After the invention of the device, rapid development and development of electronic technology is possible, its name is transistor. It is a breakthrough in science and technology. 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. Hence, gaining knowledge on this subject is an essential part of the curriculum of electronics students.
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| What is Transistor? |
Transistor is a semiconductor active electronic device consisting of two pn junctions. It can be npn junction or pnp junction. It has three terminals. It can amplify the electronic signal 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 Lillianfeld filed a patent for a field effect transistor in Canada. He filed a similar patent in the United States again in 1926/1928. This is where the idea of using solid-state devices instead of tubes came into the world of electronics. But Lillianfeld later submitted no 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 had taken place. In 1947 John Bardeen and Walter Bratton conducted the experiment under the supervision of scientist William Shockley of AT&T's Bell Labs in the United States and observed that when two gold point contacts were placed on a germanium crystal, an output signal was produced , which had a higher power. Input. Thus the first practical transistor operation was discovered. And it is the world's first practical point contact transistor. These three scientists from Bell Lab 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 Laboratories scientist Morris Tannbaum created the first working silicon transistor, and in the same year Texas Instruments of America made the first commercially available silicon transistor. In 1960, Korean-American electrical engineer Dawseong Kang and Egyptian engineer Martin Atala jointly built the first metal oxide semiconductor transistor (MOS transistor) at Bell Laboratory.
The 'trans' to 'trans' and 'resistor' to 'easter' are collectively transistors. When a new device is discovered, researchers choose an appropriate name for the device so that the name can properly describe the device. A transistor has two junctions. During transistor operation one junction is forward biased and the other is reverse biased. Forward bias junction is low resistance path and reverse bias junction is high resistance path. The weak input signal is applied to the low-resistance circuit and the output is received from the high-resistance circuit. Thus a transistor transfers the signal from the low-resistance end to the high-resistance end. Here the term 'trans' expresses the signal transfer characteristic of the transistor and the term 'ester' refers to the transistor as a resistive class of devices.
Transistors can be classified from different perspectives. New classes of transistors are currently being invented in modern research, making transistors a very rich class of devices. Therefore it is quite difficult to classify transistors properly. So a clear classification of transistors is shown:
Basically all transistors can be divided into two main categories:
- Point Contact Transistor
- Junction Transistor
Depending on the semiconductor used, transistors are of the following types:
- Germanium Transistor
- Silicon Transistor
- Polycrystalline Transistor
- Monocrystalline Transistor
- Silicon carbide Transistor
Depending on the nature of construction, junction transistors are of the following types:
- Bipolar Junction Transistor (BJT)
- Field Effect Transistor (FET)
There are two types of Bipolar Junction Transistor (BJT) according to junction nature and polarity:
- NPN Tansistor
- PNP Transistor
There are two types of field effect transistors (FETs):
- Junction Field Effect Transistor (JFET)
- Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
There are again two types of JFETs:
- N-Channel JFET
- P-Channel JFET
There are two types of MOSFET or IGFET:
- Decrease Surge Type (DE MOSFET)
- Enhancement Only Type (e-only MOSFET)
There are two types of Depletion Enhancement Type (DE MOSFET):
- N-channel de mosfet
- P-Chanel de Mosfet
There are two types of enhancement only type (e-only MOSFET) ,
- N-channel e-only MOSFET
- P-Channel E-only MOSFET,
*** We will be discussing BJT only in the ongoing article.
NPN Transistor:
Placing a thin layer of a p-type semiconductor between two n-type semiconductor regions creates two p-n junctions and thus an npn transistor. A transistor has three regions. The two homogeneous regions are the collector and emitter regions and the opposite regions are the base, eg: 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 drawn from the three types of regions of the transistor. The junction between collector and base of a transistor is called collector-base junction and the junction between emitter and base is called emitter-base junction.
PNP Transistor:
Placing a thin layer of an n-type semiconductor between two p-type semiconductor regions creates 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 drawn from the three types of regions of the transistor. The junction between collector and base of a transistor is called collector-base junction and the junction between emitter and base is called emitter-base junction.
Nomenclature of terminals of transistors:
There are three layers of doped semiconductors inside a transistor. Two broad layers of inhomogeneous semiconductor are located at either end and a thin layer of inverted semiconductor in the middle which forms two pn junctions with two adjacent layers. A metal connection terminal called the 'Base' is pulled from the middle thin layer and two identical metal connection terminals are drawn from the two side layers, one called the 'emitter' and the other called the 'collector'.
The emitter always moves towards the base and provides a large number of carriers at 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 emitter to pass maximum current between the layers of the transistor.
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 allows current to flow in the external circuit, so this layer is called the 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 light doped extremely thin (usually 10 - 6 m) layer inside the transistor's structural framework is called the base. It is composed of semiconductors with reverse polarity compared to collector and emitter. The base layer forms two PN junctions with emitter and collector on either side. 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 emitting layer is highly doped so that it can supply a greater majority of carriers to the base layer.
The base layer is lightly doped and the collector layer is doped as needed.
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.
Base-emitter junction is always forward biased and base-collector junction is always reverse biased.
The resistance of the base-emitter junction is less than that of the base-collector junction, so a very 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 either side of a p-type or n-type semiconductor, current conducts by the flow of holes in the p-type and electrons in the n-type. Hole flow is the opposite of electron flow, and this type of flow is called drift current.
Diffusion current: If the concentration of electrons or holes is high somewhere on the crystal surface of the semiconductor material, then the charge carrier diffuses from a region of higher concentration to a region of lower concentration, this is called diffusion current. Diffusion current does not require any external voltage.
Technique of Current Flow in Transistor Operation/Active Mode 1
In case of NPN transistor:
Two voltage sources (VBE and VCB) are connected to bias the transistor for active mode operation. VBE provides forward bias to the source base-emitter junction and the source VCB provides reverse bias to the base-collector junction.
Since the base-emitter junction is forward biased, many electrons (majority carriers) from the n-type emitter region diffuse into the p-type base region in the base-emitter junction, resulting in an emitter current IE. Since 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 the base current iB1 flows. The small number of holes remaining in the base layer combine with the large number of incoming electrons with very few electrons, causing a very small base current iB2 to flow through the electrons, i.e. the two components of the base current iB1 and iB2. The remaining abundant electrons that reach the base level are considered minority carriers for the base level. We know that in reverse bias current flows through the PN junction to the minority carriers. Since 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 attracted by the positive terminal of the VCB source . , due to which the current IC flows in the external circuit. Thus in an NPN transistor the electrons flow through, the very small base current due to hole flow is neglected. The direction of conventional flow of IE, IB, IC is shown opposite to the direction of electron flow 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 in the transistor is discharged through the emitter terminal.
Technique of Current Flow in Transistor Operation/Active Mode 2
In case of PNP transistor:
Two voltage sources (VEB and VBC) are connected to bias the transistor for active mode operation. Source VEB provides forward bias to the base-emitter junction and source VBC provides reverse bias to the base-collector junction.
Since the base-emitter junction is forward biased, many holes (majority carriers) from the p-type emitter region diffuse into the n-type base region in the base-emitter junction, resulting in an emitter current IE. Since 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 the base current iB1 flows. Electron-hole recombination occurs when the small number of electrons remaining in the base level joins the incoming large number of holes with a small number of holes, in which a very small amount of base current iB2 flows, i.e. the two components iB1 and iB2 of the base current IB. The remaining abundant holes at the base level are considered minority careers for the base level. We know that in reverse bias current flows through the PN junction to the minority carriers. Since the base-collector junction is reverse biased, a large number of holes (minority carriers) from the n-type base region cross the base-collector junction to the p-type collector region and are attracted by the negative terminal of the VBC source. , due to which the current IC flows in the external circuit. Thus as current flows through the hole current in the pnp transistor, the very small base current caused by electron flow is neglected. The conventional flow direction 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 ………………. (ii)
That is, all the current in the transistor is discharged through the emitter terminal.
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