Bipolar Junction Transistor Introduction
A bipolar junction transistor is a device consisting of two back-to-back PN junctions. It has three terminals. Both these junctions form three regions, which are called emitter, base, and collector. This device was invented by William Shockley, John Barden, and Walter H. Britain on 23rd December 1947. All three were awarded a noble prize for this remarkable invention.
The name transistor has been derived from “Transfer Resistor” which means that it can transfer its internal resistance to the emitter, low resistance of the base circuit to the collector, to very high resistance of the base circuit. Further, as the working of the transistor depends on the flow of majority and minority carriers (i.e. holes and electrons), therefore, it is called a bipolar device. If two PN junctions’ diodes are interconnected in mutually reverse directions, a transistor is formed. (Figure 4.1)
Advantages of Transistor
Transistors have the following advantages compared to a vacuum tube
- They have a longer life. Thus, electronic instruments do not develop defects for a long time.
- There is no need of heating it up. Nor do they require any power for this purpose. They start working immediately, which results in the conservation of electrical energy.
- They can function on low voltages.
- They are lighter in weight and physically small. Due to the small space requirement, instruments composed of it are usually small.
- Strong from the construction point of view, therefore can withstand jolts and trembling situations.
- Are comparatively cheaper than vacuum tubes
As characteristics of a transistor change with variations in temperatures, therefore reasonable outcomes are not possible at higher temperatures, which is one of its greatest drawbacks.
A junction transistor basically comprises two PN junctions which are shaped in the form of a single semiconductor crystal. The junction is manufactured through sandwiching layers of P and N-type semiconductors with a pair of opponent types. Both the junctions form three regions which are called emitter, base, and collector. As a transistor has three layers, therefore, three terminals are fitted with these three layers. Terminals alongside Emitter, Base, and Collector have been shown as C, B, and E in the diagram.
The two junctions which are produced between these three layers are called emitter-base (E/B) junction and collector- base Junction (C/B) (figure 4.2)
Bipolar junction transistors are available in two types. One type is called PNP while the other one is NPN transistor. In diagram 4.3 construction and symbol of the junction transistor have been illustrated. As can be seen from the figure, a PNP transistor consists of two P-type semiconductors, which are isolated through a narrow N-type semiconductor. In other words, the PNP transistor comprises an N-type material, which is sandwiched between two P-type materials. On the contrary, an NPN transistor consists of two N-type semiconductor materials separated through a narrow P-type semiconductor, or an NPN transistor is formed by sandwiching a P-type material between two N-type materials.
The arrow sign in PNP and NPN transistors symbols is also located on the emitter (i.e. collector has no arrow sign) The direction of the arrow sign reflects the direction of flow of conventional current. The direction of the arrow sign for a PNP transistor is from the emitter towards the base (i.e. towards the inner side) which means that the transistors’ emitter-base is positive with respect to the collector. For the NPN transistor, the arrow sign is from the base towards the emitter (i.e. towards the outer side) which reflects that the transistors’ base is positive with respect to the emitter. PNP and NPN transistors are called junction transistors or bipolar transistors. Remember, that transistor functions through an injection, diffusion, and collector mechanism. The detail of the transistors’ all three electrodes is as below:
This region or electrode of the transistor is most heavily doped compared to the other two electrodes and it is located on one side of the transistor. The basic function of the emitter is supplying the majority of charge carriers (electrons or holes) to the base. The emitters of both PNP and NPN transistors are forward biased related to the base so that it supplies the majority of carriers easily to the base. The emitter of the PNP transistor supplies hole charges whereas the NPN transistors’ emitter supplies free electrons.
This is a central part of the transistor, which produces two PN junctions between emitter and collector. In other words, the part which combines collector and emitter together and changes it into two junctions is called the base. Transistor’s base is quite thin/ narrow (10-6m) compared to the emitter and collector and it is also very lightly doped. The function of the base is to control the flow of charges. The base-emitter junction is forward biased whereas the base-collector junction is reverse biased. In the first case, there is very low resistance while in the second case, resistance is very high.
As the name implies, its fundamental function is the collection of majority charge carriers. The collector is always located on the opposite side of the emitter. The Collector region of most of the transistors is large compared to the emitter region (i.e. collector is larger in size compared to the base and emitter) because the collector has to dissipate high power. On account of this basic reason and difference, the collector of a transistor is never used as an emitter nor emitter of the transistor used as a collector. Collection base junction of PNP and NPN transistors is always reverse biased. Collector of PNP transistor receives holes whereas collector of NPN transistor receives electrons. Remember, the doping level of the collector lies between the emitter’s heavy doping and the base’s light doping (i.e. collector is neither heavy doped like an emitter nor light doped like an emitter, rather its doping level is less than the emitter’s doping level and higher than the doping level of base)
In figure 4.4, the transistor’s construction, symbols, apparent shape, and terminal connections have been shown
Types of Bipolar Junction Transistors
As discussed above, there are following two types of BJT.
- PNP Transistor
- NPN Transistor
It consists of an N-type material, which is sandwiched between two P-type materials. In other words, the transistor, which is formed through cramming an N-type material between two P-type materials, is called a PNP transistor.
Biasing and Working of PNP Transistor
For the normal operation of a transistor, it is necessary that correct polarity voltages be provided to both of its junctions. It has to be kept in mind that for correct and normal transistor operation, its emitter-base junction must always be forward biased, while its collector-base junction should be reverse biased. In figure 4.5, the flow of current through a PNP transistor has been explained.
In this method of PNP transistor’s biasing, emitter and base junction are forward biased, whereas collector and base junction reversed bias. Due to the forward biasing emitter base junction, the majority of carriers (holes) from the emitter flow towards the base. Thus, quite a large number of holes across the junction and enter the base. Holes recombine with electrons after entering the base. As the base is narrow and lightly doped, therefore, recombination of holes and electrons in the base is quite small (approximately 2-5 percent). Therefore, of all the holes coming from the emitter, nearly 89-95 percent do not recombine with electrons present in the base. As the collector-base junction is reverse bias, therefore, the flow of holes from the collector to the base cannot occur due to high resistance. However, in the opposite direction, i.e. from base to collector, the flow of holes starts, as collector voltage attracts nearly 95 percent of holes which are present in the base region, towards it. Thus, holes provided through emitter move to base-collector junction via base, and holes current continue in PNP transistor. Remember, that collector current and emitter current are nearly equivalent, but the value of base current is quite low.
NPN transistor comprises two N-type semiconductor materials that have been isolated through P-type semiconductor material. In other words, a transistor that is manufactured by sandwiching a P-type material between two N-type materials is called an NPN transistor.
Biasing and Working of NPN Transistor
The emitter-base junction of the NPN transistor is forward bias during its normal operation. And its collector-base junction is reverse biased. Figure 4.6
As emitter-base junctions are forward biased, the majority of carriers (electrons) flow towards the base, thus, a large number of electrons arrive at the base crossing junction. As the base is very narrow and lightly doped thus, electrons coming from the emitter pass in or diffuse and combine with some holes in the base (remember, this recombination is very small), and more than 95% of electrons enter the collector due to an effect of the positively charged collector. This becomes possible due to the attraction of collector voltage. As collector-base junctions are reverse biased, therefore, the flow of electrons from the collector towards the base does not occur (due to high resistance), rather electrons always flow from the base towards the collector. Thus, the flow of electron current commences in the NPN transistor due to the collection or diffusion of emitter-provided electrons in the base-collector junction. The direction in the figure reflects the flow of conventional current (holes current). Remember, the transistor does not conduct (i.e. does not allow passage of current from within it) until its emitter-base junction is forward bias.
Three types of current pass through an appropriately biased transistor (i.e. emitter current IE, base current IB, and collector current IC)In figure 4.7 (a), the quantities and directions of these currents have been shown in a PNP transistor that is fitted on a common base arrangement. As the source of charging in the transistor is the emitter, therefore, current passing via the emitter is called emitter current. The emitter currents IE, are divided into two parts while flowing towards collector and base. The current which flows from the base towards the collector is called base current IB, the value of which is very small (i.e. 2-5 %). As a major chunk of charge flows towards the collector through the base and causes collector current, therefore, the current passing through the collector is called collector current. The value of this current is quite high (95-98%). Thus, the total collector current and base current equal emitter current, as has been illustrated in the diagram. i.e.
IR= IB + IC
Moreover, IE transmits through the transistor, whereas IB and ICboth flow outside the transistor. In figure (b), the current passing through a PNP transistor fitted on common-emitter series. Here too, the emitter current is equal to the total of base and collector currents. i.e.
IE= IB+ IC
However, it must be kept in mind that current flowing towards the transistor or current passing from within a transistor is considered positive. Whereas, the current flowing out from a transistor or current flowing outside, is considered negative. Thus, IE is positive, while both IB and IC are negative. According to Kirchhoff’s current law (i.e. total of currents flowing in and out towards a point in a closed circuit equal to zero), transistor currents can also be explained as below.
IE+ (-IB) + (-IC) =0
Or IE– IB-IC = 0
Or IE= IB+ IC
Whatever the nature of the transistor or arrangement of its connections, the afore-mentioned fact (i.e. emitter current equals base and collector currents) always remains true. Base current, controls the transistor’s collector current and when its value is increased, the collector current also increases. However, these currents also depend on the value of forwarding voltage. If the forward bias voltage is increased, base current also increases, due to which collector current also witnesses an increase.
The ratio between output and input of a transistor circuit is called transistor gain.
(1) Whenever biasing of a transistor is done, the major share of the majority carriers (usually 95-98%), move on from the emitter and reach the collector, thereby producing collector current. At the same collector current transmits through the load. The ratio between collector current and emitter current is called the current gain of a transistor. It is denoted by Ai or α dc. Its value is always less than 1.Current Gain= α dc= Ai = -IC/IE= I out/ I in
IC has a negative sign because IC is rolling out of the transistor, whereas IE is arriving in the transistor. Therefore, IE has been taken positively while IC is negative. Thus, the above relationship can be described in a simple term as below:
α dc = IC/IE= collector current/ emitter current
(Negative sign has been removed here because we make a comparison between quantities of current instead of the direction of current)
α dc can also simply be mentioned as just α and it is also called forward current transfer ratio or amplification factor as well and is denoted by h FB. Here F means forward and B means Common-Base. Remember, that alpha always passes through a common-base circuit) Further, writing dc with alpha signifies that the ratio between IC and IF consists only of dc values.
The alpha of a transistor (α) actually is a measurement of its advantage/ benefit. A transistor with a high alpha value is considered good because the collector current in such a transistor nearly equals its emitter current.
A transistor may also have a-c alpha (αac), which shows the ratio of change in collector current to a change in emitter current i.e.
αac= ∆IC/∆IE= change in collector current/ change in emitter current.
Ac alpha is also called short circuit gain of some transistor and it is denoted by h fb. Remember, if FB is written in capital letters, it indicates d-c values, whereas written in small letters fb, denotes a-c values. Practically, we can also denote α dc and α dc by the letter α.
(2) The ratio between dc collector current and dc base current is also called current gain and it is denoted by beta (β) i.e.
Βdc= – IC/ IB= IC/IB or IC= βIB
It is also called the common-emitter d-c forward transfer ratio and can also be indicated as h FE. Remember, that h FE can only be achieved via common-emitter circuits which have an emitter ground and a maximum value of beta is 500. An increase in dc beta also normally tends to increase alpha. Further, no load is exerted on the transistor’s output for achieving alpha or beta.
When the a-c operation of some transistor has to be analyzed, we apply a-c beta (βac) which is equal to the following.
Β ac= ∆IC/∆IB
βac can also be represented as hfc
We know that the maximum part of emitter current (95 – 98 percent) passing directly through the collector, causes a collector current and a very small quantity of emitter current (1- 2 percent) to convert into base current. Thus, the current gain (ratio of output current and input current) of a transistor is always less than unity. The question arises that if currents’ gain or value of currents’ forward amplification is less than unity, how can a transistor amplify some signal. This becomes possible due to the difference found between input and output circuits’ resistance, which is called the resistance ratio. As a transistor functions as a transfer of resistance, it is briefly called a transistor, and this name has been given to it due to its very property. If the emitter-base is forward biased while the collector-base is reverse biased, the value of emitter-base resistance due to its being forward bias ranges between 40- 800 ohms. And the resistance of the collector base, due to being reversed bias, is very high i.e. 1 million ohms. As the resistance of emitter circuits is less, therefore, a relatively large current passes through it. There are low voltage drops parallel to emitter-base resistance (REB). Contrarily, as the resistance of the collector-base circuit is quite high, therefore, a relatively small current (αIE) passes through the collector. However, high voltage drops occur parallel to collector–base resistance (RCB), thus, the voltage gain of a transistor can be determined by comparing voltage drops parallel to input and output resistors. In other words, the ratio between a transistor’s output voltage and the input voltage is called voltage gain of that transistor i.e.
Voltage Gain= Aυ=αIE RCB/IE REB= Voltage across RCB/ Voltage across REB
If the term IE is mutually canceled in the above equation, it can also be written as
Aυ= αx RCB/ REB or αx resistance ratio
In other words, voltage gain can also be written as,
Voltage gain= Output voltage/ Input voltage = V/V
Or output current x output resistance/ Input current x input resistance = I R0/ IERI= α Rout/ Rin
Voltage gain can also be obtained through multiplying alpha (α) with output and input resistance (also called resistance gain)
Voltage gain= αx Ar
Remember, a small change in the voltage of the emitter-base circuit causes a very large change in collectors’ voltage. The gain of an ordinary transistor is approximately up to 2000 times.
We know that power equals the product of square current and its resistance (I2R), therefore, the power gain of a transistor can also be indicated by the following formula.
Power Gain= Pout/ Pin= (output current) 2x output resistance/ (Input current)2 x input resistance
Or = (αIE) 2 x RCB/ I2E x RFB= α2x RCB/ RFB = αx2 Rout/ Rin
Or AP= α2 x Ar
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