PHY 201 (Blum)

Transistors and Logic Gates

References:http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page1.html

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Transistors There are various kinds of transistors:

bipolar, field-effect, etc. They differ in stability, energy usage, and so

on, but they serve a similar purpose. They are used to amplify a signal or to act

as a switch. It is as switches that they are used in computers. The change from vacuum tubes to transistors

meant dramatic differences in the physical size of computers and ultimately dramatic differences in their speed, capacity and ubiquity.

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Generations

Computers are thought of as belonging to generations.

For the ENIAC and other computers of the first generation, the processor was comprised of vacuum tubes.

The processor’s individual vacuum tubes were replaced by individual transistors in the second generation of computers.

Several transistors could be placed close together on an integrated circuit (IC) or chip. In third generation computers the processor consisted of several IC’s.

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Processor Microprocessor Eventually the entire processor was

placed on a single chip. When this became standard computers were said to enter the fourth generation. In this case, the processor is known as a

microprocessor. Some large machines today may have

the processor spread over more than chip. But PCs typically have a single processor.

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Tubes

A computer of the first generation consisted of tubes.

Tubes started with an effect Thomas Edison noticed while experimenting with light bulbs.

John Ambrose Fleming discovered that one could exploit the effect to detect radio waves and convert them to electricity, but the signal was too small.

Lee de Forest added to the device, making the triode; Edwin Armstrong pointed out it could be used to amplify signals.

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Transistors

A computer of the second generation consisted of transistors.

William Shockley, John Bardeen and Walter Brattain developed the transistor while working at Bell Labs in 1947. (Nobel Prize 1956)

The transistor could play the same role as the vacuum tube but was significantly smaller – and thus faster and less power consuming.

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Integrated Circuit

A computer of third generation consisted of integrated circuits.

The problem with computers is that they required so many transistors connected to one another – the so-called “tyranny of numbers.”

This problem was solved by the “monolithic idea” – the idea the many circuit elements (mainly transistors) could be placed on the same piece of semiconductor, i.e. an integrated circuit (IC).

In 1958 Jack Kilby of Texas Instruments invented the IC. In 1959 Robert Noyce of Fairchild Semiconductor independently developed a better-designed IC. (Nobel Prize 2000.)

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Microprocessor A computer of fourth generation has a

microprocessor. What distinguishes a microprocessor

from other integrated circuits is that a microprocessor can be programmed.

So along with the idea of the microprocessor comes the idea of the instruction set – the set of actions the programmer can have the microprocessor perform.

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Intel Intel was founded by Bob Noyce and

Gordon Moore who formerly worked for Fairchild Semiconductor.

In a collaboration between Busicom (a Japanese firm) and Intel to make calculators, Ted Hoff devises a plan to put all the main circuitry on one chip instead of the original plan of twelve (in 1968).

The microprocessor was born.

Noyce Moore

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The Microprocessor is born

Intel made the 4004 (the first microprocessor) for Busicom. Knowing they had a good thing, they bought the rights to the 4004 from Busicom for $60,000.

Ted Hoff

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4004

One of the first commercially available microprocessors.

But returning to the matter at hand transistors.

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Diode Review Recall that a pn junction — the

joining together of p-doped (“too few” electrons) and n-doped (“extra” electrons) — makes a diode.

A diode is a circuit element that allows current to flow in one direction (forward bias) but not in the other (reverse bias).

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Diode Review (Cont.) Recall that a semiconductor had a full

valence band and an empty conduction band separated by a gap.

We could improve the conductivity of a semiconductor by doping it. N-doping put some electrons into the

conduction band which were free to move about.

P-doping freed up some places in the valence band – these “holes” were also free to move about.

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Diode Review (Cont.) In a diode one brings together n-doped

and p-doped material. Some of the “extra” electrons from the n

side’s conduction band fill the empty levels in the p side’s valence band, forming a region in which the valence band is filled and conduction band is empty.

This region (called the depletion zone) is a poor conductor.

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Applying a voltage Connecting a p-n junction to a battery as shown below

adds positive charges on the n-doped side making the region of poor conductivity larger.

This is called reverse bias. Current doesn’t flow in this direction.

p-doped - - -- - -

+ + + + + + n-doped

+ + +

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Applying a voltage II Connecting a p-n junction to a battery as shown below

adds positive charges on the p-doped side making the region of poor conductivity smaller (oversimplified).

This is called forward bias. Current does flow in this direction.

p-doped - - -- - -

+ + + + + + n-doped

+ + +

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Bipolar transistors A bipolar transistor starts with two back-

to-back diodes (pn junctions). There are two kinds: NPN and PNP

The middle region is usually small

N-doped

P-doped

N-doped

P-doped

N-doped

P-doped

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Third lead So far the device seems useless;

two back-to-back diodes wouldn’t conduct in either direction.

But we add a third lead (connection) directly to the middle portion.

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Not symmetric The transistor would seem to be

symmetric with the two N-doped regions being the same, but actually these regions differ in their amount of doping and serve different purposes in the transistor.

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Collector, base, emitter

Collector

Base

Emitter

In bipolar transistors, the various regions are referred to as the collector, base and emitter.

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Collector, base, emitter

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Connecting the transistor Imagine applying a

potential difference (voltage) across the base-collector leads with the collector higher, this reverse biases that pn junction so there would be no current flow.

C (N)

B (P)

E (N)

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No flow There is no flow because of the

depletion zone (the region in which the valence band is filled and the conduction band empty).

Reverse bias voltages tend to make the depletion zone a bit larger.

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Connecting the transistor (Cont.)

Now consider applying a (smaller) voltage across the base-emitter leads with the base higher, this forward biases that pn junction so current will flow.

C

B

E

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Flow Forward biasing a pn junction tends to

eliminate the depletion zone (in this case putting electrons into the conduction band).

Because the transistor has one shared depletion zone that has been eliminated by the base-emitter forward bias, both currents (collector-base and base-emitter) can flow.

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NPN in a circuit The arrow on an

NPN points from base to emitter indicating the forward-bias direction that turns the transistor “on.”

CBE

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Characteristics of the Off State

B

C

E

Little to no current going into the base and a large voltage drop across the collector/emitter leads.

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Off Base-emitter circuit

Little to no current flowing. Most of the voltage dropped across the

base-emitter as opposed to the resistor in the circuit.

Collector-emitter circuit Little to no current flowing Most of the voltage dropped across the

collector-emitter as opposed to the resistor in the circuit.

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Characteristics of the On State

C

B

E

Current through the base lead, small voltage drop across collector/emitter leads.

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On Base-emitter circuit

Current flowing. Most of the voltage dropped across the

resistor as opposed to the base-emitter in the circuit.

Collector-emitter circuit Current flowing. Most of the voltage dropped across the

resistor as opposed to the collector-emitter in the circuit.

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Logic gates The on-off nature of diodes and

transistors make them ideal for building logic gates.

Logic gates have input which is interpreted as a logic value (0 or 1, low or high, false or true) and have output which can also be interpreted logically.

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Logic gates

Logic Circuit Symbol

NOT

AND

OR

NAND

NOR

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AND Gate

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OR Gate

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NOT Gate

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NOR Gate

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NAND Gate

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Logic Gate Example (00)

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Logic Gate Example (01)

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Logic Gate Example (10)

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Logic Gate Example (11)

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NOR

A B Out

0 0 1

0 1 0

1 0 0

1 1 0