7.1.1. Lecture - 07

So, dear students, welcome back to this analog electronic circuits, one of the early modules of the course. Myself Dr. Pradip Mandal from E and ECE department associated with IIT, Kharagpur. So, today’s discussion, it will be on BJT characteristic. From semiconductor device, you may be aware about the BJT, but today what will be discussing is that its basic characteristic, what are the characteristics are necessary for understanding analog electronic circuit.

In this introduction, Dr. Pradip Mandal welcomes students and sets the stage for the discussion on Bipolar Junction Transistors (BJTs). He emphasizes that while students may have some prior knowledge of BJTs from semiconductor studies, the focus today is to deepen that understanding by exploring essential characteristics fundamental to analog electronic circuits. Understanding the characteristics of BJTs—particularly the current-voltage (I-V) relationship—is crucial for designing and analyzing electronic circuits.

Think of BJTs like faucets in your home. Just as a faucet controls the flow of water based on the position of its handle, a BJT controls the flow of current in electronic circuits. Understanding how the faucet works (i.e., its characteristics) helps plumbers fix it when it leaks, similar to how knowing BJT characteristics aids engineers in troubleshooting electronic designs.

So, let us see the plan overall plan. So, today’s plan is to cover the basic structure of BJT, and typically what are the bias conditions are followed for BJT particularly in analog operation. And then will be starting with current equation of normal or standard p-n junction it may be silicon or germanium. And we will start with isolated junction then will be gradually moving towards what are the two junctions, and particularly if the two junctions of BJTs are in the near vicinity what are the interactions are happening between the two junctions current.

Dr. Mandal outlines the goals for the lecture, which include discussing the basic structure of the BJT and the bias conditions used in analog applications. He indicates that the lesson will begin with standard p-n junction current equations and progressively examine the interactions between the two junctions that make up a BJT. This progression from isolated junctions to examining the relationship between them is key to understanding how BJTs operate under different conditions.

Picture building a sandcastle at the beach—first, you need to create a solid base (the structure) before decorating it with towers (the biasing conditions). Similarly, we need to understand the underlying structure of the BJT before diving into more complex behaviors and interactions.

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.

Dr. Mandal explains the structure of the BJT, which consists of two n-p junctions: one forms the emitter and the other the collector, while the middle section is p-type material known as the base. This structure is crucial because it dictates how the BJT operates, especially in amplifying signals. The emitter region is heavily doped n-type, which facilitates a large flow of charge carriers (electrons), whereas the base is lightly doped to control carrier movement efficiently.

Consider the BJT structure like a sandwich: the bread on the outside (the n-regions) is thicker (heavily doped) to hold everything together (like the emitter and collector), while the filling (the p-region or base) is thinner. The more balanced the ingredients, the better the sandwich stays together, similar to how the doping levels affect BJT functionality.

So, 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-J1 it will be forward biased by a voltage called base to emitter voltage.

Dr. Mandal explains that in typical operation, the base-emitter junction of the BJT must be forward biased. This means the base (p-region) is connected to a positive voltage relative to the emitter (n-region). Forward biasing reduces the barrier for charge carriers, allowing current to flow easily into the emitter, facilitating amplification of signals.

Imagine a water tank with a valve. If the valve is slightly open (forward bias), water will flow through easily (current flow). If the valve is completely shut (reverse bias), hardly any water can escape. Thus, forward biasing behaves like allowing water in while controlling its flow.

So, on the other hand, base to collector junction again for normal operation, so this junction-J2, it is reverse bias which means that this n-region it is having higher potential than the p-region.

In contrast to the forward-biased base-emitter junction, the base-collector junction (junction J2) operates under reverse bias during normal operation. This condition raises the potential of the collector n-region above that of the base p-region, which helps in collecting the charge carriers that have diffused into the base, thus allowing for efficient transistor action.

Think of the collector-base junction like a drain in a sink: when the water (current) reaches the base, the drain (collector) needs to be at a lower height to quickly pull the water away from the base area, preventing overflow (current buildup).

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.

The behavior of current flow through the forward-biased junction shows an exponential dependence on voltage. This characteristic behavior is defined by the diode equation, where the current increases sharply as the forward bias voltage rises. It emphasizes the non-linear nature of BJTs where small changes in voltage can lead to significant changes in current.

Consider turning on a tap very slightly: at first, only a trickle of water flows out (low current), but as you turn the tap more and more, a deluge gushes out (high current). The exponential relationship is similar—small increments in voltage lead to large changes in current.

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