We already know that the diodes which are simple diodes are made up from two pieces of semiconductor material to form a simple pn-junction here, and we will also learn about their characteristics and properties.
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Joining the individuals together which are of two signal diodes back-to-back. This will result in providing us with two PN-junctions which are connected in series that are shared with a common N and P terminal. The diodes which are two in number fusion produce a three layer of the two junction and a three-terminal device which forms the basis of a Bipolar Junction Transistor or we can say BJT for short.
The ability or we can say that the capacity of the transistor’s to change between these two states that are n and p enables it to have two basic functions: that is “switching” to digital electronics or the “amplification” that is analogue in electronics.
There are two types of basics of bipolar transistor construction. The one is PNP and the other one is the NPN, which describes basically the physical arrangement of the P-type as well as the N-type semiconductor materials from which they are made.
The transistor which is said as the Bipolar Transistor is the basic construction which consists of two PN-junctions which produce three terminals which are connecting and that are each terminal being given a name to identify it from the others. These three terminals are known and labelled as the Emitter that is denoted by letter E , the Base which denoted by letter B and the Collector denoted by letter C respectively.
Bipolar Transistor Construction
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The circuit of the symbols for both the NPN and the PNP bipolar transistor are given above in the diagram with the arrow which is in the circuit symbol. It is always showing the direction of “conventional current flow” that is between the emitter terminal and the base terminal.
Bipolar Transistor Configurations
As a transistor which is the Bipolar Transistor is a three terminal device. There are three types of ways which are possible to connect it within a circuit. Which when is an electronic circuit with one terminal being common to both the output and input. The very Common Base Configuration – which has gain of Voltage but no Current Gain.
The Common configuration of Emitter – which has both voltage and Current Voltage Gain.
The Commons Configuration Collector – has the gain of Current But no Voltage Gain.
The Common Base (CB) Configuration
As the name itself suggests, in the very base of Common or we can call grounded which is base configuration. The base connection is very common to both the output signals and input signal. The signal which is input signal is applied between the transistors base and the emitter terminals. While on the other hand corresponding output signal is taken from between the base and the collector terminals.
The current of the collector output is less than the emitter current input which is resulting in a current gain for this type of circuit of “1” that is said as unity or less on the other hand words and the common base configuration that “attenuates” the input signal.
The Common Base Transistor Circuit
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As the above figure is shown this type of amplifier configuration is a non-inverting voltage amplifier circuit. In that the voltage which is signal voltages that is Vin and Vout are “in-phase”. These types of transistors have an arrangement which is not very common due to its unusually high voltage and the characteristics gain. The input of the whole thing or characteristics represent that of a base which is forward biased diode while the output characteristics represent that of an illuminated diode of photo.
Structure
These are the regions which are respectively of p-type and n-type and p type transistor in a PNP transistor and a n type as well as p type and n type in an NPN transistor. Each region of the semiconductor is connected to a terminal which is appropriately labeled as emitter which is denoted by E, base denoted by B and collector denoted by C.
The base is located physically between the collector and the emitter. And is made from lightly doped which is material of high-resistivity. Resulting in making the resulting value of α which is very close to unity. And it is also giving the transistor a large β. The junction which is bipolar junction transistor is unlike other transistors and is usually not a device which is symmetrical.
FAQs on Bipolar Junction Transistor
1. What is a Bipolar Junction Transistor (BJT)?
A Bipolar Junction Transistor, or BJT, is a three-terminal semiconductor device that functions as either an amplifier or a switch in electronic circuits. It is a current-controlled device, meaning a small current at its input (base) controls a much larger current at its output (collector). It consists of two P-N junctions formed back-to-back, and it is called 'bipolar' because its operation involves two types of charge carriers: electrons and holes.
2. What are the three terminals of a BJT and their specific roles?
A BJT has three terminals, each with a distinct function based on its size and doping concentration:
- Emitter (E): This region is heavily doped and its primary role is to inject or 'emit' a large number of majority charge carriers into the base.
- Base (B): This is the central region, which is very thin and lightly doped. Its function is to control the flow of charge carriers from the emitter to the collector.
- Collector (C): This region is the largest in size and is moderately doped. Its purpose is to 'collect' the charge carriers that pass through the base.
3. What are the two main types of Bipolar Junction Transistors?
The two primary types of BJTs are distinguished by their semiconductor layer arrangement:
- NPN Transistor: This type consists of a thin layer of P-type semiconductor sandwiched between two layers of N-type semiconductor. In an NPN transistor, the majority charge carriers are electrons.
- PNP Transistor: This type consists of a thin layer of N-type semiconductor sandwiched between two layers of P-type semiconductor. In a PNP transistor, the majority charge carriers are holes.
Due to the higher mobility of electrons, NPN transistors are generally faster and more commonly used in high-frequency applications.
4. How does a BJT work as an amplifier?
A BJT amplifies a signal by using a small input current to control a much larger output current. For amplification, the transistor is operated in the active region. This is achieved by forward-biasing the emitter-base junction and reverse-biasing the collector-base junction. A small change in the input signal voltage causes a small change in the base current (IB). This small base current allows a proportionally much larger collector current (IC) to flow. The ratio of the change in collector current to the change in base current is known as the current gain (β). This gain results in a magnified version of the input signal at the output.
5. What is the fundamental difference between a Bipolar Junction Transistor (BJT) and a Field-Effect Transistor (FET)?
The most fundamental difference lies in how they are controlled. A BJT is a current-controlled device, where the output collector current is controlled by the input base current. In contrast, a FET is a voltage-controlled device, where the output drain current is controlled by the voltage applied to its gate terminal. Consequently, BJTs have a relatively low input impedance, while FETs have a very high input impedance.
6. What are the two primary functions of a BJT in electronic circuits?
A Bipolar Junction Transistor is primarily used for two main functions:
- Signal Amplification: It can take a weak input signal (like an audio signal) and produce a much stronger, amplified version of it at the output.
- Electronic Switching: It can act as a high-speed switch. By controlling the base current, the transistor can be turned completely ON (saturation region) or OFF (cut-off region), allowing or blocking the flow of current in a circuit.
7. Why is the base region of a BJT made very thin and lightly doped?
The structural design of the base is critical for transistor action. It is made thin and lightly doped to ensure that most of the charge carriers injected from the emitter travel across to the collector with minimal recombination in the base. A thin base reduces the transit time for carriers, and light doping means there are fewer majority carriers in the base to recombine with. This design maximises the collector current and is essential for achieving a high current gain (β).
8. Can a BJT be constructed by simply joining two P-N diodes back-to-back? Why or why not?
No, a functional transistor cannot be made by joining two separate diodes. A BJT is manufactured from a single, continuous piece of semiconductor crystal with three distinct doped regions. The key to its operation is the extremely thin base region that allows charge carriers from the emitter to diffuse across it and be collected by the collector. Joining two diodes creates two separate depletion regions with a physical and electrical break in between, which prevents this crucial transistor action from occurring.
