The hot-carrier effect is a reliability issue that occurs when hot (energetic) carriers damage the Si-SiO2 interface and/or trap oxide. This causes the transistor’s current drive capability to deteriorate, eventually resulting in circuit failure. Hot-carrier effects occur when unwanted electron energy accumulates in specific regions of transistors, causing devices to perform unreliably.
A University of Surrey undergraduate student has discovered a method to suppress hot-carrier effects, which have plagued devices that use thin-film transistor architecture, such as smartwatches and solar panels.
Lea Motte studied a new device, the multimodal transistor, as an alternative to conventional thin-film transistors in her final-year project at Surrey, invented and developed by Ph.D. candidate Eva Bestelink and supervisor Dr. Radu Sporea.
An undergraduate student has discovered a way to suppress hot-carrier effects that have plagued devices that use thin-film transistor architecture – such as smartwatches and solar panels.
Lea took advantage of a distinguishing feature of multimodal transistors: the separation of controls for introducing electrons into the device and allowing them to move across the transistor. Lea discovered through computer simulations that applying the correct voltage to the transport control region can prevent unwanted hot-carrier effects. Furthermore, it ensures that the current flowing through the transistor remains constant across a wide range of operating conditions.
The term ‘hot carriers’ refers to holes or electrons (also known as ‘hot electrons’) that have gained extremely high kinetic energy after being accelerated by a strong electric field in areas of high field intensities within a semiconductor (particularly MOS) device. Hot carriers, due to their high kinetic energy, can be injected and trapped in areas of the device where they should not be, forming a space charge that causes the device to degrade or become unstable. As a result, the term “hot carrier effects” refers to device degradation or instability caused by hot carrier injection.
When carriers (electrons or holes) gain a large amount of kinetic energy as a result of the presence of a strong electric field within a semiconductor device. Hot electrons are more likely than hot holes because they are more mobile to begin with. Hot carriers are injected/trapped in specific areas, causing undesirable device behavior and/or degradation, resulting in Hot Carrier Effects.
Ph.D. student Eva Bestelink systematically studies Lea’s discovery of the unusual behavior in multimodal transistors in a paper published in the journal Advanced Electronic Materials, confirming it with measurements in microcrystalline silicon transistors and performing extensive device simulations to understand the device physics that underpins its unique ability.
This discovery implies that future multimodal transistor-based technologies may be more power-efficient, and it may lead to high-performance amplifiers, which are critical for measuring signals from environmental and biological sensors.
The high drain-effect transistor characteristic observed after hot-carrier injection and trapping in the oxide was discovered to be caused by an uneven trapped-carrier distribution near the drain, which causes the threshold voltage to vary as a function of drain voltage.
“We now have a better understanding of what the multimodal transistor can offer when made with materials that cause numerous challenges to regular devices,” said Eva Bestelink, lead author of the study from the University of Surrey.
“This work provides circuit designers with insight into how to operate the device for maximum performance. Long term, the multimodal transistor provides an alternative for emerging high-performance materials where traditional solutions are no longer feasible.”
