7.4. Basic Structure of BJT

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.

A Bipolar Junction Transistor (BJT) consists of three layers of semiconductors, forming two distinct junctions. The first junction is an n-p junction where electrons are the majority carriers, and the second junction is a p-n junction where holes are the majority carriers. The n-p junction is commonly referred to as the emitter, and the p-n junction is referred to as the collector, with the material in between them known as the base.

Imagine a sandwich with layers of bread and filling. The bread represents the n-type and p-type materials, while the filling is analogous to the base. Just as the bread layers allow different ingredients to pass through when pressed together, the BJT allows current through its layers when voltage is applied.

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 BJT has three terminals: the emitter (n-region) where current enters, the collector (another n-region) where current exits, and the base (p-region) which controls the amount of current flowing through the transistor. The emitter is heavily doped to inject carriers (electrons or holes), while the base is lightly doped to allow control of the current.

Consider a water faucet (emitter) where water (current) flows out. The valve (base) controls how much water flows through the faucet. If you tighten the valve, less water flows; if you loosen it, more water flows.

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.

For the BJT to function correctly, the base-emitter junction must be forward biased, meaning that the base (p-region) is at a higher voltage than the emitter (n-region). This forward bias allows current to flow easily from the emitter into the base, enabling the transistor to amplify signals.

Think of a door that opens inward (base-emitter junction). When you push on the door (apply positive voltage), it swings open easily, allowing people (carriers) to flow into the room (the base area).

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.

When the base-emitter junction is forward biased, current flows from the emitter to the base. The amount of current that flows through the junction is exponentially related to the voltage applied. This means that a small increase in voltage can result in a much larger increase in current, which is a key mechanism of how BJTs amplify signals.

Imagine a small faucet leak (current) that becomes a torrent when you apply high pressure (voltage). Just a little bit of pressure can turn a trickle into a rush of water.

The expression of the both base current and emitter current it is given as say some constant we will see that what is the constant involved multiplied by e power the forward bias voltage divided by thermal equivalent voltage ‒ 1.

The currents at the base and emitter can be mathematically represented using the diode equation, which states that the current is proportional to the exponential of the voltage across the junction divided by the thermal voltage. This relationship helps to predict how the BJT will behave under different voltage conditions.

Think of this relationship like a dimmer switch for a light bulb. As you gradually increase the switch (voltage) from off to bright, the brightness increases dramatically right at the start, similar to how current increases exponentially with voltage.

So, as the electrons are moving inside the base region, the current carried by electrons it may drop as it is getting recombined.

As electrons flow from the emitter into the base, they can recombine with holes in the base region, reducing the overall electron current. Not all carriers make it through to the collector, as some are lost in the base due to recombination with opposite charge carriers, which is crucial for maintaining the current flow in the transistor.

Think of a crowded concert where some people (electrons) try to move forward but get distracted and stop to chat with friends (recombine). Not everyone makes it to the front (collector), which affects how many can get there.

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