Saturday, February 23, 2019

Transistors - The Invention That Changed The World. History, Working And Types Of Transistor.



We are living in a time of incredible growth. One that has already started to transform the way we live our lives. The edge of information with 40% of the world's population currently connected to the Internet. The human race is more intertwined than ever before. But what led to this amazing tool? What single invention gave rise to our smartphone equipped generation?


It is a transistor, the transistor is information itself. The transistor is so simple but it is the foundation of all our modern computers. To understand its impact we need to understand the history and science behind this.

History Of Transistor

Before the transistor existed we used vacuum tubes which are bulky evacuated glass bulbs. The triode vacuum tube consisted of three parts: the cathode, grid and anode.

The current is passed through the cathode and it begins to heat up causing it to release electrons. As gases have been removed from the tube the electrons have very little resistance to their movement and they are attracted to the positively charged anode. This completes the circuit and current flows. But we can manipulate this flow of electrons in many useful ways with the grid

For example, we can use it as a switch, if we place a light bulb in it, it will only light up when there's a positive voltage across the grid. If we apply a negative voltage the negative charge will repel electrons trying to pass through. This is the foundation of binary coding which is the ones and zeros that gave birth to the edge of information. Here one is a positive voltage and zeros are negative. One turns the light on and zero turns it off.

The world's first general-purpose electronic computer the ENIAC used 18,000 vacuum tubes to perform calculations designed by John Mauchly and J. Presper Eckert. It was completed in 1945, it was purpose-built to calculate trajectories for artillery during World War II. A calculation that would take a human a day to calculate took ENIAC, 30 minutes but this thing weighed 30 tons and took up an entire room.

It was incredibly power hungry as the vacuum tube cathodes needed to be heated to work which also meant that the vacuum tubes burnt out regularly and needed to be replaced. All this to perform a function that your phone basically does with Angry Birds. Today it's computing power could be contained on a silicon chip no larger than a grain of sand that's thanks to the transistor. A modern form has around 2 billion transistors which performed the exact same job as the vacuum tube but on the nanoscale. Let's look at how it works?

How Transistor Work?

The transistor is in your CPU or microscopic and is manufactured with incredible precision with machines on thin wafers of silicon crystals that are sliced off silicon ingots. So what makes silicon so special that an entire section of the San Francisco Bay Area has been nicknamed after the material?

Silicon is a semiconductor which means it's conducting properties can be tailored by introducing impurities to the crystal structure. Silicon has four electrons in its valence shell and this is the outermost orbit for electrons and it determines many of the chemical properties of the atom. Atoms want eight electrons in that shell as this makes them very stable so silicon readily forms covalent bonds with four neighboring silicon atoms to gain those extra electrons.

Now if we introduce those impurities to this pure silicon crystal we can change how it conducts the current. If we add phosphorus which has five electrons in its valence shell the extra electron is left free to roam the crystal structure. This extra electron makes the N-type negatively charged which is where the name comes. From the P-type is positively charged because it is doped with Boron which has three electrons in its valence shell. This structure wants to gain its final electron and will steal electrons from its neighboring atoms. This creates a mobile positive charge called a hole.

The conductivity of the material has thus been increased as we've increased the number of mobile charges. When we arrange N-type and P-type semiconductors as shown in the image and attach terminals to each we create the world's most prevalent transistor, the NPN transistor. The transistor works due to the interaction of those free electrons and holes at the N-type and P-type Junction.

Free electrons in the N-type were migrated over to fill those holes in the P-type. This creates a boundary layer called the depletion layer which prevents more electrons passing through due to the negative charges repelling each other but when a positive voltage is applied to the base it negates the depletion layer and allows current to flow through completing the circuit. As you can see this is very similar to the function of the vacuum tube so how exactly does this allow computers to perform all these complex functions.

Let's look at a very basic example let's add two numbers together first we need to learn how numbers are represented in binary that's the ones and zeros that are used to sort data. The number shown in the image is number 15 which is the largest number you can represent with 4 bits.

The first bit represents 1 the next 2 then 4 and finally 8 added up that equals 15. This pattern continues with each successive bit representing double the previous so we can add an additional bit if we want to count up to 31

Let's add 5 and 6 together to do this we want the circuit that will hold a 1 in the position as shown in the image. When either 1 and carry the 1 forward when both are 1. As you can see this will give us the number 11. The simplest circuit that can do this is a half adder which contains two types of logic gates. These are devices that can modify the binary code they are built using transistors.

The first is the XOR logic gate which gives a 1 only when one of the inputs is 1 if both are 0 or 1 it gives a 0.

The second logic gate is an AND gate which gives a 0 for everything except when both inputs are 1.

If we worried these logic gates like this we create a half adder which gives two outputs our sum and our carry. This allows us to add our binary number one bit at a time. A more complicated circuit is needed to perform the calculation in one step. Modern computers can perform millions of these calculations per second and they're still getting faster.

The co-founder of Intel, Gordon E. Moore noticed a trend in 1965 that the density of transistors on integrated circuits doubles every two years that trend has held until very recently but it is starting to slow down. One of the reasons for this is the less well-known of Moore's predictions. Moore second law or rocks law which states that the cost of manufacturing these devices will double every four years.

Intel made an announcement last year that the rate of advancement was slowing for these reasons. It's getting more and more difficult for chip manufacturers to shrink their product while maintaining profit.

Another problem that transistors are facing is quantum tunneling. As these transistors get smaller so do the barriers between different sections. The barriers between each section of the transistor are getting so thin that electrons can pass right through them. With no definitive successor to the silicon transistor lined up this incredible period of growth over the last 50 years could patrol in the near future.

Some want to harness quantum mechanics to perform calculations faster than any transistor ever could. Others want to decentralize computing power and create the so-called Internet of Things. Intel has said themselves that they plan to shift their focus from increases in speed to decrease in power consumption. One thing is for sure the computer industry will have to redefine itself in the near future.