Recently, researchers have been able to develop functional complementary organic transistors and successfully demonstrated a NOT gate. Why is complementary logic so important, what did the researchers develop, and how could it help the field of flexible and wearable electronics?

Why is complementary logic so important?

Semiconductors come in all shapes and sizes, but the development of new semiconductor materials is helping to push electronics into new fields never before thought possible. For example, organic-based semiconductors are generally flexible and can be potentially useful in wearable electronics.

However, simply creating a transistor out of a semiconductor does not make that semiconductor a practical solution if the underlying logic circuits made using that semiconductor consumes large amounts of power.

Of all logic families, complementary logic gates use the least amount of power as they utilise complementary transistors to connect the output of the gate to either the power rail or ground. Other technologies such as NMOS, PMOS, and TTL do not use complementary devices and, as a result, are often power-hungry. This is why the electronics industry has shifted chiefly towards CMOS as the dominant logic technology.

Therefore, if new semiconductor materials cannot produce complementary transistors on the same substrate, their ability to create complex logic circuits is severely limited and highly impractical. Researchers worldwide are racing to achieve this with some results demonstrating complementary devices, but most are still far from practical applications.

Researchers develop vertical organic complementary transistor logic

Recently, researchers at Technische Universitat Dresden, Helmholtz-Zentrum Dresden Rossendorf (HZDR) and Northwestern Polytechnical University have developed complementary vertical organic transistors that can be used to produce low-power logic circuits. Up until recently, vertical organic transistors have not been able to operate with complementary logic. Still, their tunable threshold voltages and shorter channel lengths have made them a potential transistor technology.

The researchers who developed the new design worked from their previous research into dual-gate vertical transistors and connected an OPDBT n-type transistor and a p-type OPBT transistor to create a NOT gate.

Furthermore, the n-type transistor has a secondary input for controlling the threshold voltage, which allows for tuning of the inverter to operate at various voltages. In this case, the researchers demonstrated their devices operating at 4V with a signal propagation delay of 11ns while also proving that their devices could reliably operate on input voltages less than 2V. The researchers also demonstrated their devices operating at 10MHz with rise and fall times of around 6ns making it practical for low-end electronics (i.e. microcontrollers).

How could such electronics help with wearable devices?

The development of complementary logic using organic transistors is important for wearable devices as organic-based electronics are often flexible in nature. This flexibility is essential as current wearable devices are ridged, generally uncomfortable, and genuinely flexible electronics move with the body and skin.

Organic, flexible devices already exist, but there are few examples of functioning logic circuits using complementary logic, meaning that practical, flexible devices cannot currently be manufactured. Wearable devices would also benefit from the low-power consumption of complementary logic, enabling power sources to be reduced in size. The reduction in the size of the power source further improves wearability and may even open up new power sources (such as wearable solar).

Overall, the creation of flexible complementary organic transistors brings the electronics industry one step closer to practical printable electronics that are fully flexible, easily manufactured, and highly mobile.