7.6.1. Junction Current

So, if you see the BJT as you may be aware from semiconductor device, what it is having it is the basic structure it is having two junctions, say for example, n-p junction and then p-n junction. And in this n-region, we do have electrical connection; we may be aware of this called say emitter. So, likewise in the other side of the device the other n-region, it is having a terminal called collector terminal, then the middle portion in between which is p-type. And in this p-region, it is also having one terminal through which you can apply voltage and you can observe the current and this terminal it is referred as base.

The Bipolar Junction Transistor (BJT) consists of three regions: the emitter (n-region), the base (p-region), and the collector (n-region). The emitter is responsible for injecting carriers (electrons or holes) into the base, while the base controls the flow of these carriers. This structure is fundamental for understanding how the BJT operates in electronic circuits.

Think of the BJT as a water tap. The emitter is the water source, the base is the tap's control that allows water flow, and the collector is the outlet where the water exits. The amount of water flowing through the outlet (collector) depends on how much you open the tap (base).

In normal circumstances, particularly for analog operation unless otherwise it is stated, base emitter junction the junction-1 it is forward biased which means that the p-region it is having a +ve voltage with respect to the emitter n-region. So, this junction it will be forward biased by a voltage called base to emitter voltage. On the other hand, base to collector junction again for normal operation, so this junction it is reverse bias which means that this n-region it is having higher potential than the p-region.

For typical analog operation, the base-emitter junction (junction-1) is forward biased, allowing current to flow easily. This setup means that the base (p-region) has a higher voltage than the emitter (n-region). Conversely, the base-collector junction (junction-2) is reverse biased, meaning the collector (n-region) has a higher voltage than the base (p-region), which limits current flow. This biasing arrangement is crucial for the BJT to function correctly in amplifying signals.

Imagine a road that flows easily downhill. The downhill part (the forward bias) allows cars (current) to pass quickly from base to emitter. On the other hand, the uphill part (the reverse bias) makes it hard for cars to go from the collector to the base, controlling the traffic flow (current).

Now, we know that through a p-n junction if this junction is say a forward bias, and if this second junction if it is far away from this junction, then we know that this current it will be having exponential dependency of this forward bias on the forward bias voltage. This junction current it is having two components; one is due to the movement of the electron another one is due to the movement of the holes.

When a p-n junction is forward biased, current can flow easily through the junction. This current demonstrates an exponential relationship with the applied voltage. Additionally, the total junction current consists of contributions from both electrons (negative charge carriers) and holes (positive charge carriers). The presence of both carriers is essential for the proper operation of the BJT, as they contribute to the overall current flowing through the device.

Think of a river flowing over a dam (the forward biased junction). The water (current) can flow easily when you reduce the dam's height (increase voltage). The water molecules can be thought of as both small (electrons) and large (holes) particles that together create the flow needed to turn a water wheel (power the circuit).

As the electrons are moving inside the base region, the current carried by electrons it may drop as it is getting recombined. And the responsibility of carrying the current of course, it will be, partially it will be done by the majority carrier concentration.

As electrons enter the base region, some recombine with holes, reducing the amount of current carried by electrons. However, holes (the majority carriers in the p-type base) will also contribute to the overall current. This balance of minority and majority carriers is essential for the BJT's functionality, as it affects how well the transistor can amplify or switch signals.

Consider a game of tag in a park. The chasers (electrons) can try to run fast and tag others (holes) but might get fatigued (recombine) after a certain point. Meanwhile, some still want to join in, and the group continues because other players (majority carriers) are ready to keep the game going vigorously. This dynamic keeps the activity (current) alive.

To find the net current what you can do you can find what may be the current at this point coming due to the diffusion of the electron plus whatever the currents are coming at this point due to diffusion of the holes. The total current, it may be obtained just by considering diffusion current at x = 0 plus diffusion current due to the holes.

The net current in a BJT can be calculated by summing the diffusion currents of both electrons and holes at any given point (like at the junction). This is crucial for understanding how the transistor amplifies signals, as the behavior of these charge carriers dictates the overall current flowing through the device.

Imagine a bustling marketplace where both buyers (electrons) and sellers (holes) are continuously moving in and out. The total activity (current) in the market is the combined efforts of both groups. If more buyers show up, the marketplace gets busier (more current), which is crucial for its overall performance just like how the total current depends on both carrier types.

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