29-07-2021 | | By Robin Mitchell
Recently, researchers have been able to develop complementary vertical organic transistors that may reach GHz speeds in the future. Why is CMOS logic so crucial to modern electronics, what did the researchers develop, and how can such research help push organic transistors into the commercial market?
When it comes to creating logic circuits, a wide range of technologies can be used. The 1960s saw the use of RTL technology (Resistor-Transistor-Logic) due to its simplicity. The 1970s saw the use of NMOS due to its ease of integration into silicon, and the 1980s saw large scale use of TTL (Transistor-Transistor-Logic) thanks to its high speed.
Complementary Metal Oxide Semiconductor (CMOS) is a digital logic technology that uses two pairs of opposite transistors (an N and P-type) that work together to either provide an output with a connection to VDD or 0V. While CMOS has existed since the late 1960s, it was only in the 80s where its popularity began to increase. The first CMOS devices were slow (operating in the kHz instead of MHz), meaning they were not desirable for computing applications. However, reducing the size of CMOS transistors makes them operate at lower voltages at higher speeds, and this advantage finally overtook NMOS and TTL in the 1980s.
The first significant advantage of CMOS is that outputs are either directly connect to VDD or 0V through fully saturated transistors. Meaning that the resistance between VDD/0V and the output is minimal, resulting in very little power dissipation by the CMOS circuit. The input impedance to CMOS devices is also incredibly high, meaning that CMOS devices consume virtually no input current. The high impedance also means that CMOS devices can drive many other devices in parallel (CMOS has a drive ratio of around 1:50). CMOS is also entirely dependent on transistor arrangements and does not rely on passive components such as resistors or capacitors to function. Thus, a design can mainly consist of transistors miniaturised to extraordinary sizes.
Recently, researchers from the University of Dresden have developed vertical organic complementary organic transistors on a singular substrate. Many organic transistor designs include complementary circuits, but these use 2D planar transistors whose sizes are large and slow. By making the transistors vertical, the researchers have significantly sped up the transistors' operation.
The new transistors have also been shown to operate at 4V, which is an essential step towards reducing operating voltages and increase device speed. While higher voltages present increased noise immunity, lower voltages enable higher-speed devices due to the effects of slew rate (i.e. the maximum voltage change per unit time).
To demonstrate the capabilities of the new devices, the researchers constructed a ring oscillator using multiple inverters. The 11ns rise and fall time of the device shows promise that the organic transistors developed by the team could soon approach the GHz barrier.
Organic transistors have a long way to come, and one may wonder what the point is in developing such technologies when current semiconductor technology is already able to operate at speeds greater than 5GHz for computing alone. While organic electronics will unlikely replace the processor in a desktop PC, they possess one attribute that silicon semiconductors never will; flexibility.
Organic electronics are often based on flexible polymer chains that can undergo stress, strain, and constant manipulation while retaining their electrical properties. Electronics based on such polymers are inherently flexible, making them ideal for movement applications. Of all markets that would benefit from flexible electronics, the wearable industry would by far be the biggest customer.
Wearable electronics face many challenges today due to the dependency on non-flexible electronics. The development of flexible GHz speed transistors would enable devices to integrate into clothing and wearables. It would also enable the development of Wi-Fi and Bluetooth devices (this can only be achieved with transistors that can operate at GHz speeds). Instead of relying on multiple inflexible silicon semiconductors, an entire design could be printed onto a single piece of material, including a processor, display, and wireless modem, fully realising truly wearable electronics.
With the demonstration of CMOS logic in vertical organic transistors, researchers are now closer than ever before to creating practical wearable electronics.