The new property allows extracting semiconductor characteristics in greater detail, which will give a considerable boost to the development and characterization of new semiconductor materials.
Over the course of decades, researchers have experimented on semiconductor materials and semiconductor devices to fully understand their physical characteristics. By determining the physical limits of a material or device, the industry learns what performance improvements are possible using any given material, and can plan product roadmaps accordingly. It's a truism that most such experiments reveal limitations, but in a new study in journal Nature by IBM-led collaboration, the results were the opposite; the research partners discovered a 140-year-old secret associated with the Hall effect, a previously unknown property that is expected to open up entirely new avenues for improving semiconductor performance.
The Hall effect
First, let's review the Hall effect. The fundamental properties of the charge carriers (positive or negative) inside a semiconductor material are their speed under an applied electric field and how densely they are packed in the material. In 1879, physicist Edwin Hall found a way to determine these properties. He discovered that a magnetic field diverted the movement of electronic charges within a conductor and that the amount of deflection can be measured as a voltage known as Hall voltage. This voltage combined with standard conductivity measurement, provides information on the charge type, their mobility (μ) and density inside the semiconductor.
Figure 1: Layout of the Hall effect. A n-type semiconductor with an applied current I and a magnetic field B perpendicular to the surface (z-axis). The difference in potential VH generated is known as the Hall effect
The Hall effect refers to the measurable voltage through a conductor (or semiconductor), when a magnetic field influences an electric current flowing through it. A transverse voltage is generated, which is perpendicular to the applied current due to the balancing of the Lorentz force and the electric one. This physic effect finds application in many solutions, particularly in the field of modern appliances and automotive applications. The implementation of Hall sensors improves reliability and durability, eliminating the mechanical wear of the movements (figures 1). Figure 1 shows what happens to the charges moving through a semiconductor in a magnetic field. The Hall voltage (VH) is perpendicular to the current flow direction:
...where H is the Hall coefficient, which is negative if the majority carriers are electrons and positive if the majority carriers are holes. I is the current, Bz is the perpendicular magnetic field and w is the thickness of the sample. If there are two carriers present, the Hall coefficient is given as:
...where n is the concentration of electrons, p the concentration of holes, μN the mobility of electrons, and μP the mobility of the holes, q the charge of the electron. The properties of both majority and minority carriers such as type, density, and mobility are fundamental and govern the performance of semiconductor devices. Obtaining this information simultaneously under the light would unlock many critical parameters which are vital to optoelectronic devices and solar cells, but this goal has not been clearly achieved yet.
The progress of physics
The researchers from IBM, KAIST (Korea Advanced Institute of Science and Technology), KRICT (Korea Research Institute of Chemical Technology) and Duke University were able to extract these properties by exploiting the Hall effect with light as a test source, simultaneously obtaining information on the majority and minority carriers such as their density and mobility, carrier lifetimes and the diffusion lengths. Practical applications include new and faster characterization for semiconductors, better-performing optoelectronics, and new materials and devices for artificial intelligence technology.
“The advances that we made rest on a new insight in the Hall effect with light which can be summarized in a succinct equation: Δμ=d(σ2H)/dσ, which is unknown for 140 years since the discovery of the Hall effect in 1879. The formula tells you new information of the mobility difference of holes and electrons. It helps us solve a long-standing problem of how to extract the hole and electron carrier information simultaneously in a semiconductor. I think this an exciting advance as we can now study semiconductor material in greater details,” said Oki Gunawan from IBM, the lead author of the paper.
In this experiment, both carriers contribute to changes in conductivity (σ) and Hall coefficient (H, which is proportional to the ratio between the Hall voltage and the magnetic field). The key intuition comes from the measurement of the conductivity and the Hall coefficient as a function of light intensity and then analyzing the problem by looking into the σ-H plot (figure 2) to extract various parameters using the new formula.
The team called the new technique, carrier-resolved photo-Hall (CRPH) measurement. The technique requires a clean measurement of the Hall signal. For this purpose, it is necessary to perform the Hall measurement with an oscillating magnetic field (AC). In this way, it’s important to extract the signal that has the same phase as the oscillating magnetic field using a technique called lock-in detection.
Taking advantage of a previous IBM Research study, it is possible to obtain strong oscillations of the unidirectional and pure harmonic magnetic field. This study is related to a new effect of confinement of the magnetic field, nicknamed the "camelback" effect, which occurs between two lines of transverse dipoles when they exceed a critical length (figures 2 and 3).
Figure 2: (a) Hall effect (b) Carrier-resolved photo-Hall effect (CRPH). (c) The CRPH analysis. The key insight comes from measuring the conductivity and Hall coefficient as a function of light intensity. Hidden in the trajectory of the conductivity- Hall coefficient (σ-H) curve, reveals a crucial new information: the difference in mobility of both carriers. (Courtesy of Oki Gunawan/IBM Research)
Figure 3: (a) The camelback field confinement effect. (b) The parallel dipole line (PDL) trap system. (c) The IBM PDL Hall system. (Courtesy of Oki Gunawan/IBM Research)
“The traditional way people perform Hall effect, they apply a static magnetic field using huge coil, called a Helmholtz coil. It is not as efficient for AC field generation because it is a huge inductor. In this experiment, we used a novel system to generate ac magnetic field which is based on a magnetic trap system called parallel dipole line (PDL), that exhibits a new type of field confinement effect called “camelback effect” as shown in Fig. 3(a)&(b). When you rotate the PDL system it serves as an ideal system to generate ac field magnetic field for our photo-Hall experiment because the field is unidirectional, pure harmonic and there is plenty of space to shine light (Fig. 3c),” said Gunawan.
The new technique suggested by the IBM collaboration allows extracting a surprising amount of information from semiconductors. Contrary to the only few (three) parameters obtained in the classical Hall measurement, this new technique allows measuring other parameters of both electrons and holes at various light intensity levels such as the mobility, diffusion length, density, and recombination lifetime. The main goal of this experiment is to measure the Hall signal under an oscillating magnetic field at a constant rate for different light intensities.
“Typically, we use one rotation per minute, which is actually quite slow because if you spin the magnets too fast, there could be additional parasitic effect like the Faraday emf voltage, that could compete with the desired Hall effect. The true photo-Hall signal is the signal with the same frequency and phase as the oscillating magnetic field. So, if you do this experiment with dc (static) magnetic field, your desired Hall signal will get buried. So, we believe this is also another reason why people couldn't solve this problem for more than a hundred years because you really have to do this with ac magnetic field to get clean experimental data," according to Gunawan.
This new discovery and technology will help push the progress of semiconductors forward thanks to the knowledge and tools that allow extracting the physical characteristics of semiconductor materials in great detail. The Hall technology has replaced many traditional surveying techniques in various applications, including level measurement and motor control. Several techniques are available to determine the position: for example, if the application requires a limited and discrete position, simple switches such as the Allegro A1120 or A321x can be used. Figure 4 shows a possible circuit for detecting the breakage of a belt, which operates using a fixed magnet to a rotating drum and a stationary Hall switch.
Figure 4: circuit application using a Hall sensor
The current consumption of the motor is directly proportional to the motor torque exerted. Therefore, a typical method of controlling the speed and force applied to a motor is the measurement of the current consumption in a microprocessor. The microprocessor can then calculate whether current must be applied to the motor to reach the desired speed. Hall effect current sensors can be placed directly in series with the motor because they have a very low resistance. The automotive sector has stood out as a leader in the global magnetic field sensor market, accounting for over 40% of market share.