8.2.2. Junction current of BJT

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.

A Bipolar Junction Transistor (BJT) consists of three regions: the emitter (n-region), the base (p-region), and the collector (n-region). There are two junctions formed where these regions meet: junction-1 between the emitter and base, and junction-2 between the base and collector. This structure is critical for the functioning of the BJT, as it allows for the control of current flow through the device.

Think of a BJT like a water valve. The three regions represent parts of a plumbing system: one region supplies water, one region is the control mechanism, and the final region allows water to flow out. Just as the valve controls the water flow, the BJT uses the electric current to control output current.

For active region of operation, one of these junctions is 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, the base-emitter junction (junction-1) is forward biased, meaning that the voltage applied allows current to flow easily through the junction. Conversely, the base-collector junction (junction-2) is reverse biased, which restricts current flow. This unique biasing setup is essential for the transistor to amplify signals, as it allows the control of current flowing through the collector.

Imagine a one-way street where cars can flow freely into a neighborhood (the forward-biased junction) but cannot exit back onto the main road (the reverse-biased junction). This setup lets you control traffic in a way that optimizes flow into the neighborhood.

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 the BJT, minority carriers (electrons in the p-region of the BJT) exhibit a change in concentration based on the distance from the junction. This relationship is characterized by an exponential function, indicating that the concentration decreases quickly as you move away from the junction. This behavior is crucial for the operation of the transistor, particularly in terms of how well it can amplify signals.

Consider how light intensity diminishes as you move away from a light source. Similarly, the minority carrier concentration decreases rapidly as you move away from the junction within a BJT. Just as light is less intense further from the bulb, minority carriers become less available further from the junction.

The current carried by electrons and the current carried by holes. So likewise I; it is also having two current components namely we do have I and then we do have I.

In a BJT, current is primarily due to two types of charge carriers: electrons and holes. The electron current, denoted as 'I', moves through the n regions, whereas hole current, denoted as 'p', moves through the p region. The BJT operates by balancing these currents, and under different biasing conditions, one or the other component may dominate the overall current flow.

Think of the BJT as a traffic system where cars (electrons) and bicycles (holes) are flowing in opposite directions on a road. The balance of how many cars versus bicycles are on the road affects overall traffic flow, just like the balance of charge carriers affects current flow in a BJT.

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

When the two junctions of the BJT are brought closer together, the effect of the depletion region becomes significant. The movement of minority carriers across the junction changes because they experience the influence of the nearby junction’s electric field. This affects both the injection current (current from the emitter) and the recombination current (current that corresponds to carrier loss), ultimately altering the overall current characteristics of the BJT.

Imagine two magnets placed close together. The field from one magnet influences the other’s operations. Similarly, when the BJT’s junctions are near each other, the electric field from one junction affects the flow of carriers at the other junction.

So, the current carried by electrons it is actually it is having two components; one is it is getting recombined with the holes...the other component it is basically electrons are moving here which is contributing additional current of the collector terminal.

The current associated with electrons can be divided into two parts: the injected current, which flows into the collector, and the recombination current, which happens when electrons recombine with holes in the base. The balance between these currents is vital for the operation of the transistor, enabling it to amplify signals effectively.

Think about a factory where workers (electrons) can either produce goods (inject current) or leave the factory due to breaks (recombine). The efficiency of the factory depends on how well these workers are managed to maintain a steady production flow.

As I say that the base terminal current; it is summation of the two current components of this one; namely I and I.

The total current at the base terminal of the BJT is the sum of various components: the current carried by holes from junction-1 and the recombination current. Understanding how these currents combine helps us to derive meaningful expressions for total terminal currents, and thus helps predict the transistor's behavior under different biases.

Consider a multitasking worker in an office who handles multiple tasks at the same time. Just like the total output of that worker is a combination of all the tasks they're handling, the total current in the BJT at a specific terminal results from the contribution of different current components.

The value of this β typically it is getting increased by decreasing this base width... increase this N and N we can decrease so that the β of the transistor it can be increased.

The current gain of a BJT, denoted as β, is a crucial parameter that indicates how much the collector current can be amplified compared to the base current. Factors affecting β include the dimensions of the transistor, specifically the base width, and the doping concentrations in different regions of the BJT. Higher β values are desirable as they indicate better amplification capabilities.

Think about a microphone and speaker system. The gain of the system can be thought of as analogous to β. Just as you can increase the volume output by optimizing microphone placement and sensitivity, the effectiveness of a BJT can be enhanced by carefully controlling its physical characteristics.

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