NaMLab achieves a breakthrough.
Scientist at the Nanoelectronic Materials Laboratory (NaMLab) have demonstrated the world’s first complementary electronical circuit built without dopants. Based on silicon nanowires, a novel type of transistor that can be flexibly tuned as n- or p- type has been developed. First inverter circuits with complementary technology have been fabricated from these nanoscale building blocks and demonstrated functionality.
Most of today’s digital circuits are based on complementary logic, where p- and n- type transistors are combined to reduce standby-power consumption. The device polarity, i.e. its p- and n- type properties has been adjusted by doping silicon with other elements. However, as the device dimensions are being scaled down, accurate and reliable control of the dopant distribution required for future generations is a technological challenge. As an alternative approach to doping, scientists at NaMLab have devised a novel nanowire transistor which can be operated either as p- or n-type, explicitly avoiding the use of dopants. In the group of Dr. Walter M. Weber at NaMLab gGmbH an operating inverter could be demonstrated by the combination of two of such transistors.
The application in mind is not a replacement of CMOS devices but rather the implementation in low power flexible electronics with small circuit complexity. The temperatures used for processing can all be kept below 400°C, principally enabling processing on temperature sensitive substrates. The working principle is highly facilitated by the small size of the contacts. However, the devices and circuits demonstrated are principally not limited to the case of nanowires.
A single gate stack provides electrical control of the transistor for gate lengths as short as 32 nm. Extensive electrical characterization revealed the underlying transport mechanism: The conductance is primarily tuned by the injection of charge carriers through the Schottky contacts, the gate field steers this injection by locally changing the effective Schottky barrier heights.
Contrary to the design of classical FETs, two independent gate electrodes were used, instead of one. Top gate structures were aligned to independently tune the individual Shottky barriers. These novel devices enable a flexible configuration: while one energy barrier is used to block one type of charge carriers the other one can tune the injection of the other type of carriers. Accordingly, these transistors provided unipolar p- and n- type transport. These transistors were used to build a nanowire based inverter. The circuit was built from two Silicon nanowires that share one top gate contact the inverters input. Digital inversion operation was proven with this technology.
The work for the first doping free inverter circuit was part of the NODE project funded by the EU-Commission. The NODE project focuses on an innovative bottom-up approach to fabrication and integration of nanoelectronic devices, based on self-assembling semiconductor nanowires. The primary target is to deliver replacement and add-on technologies to silicon CMOS, such as FET devices for logics and III-V bipolar transistors for RF applications.
