﬁcant fraction (e.g., 25% ) of the wavelength. Unfortunately, a frequency range of 20 Hz to

20 kHz translates to a wavelength 1 of 1:5 10 7 mto 1:5 10 4 m, requiring gigantic antennas

for each cellphone. Conversely, to obtain a reasonable antenna length, e.g., 5 cm, the wavelength

must be around 20 cm and the frequency around 1.5 GHz.

Recall that the wavelength is equal to the (light) velocity divided by the frequency.

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Examples of Electronic Systems

How do we “convert” the voice band to a gigahertz center frequency? One possible approach is

to multiply the voice signal, x(t) , by a sinusoid, A cos(2 f c t) [Fig. 1.2(a)]. Since multiplication

in the time domain corresponds to convolution in the frequency domain, and since the spectrum

( )

cos( 2

f C t

)

Output Waveform

Voice

Signal

(a)

Spectrum of Cosine

Output Spectrum

Voice

Spectrum

−

f C

−

f C

(b)

Figure 1.2 (a) Multiplication of a voice signal by a sinusoid, (b) equivalent operation in the frequency

domain.

of the sinusoid consists of two impulses at f c , the voice spectrum is simply shifted (translated)

to fc [Fig. 1.2(b)]. Thus, if fc =1 GHz, the output occupies a bandwidth of 40 kHz centered

at 1 GHz. This operation is an example of “amplitude modulation.” 2

We therefore postulate that the black box in the transmitter of Fig. 1.1(a) contains a

multiplier, 3 as depicted in Fig. 1.3(a). But two other issues arise. First, the cellphone must deliver

Power

Amplifier

cos( 2

f C t

)

Oscillator

(a)

(b)

Figure 1.3 (a) Simple transmitter, (b) more complete transmitter.

a relatively large voltage swing (e.g., 20 Vpp ) to the antenna so that the radiated power can reach

across distances of several kilometers, thereby requiring a “power ampliﬁer” between the mul-

tiplier and the antenna. Second, the sinusoid, A cos 2 fct , must be produced by an “oscillator.”

We thus arrive at the transmitter architecture shown in Fig. 1.3(b).

Let us now turn our attention to the receive path of the cellphone, beginning with the sim-

ple realization illustrated in Fig. 1.1(b). Unfortunately, This topology fails to operate with the

principle of modulation: if the signal received by the antenna resides around a gigahertz center

frequency, the audio speaker cannot produce meaningful information. In other words, a means of

Cellphones in fact use other types of modulation to translate the voice band to higher frequencies.

Also called a “mixer” in high-frequency electronics.

( )

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translating the spectrum back to zero center frequency is necessary. For example, as depicted in

Fig. 1.4(a), multiplication by a sinusoid, A cos(2 f c t) , translates the spectrum to left and right by

Output Spectrum

Received Spectrum

Spectrum of Cosine

−

f C

−

f C

−2

f C

(a)

Low−Noise

Amplifier

Low−Pass

Filter

Low−Pass

Filter

oscillator

(b)

(c)

Figure 1.4 (a) Translation of modulated signal to zero center frequency, (b) simple receiver, (b) more

complete receiver.

c , restoring the original voice band. The newly-generated components at 2fc can be removed

by a low-pass ﬁlter. We thus arrive at the receiver topology shown in Fig. 1.4(b).

Our receiver design is still incomplete. The signal received by the antenna can be as low as

a few tens of microvolts whereas the speaker may require swings of several tens or hundreds

of millivolts. That is, the receiver must provide a great deal of ampliﬁcation (“gain”) between

the antenna and the speaker. Furthermore, since multipliers typically suffer from a high “noise”

and hence corrupt the received signal, a “low-noise ampliﬁer” must precede the multiplier. The

overall architecture is depicted in Fig. 1.4(c).

Today’s cellphones are much more sophisticated than the topologies developed above. For

example, the voice signal in the transmitter and the receiver is applied to a digital signal processor

(DSP) to improve the quality and efﬁciency of the communication. Nonetheless, our study reveals

some of the fundamental building blocks of cellphones, e.g., ampliﬁers, oscillators, and ﬁlters,

with the last two also utilizing ampliﬁcation. We therefore devote a great deal of effort to the

analysis and design of ampliﬁers.

Having seen the necessity of ampliﬁers, oscillators, and multipliers in both transmit and re-

ceive paths of a cellphone, the reader may wonder if “this is old stuff” and rather trivial compared

to the state of the art. Interestingly, these building blocks still remain among the most challenging

circuits in communication systems. This is because the design entails critical trade-offs between

speed (gigahertz center frequencies), noise, power dissipation (i.e., battery lifetime), weight, cost

(i.e., price of a cellphone), and many other parameters. In the competitive world of cellphone

manufacturing, a given design is never “good enough” and the engineers are forced to further

push the above trade-offs in each new generation of the product.

1.2.2 Digital Camera

Another consumer product that, by virtue of “going electronic,” has dramatically changed our

habits and routines is the digital camera. With traditional cameras, we received no immediate

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Sec. 1.2

Examples of Electronic Systems

feedback on the quality of the picture that was taken, we were very careful in selecting and

shooting scenes to avoid wasting frames, we needed to carry bulky rolls of ﬁlm, and we would

obtain the ﬁnal result only in printed form. With digital cameras, on the other hand, we have

resolved these issues and enjoy many other features that only electronic processing can provide,

e.g., transmission of pictures through cellphones or ability to retouch or alter pictures by com-

puters. In this section, we study the operation of the digital camera.

The “front end” of the camera must convert light to electricity, a task performed by an array

(matrix) of “pixels.” 4 Each pixel consists of an electronic device (a “photodiode” that produces

a current proportional to the intensity of the light that it receives. As illustrated in Fig. 1.5(a),

this current ﬂows through a capacitance, CL , for a certain period of time, thereby developing a

Amplifier

Light

I Di o de

V out

Photodiode

Signal

Processing

(a)

(b)

(c)

Figure 1.5 (a) Operation of a photodiode, (b) array of pixels in a digital camera, (c) one column of the

array.

proportional voltage across it. Each pixel thus provides a voltage proportional to the “local” light

density.

Now consider a camera with, say, 6.25-million pixels arranged in a 2500 2500 array [Fig.

1.5(b)]. How is the output voltage of each pixel sensed and processed? If each pixel contains

its own electronic circuitry, the overall array occupies a very large area, raising the cost and the

power dissipation considerably. We must therefore “time-share” the signal processing circuits

among pixels. To this end, we follow the circuit of Fig. 1.5(a) with a simple, compact ampliﬁer

and a switch (within the pixel) [Fig. 1.5(c)]. Now, we connect a wire to the outputs of all 2500

pixels in a “column,” turn on only one switch at a time, and apply the corresponding voltage

to the “signal processing” block outside the column. The overall array consists of 2500 of such

columns, with each column employing a dedicated signal processing block.

Example 1.2

A digital camera is focused on a chess board. Sketch the voltage produced by one column as a

function of time.

The term “pixel” is an abbreviation of “picture cell.”

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