"From Vacuum Tubes to FinFETs" : The Evolution of Transistor Technology

 The evolution from Bipolar Junction Transistors (BJT) to Fin Field-Effect Transistors (FinFET) is a significant milestone in the field of semiconductor technology. The evolution of Transistor technology has been a fascinating journey that has revolutionized the world of electronics. Transistors replaced bulky and unreliable vacuum tubes, leading to the miniaturization of electronic devices and paving the way for the digital age. However, as the demand for smaller and more efficient devices increased, new transistor technologies had to be developed to meet these requirements. One such technology is the FinFET, which has become the standard in modern integrated circuits. In this article, we will explore the evolution of transistor technology from vacuum tubes to FinFETs, highlighting the advantages and disadvantages of each technology and their impact on the world of electronics. 

Vacuum Tube

What is a Point Contact Device?

• Prior to the development of bipolar junction transistors, vacuum tubes, which were very expensive, were employed in electronic circuits. At the time, vacuum tubes were also available in the form of triodes, which were three-terminal devices similar to transistors.
• The hype surrounding vacuum tube triodes persisted for nearly 50 years, but they took up a lot of space, weren't as reliable when used, and added complexity to the circuit simply by having more of them. These complications included issues with current, voltage, and other variables.In order to run and control circuits, scientists first devised means to manage the rogue behaviour of the electrons inside vacuum tubes.
• A crude two-point contact device was finally created in 1947 thanks to the work of John Walter and Bardeen. It was far from the contemporary bipolar junction transistor, but it provided the groundwork for its creation at a time when everything was vacuum! Following this not-so-famous endeavour, William Shockley succeeded in creating a bipolar junction transistor by pressing semiconductor material wafers together.And guess what? Bardeen, John Walter, and William Shockley received the Noble Prize in 1956 for their contributions.Bipolar junction transistors brought about an unimaginable transformation in the electronics industry.
• Bipolar junction transistors were produced separately as distinct parts and individual devices up to the closing decades of the 19th century, but later, with the development of integrated circuits, the world experienced another electronic revolution.

Point Contact Diode

Problems of the point-contact transistor

• The point-contact transistor is a surface-effect device, i. e. important effects occur at the semiconductor surface. Surfaces are easily contaminated. Surface-effect devices are unstable (e. g. mechanical vibrations).
• Remedy: William Shockley invented the pn-junction transistor. The physically relevant region is moved to the bulk of the material. The p-n junction transistor is a bulk device. Shockley, Bardeen, and Brattain were awarded the Nobel Prize in 1956.

Bipolar Junction Transistor

BJT, which is a type of transistor, was the dominant technology until the early 2000s. It is a three-layer device consisting of two doped semiconductor regions (p-type and n-type) separated by a thin layer of the opposite type of semiconductor. When a voltage is applied across the device, current flows through the device, making it useful for amplification and switching applications. However, as the size of BJT transistors decreased, it became increasingly difficult to control their behaviour due to the limited control over the carrier flow. Moreover, as the number of transistors on a chip increased, the power consumption increased, and the device became less efficient. 

Working Principle of BJT

• A Bipolar Junction Transistor (BJT) is a type of transistor that uses both electrons and holes as charge carriers to amplify current. It consists of three layers of semiconductor material: a P-type layer sandwiched between two N-type layers, or an N-type layer sandwiched between two P-type layers. 

• The three regions of the BJT are called the emitter, the base, and the collector. The base is lightly doped compared to the emitter and collector, and it is located between the emitter and collector. When a voltage is applied across the base-emitter junction, it creates a forward-biased junction, allowing electrons to flow from the emitter to the base. 

• The emitter supplies electrons to the base, which then forms a thin layer of negative charge carriers near the base-collector junction. This layer is called the depletion region, which acts as a barrier to the flow of electrons from the base to the collector. 

• However, when a voltage is applied across the base-collector junction, it creates a reverse-biased junction, which widens the depletion region and reduces the current flow. The collector current is controlled by the amount of current flowing into the base, which is determined by the voltage applied to the base-emitter junction. 

• The BJT operates in two modes: the active mode and the saturation mode. In the active mode, the base-emitter junction is forward-biased, allowing a significant amount of current to flow through the transistor. In the saturation mode, the base-emitter junction is fully forward-biased, allowing maximum current to flow through the transistor. 

Overall, the BJT is a versatile and widely used transistor technology, and its operation can be further enhanced by the addition of resistors and capacitors to the circuit.

Here are some advantages and disadvantages of BJT:

Advantages of BJT:

• High gain: BJT can provide high voltage gain with low power input, making it useful in amplification applications.

• Low input resistance: BJT has low input resistance, which makes it easier to drive the device with a low-power signal source.

• Simple construction: BJT has a simple structure and can be easily manufactured, making it an affordable option for many applications.

• Fast switching: BJT can switch quickly between the on and off states, making it suitable for switching applications.

• Wide operating temperature range: BJT can operate over a wide range of temperatures, making it suitable for use in harsh environments.

Disadvantages of BJT:

• High power consumption: BJT can consume a significant amount of power, making it unsuitable for low-power applications.

• Limited frequency response: BJT has a limited frequency response, which can make it unsuitable for high-frequency applications.

• Temperature sensitivity: The performance of BJT is sensitive to temperature changes, which can lead to changes in the device's characteristics over time.

• Large size: BJT can be relatively large in size, which can limit the device's usefulness in applications where space is limited.

• Noise: BJT can generate noise that can interfere with the signal being amplified or switched.

In summary, BJT can provide high gain and fast switching, but it consumes a significant amount of power, has limited frequency response, and can be sensitive to temperature changes.This led to the development of MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) technology. MOSFETs use a metal gate separated from the semiconductor by an insulating layer. They offer better control over the flow of current and consume less power, making them a better option for small transistor sizes.  

Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET)

MOSFETS are widely used in electronic devices as they offer several advantages over other types of transistors. However, like any technology, they also have some drawbacks.

Working Principle of MOSFET

• A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a type of transistor that is commonly used in electronic circuits due to its high input impedance, low power consumption, and fast switching speed. MOSFETs are composed of a gate, source, and drain, with a thin layer of insulating material (usually silicon dioxide) separating the gate from the semiconductor material. 

• When a voltage is applied to the gate, it creates an electric field that controls the flow of charge carriers between the source and drain. MOSFETs operate in two modes: the depletion mode and the enhancement mode. 

• In the depletion mode, the MOSFET is naturally conducting because a channel of charge carriers (electrons for N-channel MOSFETs and holes for P-channel MOSFETs) already exists between the source and drain. The gate voltage controls the width of the channel and the amount of current that flows through it. By applying a negative voltage to the gate in the case of an N-channel MOSFET or a positive voltage in the case of a P-channel MOSFET, the width of the channel can be reduced, causing the MOSFET to turn off.

• In the enhancement mode, the MOSFET does not have a channel between the source and drain when the gate voltage is zero. However, by applying a positive voltage to the gate of an N-channel MOSFET or a negative voltage to the gate of a P-channel MOSFET, an electric field is created that attracts charge carriers to the area beneath the gate, creating a channel between the source and drain. The amount of current that flows through the channel is again controlled by the gate voltage. 

Overall, MOSFETs are highly efficient and reliable components that are widely used in a variety of electronic circuits, including power amplifiers, switching regulators, and microprocessors. The ability to operate in both depletion and enhancement modes makes MOSFETs highly versatile, and they are capable of handling high voltages and currents while consuming very little power. 

Here are some advantages and disadvantages of MOSFET: 

Advantages of MOSFETS:

•       Low power consumption: MOSFETs consume less power compared to other types of transistors, making them ideal for battery-operated devices.

•      High input impedance: MOSFETs have a very high input impedance, which makes them easy to interface with other electronic components.

•       High-frequency response: MOSFETs have a wide frequency response, which makes them suitable for high-frequency applications.

•        High power handling capacity: MOSFETs can handle high levels of power, making them suitable for high-power applications such as power amplifiers.

•      Small size: MOSFETs can be made very small, which makes them ideal for use in compact electronic devices.

 Disadvantages of MOSFETS:

•      High gate capacitance: MOSFETs have a high gate capacitance, which can limit their switching speed and make them unsuitable for some applications.

•       Sensitive to static electricity: MOSFETs can be easily damaged by static electricity, which can be a concern in some manufacturing environments.

•      Cost: MOSFETs can be more expensive than other types of transistors, especially for high-power applications.

•       Temperature sensitivity: MOSFETs can be sensitive to temperature changes, which can affect their performance.

•      Gate voltage limitations: MOSFETs have a limited range of gate voltage, which can limit their suitability for some applications.

In summary, MOSFETs offer low power consumption, high input impedance, high-frequency response, high power handling capacity, and small size. However, they also have high gate capacitance, can be sensitive to static electricity and temperature changes, can be expensive, and have a limited range of gate voltage. 

For more detailed explanation about MOSFETS refer "MOSFET: THE GATEKEEPER OF THE CIRCUIT WORLD"

Main variations between BJT and MOSFET transistors  

MOSFET Vs BJT

• A MOSFET is a metal oxide semiconductor field-effect transistor, whereas a BJT is a bipolar junction transistor.

• A MOSFET has three terminals: source, drain, and gate, compared to a BJT's three: base, emitter, and collector.

• MOSFET is utilised for high power applications, while BJT is used for low current applications.

• Modern analogue and digital circuits tend to use MOSFETs more frequently than BJTS.

• The current at the base terminal determines how well a BJT operates, whereas the voltage at the oxide-insulated gate electrode determines how well a MOSFET operates.

• The MOSFET is a voltage-controlled device, whereas the BJT is a current-controlled device.

• In the majority of applications, MOSFETs outperform BJTs.

• Compared to BJT, the MOSFET structure is more intricate.

FinFETs 

FinFETs, which are a further evolution of MOSFETs, were developed to address the challenges of scaling down transistor size even further. FinFETs have a fin-shaped channel structure with a gate wrapped around the fin on three sides, which provides better control over the flow of current. This design allows for higher device density, lower power consumption, and improved performance. 

FinFET technology has been widely adopted by semiconductor manufacturers and is used in modern processors, mobile devices, and other electronic devices. It is expected to remain the dominant technology for the foreseeable future, as manufacturers continue to find ways to improve its performance and efficiency. 

Structure Of FinFET 

Advantages of FinFET:

• Improved performance: FinFETs provide better performance than traditional MOSFETs in terms of speed, power consumption, and leakage current.

• Low power consumption: FinFETs consume less power compared to other transistors, making them ideal for battery-operated devices.

• Scalability: FinFETs can be easily scaled down to smaller sizes, making them ideal for use in advanced integrated circuits.

• Reduced leakage current: FinFETs have reduced leakage current compared to planar MOSFETs, which results in improved efficiency.

• High-density integration: FinFETs can be integrated at high density, making them ideal for use in high-performance computing applications.

 Disadvantages of FinFET:

• Complex manufacturing process: The manufacturing process for FinFETs is more complex than for traditional MOSFETs, which can result in higher production costs.

•  Higher manufacturing variability: FinFETs have more variability in their manufacturing process, which can lead to differences in performance between individual devices.

•  Increased design complexity: The design of FinFET-based circuits can be more complex compared to traditional MOSFETs due to their unique structure.

• Limited understanding: FinFETs are a relatively new technology, and there is still much to learn about their behavior and reliability over time.

• Limited availability: FinFETs are not yet widely available and may not be suitable for all applications due to their unique characteristics. 

Difference between MOSFET and FinFET:    

• Drive strength: While the drive strength of a FinFET can be enhanced by inserting more or longer fins, the drive strength of a conventional MOSFET is defined by the channel width.

• Speed:A larger driving current can be the cause of FinFETs' quick switching times. Otherwise, compared to planar MOSFETs, three-dimensional FinFET is a high-speed device.

• Construction: Multi-gate devices are easily made thanks to FinFET technology. Multi-gate construction is challenging in MOSFET planner construction.

•  Leakage current:In switching devices, leakage power is caused by leakage current and leakage voltage. FinFET has a low leakage current, its power consumption is lower than that of MOSFET.

•  Leakage voltage and leakage current are the two main factors that contribute to leakage power in switching devices. Due to its low leakage current, a finFET consumes less energy than a MOSFET.Leakage power: Because of leakage power, the length of the gate significantly affects how much leakage current is produced. Because the gate is wrapped around the drain-source channel and there is no leakage current when the gate is not energised, FinFETs have adequate gate length.

• Scalable: Many transistors can fit onto a single chip. Because it is more scalable per footprint area than MOSFET, FinFET technology is suitable for the fabrication of integrated circuits.

• Applications: Planar MOSFETs frequently employ channel doping to enhance short-channel behaviour. FinFETs have dopant-induced fluctuations because of the wrap-around gate over the thin body, which makes channel doping optional. Because of this compactness, the drain and source are closer together and the gate control over the channel carries is decreased. Size: Transistor shrinks as chips get smaller. In MOSFETs, this kind of short channel effect has the potential to be extremely problematic.

• FinFET performs better than planar MOSFET in terms of voltage gain and subthreshold slope.

The Desirable Characteristics of FinFETs 

The three-dimensional structure of FinFETs provides many advantages over MOSFETS.  Some of these merits include:

• A single chip can have a considerable number of transistors. Due to its higher scalability than MOSFETs for a given footprint size, FinFET technology is appropriate for the production of integrated circuits.

• Transistors reduce in size along with chips. This compactness reduces the gate's control over the channel carriers and brings the drain and source closer together. In MOSFETs, this kind of short-channel effect can lead to significant problems. FinFETs have better short-channel behaviour due to the presence of fins. 

• Planar MOSFETs frequently employ channel doping to enhance short-channel behaviour. Channel doping in FinFETs is optional due to the wrap-around gate covering the thin body. Therefore, FinFETs don't have any dopant-induced fluctuations.

• In order to reduce leakage current and, in turn, leakage power, the gate's length is important. In FinFETs, the gate is long enough to wrap around the drain-source channel, preventing leakage current when the gate is not energised. However, as the gate is shrunk in MOSFETs, leakage current occurs. 

• In switching devices, leakage power results from leakage current and voltage. FinFETs consume less electricity than MOSFETs because they have low leakage current devices.

• While the drive strength of a FinFET transistor can be increased by incorporating more or longer fins, the drive strength of a planar MOSFET is dependent on the channel width. 

• Faster switching times in FinFETs can be a result of a larger drive current. In contrast to planar MOSFETs, three-dimensional FinFETs can be regarded to be high-speed electronics. 

• FinFET technology makes multi-gate devices simple to manufacture. Multi-gate fabrication in MOSFETs is laborious due to planar architecture. 

• Compared to planar MOSFETs, finFETs have a better subthreshold slope and a larger voltage gain.  

Conclusion

In summary, FinFETs provide improved performance, reduced power consumption, scalability, reduced leakage current, and high-density integration. However, they also have a complex manufacturing process, higher manufacturing variability, increased design complexity, limited availability, and a limited understanding of their long-term behavior.