Researchers devised and tested a highly sensitive method of detecting and counting defects in transistors — a matter of urgent concern to the semiconductor industry as it develops new materials for next-generation devices. These defects limit transistor and circuit performance and can affect product reliability.
A typical transistor is, for most uses, basically a switch. When it’s on, current flows from one side of a semiconductor to the other; switching it off stops the current. Those actions respectively create the binary 1s and 0s of digital information.
Transistor performance critically depends on how reliably a designated amount of current will flow. Defects in the transistor material, such as unwanted “impurity” regions or broken chemical bonds, interrupt and destabilize the flow. These defects can manifest themselves immediately or over time while the device is operating.
Over many years, scientists have found numerous ways to classify and minimize those effects. But defects become harder to identify as transistor dimensions become almost unimaginably small and switching speeds very high. For some promising semiconductor materials in development — such as silicon carbide (SiC) instead of silicon (Si) alone for novel high-energy, high-temperature devices — there has been no simple and straightforward way to characterize defects in detail.
The new method works with both traditional Si and SiC, allowing researchers to identify not only the type of defect, but also the number of them in a given space with a DC measurement. The research focuses on interactions between the two kinds of electrical charge carriers in a transistor: negatively charged electrons and positively charged “holes,” which are spaces where an electron is missing from the local atomic structure.
When a transistor is functioning correctly, a specific electron current flows along the desired path. If the current encounters a defect, electrons are trapped or displaced and can then combine with holes to form an electrically neutral area in a process known as recombination. Each recombination removes an electron from the current. Multiple defects cause current losses that lead to malfunction. The goal is to determine where the defects are, their specific effects, and — ideally — the number of them.
In the new work, the researchers concentrated on one region that is typically only about 1 billionth of a meter thick and a millionth of a meter long: the boundary, or channel, between the thin oxide layer and the bulk semiconductor body. To focus exclusively on activity in the channel, researchers use a technique called bipolar amplification effect (BAE), which is achieved by arranging the bias voltages applied to the source, gate, and drain in a particular configuration.
The exact mechanism by which BAE operates was not known until the team developed its model. Before the model of BAE, the scheme was used strictly as a resource for applying voltages and controlling currents for EDMR measurements, which is useful for a more qualitative defect identification. The new model enables BAE as a tool to quantitatively measure the number of defects and to do so with just currents and voltages.
The model, which the researchers tested in a set of proof-of-concept experiments on metal oxide semiconductor transistors, makes quantitative measurements possible. The technique can provide insight into the presence of destabilizing transistor defects and a path to mechanistic understanding of their formation. With such knowledge, there would be greater opportunity to control and reduce them in order to improve transistor performance and reliability.