What is a transistor?

 What is a transistor?

A transistor is a  Semiconductor device that regulates or controls current or voltage flow in addition to amplifying and generating these electrical signals and act as a switch or gate for them. Typically, 

 

transistors consist of three layers or terminals (NPN) or (PNP) of a semiconductor material, each of which can carry a current. A transistor basically acts as a switch and an amplifier. Transistors are one of the key components in most of the electronic devices.

Parts of a Transistor

A typical transistor is composed of three layers of semiconductor materials .It has three terminal A voltage or current that is applied to any one pair of the terminals of a transistor controls the current through the other pair of terminals. There are three terminals for a transistor. They are listed below:

·        Emitter (E) : The outermost layer of a transistor, is  commonly known as the emitter, it is heavily doped with N- type or  P-type semiconductor material. P-type material is often employed to produce the emitter of a PNP transistor, whereas N-type material is mostly utilized for  creating the emitter of a NPN transistor.

·        Base (B): The intermediate layer is called the base, it forms a narrow zone between the emitter and collector and is slightly doped. Its made up of different material than the emitter. A NPN transistor's base is P-type, while a PNP transistor's base is of  N-type.

·        Collector (C): The inner layer is called as collector, and it has a modest doping level. The type of semiconductor used in the designing  is the same as that of the emitter. An NPN

transistor's collector is formed from N-type material, whereas a PNP transistor's collector is made of P-type material.

Types of transistor:

The two main types of transistors are bipolar junction transistors (BJTs) and field-effect transistors (FETs). The main difference between the two is that BJTs are current-controlled, while FETs are voltage-controlled. 

  

BJT(Bipolar Junction Transistor):

• A Bipolar Junction Transistor, also known as   BJT. A BJT is a type of transistor that carries charges using both electrons and holes. , is a semiconductor device with three terminals and two p-n junctions that can amplify or magnify a signal. 
• It is controlled by current. The base, the collector, and the emitter are the BJT’s three terminals. A BJT may also be implemented as a true solid-state switch in electrical circuits and can also be used to increase the strength of a weak signal

There are two junctions formed in BJT . 

They are -

·        Emitter-Base junction: The Emitter-Base junction of a BJT refers to the junction that is  formed between the emitter and base. During normal operation , this junction is forward-biased.

·        Collector-Base junction : Collector-Base junction is the name given to the junction that is  formed between the emitter and base. During normal operation , this junction is reverse-biased.

Types of BJTs

BJT could possibly be divided into two broad groups based on how the p-type and n-type semiconductor materials are arranged physically:

·        PNP Bipolar Junction Transistor

·        NPN Bipolar Junction Transistor

NPN Transistor

P-type material is sandwiched between the two n-type of  semiconductors in NPN type semiconductor. The two semiconductors of the n type act as the emitter and collector, respectively, while of p-type acts as the base.

P-type material is sandwiched between the two n-type of  semiconductors in NPN type semiconductor. The two semiconductors of the n type act as the emitter and collector, respectively, while of p-type acts as the base. The Emitter-base junction is forward bias and collector-base is reverse bias during the forward active state of NPN transistor. Only if voltage  exceeds the barrier potential, which is 0.3 volts for germanium transistors and 0.7 volts for silicon transistors, is the emitter-base junction forward biased.

Due to forward bias on the emitter- base junction the free electrons move towards the base region in N –type emitter. This constitutes emitter current . Direction of conventional current is opposite to the flow of electrons. Electrons after reaching the base region tend to combine with the holes. If these free electron combine with holes in the base, they constitute base current .

Most of the free electrons do not combine with the holes in the base as base and the width is made extremely small and electrons do not get sufficient holes for recombination. Thus most of the electrons will diffuse to the collector region and constitutes collector current . There is another component of collector current due to the thermal generated carriers. known as reverse saturation current and is quite small.

PNP Transistor

The two p-type of semiconductors are sandwiched between the n-type semiconductor in a PNP BJT. The two  p-type semiconductors act as the emitter and collector, respectively, and the n-type acts as the base. In a PNP transistor, current is due to  the movement of holes, as opposed to the mobility of free electrons in an NPN transistor. The collector base junction of a PNP transistor is reverse biased, whereas the emitter base junction is biased forward. The holes within the emitter area flow into the base region due to the forward bias at the emitter-base junction. This constitutes the emitter current .

The holes after reaching the base region, combine with the electrons in the base and constitutes base current. Most of the holes do not combine with the electrons in the base region as base width is made extremely small, and holes does not get sufficient electrons for recombination.

Thus most of the holes diffuse to the collector region and constitutes collector region. There is small component of collector current due to the thermally generated carriers . This is called reverse saturation current.

The transistor is fundamentally a current-controlled device, hence the voltage between its terminals has significant effect on its behaviuor.

·        Voltage Control: If we talk of input, the BJT is primarily a voltage-controlled device. The  base-emitter junction voltage determines the   current flows between the emitter and the collector. And the  base-emitter junction voltage  determines whether the transistor is "on" or "off" and hence influences the amount of current that may travel through it.

·        Charge Control: The current between the collector and emitter is determined by the movement of charge carriers, such as electrons or holes, in the base area. Between the collector and emitter, a considerably bigger current is controlled by a much smaller current at the base. Holes are charge carriers in PNP transistors , whereas NPN transistors have electrons. The transistor's  ability to work as a switch or an amplifier are determined by the number of charge carriers delivered into the base region.

·        Current Flow: The current that flows between the collector and emitter of a BJT is its main output. The voltage across the base-emitter junction controls base current, which  influences the collector current.