TRANSISTOR – Use, Definition, BJT, is MOSFET good?

We are getting more and more depending on computers day by day and modern computers are based on the transistor. So, transistors are incredibly important!

INDEX

1. What is a Transistor
2. Importance of Transistor
3. History of Transistor
4. How Transistors are made
5. Use of a Transistor
6. Types of Transistors
7. What is MOSFET And how it is better than BJT
8. Conclusion
9. FAQs

What is a Transistor

We can pull out two words from the word “Transistor” and that words are “Transfer” and “Resister”. In other words, transistor is a device made up of semiconductor which controls the voltage or current flow by transferring its resistance from high to low.

Here is the meaning of transistor given by Bell Labs (transistor was invented at bell labs)

Transistor is an abbreviated combination of the words “transconductance” or “transfer”, and “varistor”. The device logically belongs to the varistor family, and has the transcondutance or transfer impedance of a device having gain, so that this combination is descriptive.

Importance of Transistor

Imagine one day you woke up and found that your smartphone is not there. now, you will go to your sibling’s room and try to make a phone call from their mobile to your mobile so you can found it easily. But your parent’s phone is also missing! All around the world all computers are gone invisible.

Just imagine the world without computers! All robots in the companies, all printers, all machines are not more available. What will happen?  

History of Transistor

Transistor was invented on Dec. 23, 1947 by John Bardeen, Walter Brattain, and William Shockley at the Bell Laboratories.

Before the transistor’s invention, all computers were based on ‘Vacuum Tubes’. But, in April 1955 IBM announced the first completely transistorized computer “IBM 608 calculator” which was a calculator (shown at right). It contained more than 3000 transistors and in 1957, customers could purchase it for $83,210 (or rent it for $1,760 a month).

Now, the world’s fastest supercomputer (“Sunway TaihuLight”) has 400 trillion transistors in its processing part of the hardware. Transistor’s weight is just under 0.000000000000000000000000000001 kg! Gordon Earle Moore, the co-founder and chairman emeritus of Intel Corporation said that

the number of transistors in a dense integrated circuit doubles approximately every two years. His this statement is famous as “Moore’s law”.

How Transistors are made

Transistors are made with silicon and silicon is a semiconductor and Semiconductors are not completely conductor (flow the current easily) and not even completely nonconductors or insulators (don’t allow to flow current) they are intermediate. However, this means that they don’t allow current to flow in normal condition. but, if we give them the energy required to free the electrons then they act as a conductor.

• Firstly, we add impurities into the semiconductors. (This process is called doping) and Doping is required. because, after doping semiconductor has more free electrons or holes. so, the flow of current and conductivity of semiconductor increases.

P-type semiconductor:

If we add a trivalent impurity in a pure semiconductor which has 3 valence electrons, so a large number of holes will be created and current flow will be increased.

Here are some examples of trivalent impurities

• Boron (B), Indium (In), Gallium (G), Aluminum (Al), etc.

N-type semiconductor:

If we add a pentavalent impurity in a pure semiconductor which has 5 valence electrons, so a large number of a free electron will be created and current flow will be increased.

Here are some examples of pentavalent impurities

• Phosphorus (P), Arsenic (As), Antimony (Sb), etc.

NOTE:   P and N-type semiconductors are not positively or negatively charged. however, they are neutral.

The N-type semiconductor just indicates that it has electrons as a majority charge carrier and P-type has holes. Electron denotes as negative sign and holes are just an absence of electron so it denotes as a positive sign that’s why it’s called as P-type and N-type. (Here, P stands for positive and N for negative)

• Now, we have to create a PN junction diode to make Bipolar junction transistor (BJT)
  • If we join P-type material with N-type, the electrons at the N-type side will start diffusing to P-type and the holes at the P-type side will start diffusing the N-type side.

Depletion Layer

Because of this, A space charge region will get created between them. it’s called “Depletion Layer”

• So, this way a junction is created between them. Therefore, we know it as a PN junction diode.

Actually, BJT is like two back to back diodes as shown in the below, figure there is two diode-connected back to back. Because of two diode junction transistor has two junctions, first between the emitter and the base and second between the collector and the base. The left side diode is called a collector-base diode and the right side diode called emitter-base diode.

Now, we are going to bias the BJT. So, this type of transistor can bias in two ways. First is forward bias and the second one is reverse bias

• In forward bias, collector and emitter (both P-type) are connected with the positive terminal of the battery and base (N-type) is connected with the negative terminal of the battery.
• In reverse bias, collector and emitter (both P-type) are connected with the negative terminal of the battery and base (N-type) is connected with the positive terminal of the battery.

Types of Transistors

There are many types of transistors are available also. Here is the list of them:

1. Junction Transistor
2. NPN Transistor & PNP Transistor
3. Field Effect Transistor(FET) & Junction Field Effect Transistor(JFET)
4. MOSFET(Metal-oxide Semiconductor Field-effect Transistor)
5. N-channel MOSFET and P-channel MOSFET, etc.

Use of a Transistor

It is mainly used as Amplifier and Switch.

• When a transistor act as a switch it’s called Gate. But how can a it work as a switch?

The switch is ON when the base of transistor get voltage and OFF when the base voltage is absent. Therefore, the transistor is used in the computer as a Gate. which understands the binary language (1 and 0) here the 1 means ON and 0 means OFF and all of our data like videos, audios, images, text, etc. are stored in the memory in this 1 and 0’s form.

• Transistors are also used for memory storage and Each of one gate can store one unit 1 or 0. Two transistors can make one stable memory cell, so that can stores one bit.

So let’s find out how many transistors you need to store 1 GB data:

1 Gb = 1,024 Mb

1,024 Mb = 1,048,576 kb                       [ 1 Mb = 1024 kb ]

1,048,576 kb = 1,073,741,824 bytes                  [ 1 kb = 1024 bytes ]

1,073,741,824 bytes = 8,589,934,592 bits                       [ 1 byte = 8 bits ]

We need 17,179,869,184 transistors to store 1 GB of data!      [ 2 transistor = 1 bit ]

NOTE: it’s not that accurate because it is not fixed that always two transistors per bit stores 1 bit, each bit is stored individually. That’s a little complex part and even that’s not our topic.

• First-ever time It was used as an amplifier. You must have seen the hearing aids, that used transistor as an amplifier because when it work as an amplifier, it takes a small amount of current and produce a big amount of current and that’s why its use in hearing aids. Hearing Aids convert small sound into loud sound so the person can hear clearly.
• Transistor’s high-power radio frequency (RF) abilities are used in radar. All cell phones use a transistor as an amplifier.

Transistor has use in Integrated circuits and many more circuits so we can say that transistors are used in almost every electronics devices!

What is MOSFET And how it is better than BJT

MOSFET is actually Metal-oxide Semiconductor Field-effect Transistor. And it is just another type of Transistor.

• As we discussed above BJT has a collector, base, and emitter the same as MOSFET has a source, drain, and gate but its Input impedance does not change if we change DRAIN or SOURCE bias conditions.
• MOSFET is smaller than BJT in size. So, we can place more MOSFET in circuits and it will be faster and more powerful!
• MOSFET consumes less electricity than BJT because MOSFET can be directly connected in parallel but BJT can’t connect directly, it requires emitter resistance which consumes more power.
• The structure of MOSFET is more complex than BJT that’s why MOSFET is more costly than BJT.
• MOSFET can work well even if the temperature is little high. So, it has very fewer chances to get burn out.
• MOSFET introduce less noise into the signal than BJT.
• MOSFET switches faster than BJT, so MOSFET is a lot faster than BJT and it is MOSFET’s main advantage over BJT.

Conclusion

Transistor is a semiconductor device having three layers, three terminals and two junctions. Since we have only two types of semiconductors i.e. the p-type and n-type, there are two types of transistors: they are NPN transistor and PNP transistor.

FAQs