7 - Overview of Bipolar Junction Transistors (BJTs)

A Bipolar Junction Transistor, or BJT, is a type of electronic device that can control current. It is described as a 'bipolar' device because it uses two types of charge carriers: electrons (negatively charged) and holes (positively charged). This unique feature distinguishes it from other types of transistors that might only use one type of carrier.

BJTs serve mainly two purposes: amplification and switching. Amplification refers to the ability of the BJT to take a small input current and increase it to a larger output current, which is valuable in audio and radio signals. Switching refers to the ability to turn on and off digital signals, which is crucial in logic circuits and computers.

A BJT has three main parts:
1. Emitter (E): This terminal is heavily doped with impurities, which increases the number of charge carriers available. Its job is to send out electrons or holes (depending on the type of transistor).
2. Base (B): This is a very thin and lightly doped section that controls the flow between the emitter and collector. Due to its thin structure, it allows most charge carriers from the emitter to pass on to the collector.
3. Collector (C): This terminal is moderately doped and physically larger. It collects the charge carriers that have moved through the base from the emitter. The size and doping level allow it to handle the maximum current from the circuit.

There are two primary types of BJTs: NPN and PNP, each with distinct characteristics:
- NPN Transistor: In this type, electrons are the majority carriers. The current flows from the collector terminal to the emitter. To operate, a positive voltage must be applied to the base-emitter junction.
- PNP Transistor: Here, holes are the majority carriers. The current flows in the opposite direction, from the emitter to the collector. This type requires a negative voltage at the base-emitter junction relative to the emitter to function effectively.

The BJT operates based on two p-n junctions that act like two diodes stacked back-to-back. Specifically:
1. Emitter-Base Junction: When the voltage is applied in the correct direction (forward biased), it allows current (electrons) to flow from the emitter into the base.
2. Collector-Base Junction: In contrast, this junction is set up with a reverse bias, which means it opposes the flow of current. This arrangement creates a scenario in which, when electrons flow into the base, they predominantly continue on into the collector.
3. The efficiency of this process is key: a small input current at the base can control a much larger current flowing from collector to emitter. Hence, a small signal can effectively control a larger signal.

In a BJT, there are relationships that describe how the currents behave:
- IE (Emitter Current) is the sum of the Collector Current (IC) and Base Current (IB). This shows that whatever current comes into the emitter is divided between the collector and base.
- β (Beta) represents current gain and is calculated as the ratio of collector current (IC) to base current (IB). Typically, this value ranges from 20 to 200, indicating how effectively the BJT can amplify the current.
- α (Alpha) is the ratio of collector current to emitter current and is usually close to 1, showing the efficiency of conversion between these currents.
This mathematical understanding enables engineers to predict how effectively a BJT will operate under various conditions.

BJTs can operate in different modes based on their biasing conditions:
- Active Mode: Here, the emitter-base junction is forward biased, while the collector-base junction is reverse biased. This allows the transistor to amplify signals effectively.
- Cut-off Mode: In this state, both junctions are reverse biased, meaning no current flows through the transistor. The BJT is effectively OFF.
- Saturation Mode: Both the emitter-base and collector-base junctions are forward biased. This means the transistor is fully ON, allowing maximum current flow, functioning as a switch.
- Inverse Active Mode: Less commonly used, where the roles of the terminals are reversed. It is not typically applied in practical circuits due to inefficiencies.

BJTs can be configured in three main ways:
1. Common Base Configuration: This setup has low input impedance and high output impedance, offering high voltage gain without changing the phase of the input signal. It's useful when low input current is needed.
2. Common Emitter Configuration: This is the most popular configuration. It has moderate input and output impedances, delivers high voltage and current gains, but introduces a 180-degree phase shift meaning the output signal is inverted compared to the input.
3. Common Collector Configuration (also known as Emitter Follower): Typically features high input impedance and low output impedance with a gain that is less than one. It provides no phase shift and is used for buffering, ensuring that the output doesn't affect the input signal.

BJTs can operate as switches, functioning effectively based on their modes:
- In the Cut-off Region, the BJT is in the OFF state, meaning no current flows through it. It acts like an open switch.
- In the Saturation Region, the BJT is fully ON, allowing current to flow freely. It mimics a closed switch. This switching ability makes BJTs suitable for various applications, such as digital logic circuits, where they can control signals on and off, and microcontrollers that interface with other components.

BJTs are highly effective as amplifiers when they operate in the Active Region. In this state, a small current at the base can significantly control a larger current through the collector. This characteristic is especially beneficial in applications like audio amplifiers (enhancing sound quality), RF amplifiers (boosting radio signals), and sensor signal conditioning (making weak sensor signals usable).

BJTs come with both pros and cons:
Advantages:
- High gain-bandwidth product means they can amplify signals very effectively across a wide range.
- Simple biasing makes them easier to set up in circuits compared to some alternatives.
- Good linearity means the output signal closely resembles the input, which is crucial for various analog applications.

Disadvantages:
- BJTs consume more power than MOSFETs, making them less efficient for certain applications.
- They have lower input impedance, which can lead to more power loss in some circuits.
- Thermal runaway risk occurs when increased temperature causes more current flow, leading to even more heat and potentially damaging the transistor.

BJTs and MOSFETs are two different types of transistors with unique characteristics:
- Type: BJTs are bipolar, which means they use both electrons and holes. MOSFETs are unipolar and rely on one type of charge carrier, making them generally more efficient.
- Control: BJTs are controlled by current, while MOSFETs are controlled by voltage, leading to differences in their operation.
- Input Impedance: BJTs have low input impedance, which affects their power consumption, whereas MOSFETs have very high input impedance. This trait allows them to use less power.
- Switching Speed: MOSFETs typically switch faster than BJTs, making them better suited for rapid on/off applications.
- Power Consumption: BJTs generally consume more power than MOSFETs, especially under high switching speeds.
- Preferred Use: BJTs are often used in analog circuits, while MOSFETs are more common in digital and integrated circuits.

BJTs find use in a variety of applications due to their versatile and reliable operating characteristics:
- They are employed as low-noise amplifiers in audio equipment to ensure clear sound reproduction.
- BJTs serve as amplifiers in audio and radio circuits, enhancing weak signals so they can be used effectively.
- Historically, they were pivotal in digital logic gates, particularly in TTL (Transistor-Transistor Logic) circuits.
- BJTs are used in switching regulators to efficiently manage power supply conversion.
- They also control larger loads as relay drivers and help illuminate LEDs in various circuits.

In summary, a BJT is a current-controlled device that uses both types of charge carriers (electrons and holes). Its three primary operating regions—active for amplification, cut-off where it is OFF, and saturation where it is fully ON—define its function as a switch or amplifier. The three configurations (Common Emitter, Common Base, and Common Collector) determine how the device is used in different applications, especially with respect to gain and phase shifts. Despite the growing preference for MOSFETs in digital applications, BJTs remain crucial in many analog circuit designs.