Working of a Transistor | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

Transistors

Silicon, a chemical element often found in sand, isn't normally a conductor of electricity. A chemical process called doping -- in which impurities are introduced into a semiconductor to modulate electrical, optical and structural properties -- enables silicon to gain free electrons that carry electric current. The silicon can be classified as an n-type semiconductor -- electrons flow out of it -- or a p-type semiconductor -- electrons flow into it. Either way, the semiconductor enables the transistor to function as a switch or amplifier.

There are different types of transistors available, but we’ll focus on the NPN transistor in common emitter mode. This type has a heavily doped and wide emitter region, which contains many free electrons (majority carriers).

The collector region is wide and moderately doped, so it has fewer free electrons than the emitter. The base region is very thin and lightly doped, with a small number of holes (majority carriers). Now, we connect one battery in between emitter and collector. The emitter terminal of the transistor is connected to the negative terminal of the battery. Hence the emitter-base junction becomes forward biased, and base-collector junction becomes reverse biased. In this condition, no current will flow through the device. Before going to the actual operation of the device let us recall the constructional and doping details of an NPN transistor. Here the emitter region is wider and very heavily doped. Hence the concentration of majority carriers (free electrons) in this region of the transistor is very high.The base region, on the other hand, is very thin it is in the range of few micrometers whereas emitter and collector region are in the range of millimeter. The doping of the middle p-type layer is very low, and as a result, there is a very tiny number of holes present in this region. The collector region is wider as we already told and doping here is a moderate and hence moderate number of free electrons present in this region.

• Transistors can be combined to form a logic gate, which compares multiple input currents to provide a different output. Computers with logic gates can make simple decisions using Boolean algebra. These techniques are the foundation of modern-day computing and computer programs.
• Transistors also play an important role in amplifying electronic signals. For example, in radio applications, like FM receivers, where the received electrical signal may be weak due to disturbances, amplification is required to provide audible output. Transistors provide this amplification by increasing the signal strength.
• The voltage applied between the emitter and collector drops in two places. First, the emitter-base junction has a forward barrier potential of about 0.7 volts in silicon transistors. The rest of the voltage drops across the base-collector junction as a reverse barrier.
• Whatever may be the voltage across the device the forward barrier potential across emitter-base junction always remains 0.7 volts and the rest of the source voltage is dropped across the base-collector junction as reverse barrier potential.
• This means the collector voltage can't overcome the forward barrier potential. So, the free electrons in the emitter can't cross into the base. As a result, the transistor behaves like an off switch.
• NB: - As at this condition the transistor does not conduct any current ideally, there will be no voltage drop at the external resistance hence entire source voltage (V) will drop across the junctions as shown in the figure above.
• Now let us see what happens if we apply a positive voltage at the base terminal of the device. In this situation, the base-emitter junction gets forward voltage individually and certainly, it can overcome the forward potential barrier and hence the majority carriers, i.e., free electrons in the emitter region will cross the junction and come in the base region where they get very few numbers of holes to recombine.

But due to the electric field across the junction, the free electrons migrating from emitter region get kinetic energy. The base region is so thin that the free electrons coming from emitter do not get sufficient time to recombine and hence cross the reverse biased depletion region and ultimately come to the collector zone. As there is a reverse barrier present across the base-collector junction, it will not obstruct the flow of free electrons from the base to the collector as the free electrons in the base region are minority carriers.
In this way, electrons flow from emitter to collector and hence collector to emitter current starts flowing. As there are few holes present in the base region some of the electrons coming from emitter region will recombine with these holes and contribute base current. This base current is quite smaller than collector to emitter current.

Some electrons from the emitter contribute to the base current, while most go through the collector. The emitter current is the total of the base and collector currents. So, the emitter current is the sum of the base and collector currents.

Types of Transistors:

Transistors are classified into two major types:

1. Bipolar junction transistor (BJT)
2. Field-effect transistor (FET)

A BJT is one of the most common types of transistors, and can be either NPN or PNP. This means a BJT consists of three terminals: the emitter, the base and the collector. By joining these three layers, a BJT can amplify an electrical signal or switch the current on or off.

Two kinds of electrical charge -- electrons and holes -- are involved in creating a current flow. In its normal operation, the BJT's base-emitter junction is forward-biased with a very small emitter resistance, while the base-collector junction is reverse-biased with a large resistance.

In a PNP-type BJT, conduction happens through holes or the absence of electrons. The collector current is slightly less than the emitter current. Changes in the latter affect the former. The base controls the current flow from the emitter to the collector. In this case, the emitter emits holes, which are then collected by the collector.

• In an NPN-type BJT, electrons pass from the emitter to the base and are collected by the collector.
• When this happens, conventional current flows from the collector to the emitter.
• The base controls the number of electrons emitted by the emitter.
• A field-effect transistor (FET) also has three terminals -- source, drain and gate -- which are analogous to BJT's emitter, collector and base, respectively.
• In the FET, the n-type and p-typesilicon layers are arranged differently from those of the BJT.
• They are also coated with layers of metal and oxide to create the metal-oxide semiconductor field effect transistor (MOSFET).
• In the FET, field effect refers to an effect that enables the flow of current and switches the transistor on.
• Electrons can't flow from the n-type source to the drain because the p-type gate between them contains holes.
• But attaching a positive voltage to the gate creates an electric field that enables electrons to flow from the source to the drain.
• This creates the field effect, which facilitates the flow of current in the FET.

FETs are commonly used in low-noise amplifiers, buffer amplifiers and analog switches. The metal-semiconductor field-effect transistor (MOSFET) is commonly used for high-frequency applications, such as microwave circuits.

Other transistor types include the following:

• junction field effect transistor (JFET), a three-terminal semiconductor essential in precision-level, voltage-operated controls in analog electronics;
• thin-film transistor (TFT), a type of FET often used in liquid crystal displays (LCDs);
• Schottky transistor, which combines a transistor and a Schottky diode known for extremely fast switching to keep the transistor from saturating by diverting excessive input current; and
• diffusion transistor, which is a type of BJT formed by the diffusion of dopants onto a substrate