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It is is a three-terminal semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Most transistors are made from very pure silicon, and some from germanium, but certain other semiconductor materials are sometimes used. A transistor may have only one kind of charge carrier, in a field-effect transistor, or may have two kinds of charge carriers in bipolar junction transistor devices.

1) Bipolar Junction Transistors: A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching. BJTs are of two types known as NPN and PNP transistors. In NPN transistors electrons are the majority charge carriers. It consists of two n-type layers separated by a p-type layer. On the other hand, PNP transistors use Holes as their majority charge carriers and It consists of two p-type layers separated by an n-type layer.

Transistors are available in various types of packages such as 2N series or the Surface mount MMBT series they all have their specific advantages and applications. Out of these, there is another kind of Transistor series the BD series which is a power transistor series. The Transistors of this series are generally designed to generate extra power and hence they are a bit bigger than other transistors.

BD 139 transistors are NPN transistors and BD140 transistors are PNP transistors. Similar to other transistors they also have 3 pins and their pin configuration is shown in the image above.

Advantages of Power Transistors:-

1) It is very easy to turn ON and turn OFF the power transistor.

2) The power transistor can carry large currents in ON state and block very high voltage in OFF state.

3)The power transistor can be operated at switching frequencies in the range of 10 to 15 kHz.

4)ON-state voltage drops across the power transistor are low. It can be used to control the power delivered to the load, in inverters and choppers.

1) Transistor as an Amplifier:

A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter-base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The low resistance in the input circuit lets any small change in the input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which then flows through the load resistor RL, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier.

2) Transistor as a Switch:

Transistor switches can be used to switch and control lamps, relays, or even motors. When using the bipolar transistor as a switch they must be either “fully-OFF” or “fully-ON”.Transistors that are fully “ON” are said to be in their Saturation region. Transistors that are fully “OFF” are said to be in their Cut-off region. When using the transistor as a switch, a small Base current controls a much larger Collector load current. When using transistors to switch inductive loads such as relays and solenoids, a “Flywheel Diode” is used. When large currents or voltages need to be controlled, Darlington Transistors can be used.

Though the best choice for power transistors is BD139 and BD140 but if they are not available then you may also go for BD135 and BD136 which are NPN and PNP transistors respectively but preference must be given to BD139/140 pair. So that's it for the tutorial hope it was helpful to you.

So, now after so much of the theoretical part, we will be discussing an application of the BD139 and BD140 Transistor packages. This application is the H-Bridge Circuit which is used in motor driver circuits. When we need to run DC motors, it is required that a high amount of power is delivered to the motors which can not be fulfilled by the microcontroller alone so we need to attach a transistor circuit between the controller and the motor which works as an amplifier and helps in running the motor smoothly. The circuit diagram for this application is shown in the image above. With this H-bridge circuit, enough power is delivered to run two DC motors smoothly and with this, we can also control the direction of rotation of the motors. One thing we need to keep in mind while using BD139/140 or any other power transistors is that the power transistors generate a large amount of power which is also generated in the form of heat so to prevent overheating problem we need to add a heatsink to these transistors for which a hole is already provided on the transistor.