What is Transistor? | History of Transistor Development

The rapid growth and development of electronic technology is possible after the invention of the device, its name is transistor. The use of transistors has made it possible to design electronic devices with small size, low power dissipation, and low heat dissipation circuits. Transistors are the primary and fundamental components of modern electronic devices and components. Therefore, gaining knowledge on this subject is an essential part of the curriculum for electronics students.

A transistor is a semiconductor active electronic device that has two PN junctions inside it. It can be NPN junction or PNP junction. It has three terminals. It can amplify electronic signals and switch any signal or electronic power.

In 1904, Sir Ambrose Fleming invented the vacuum tube diode. This was followed by the invention of the thermionic triode in 1907, which was then used in radio and telephony circuits. But the tubes were heavier, hotter and dissipating more power. In 1925, American Jewish scientist Julius Edgar Lilienfeld filed a patent for the field effect transistor in Canada. He filed a similar patent again in the United States in 1926/1928. From here came the idea of using solid state devices instead of tubes in the world of electronics. But Lilienfeld subsequently did not submit any practical papers in support of his device or patents or working prototypes because high-quality semiconductor production had not yet begun. In 1934, German electrical engineer Dr. Oskar Heil filed a patent for a similar device, but no practical construction took place. In 1947, John Bardeen and Walter Bratton, under the supervision of scientist William Shockley at AT&T's Bell Labs in the United States, observed that when two gold point contacts were placed on a germanium crystal, an output signal was generated, Whose power is more than germanium crystal. Input. Thus the first practical transistor operation was discovered. And it is the world's first practical point contact transistor. These three Bell Lab scientists were awarded the Nobel Prize in Physics in 1956 in recognition of their research, discoveries and work. In 1948, William Shockley invented the junction transistor. In 1954 Bell Labs scientist Morris Tenenbaum created the first working silicon transistor, and in the same year, Texas Instruments of the US marketed the first silicon transistor commercially. In 1960, Korean-American electrical engineer Daewong Kahang and Egyptian engineer Martin Atala jointly created the first metal oxide semiconductor transistor (MOS transistor) at Bell Laboratories.

The word Transistor is made up of two words 'Transfer' and 'Resistor'. The 'trans' from transfer and the 'instor' from 'resistor' are collectively transistors. When a new device is discovered, researchers choose an appropriate name for the device so that the name accurately describes the device. There are two junctions in a transistor. One junction is forward biased and the other is biased during transistor operation. The forward bias junction is the low-resistance path and the reverse bias junction is the high-resistance path. The weak input signal is applied to a low-resistance circuit and the output is obtained from a high-resistance circuit. Thus a transistor transfers the signal from the low-resistance end to the high-resistance end. Here the word 'trans' expresses the signal transfer characteristic of the transistor and the word 'instor' refers to the transistor as a resistive class of devices.

Transistors can be classified from different points of view. New classes of transistors are currently being invented in modern research, making transistors a very rich class of devices. Therefore, it is very difficult to classify transistors correctly. So a clear classification of transistors is shown:

Basically all transistors can be divided into two main categories:

Point contact transistor
Junction transistor

Based on the semiconductor used, transistors are of the following types:

Germanium transistor
Silicon transistor
Polycrystalline transistor
Monocrystalline transistor
Silicon carbide transistor

On the basis of nature of construction, junction transistors are of the following types:

Bipolar Junction Transistor (BJT)
Field Effect Transistor (FET)

Bipolar Junction Transistors (BJT) are of two types according to the nature and polarity of the junction:

NPN Transistor
PNP Transistor

There are two types of field effect transistors (FETs):

Junction Field Effect Transistor (JFET)
Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

JFETs are again of two types:

N-Channel JFET
P-Channel JFET

There are two types of MOSFET or IGFET:

Depletion Enhancement Type (DE MOSFET)
Enhancement Only Type (E-only MOSFET)

There are two types of depletion enhancement type (DE MOSFET):

N-Channel DE MOSFET
P-Channel DE MOSFET

There are two types of enhancement only type (E-only) MOSFETs:

N-Channel E-Only MOSFET
P-Channel E-Only MOSFETs

*** We will only discuss BJTs in the ongoing article.

By placing a thin layer of p-type semiconductor between two n-type semiconductor regions, two p-n junctions are formed and thus an npn transistor is formed. A transistor has three regions. The two homogeneous regions are the collector and emitter regions and the opposite regions are the base, for example: in an NPN transistor the base is the P-type region and the N-type regions are the collector and emitter. The three connection terminals emitter, collector and base are derived from the three types of regions of the transistor. The junction between the collector and the base of the transistor is called the collector-base junction and the junction between the emitter and the base is called the emitter-base junction.

Placing a thin layer of an n-type semiconductor between two p-type semiconductor regions forms two p-n junctions and thus a PNP transistor. A transistor has three regions. The two homogeneous regions are the collector and emitter regions and the opposite region is the base. For example, in a pnp transistor, the base is the n-type region and the p-type region is the collector and emitter. The three connection terminals emitter, collector and base are derived from the three types of regions of the transistor. The junction between the collector and the base of the transistor is called the collector-base junction and the junction between the emitter and the base is called the emitter-base junction.

Inside a transistor are three layers of doped semiconductors. Two broad layers of inhomogeneous semiconductor are located at either end and a thin layer of inversion semiconductor in between which forms two PN junctions with two adjacent layers. A metal connection terminal called 'Base' is drawn from the middle thin layer and two identical metal connection terminals are drawn from the two side layers, one called 'Emitter' and the other 'Collector'.

The emitter is always forward biased towards the base and supplies a large number of carriers to the base level. That is, the base-emitter junction of a PNP transistor is always forward biased and the p-type emitter layer provides a large number of holes to the base layer. Similarly, the base-emitter junction of an NPN transistor is also always forward biased and the n-type emitter layer supplies a large number of free electrons to the base layer. Too much doping of the emitter layer causes the maximum current to flow through the emitter between the transistor's layers.

The collector layer is located at the opposite end of the emitter. It always provides reverse bias. It collects most of the charge carriers from the base-collector junction and conducts the current to the external circuit, hence this layer is called collector. Generally, the collector layer is made wider than the base and emitter and this layer is lightly doped compared to the emitter. Because more power is wasted at this stage.

The lightly doped very thin (typically 10 - 6 µm) layer inside the structural frame of a transistor is called the base. It is made of semiconductors with reverse polarity compared to the collector and emitter. The base layer forms two PN junctions with emitter and collector on the two sides. Forward bias is applied to the base-emitter junction which creates a low resistance in the emitter circuit and reverse bias is applied to the base-collector junction which creates a high resistance in the collector circuit.

A transistor has three layers emitter, base and collector.

The base layer is the thinnest, the emitter layer is the medium and the collector is the thickest layer.

The emitter layer is heavily doped so that it can supply more majority carriers to the base layer.

The base layer is lightly doped and the collector layer is further doped as required.

A transistor has two pn junctions like two diodes connected back-to-back, the junction between base and emitter is called base-emitter junction and the junction between base and collector is called base-collector junction Is.

The resistance of the base-emitter junction is lower than that of the base-collector junction, so a small amount of forward bias is applied to the base-emitter junction and a high reverse bias is applied to the base-collector junction.

Drift Current: When an external electric field is applied to a p-type or n-type semiconductor, the current conducts due to the flow of holes in the p-type and the flow of electrons in the n-type. Hole flow is the opposite of electron flow, and this type of current flow is called a drift current.

Diffusion Current: If the concentration of electrons or holes is high somewhere on the crystal surface of the semiconductor material, the charge carriers diffuse from the region of high concentration to the region of low concentration and form a current flow, it is called diffusion current. Diffusion current does not require any external voltage.

What is NPN Transistor?

Two voltage sources (VBE and VCB) are connected to bias the transistor for active mode operation. The source VBE provides forward bias to the base-emitter junction and the source VCB provides reverse bias to the base-collector junction.

As the base-emitter junction is forward biased, many electrons (majority carriers) diffuse from the base-emitter junction to the p-type base region, resulting in emitter current IE. Because the base layer is a very thin and lightly doped p-type semiconductor, it contains very few majority carriers (holes). From this small number of holes, some holes move to the emitter region and base current iB1 flows. The small number of remaining holes in the base layer connect with the large number of incoming electrons with very few electrons, so that very little base current iB2 flows through the electrons, i.e. the two components of the base current iB iB1 and iB2. The remaining superabundant electrons coming to the ground level are considered as minority carriers for the ground level. We know that in reverse bias the current flows through the PN junction for the minority carriers. As the base-collector junction is reverse biased, many electrons (minority carriers) from the p-type base region cross the base-collector junction and are accepted into the n-type collector region and are attracted by the positive terminal of the VCB source. Huh. , causing current to flow in the IC external circuit. Thus electron current flows through the NPN transistor, the very small base current caused by hole flow is neglected. The direction of conventional current flow Ie, Ib, Ic is opposite to the direction of electron flow as shown by arrows in the figure. On applying KCL to the base point of the transistor, we get,

Ie = Ib + Ic ………………………. (I)

That is, all the current of a transistor is discharged through the emitter terminal.

What is PNP Transistor?

Two voltage sources (VEB and VBC) are connected to bias the transistor for active mode operation. The source VEB provides forward bias to the base-emitter junction and the source VBC provides reverse bias to the base-collector junction.

As the base-emitter junction is forward biased, many holes (majority carriers) from the p-type emitter region diffuse from the base-emitter junction to the n-type base region, resulting in emitter current IE. Because the base layer is a very thin and lightly doped n-type semiconductor, it contains very few majority carriers (electrons). From this small number of electrons, some electrons move to the emitter region and base current iB1 flows. Electron-hole recombination occurs when the few remaining electrons in the base level combine with the incoming large number of holes to cause a very small amount of base current iB2 to flow, i.e. the two components iB1 and iB2 base current Ib . Many of the remaining holes at the entry level are considered minority careers for entry level. We know that in reverse bias the current flows through the PN junction for the minority carriers. As the base-collector junction is reverse biased, many holes (minority carriers) from the n-type base region cross the base-collector junction into the p-type collector region and are attracted to the negative terminal of the VBC source, causing a current is IC to flow in the external circuit. Thus in pnp transistor current flows through hole current, the very small base current due to electron flow is neglected. The direction of conventional current flow of Ie, Ib, Ic is shown by arrows in the figure which is the direction of hole flow. On applying KCL to the base point of the transistor, we get,

Ie = Ib + Ic ………………………. (Second)

That is, all the current of a transistor is discharged through the emitter terminal.

What is Transistor? | History of Transistor Development