THE TRANSISTOR

Lecture Notes


CSC 5  
Prof. D. C. Cassidy

Nat. Sci. Program

www.dcassidybooks.com


 

Introduction

 

The transistor is the fundamental basis of all modern computer and electronic hardware today.  It replaced the vacuum tubes used in the first generation of modern computers, built in WW2, discussed in the textbook, as well in radios, TVs, stereos, etc.

 

Advantages over vacuum tubes are:

C           They don’t burn out as fast.

C           easier to mass produce (no vacuum in glass required),

C           much smaller, so the electronic components are much smaller and faster (electrons in a signal don’t have to travel very far).

 

As we will see later, while vacuum tubes are several centimeters in size, transistors are now in the nanometer range (1 billionth of a meter, 10-9 m), which is approaching the size of atoms.

 

This fundamental electronic device was invented just over 50 years ago in 1947 at Bell Telephone Labs in Murray Hill, NJ – now associated with Lucent Technologies

 

The inventors called it the “transistor” from “transmit - resistor”, i.e, it can be transformed from a resistor to a transmitter of electrons through our outside input.

 

Let’s look very briefly at 4 questions:

1. What is it?

2. What does it do?

3. How does it do it?

4. Who invented it and why?

 

 

1. What is it?

 

Basically: It’s an on/off switch without moving parts.

It conducts or blocks current flow between 2 points depending upon whether or not there is a voltage at a point in between called the Base (sometimes called the gate, which is very confusing).  (See Handout)

 

This is ideal for computers, because computers work with the digits 0 and 1–the binary code. The 0s and 1s can be represented electronically by the on or off switch.

           

 

2. What Does it Do?

 

Figure 4.8 on p. 97 of the text shows a schematic picture of a transistor (hand out).

 

Let’s take this apart, piece by piece :

1. Source charge on insulated wire, so it remains +.

 

2.  Tap into the source to see if it is charged. We can tell by the voltage on the “output” wire (if there is a shock or not). If it has + charge on source, there will be a voltage. Call this 1.

3. Ground the wire: charge flows to ground, none left, so has 0 voltage (no shock)–call this 0.

4. Put in a Base with a wire that carries a voltage: the base can be made to conduct or insulate the source from the ground, depending upon the voltage from a wire connected to the base. 

5. If there is a voltage (of about +0.5 V) on the base wire, the Base conducts. If there is no voltage, it does not conduct. It blocks the flow of charge from the source to the ground. It insulates. 

6.  We call the voltage on the base the input voltage, because we can tell the base to conduct or not by inputting a voltage or not.

 

7. Let’s put this all together.

A. Put a + charge on the source. Input a voltage of about +0.5 volts on the base. What happens?  The base conducts, so the + charge flows to the ground, leaving 0 charge on the source. What does the Output voltage read? 0.

 

B. Now put a + charge on the source. Input no voltage to the base. What happens? The base blocks the flow of charge from the source to the ground. What does the Output voltage read?  A positive voltage.

 

A positive voltage is read by the computer as a 1. A zero voltage is read as a 0.

So we can summarize what happens with the transistor by the following table:

 

            Vin                  Vout

            1                      0

            0                      1

 

Notice that Vout is the opposite of whatever Vin is.  In other words, the transistor takes the input signal of 0 or 1 and reverses it to 1 or 0.

In logic, this is called a negation. In computers this is called a NOT GATE,

because it takes the input and turns it into what it is NOT.

 

Can build up other logical results (NOR, NAND etc) in circuits using this simple device.

 

 

3. How does it do it?

 

The behavior of the base in the transistor derives from the properties of certain elements called semiconductors–silicon and germanium.

The properties of semiconductors are based on the principles of quantum mechanics.

 

We can’t go into all of the details of course. Just a little.

QM is a new physics of atomic processes developed about 75 years ago.

 

Based on the idea that energy of electrons in atoms and in metals is not continuous but limited to only certain values.

Bohr atom (hand out): Not like planets, since only certain allowed orbits.

Combine atoms to form a solid, then electrons exist in bands of energy. 

            Insulator: bands are filled, electrons can’t move.

            Conductor: band is partially empty, electrons can move

            semiconductor: filled bands, but narrow gap to empty band.

 

One way to make semiconductor into a conductor is to take silicon and mix in some phosphorous atoms. (See periodic table) Compared to silicon each of these has 1 extra negative electron, which must then go into the conduction band:  Called n-type.

 

Or instead mix in some aluminum atoms in a silicon crystal. These have 1 electron too few, thus creating some spaces in the filled band, which behave like + charges. Called p-type.

 

In 1947, researchers at Bell Labs put these 2 types of “doped” semiconductors together in a “sandwich” of n - p - n semiconductors.

They found it could transmit or resist the flow of current through the device depending upon the charge on the center piece, called the Base (gate). (Handout)

They invented what is now called the n-p-n junction transistor.

 

Show voltages.  Drawing etc

 

Schematic representation of DRAM cell (handout).

 

 

4. Who Invented It and Why?

See Websites given on handout

 

ATT Bell Labs. ATT had long standing problem that had to be solved in 1930s.

Long distance phone service needed amplifiers to get signal across the country. Small signal dies out in wire after short distance.

 

Late 1930s, Lee DeForest invented the Triode, a vacuum tube used to amplify the signal on phone lines. But too hot, broke easily etc.

So ATT began looking at solid state devices.

 

WW2: heavy military investment in radar and electronics, including computers and semiconductor devices.  But needed a solid-state switch (on/off)

 

Bell Labs, set out to develop such a switch using semiconductors. Named William Shockley to head the team.

 

Shockley, an engineer, hired Walter Brattain, an exp physicist, and John Bardeen, a theoretical physicist, as well as a support staff

Dec 1947, Bardeen and Brattain working alone realized that the barrier between n and p semiconductors was a problem.  Invented a point-contact type of transistor.

 

Shockley angered that he had been left out.

Went off to a hotel room in Chicago and in 4 weeks over Christmas and New Years 1947-48, he invented the n-p-n sandwich transistor. (Can also be p-n-p)

 

All 3 received the Nobel Prize in physics in 1956 for the invention of the transistor.

But internal competition broke the team apart.

 

Brittain–left Bell Labs to teach physics at Whiman College in Walla Walla, Wash.

 

Bardeen --went on to teach physics at Univ Ill. In 1972 he received his second Nobel Prize in physics, together with 2 other people, for the theory of superconductivity.

 

Shockley moved to Palo Alto, Calif and founded his own company, Shockley Semiconductor, which became the nucleus of Silicon Valley.

 

8 of his employees eventually left and founded Fairchild Semiconductor.

1968--2 members of that company–Bob Noyce and Gordon Moore–co-invented the integrated circuit for computer devices.

 

1971–they invented the world’s first microprocessor: A miniature logic circuit of 2300 tiny transistors and wires all on a single chip of silicon.  Using photoresist method. (Explain) In the same year they founded the Intel Corporation to manufacture the new microchip computer processors.

 

The microprocessor circuits have gotten smaller and smaller and faster and faster. Today, now reaching the atomic scale–single atoms with single electrons moving through the circuit.

 

The latest Intel Pentium 4 processor contains 55 million transistors and connections etched onto a single chip of just 146 mm2.  Each transistor is just 70 nm in size, with a   base of only 1.5 nm.  (See square on handout)

 

Economics of processors

But from the 1950s on, the market for integrated circuits in the US was dominated by military funding and applications–i.e., for rockets, jets, and electronics.

 

While US preoccupied with such applications, 2 Japanese entrepreneurs founded Sony Electronics for consumer products with strong backing of the Japanese government.

 

Mass produced transistor radios in 1950s, which opened up the consumer electronics market world wide.

 

Japanese and European firms began buying up US electronics companies to the point that by 1990 consumer electronics and computer microchips no longer produced in the US.

This was perceived as a threat, not to the economy, but to the military which needed electronics for high-tech weapons and communications.

 

Era of first President Bush. Despite laissez faire, free-market ideology, US funded the National Semiconductor Initiative, which greatly benefitted companies like Intel and AMD.

 

With the popularity of the PC and the rise of the Internet, the US has regained some market share in semiconductor chips.

 

But foreign companies still dominate in the areas of consumer electronics, game consoles (Play Station, Nintendo etc) and computer peripherals, such as random access memory.