7.2.2. Derivation of Current Flow

Jumping into this one, and it may create abundant availability of the electrons and contributing significantly to this collector current. In other words, based on this voltage the electrons are getting injected into base region, and they are nicely collected by the collector terminal by the virtue of this strong reverse bias voltage.

The section discusses how the voltage applied in a transistor circuit influences the flow of electrons. When a voltage is applied across the base-emitter junction, it creates an environment where electrons from the emitter can be injected into the base region. This injection is crucial as it forms the basis of the collector current, which is the current that flows from the collector terminal. The strong reverse bias on the collector terminal also ensures that these electrons are effectively collected, enhancing the overall current flow in the circuit.

You can think of this electron injection process like water flowing through a pipe. When you increase the pressure (analogous to the applied voltage), more water (electrons) is pushed into the pipe (the base region). The collector terminal acts like a drain, where all the water is collected after flowing through the pipe.

So, if these two junctions are remaining isolated, we cannot get BJT operation, it will be rather working as two back to back diodes. And this will be getting converted only when these two junctions are moving close to each other in the near vicinity.

The text points out that for a Bipolar Junction Transistor (BJT) to function effectively, the two junctions (base-emitter and base-collector) must not be isolated. If they are isolated (like two separate diodes), the transistor cannot operate as intended. Instead, it will behave more like two diodes connected back to back, which limits the control of current. However, when these junctions are brought closer, the interactions between them allow for the transistor action to commence, facilitating controlled current flow.

Imagine two friends standing apart and having a conversation. If they are too far away, they won’t be able to communicate well (isolated operation). But as they get closer, they can hear each other clearly and respond quickly, much like how the junctions in a BJT need to be close for effective operation.

So, instead of having an exponential fall of the minority carrier rather it will be having going to drop to 0, because of the reverse bias.

In a BJT, the behavior of minority carriers (electrons in the p-type material, for example) is essential for current flow. Under normal conditions, these carriers decay exponentially with distance. However, when there's a reverse bias applied, the minority carriers do not decrease exponentially; rather, they drop sharply to 0. This means fewer carriers are able to contribute to the current unless brought close to the active junction.

Think of minority carriers as grass in a garden. When plants grow far apart, the grass can thrive in specific spots (exponential fall). But if a gardener applies heavy mulch (reverse bias), the grass has no chance to grow because the conditions are too tough, leading to a sharp decline in growth.

This is where we are talking about the current particularly current carried by electron. I like to mention here a small correction; please make a note of that. Whenever we are taking say, then we do have , so that L part it will be coming here. So, this is a correction.

The section addresses how to calculate the current specifically carried by electrons in the BJT. A formula is mentioned, which incorporates a correction term related to the diffusion length of electrons (denoted as L). This correction is important because it accurately represents how far electrons can diffuse and still contribute to the overall current. Understanding these parameters is crucial for analyzing transistor behaviour and performance.

Imagine a marathon runner (the electron) who can only run a certain distance before getting tired (diffusion length, L). If the track is longer than this distance, the runner won’t make it to the finish line and thereby won't count toward the timing (current). Accurately calculating this run helps understand how many racers can finish and affect the overall race time (current flow).

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