Communication and Device Technology

Introducing cutting-edge technologies that realize new services and societies

Overview of transistor technology and NTT's R&D on the world's fastest transistor

What is a transistor?

A transistor is a component that controls current and voltage in many electronic circuits. It has two functions which are amplification of electric signals and switching them.

Although it is difficult for us to see transistors directly because it is very small and hidden in packages of appliances, but they play a very important role to support and enrich our lives. In fact, transistors are used in almost every electrical appliance around us, including smartphones, personal computers, and televisions, and have a significant impact on their performances. Therefore, transistor technologies are being improved every day to achieve higher performance (higher operating speed, lower power consumption, smaller size, etc). Recently, there are cases in which more than 10 billion transistors are integrated in one chip and operated at over a few gigahertz (GHz).

If we look at the structure of a transistor, it consists of p-type semiconductors, which have positively charged electricity (holes) moves through them, and n-type semiconductors, which have negatively charged electricity (electrons) moves through them. A transistor with an n-type semiconductor sandwiched between p-type semiconductors is called a PNP transistor, while a transistor with a p-type semiconductor sandwiched between n-type semiconductors is called an NPN transistor. The semiconductor center layer is called a base, while the other two outer layers are called a emitter and collector. In PNP transistors, holes injected side (the layer where current is input to the transistor) is called the emitter, and the other side (where the current is output from the transistor) is called the collector.

In a transistor, a base current controls a collector current. For example, when the base current flows from the base to the emitter, the transistor becomes ON state (and the collector current flows from the emitter to the collector). In contrast, when the base current does not flow, the transistor becomes OFF state (and the collector current does not flow). Because the collector current drastically changes by a small change of base current (= input current), transistors can also amplify current signal. Using these functions of transistors, complex calculation and various useful functions can be realized by integrating transistors and connecting them with electrical wiring.

Strictly speaking, transistors with the three-layer structure described above are called bipolar transistors because both positive and negative electric signals (holes and electrons) flow through them. Because a bipolar transistor is the first invented transistor and first puts into practical use, it is sometimes referred to simply as a “transistor”. In contrast, field-effect transistors (FET) that compose complementary metal oxide semiconductor (CMOS) integrated circuits, which are mainstream for present circuits, have only either negative electricity (electrons) or positive electricity (holes) flowing through them, so they are classified as unipolar transistors.

Issues in transistor technology

Fig. 1. Types and characteristics of transistors and limits on operating speed
Fig. 1. Types and characteristics of transistors and limits on operating speed

In the future, smart society is expected by using internet of things (IoT) and artificial intelligence (AI) widely. However, it is concern that the network capacity will be insufficient because the data volume will explosively increase when large amount of electronic devices are connected via networks. To solve this issue, one of the key technologies is improving transistor performance.

When switching speed of transistors becomes faster, integrated circuits can process more information per unit time. Therefore, it is possible to increase the capacity of optical/wireless communication networks by replacing integrated circuits with faster transistors.

However, current gain cutoff frequency (fT), which is the figure-of-merit of switching speed for transistors, is saturated trend in the 700 GHz range for more than a decade, regardless of the type of transistors or their materials. This is a serious problem for realizing future large-capacity communication networks and terahertz (THz) band applications. In addition, to boost performance of integrated circuits, it is also necessary to ensure not only high-speed performance of transistors, but also high performance of current gain and breakdown voltage.

The history of improvements to transistors and NTT’s research findings

Transistors were initially invented using germanium (Ge), but silicon (Si) was subsequently used to improve performance of circuits. Silicon Valley is famous for a place where IT companies gather, and its name comes from the fact that many semiconductor manufacturers gathered there.

After that, research of compound semiconductors, which consist of two or more elements, has become active. It is because compound semiconductors have many advantages of improving the performance of transistors compared to silicon. Now, various type of transistors using compound semiconductors such as indium phosphide (InP) or gallium nitride (GaN) have been developed for high-speed communication networks to achieve high-speed switching and high output power superior to silicon-based transistors.

For over 20 years, NTT has developed InP-based transistors because of their potential of high-speed performance suitable for optical fiber communication systems. To improve transistor performance, we need progress of both crystal growth technology for making high-quality semiconductor materials and device process technology for fabricating fine transistor structures. NTT has developed crystal growth technology that forms electron acceleration structure in an extremely thin 10-nm-thick semiconductor layer (1 nm = 1/1,000,000 mm), and device process technology that forms extremely narrow electrode structures with a width of about 50 nm (about 1/2,000 of the diameter of a hair) in InP-based heterojunction bipolar transistor (HBT) structures. By combining these technologies, we have demonstrated the world’s fastest transistors with a current gain cutoff frequency of over 800 GHz.

  1. One of bipolar transistors which consists of different semiconductors.

What we can realize through this research

This transistor will enable ultra-high-speed integrated circuits operating at over 300 GHz (>300 billion operation per second). This is expected to boost information communication systems, such as multi-terabit per second (>Tbps) class optical transmission and high capacity wireless communication networks using the terahertz (THz) band. 5G wireless communication commercialized in 2020, using the 3.5 GHz band, 4.5 GHz band, and 28 GHz band, provides downlink speeds of over 1 Gbps as measured on mobile devices. Even 5G is expected ultra-high speed and ultra-low latency suitable for novel services such as automatic driving and remote surgery. However, future research based on our ultra-high speed InP-based transistor technology has great potential to realize even faster communication speeds far exceeding 5G.

NTT will continue to realize a prosperous, sustainable and secure society through developing IOWN, Beyond 5G and innovative services by utilizing characteristics of the unused THz band. We will also continue research and development novel information and communication services by further improving the speed of transistors and integrating state-the-art optical devices.

References

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