What is Transistor (BJT) and how it works?

Transistor word is combination of two words i.e. Transfer and Resistor, means it is a component which can transfer signal from lower resistance region to higher resistance region. As we move ahead we will understand this statement with more clarity when we will understand the working of a transistor. This a an active semiconductor device, means it produces voltage/current or amplify or increase the voltage or current, in simple words increases the power of input signal. Opposite to this, whatever components we saw till now only consumes the signal, either reduce the strength or absorb in some way, hence they are termed as passive component like diode. Transistors are divided in two main categories that is N-P-N transistor and P-N-P transistor. This categorization is based on the construction of transistor. Let us see how a transistor is made.

Construction:

NPN Transistor

Here is an image of N-P-N transistor. There is a layer of P-type material sandwiched between two layers of N-type material. Base material is again silicon or germanium doped with different impurities as we discussed in diode chapter. Three regions and the connected terminals are named as “Base”, “Emitter” and “Collector”, marked as ‘B’, ‘E’ and ‘C’ in image. This type of transistor is also called BJT or “Bipolar Junction Transistor”.

• Emitter: It is heavily doped region. Its job is to emit or inject current carriers, hence is termed as emitter.
• Base: It is very lightly doped region, it absorb very small amount of current carriers injected from emitter and passes rest towards collector.
• Collector: It is little less doped region than emitter, and its main role is to collect the current carrier coming from collector and passed on by base region. Hence it is called the collector.

P-N-P Transistor

Similar to construction of N-P-N transistor, we have P-N-P transistor where the base layer is N-type material, emitter and collector region made of is P-type material. Doping level is same as we saw in N-P-N transistor and the function of each region is same as that of N-P-N transistor. We will see now what is the difference in their working and how they can be used in circuit. 

Symbol

Symbol of transistor is as shown here, the outer circle may or may not be shown in some circuits, which is OK. The emitter and collector is shown as connected to base but are also separated as it is there in its physical construction. Emitter terminal is indicated with an arrow sign, whose direction actually indicates the direction of current flow when it is used in any circuit. This arrow differentiate the collector terminal from emitter terminal. 

Note: In P-N-P transistor it points inward and in N-P-N transistor it points outwards. Base and emitter junction is always forward biased but the base and collector junction is always reversed biased in any circuit.

Transistor Operation

Now let us see how we operate a transistor in circuit. Refer this connection diagram using N-P-N transistor where the base is connected to positive of left cell and emitter is connected to negative of the cell. Hence base and emitter junction is forward biased. On the other hand the collector is connected to positive terminal of right side battery and base is connected to negative terminal of the battery, hence this junction is reversed biased. Note: In forward biased condition P-type region should be supplied with positive supply and N-type region should be connected to negative terminal. This we read in topic of diode. When a P-N junction is forward biased the current flow through junction, which also suggests that the resistance of P-N junction is low which is allowing current to flow. But in reversed biased condition, the current doesn’t flow thru P-N junction means the junction resistance is very high, stopping the current to flow through it.

Here a small base current (from base to emitter), forces a larger current to flow thru collector base junction and as we see in diagram emitter current is sum of base current and collector current i.e. I_e = I_b + I_c. Any small change in base current produces large change in collector current. So if any signal is applied to base of transistor, will be amplified or its magnitude will be increased and will appear in form collector current. This is called the current gain and is denoted by “hfe = I_c / I_b” i.e. ratio of collector and base current.

Means the signal applied between forward biased low resistance region of base and emitter will be transferred to high resistance region of reversed biased collector and base, hence we used the word Transfer + Resistor = Transistor. The working of P-N-P transistor is also the same except the polarity of supply is reversed i.e. negative terminal and positive terminal of cell supplying current to transistor is exactly reversed.

So, in simple words the base current works as controller to control the collector current, a small change in base current forces a large change in collector current. Hence we can compare base as valve controlling flow of water through a pipe.

• If valve is closed the flow is zero, means if base current is zero, collector current is also zero. Transistor is in off condition and is said to be working in cut-off region.
• If valve is fully open the flow is maximum, means if the base current is maximum, the collector current is also maximum and transistor is said to operate in saturated region i.e. no more current can flow from collector.
• When tap is partially open the flow is medium, means if the base current is in middle of saturation and cut-off region, the collector current is also some where in middle of cut of and saturation region.

So a transistor can also act as a switch, this feature is used in digital circuits which we will go through in later topics.

Low Power
Transistor

Transistors comes in many different shapes and sizes depending on the power handling capacity. Smaller transistors may be able to handle power in miliwatts and bigger ones can handle power in some hundreds of Watts. For high power transistor we need metal heat sinks to absorb extra heat generated due to handling high current through it. Smaller once can be run without heat sink. Sample small power transistor and a high power transistor with heat-sink, is shown in the picture.

High Power
Transistor

As transistor has three terminals with specific function, we should connect correct leg to the correct position in circuit. Every transistor has different pin configuration. To know the correct pin name of  a particular transistor, we can search for its datasheet on internet provided by manufacturer or simply if we search on google like “pin out of 2N2222” it will show results in most of the case. But the best place is datasheet.

Picture shows here a simple circuit of an amplifier using a single N-P-N transistor. Base-emitter junction is forward biased and base-collector junction is reversed biased as it is the basic requirement to operate a transistor as explained earlier.

Transistor Amplifier

A load resistor (R_L) is connected to collector pin which is actually the resistance of the coil of a speaker in practical scenario. A low amplitude input AC signal is applied to base pin which is amplified and produced across the load resistor. If we observe the output AC signal it looks inverted as compared to input signal, this means input and output waves will be out of phase.

The maximum load current through load resistor can be calculated as  I_L = V_ce / R_L (in amperes).

Transistor Graph

If we plot the graph of transistor parameters as shown here i.e. V_ce vs I_c at different values of base current I_b, it will look like as shown in this sample graph. The transistor will not operate below a particular base current, hence it is said to be in Cut Off state. As we increase the base current, the collector current also increase, but after a limit even if we increase the base current, the collector current will not increase and the transistor is said to be in Saturation state.

The operation of a transistor between cut off region and saturation region is said to be in Active Region. When a transistor is operated as amplifier, it is used in Active region, and when operated as a switch, it operates in cut-off and saturation region. In cut-off region, it act as an open switch and in saturation mode it act as a closed switch. This is used in digital gates or circuits which we will see later when we discuss about digital gates. 

This is all about Bipolar Junction Transistor (BJT), next we will learn about Metal Oxide Semiconductor Field Effect Transistor (MOSFET). These two types of transistors are mostly used in digital circuits as well as in IC (Integrated Circuit). Hence understand the switching function of transistor as we saw in analogy with a valve controlling flow of water is important.

Keep visiting for the upcoming topics or subscribe over email to get mail alert when a new topic is posted. For beginners it is highly recommended to read in steps from the first post for better understanding. Do post comment for any more query on this topic.

Return to List of content