For decades, researchers have conducted experiments on semiconductor materials and devices to fully understand their physical properties. By determining the physical limits of a material or device, the industry understands what performance improvements can be achieved using any given material and can plan a product roadmap accordingly. This is a fact that most of these experiments reveal limitations, but in a new study published in the journal Nature, a collaborative organization led by IBM, the results are exactly the opposite; Research partners have discovered a 140 year old secret related to the Hall effect, a previously unknown characteristic that is expected to open up new avenues for improving semiconductor performance.

Firstly, let's review the Hall effect. The basic properties of charge carriers (positive or negative charges) in semiconductor materials are their velocity under an external electric field and their density in the material. In 1879, physicist Edwin Hall found a method to determine these properties. He found that the magnetic field changed the movement of electron charges inside the conductor, and the deflection could be measured by Hall voltage. This voltage, combined with standard conductivity measurements, can provide information on the type of charge, mobility (μ), and density inside the semiconductor. The Hall effect refers to the measurable voltage across a conductor (or semiconductor) when a magnetic field affects the current flowing through it. Due to the balance between Lorentz force and electric force, a transverse voltage perpendicular to the applied current is generated. This physical effect has applications in many solutions, especially in the fields of modern home appliances and automotive applications. The implementation of Hall sensors improves reliability and durability, eliminating mechanical wear during motion (Figure 1). Figure 1 shows what happens through the charge of a semiconductor in a magnetic field. Hall voltage (VH) is perpendicular to the direction of current flow: where H is the Hall coefficient, which is negative if the majority of charge carriers are electrons; If the majority of charge carriers are holes, it is positive. I is the current, Bz is the vertical magnetic field, and d is the thickness of the sample. If there are two carriers, the Hall coefficient is as follows:

Among them, n is the concentration of electrons, p is the concentration of holes, μ n is the mobility of electrons, μ p is the mobility of holes, and q is the charge of electrons. The properties of most and a few charge carriers, such as type, density, and mobility, are fundamental factors determining the performance of semiconductor devices. Simultaneously obtaining this information under illumination will release many key parameters that are crucial for optoelectronic devices and solar cells, but this goal has not yet been clearly achieved.

Progress in Physics

Researchers from IBM, KAIST (Korea Advanced Institute of Science and Technology), KRICT (Korea Institute of Chemical Technology), and Duke University were able to extract these characteristics using the Hall effect of light as a test source, while obtaining information on the density and mobility of most and minority carriers, carrier lifetime, and diffusion length. Practical applications include new and faster semiconductor properties, better optoelectronic performance, as well as new materials and devices for artificial intelligence technology.

Starting from 187, we can summarize a new discovery, namely σ d. This formula tells you new information about the difference in hole and electron mobility. It helps us solve a long-standing problem of how to extract both hole and electron carrier information from semiconductors simultaneously. I think this is an exciting progress because we can now study semiconductor materials in more detail.

In this experiment, both types of charge carriers cause changes in conductivity (σ) and Hall coefficient (H, which is proportional to the ratio of Hall voltage to magnetic field). The key intuition comes from measuring conductivity and Hall coefficient (as a function of light intensity), and then analyzing the problem by viewing the σ - H graph (Figure 2) to extract various parameters using a new formula.

The research team refers to this new technology as Carrier Resolution PhotoHall (CRPH) measurement. This technology requires clean measurement of Hall signals. For this purpose, it is necessary to use an oscillating magnetic field (AC) for Hall measurement. It is important to use a technique called lock-in detection to extract signals with the same phase as the oscillating magnetic field using this method.

By utilizing IBM's previous research results, strong oscillations of unidirectional pure harmonic magnetic fields can be obtained. This study is related to a new effect of magnetic field confinement called the "hump effect", which occurs when two transverse dipole lines exceed the critical length (Figure 2 and Figure 3).

The traditional manifestation of the Hall effect is to apply a static magnetic field using a large coil called the Helmholtz coil. Because it is a huge inductor, its efficiency in generating AC magnetic fields is not high. In this experiment, we used a new system to generate an alternating magnetic field, which is based on a magnetic trap system called Parallel Dipole Line (PDL) and exhibits a novel field confinement effect called the "Hump Effect", as shown in Figures 3 (a) and (b). Gunawan said, "When you rotate the PDL system, it is an ideal system for generating an alternating magnetic field for our light hall experiment, because the magnetic field is unidirectional, purely harmonic, and has enough space to emit light (Figure 3c).

The new technology proposed by IBM collaboration allows for the extraction of astonishing amounts of information from semiconductors. Unlike the few (three) parameters in classical Hall measurements, this new technology allows for the measurement of other parameters of electrons and holes at different levels of light intensity, such as mobility, diffusion length, density, and recombination lifetime. The main purpose of this experiment is to measure Hall signals at different light intensities under a constant rate oscillating magnetic field.

Normally, we rotate once per minute, which is actually quite slow because if you rotate the magnet too fast, it may generate additional parasitic effects, such as Faraday electromotive force voltage, which can counteract the expected Hall effect. The true optical Hall signal is a signal with the same frequency and phase as the oscillating magnetic field. Therefore, if you conduct this experiment with a DC (static) magnetic field, the Hall signal you want will be buried. Therefore, we believe this is another reason why people have not been able to solve this problem for over a hundred years, because you really need to use an AC magnetic field to obtain clean experimental data, "Gunawan said.

This new discovery and technology will contribute to the advancement of semiconductors, thanks to the knowledge and tools that can extract the physical properties of semiconductor materials in detail. Hall technology has replaced many traditional measurement techniques in various applications, including horizontal measurement and motor control. There are several methods to determine position: for example, if an application requires limited and discrete positions, simple switches such as Allegro A1120 or A321x can be used. Figure 4 shows a possible circuit for detecting belt breakage, which operates using fixed magnets and fixed Hall switches.

The current consumption of an electric motor is directly proportional to the torque applied by the motor. Therefore, a typical method of controlling the speed and force applied to the motor is to measure the current consumption in the microprocessor. Then the microprocessor can calculate whether it is necessary to apply current to the motor to achieve the desired speed. Hall effect current sensors can be directly connected in series with motors because their resistance is very low. In the global magnetic field sensor market, the automotive industry has always been in a leading position, accounting for over 40% of the market share.