8.6.3. Construction and Operation Parameters

BJT particularly say n-p-n transistor it is having three regions namely n, then p-region and n-region. In between it is having junction, junction-1 and also junction-2. They may be having different cross sectional area A and A.

A Bipolar Junction Transistor (BJT) consists of three layers: the emitter (n-region), the base (p-region), and the collector (n-region). These layers are organized in such a way that there are two p-n junctions formed between them, which are crucial for the functioning of the transistor. Junction-1 connects the base to the emitter, while Junction-2 connects the base to the collector. This arrangement is essential for controlling and amplifying electrical signals.

Think of the BJT as a water pipe system. The emitter and collector represent large water tanks, while the base is a valve that controls the flow of water (current) between them. The two junctions are like bridges that link these tanks, allowing for the movement of water when the valve is opened.

For active region of operation, one of these junctions to be forward biased by this voltage; base to emitter voltage and this junction on the other hand; it will be reverse biased.

In the active region of the BJT, a specific voltage (base-emitter voltage, V_BE) is applied to enable current to flow from the emitter to the base, making Junction-1 forward biased. Meanwhile, Junction-2 (the collector-base junction) is reverse biased, which keeps the majority charge carriers in the depletion region. This configuration allows the transistor to amplify current; small changes in the base current result in large changes in the collector current.

Imagine a traffic control setup where one set of traffic lights allows cars (current) to enter from a side street (the base), while traffic from a main road (the collector) is prevented by a red light. When the side street lights are green, cars can flow freely from there to the main road and cause increased traffic (current) on the main road.

Whenever we talk about these two junctions and if we say that these two are wide apart and they are not influencing each other; then whatever the minority carrier concentration we have seen in particularly in the p-region; it is having an exponential change.

In the context of BJTs, minority carriers are charge carriers that exist in a semiconductor material type opposite to the majority carriers. In the p-region of an n-p-n transistor, the majority carriers are holes while electrons are minority carriers. The concentration of these minority carriers in the base region changes exponentially with distance from the junction, especially when the junctions do not interact. This relationship is key to understanding how BJTs operate.

Consider a sponge absorbing water. If you drop a few drops of water at one end of the sponge, the effect (or concentration) of the water seen on the other end will decrease exponentially with distance. Similarly, the farther you go from the source (the junction), the less concentration of minority carriers you find in the base region.

The junction current I is exponential function of V. So, likewise in this junction also J the I is having exponential dependency on V.

The current flowing through the junctions in a BJT, defined as the junction current, has an exponential relationship with the applied voltage. This means that a small increase in voltage results in a large increase in current. This relationship is crucial for the transistor’s ability to amplify signals, leading to variations in the collector current based on the input base current.

Imagine pressing a balloon: at first, it feels easy to press, but as you press harder, the air inside needs to escape more forcefully, causing the balloon to expand rapidly. Similarly, a small change in the voltage across the BJT junctions causes a significant change in current.

Now, if I take these two junctions close to each other; let us see what are the things are happening.

Moving the junctions closer together impacts how the minority carriers behave and how the junction currents interact. When the distance is reduced, the minority carriers in the base region experience changes in electric fields, which in turn affects the current flowing through the transistor. This proximity leads to stronger interactions between the junctions, ultimately influencing the efficiency and performance of the transistor.

Think of two friends who are having a conversation from a distance. The farther they are apart, the less they can influence each other's thoughts and ideas. However, as they move closer, their conversation becomes more intertwined, and they can share more ideas rapidly and effectively. Similarly, when the junctions are closer, they affect each other's current flows significantly.

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