what is Transistor

A transistor is a semiconductor device that is used to amplify or switch electrical impulses. The transistor is a fundamental component of contemporary electronics. It is typically made of semiconductor material and has at least three terminals for connecting to an electronic circuit.

History

 In 1947 J. Barden, W. Brattain, and W. Shockley invented the transistor.

The term transistor was given by John R. Pierce. Though initially it was called the solid-state version of the vacuum triode, the term transistor has survived. As we will go through the topic, we will know about the transistor, mainly bipolar junction transistor or BJT. Nowadays the use of BJT’s has declined towards CMOS technology in the design of IC’s.

The word “transistor” is derived from the words “Transfer” and “Resistor” it describes the operation of a BJT i.e. the transfer of an input signal from a low resistance circuit to a high resistance circuit. This type of transistor is made up of semiconductors. We know that silicon (Si) and Germanium (Ge) are examples of semiconductors. Now, why this is called junction transistor? The answer lies behind the construction.   

Now, in this type of transistor, any one type of semiconductor is sandwiched between the other types of semiconductor. For example, an n-type can be sandwiched between two p-type semiconductors, or similarly one p-type can be sandwiched between two n-type semiconductors. These are called PNP and NPN transistors respectively. We will discuss them later. Now as there are two junctions of different types of semiconductors, this is called junction transistor.

It’s called “bipolar” because the conduction takes place due to both electrons as well as holes.

BJT (bipolar junction transistor)

A bipolar junction transistor is a three-terminal semiconductor device consisting of two p-n junctions which are able to amplify or “magnify” a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector, and the emitter. A signal of small amplitude if applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process. The basic diagrams of the two types of bipolar junction transistors mentioned above are given below. From the above figure, we can see that every BJT has three parts named emitter, base, and collector. Now initially it is sufficient for us to know that emitter-based junction is forward biased and collector-base junctions are reverse biased.

N-P-N Bipolar Junction Transistor

As stated before in the n-p-n bipolar transistor one p-type semiconductor resides between two n-type semiconductors the diagram below n-p-n transistor is shown Now IE, IC is emitter current and collect current respectively and VEB and VCB are emitter-base voltage and collector-base voltage respectively. According to the convention if for the emitter, base, and collector current IE, IB, and IC current go into the transistor the sign of the current is taken as positive and if current goes out from the transistor then the sign is taken as negative. We can tabulate the different currents and voltages inside the n-p-n transistor.

Transistor type	IE	IB		IC	VEB	VCB	VCE
n-p-n	-	+		+	-	+	+

P-N-P Bipolar Junction Transistor

           Similarly, for PNP bipolar junction transistor n-type semiconductors are sandwiched between two p-type semiconductors. The diagram of a PNP transistor is shown below For PNP transistors, current enters into the transistor through the emitter terminal. Like any bipolar junction transistor, the emitter-base junction is forward biased and the collector-base junction is reverse biased. We can tabulate the emitter, base, and collector current, as well as the emitter-base, collector base, and collector-emitter voltage for PNP transistors also.

Transistor type	IE	IB		IC	VEB	VCB	VCE
p-n-p	+	-		-	+	-	-

Working principle of BJT

The figure shows an n-p-n transistor biased in the active region (See transistor biasing), the BE junction is forward biased whereas the CB junction is reversed biased. The width of the depletion region of the BE junction is small as compared to that of the CB junction. The forward bias at the BE junction reduces the barrier potential and causes the electrons to flow from the emitter to the base. As the base is thin and lightly doped it consists of very few holes so some of the electrons from the emitter (about 2%) recombine with the holes present in the base region and flow out of the base terminal. This constitutes the base current, it flows due to the recombination of electrons and holes (Note that the direction of conventional current flow is opposite to that of the flow of electrons). The remaining large number of electrons will cross the reverse-biased collector junction to constitute the collector current. 

Thus by KCL, IE = IC + IB, The base current is very small as compared to emitter and collector current.  Therefore IE ~ IC Here, the majority of charge carriers are electrons. The operation of a PNP transistor is the same as of the NPN; the only difference is that the majority of charge carriers are holes instead of electrons. Only a small part of current flows is due to majority carriers and most of the current flows are due to minority charge carriers in a BJT. Hence, they are called minority carrier devices.

Equivalent Circuit of BJT

A p-n junction is represented by a diode. As a transistor has two p-n junctions, it is equivalent to two diodes connected back to back. This is called the two diode analogy of the BJT.

Bipolar Junction Transistors Characteristics

The three parts of a BJT are collector, emitter, and base. Before knowing about the bipolar junction transistor characteristics, we have to know about the modes of operation for this type of transistor. The modes are

1. Common Base (CB) mode
2. Common Emitter (CE) mode
3. Common Collector (CC) mode

All three types of modes are shown below

Now coming to the characteristics of BJT there are different characteristics for different modes of operation. Characteristics are nothing but the graphical forms of relationships among different current and voltage variables of the transistor. The characteristics of PNP transistors are given for different modes and different parameters.

Common Base Characteristics

Input Characteristics

For p-n-p transistor, the input current is the emitter current (IE) and the input voltage is the collector-base voltage (VCB).

As the emitter-base junction is forward biased; therefore the graph of IE Vs VEB is similar to the forward characteristics of a p - n diode. IE increases for fixed VEB when VCB increases.

Output Characteristics

The output characteristics show the relation between the output voltage and output current IC is the output current and collector-base voltage and the emitter current IE is the input current and works as the parameters. The figure below shows the output characteristics for a PNP transistor in CB mode. As we know for PNP transistors 

IE and VEB are positive and IC, IB, VCB are negative. These are three regions in the curve, the active region saturation region and the cut-off region. The active region is the region where the transistor operates normally. Here the emitter junction is reverse biased. Now the saturation region is the region where both the emitter-collector junctions are forward biased. And finally, the cut-off region is the region where both emitter and the collector junctions are reverse biased.

Common Emitter Characteristics

Input characteristics IB (Base Current) is the input current, VBE (Base - Emitter Voltage) is the input voltage for CE (Common Emitter) mode. So, the input characteristics for CE mode will be the relation between IB and VBE with VCE as the parameter. The characteristics are shown below

The typical CE input characteristics are similar to that of a forward-biased of p - n diode. But as VCB increases the base width decreases. Output Characteristics Output characteristics for CE mode is the curve or graph between collector current (IC) and collector-emitter voltage (VCE) when the base current IB is the parameter. The characteristic is shown below in the figure.

Like the output characteristics of common-base transistor CE mode has also three regions named (i) Active region, (ii) cut-off regions, (iii) saturation region. The active region has collector region reverse biased and the emitter junction forward biased. For the cut-off region, the emitter junction is slightly reverse biased and the collector current is not totally cut-off. And finally for the saturation region both the collector and the emitter junction are forward biased.

Application of BJT

BJT's are used in discrete circuits designed due to the availability of many types, and obviously because of their high transconductance and output resistance which is better than MOSFET. BJT's are suitable for high-frequency applications also. That’s why they are used in radio frequency for wireless systems. Another application of BJT can be stated as a small-signal amplifier, metal proximity photocell, etc.

Bipolar Junction Transistor Amplifier

To understand the concept of Bipolar Junction Transistor Amplifier, we should look through the diagram of a PNP transistor first.

Now as the input voltage is changed a little, say ΔVi of the emitter-base voltage changes the barrier height and the emitter current by ΔIE. This change in emitter current develops a voltage drop ΔVO across the load resistance RL, where, ΔVO = - RLΔIC ΔVO gives the output voltage of the amplifier. There is a negative sign because the collector current gives a voltage drop across RL with polarity opposite to the reference polarity. The voltage gain AV for the amplifier is given the ratio between the output voltages ΔVO to the input voltage ΔVi, so,

ΔIC / ΔIE = AI is called the current gain ratio of the transistor. From the figure diagram shown above, we can see that an increase in the emitter voltage reduces the forward bias at the emitter junction thus decreasing the collector current. It indicates that the output voltage and the input voltage are in phase. Now, finally the power gain Ap of the transistor is the ratio between the output power and the input power

Transistors come in many shapes and types. A selection of typical bipolar junction transistors (BJTs) is shown in Fig

Fig  Typical Bipolar Junction Transistors

      1. BUH515

a.       High Voltage (1500V) high power (50W) NPN fast switching transistor in an ISOWATT218 package originally designed for use in analog TV time bases but also used in switched-mode power supplies.

      2. 2N3055

a.       NPN Silicon Power transistor (115W) designed for switching and amplifier applications. Can be used as one half of a complementary push-pull output pair with the PNP MJ2955 transistor.

      3. 2N2219

a.       NPN silicon transistor in a metal-cased TO-39 package, designed for use as a high-speed switch or for amplification at frequencies from DC (0Hz) up to UHF at about 500MHz.

      4. 2N6487

a.       General purpose NPN output transistor with a power rating up to 75W in a TO-220 package.

      5. BD135/BD136

a.       Complementary (NPN/PNP) pair of low-medium power audio output transistors in a SOT-32 package.

   6.      6, 7, and 8. 2N222

a.       Small signal general purpose amplifier and switching transistors like the 2N2222 and 2N3904 are commonly available in a variety of package types such as the TO-18 metal cased package (6), and the cheaper plastic TO-92 version (7) for through-hole mounting on printed circuit boards, as well as in surface mount SOT-23 versions (8).

Fig. TO-92 package variations

Transistor Package

Many transistor package types are also available with alternative connection layouts. The TO-92 package for example has variants TO-94, TO-96, TO-97, and TO-98 all with similar physical appearance, but each with a different pin configuration. Where different package variants are available, these are usually identified on the datasheet for each particular transistor type. Typical variations, such as those for the TO-92 to 98 series of packages used for transistors such as the 2N2222 are illustrated in Fig.