Electrodes In Field-effect Transistors Research Articles

Wafer-Scale Growth of Ultrauniform 2D PtSe2 Films with Spatial and Thickness Control through Multi-step Metal Conversion.

Metal conversion processes have been instrumental in advancing semiconductor technology by facilitating the growth of thin-film semiconductors, including metal oxides and sulfides. These processes, widely used in the industry, enhance the semiconductor manufacturing efficiency and scalability, offering convenience, large-area fabrication suitability, and high throughput. Furthermore, their application to emerging two-dimensional (2D) semiconductors shows promise in addressing spatial control and layer number control challenges. In this work, we designed a multi-step metal conversion process for 2D materials to synthesize a high-quality and ultrauniform film. PtSe2 is introduced to utilize its wide-band-gap tunability, which exhibits both semiconductor and metallic properties. Our multi-step-grown PtSe2 film shows extremely low roughness (Ra = 0.107 nm) and improved interlayer quality compared to the single-step PtSe2 film. Additionally, we explored the growth mechanism of the metal conversion process and how the multi-step method contributes to the thickness uniformity of the film. We demonstrated a thin PtSe2 channel field-effect transistor (FET) array with p-type behavior with a maximum on/off ratio ∼103. The FET fabricated by the MoS2 channel with the semimetallic multi-step PtSe2 electrode shows an enhanced performance in mobility and contact resistance compared to the conventional single-step PtSe2 electrode FET.

(Invited) Norfentanyl Detection with Carbon Nanotube-Based Field-Effect Transistors

The opioid crisis, propelled predominantly by synthetic opioids such as illicit fentanyl, constitutes a significant global public health threat. To meet the critical demand for a sensitive and portable analytical tool for detecting fentanyl exposure, we have developed two types of biosensors based on single-walled carbon nanotubes (SWCNTs).The first biosensor is a field-effect transistor (FET) device with semiconductor-enriched SWCNTs, functionalized with norfentanyl antibody.1 This antibody-SWCNT-FET biosensor exhibits ultrasensitive detection of norfentanyl, the primary inactive metabolite of fentanyl, in urine samples. Various sensor configurations were investigated to optimize sensing results. The use of a "reduced" antibody facilitated oriented immobilization, enhancing the proximity of the antigen-antibody interaction to the sensor surface and resulting in improved sensitivity. These norfentanyl biosensors demonstrate remarkable sensitivity, with a limit of detection in the fg/mL range in both calibration and synthetic urine samples, showcasing their potential as highly reliable detection tools.While capture probes like antibodies enhance chemical sensitivity and selectivity, the stability of nanotube-based biosensors is constrained by the activity of these capture probes. To address this limitation, we have explored an alternative approach involving sensor arrays consisting of different metal-organic frameworks (MOFs) on SWCNTs, which act as nonspecific receptors. With the assistance of machine learning-driven discrimination, such sensor arrays hold the potential to generate unique signals for different analytes.In pursuit of this objective, size-based norfentanyl detection was achieved using SWCNT@MOF composites in FET biosensors.2 Four SWCNT@MOF composites were synthesized, with SWCNT@UiO-67 demonstrating the highest sensing response to norfentanyl due to a size-matching effect within the MOF channel. Composites with smaller pore sizes such as SWCNT@UiO-66 and SWCNT@UiO-66-NH2 did not exhibit sensitivity improvement compared to bare SWCNT. We have also demonstrated selectivity of these biosensors for norfentanyl over norhydrocodone and benzoylecgonine, which are major metabolites of hydrocodone and cocaine, respectively.Moreover, we successfully applied our sensor fabrication with a flexible FET electrode and a portable sensing setup, showing significant potential for developing a portable device for on-site detection of fentanyl exposure with improved sensitivity. Therefore, we envision our norfentanyl biosensors can enhance the safety and security of both first responders and overdose victims and provide a platform technology that can contribute to the development of effective tools for addressing the opioid crisis.[1] Shao, W.; Zeng, Z.; Star A., An Ultrasensitive Norfentanyl Sensor Based on a Carbon Nanotube-Based Field-Effect Transistor for the Detection of Fentanyl Exposure. ACS Appl. Mater. Interfaces 2023, 15, 37784–37793.[2] Zeng, Z.; Islamov, M.; He, Y.; Day, B. A.; Rosi, N. L.; Wilmer, C. E.; Star, A., Size-Based Norfentanyl Detection with SWCNT @ UiO-MOF Composites. ACS Appl. Mater. Interfaces 2023, accepted.

Ultrafast switching in spin field-effect transistors based on borophene nanoribbons.

Borophene, owing to the high mobility and long spin coherent length of its carriers, presents significant opportunities in ultrafast spintronics. In this research, we investigate the spin-dependent conductance of a Datta-Das field-effect transistor (FET) based on an armchair β12-borophene nanoribbon (BNR) using the tight-binding (TB) Hamiltonian in combination with the non-equilibrium Green's function (NEGF) method. The spin FET electrodes are magnetized by ferromagnetic (FM) insulators arranged in both parallel and anti-parallel configurations. This device acts as a controllable spin filter in the presence of Rashba spin-orbit coupling (SOC) for both configurations and its spin current is well modulated by a gate voltage and the strength of the Rashba SOC. For anti-parallel configurations, an energy gap emerges within a certain range of incoming electron energy which can disappear for electrons with flipped spin under the Rashba SOC. Furthermore, our findings indicate that the electron-electron (e-e) interaction helps the spin precession of electrons injected into the spin FET channel, thereby strengthening the Rashba SOC effect. Notably, a gate voltage can adjust the current-voltage (I-V) characteristics of this device. Finally, our calculations demonstrate that under the same conditions, the current magnitude and Ion/Ioff ratio of borophene spin FETs are several times higher than those of graphene and silicene spin FETs.

Interfacial electrical properties and transport properties of monolayer black AsP alloy in contact with metal

With strong structural stability, ultra-high carrier mobility and controllable direct band gap of semiconductors, the new two-dimensional (2D) monolayer black arsenic-phosphorus (ML b-AsP) alloy has recently attracted much attention in nanoelectronic devices. Through first-principles electronic structure calculations and quantum transport simulations, the interfacial electrical properties of the ML b-AsP/metal (Ag, Al, Au, Fe, Pd, Pt) contacts and transport properties of the integral field effect transistor (FET) configurations constructed with ML b-AsP alloy are investigated first and systematically. The results of electronic structure calculations show that the ML b-AsP alloy is strongly adsorbed to metal substrates and has undergone metallization, which leads to a weak Schottky contact at all the AsP/metal interfaces. Whereas quantum transport simulations show that, with the exception of Pt electrode FET configuration, there are small lateral transport barriers at the electrode/channel interfaces for the other five configurations. The calculations show that the FET configuration with Pt electrode provides better electrical transport performance. In addition, except for the Pd electrode configuration, other configurations exhibit an excellent negative differential conductance (NDC) effect, and some configurations exhibit significant rectifying behavior. These theoretical findings provide guidance, inspiration and insight for the design of future electronic devices based on the ML b-AsP alloy.

Characteristics of an Extended Gate Field-Effect Transistor for Glucose Sensing Using an Enzyme-Containing Silk Fibroin Membrane as the Bio-Chemical Component.

The characteristics of a glucose sensor based on an ion-sensitive TiO2/Ti extended gate electrode field-effect transistor (EGFET) are reported. A glucose oxidase-containing silk fibroin membrane was immobilized on a TiO2/Ti surface as the bio-sensing component. This EGFET-type biosensor was estimated to be able to detect a glucose concentration as low as 0.001 mg/mL in an aqueous electrolyte, which enables the sensing of glucose in the saliva and sweat. The endurance of this sensor was also examined, and it was found that the retention time of the original sensitivity for repeated use at room temperature was more than 30 days, with a high heat tolerance temperature close to 60 °C.

Intrinsic limit of contact resistance in the lateral heterostructure of metallic and semiconducting PtSe2.

High contact resistance (Rc) limits the ultimate potential of two-dimensional (2-D) materials for future devices. To resolve the Rc problem, forming metallic 1T phase MoS2 locally in the semiconducting 2H phase MoS2 has been successfully demonstrated to use the 1T phase as source/drain electrodes in field effect transistors (FETs). However, the long-term stability of the 1T phase MoS2 still remains as an issue. Recently, an unusual thickness-modulated phase transition from semiconducting to metallic has been experimentally observed in 2-D material PtSe2. Metallic multilayer PtSe2 and semiconducting monolayer PtSe2 can be used as source/drain electrodes and channel, respectively, in FETs. Here, we present a theoretical study on the intrinsic lower limit of Rc in the metallic-semiconducting PtSe2 heterostructure through density functional theory (DFT) combined with non-equilibrium Green's function (NEGF). Compared with Rc in the 1T-2H MoS2 heterostructure, the multilayer-monolayer PtSe2 heterostructure can offer much lower Rc due to the better capability of providing more transmission modes.

MoTe2 Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering.

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe2) phase, metallic monoclinic (1T') MoTe2 phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T' MoTe2 homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe2 grow out of a polycrystalline 1T'-MoTe2 matrix. We further demonstrate the operation of MoTe2 FETs made on these in situ-grown lateral homojunctions with 1T' contacts. The use of a 1T' phase as electrodes in MoTe2 FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T' electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T' interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe2 homojunction FETs for use in memory and logic circuity applications.

Development of Biosensor-Based Device for Urea Concentration Measurement in Industrial Fertilizer Samples

In this presentation, we will demonstrate creation process (R&D and engineering) of biosensor and device for direct urea concentration measurement in industrial fertilizer samples. The analyzer is fully automatic, fast, not requiring professional assistance, compact and mobile. The breakthrough technology enabling the construction of such device is amperometric urea biosensor electrode, re-purposed from the biosensor suited for monitoring hemodialysis patients developed earlier by our team. There is no direct competitor for the device – fertilizer production and supply companies have to measure urea in their products by manual, labor-intensive techniques such as Kjeldahl method, where single measurement takes at least an hour, thus creating a beneficial market window for rapid and accurate industrial urea analyzer. Direct urea concentration measurement in water media has been a scientific challenge for decades. It is possible to measure urea concentration by spectrophotometry, but only in transparent, homogeneous and single compound solutions. A target media, concentrated ammonium nitrate – urea solution (UAN), is widely used fertilizer standard in agriculture, and in this case rapid determination of urea in product is complicated. A good technique for such urea measurement would be biosensors; however, urea breakdown does not involve electron exchange with external electrode, so for a long time the potentiometric electrodes were employed, where urea breakdown causes polarization changes of enzyme-covered surface. Such sensors exhibit fast electrochemical response, but have drawbacks - influence of media pH and high ionic strength to measurement errors, low sensitivity in low analyte concentrations (usually not less than 0.01 mM). Another option, ion-sensitive field-effect transistor electrodes adequately measure the current proportional to urea concentration, but usually lack reliability and are prone to permeability problems. Our suggested technology enabling the construction of such device - amperometric urea biosensor, constructed as a membrane to be fitted on working electrode surface. Its benefits - stability, insensitivity to pH or high ionic strength, low production cost, substrate selectivity, can be used constantly in enzyme-degrading fertilizer media for at least two weeks. It works at 0.2 V vs NHE potential, thus does not suffer of potentially significant electrochemical noise. It can measure urea concentration by declared accuracy of 5 %, when the automatic calibration is performed every 3 hours. The biosensor element – a membrane, fitted on the end of graphite electrode – works for 3 months, resulting in single measurement price under one cent. The reiterative measurement is implemented by magnetically stirred flow-through electrochemical cell with opening on top for sample entry. A double-flow peristaltic pump is designed to wash the cell and pre-fill the buffer solution for introduction of 50 µl of 1:1000 (v/v) pre-diluted sample. Currently, our team has developed the technology up to a final product trial batch of biosensor and analyzer device. In the presentation we will demonstrate the biosensor design and electrochemical properties, as well as design of device appliances developed such as electrochemical cell and double-flow peristaltic pump. Figure 1

Fabrication of metal nanopatterns for organic field effect transistor electrodes by cracking and transfer printing

In this paper, we demonstrate a novel method for fabricating metal nanopatterns using cracking to address the limitations of traditional techniques. Parallel crack arrays were created in a polydimethylsiloxane (PDMS) mold using a combination of surface modification and control of strain fields. The elastic PDMS containing the crack arrays was subsequently used as a stamp to prepare nanoscale metal patterns on a substrate by transfer printing. To illustrate the functionality of this technique, we employed the metal patterns as the source and drain contacts of an organic field effect transistor. Using this approach, we fabricated transistors with channel lengths ranging from 70–600 nm. The performance of these devices when the channel length was reduced was studied. The drive current density increases as expected, indicating the creation of operational transistors with recognizable properties.

Field Effect Transistor Based on Solely Semiconducting Single‐Walled Carbon Nanotubes for the Detection of 2‐Chlorophenol

We report a promoted sensitivity to 2‐chlorophenol (2‐CP) molecules of a fabricated back gated, interdigitated electrode field effect transistor based on separated semiconducting single‐walled carbon nanotubes (CNTFET). The origin of the interaction between the nanotubes that serve as channels across the source and drain contacts and organic volatile compound is suggested due to peculiarities gained after the nanotube separation technique. The semiconducting nanotubes that were used for building‐up the CNTFETs had an average diameter of 1.4 nm and have been separated by the efficient aqueous two‐phase extraction technique. The advantage of the technique is in the one‐step extraction of semiconducting type of conductivity nanotubes with high yield. Chlorophenols are widely studied due to the toxicity and hazardous health effects depending on the exposure. Previously chlorophenols were reported to be not interacting with pristine nanotubes. As prepared CNTFET device demonstrates a detection limit below 100 parts per million for the 2‐CP molecules with a reaction time of 150 s. CNTFET devices have good repeatability, low bias stress, and electron charge carrier mobility of 17.5 ± 0.1 cm2 (Vs)−1. We discuss possible physical mechanisms of the 2‐CP adsorption induced current drop and potential ways to improve sensitivity.

Phase-engineered transition-metal dichalcogenides for energy and electronics

Abstract

Metallic 1T phase source/drain electrodes for field effect transistors from chemical vapor deposited MoS2

Two dimensional transition metal dichalcogenides (2D TMDs) offer promise as opto-electronic materials due to their direct band gap and reasonably good mobility values. However, most metals form high resistance contacts on semiconducting TMDs such as MoS2. The large contact resistance limits the performance of devices. Unlike bulk materials, low contact resistance cannot be stably achieved in 2D materials by doping. Here we build on our previous work in which we demonstrated that it is possible to achieve low contact resistance electrodes by phase transformation. We show that similar to the previously demonstrated mechanically exfoliated samples, it is possible to decrease the contact resistance and enhance the FET performance by locally inducing and patterning the metallic 1T phase of MoS2 on chemically vapor deposited material. The device properties are substantially improved with 1T phase source/drain electrodes.

Evaluation study of an ion selective field effect transistor electrode for measuring quality parameters of fuel ethanol

An ion selective field effect transistor (ISFET) electrode was evaluated for measuring pH and acid number (AN) of fuel ethanol and compared to two glass electrodes with different reference filling solutions: KCl aqueous solution (glass-KCl electrode) and LiCl ethanolic solution (glass-LiCl electrode). pH was determined at different measurement times and AN was determined using automatic potentiometric titration. For pH, the glass-KCl electrode showed the best precision and stability, with an average repeatability about four times better when compared to the ISFET electrode for the measurement time of 30 s (as indicated in the ASTM D6423 standard). For AN, the glass-KCl and glass-LiCl electrodes showed similar repeatabilities, which were about three times better than that of the ISFET electrode. In addition, the results from a recovery study demonstrated better accuracy of the glass-LiCl electrode, with a recovery value of 100.1%.

Controllable Chemical Vapor Deposition Growth of Few Layer Graphene for Electronic Devices

Because of its atomic thickness, excellent properties, and widespread applications, graphene is regarded as one of the most promising candidate materials for nanoelectronics. The wider use of graphene will require processes that produce this material in a controllable manner. In this Account, we focus on our recent studies of the controllable chemical vapor deposition (CVD) growth of graphene, especially few-layer graphene (FLG), and the applications of this material in electronic devices. CVD provides various means of control over the morphologies of the produced graph ene. We studied several variables that can affect the CVD growth of graphene, including the catalyst, gas flow rate, growth time, and growth temperature and successfully achieved the controlled growth of hexagonal graphene crystals. Moreover, we developed several modified CVD methods for the controlled growth of FLGs. Patterned CVD produced FLGs with desired shapes in required areas. By introducing dopant precursor in the CVD process, we produced substitutionally doped FLGs, avoiding the typically complicated post-treatment processes for graphene doping. We developed a template CVD method to produce FLG ribbons with controllable morphologies on a large scale. An oxidation-activated surface facilitated the CVD growth of polycrystalline graphene without the use of a metal catalyst or a complicated postgrowth transfer process. In devices, CVD offers a controllable means to modulate the electronic properties of the graphene samples and to improve device performance. Using CVD-grown hexagonal graphene crystals as the channel materials in field-effect transistors (FETs), we improved carrier mobility. Substitutional doping of graphene in CVD opened a band gap for efficient FET operation and modulated the Fermi energy level for n-type or p-type features. The similarity between the chemical structure of graphene and organic semiconductors suggests potential applications of graphene in organic devices. We used patterned CVD FLGs as the bottom electrodes in pentacene FETs. The strong π-π interactions between graphene and pentacene produced an excellent interface with low contact resistance and a reduced injection barrier, which dramatically enhances the device performance. We also fabricated reversible nanoelectromechanical (NEM) switches and a logic gate using the FLG ribbons produced using our template CVD method. In summary, CVD provides a controllable means to produce graphene samples with both large area and high quality. We developed several modified CVD methods to produce FLG samples with controlled shape, location, edge, layer, dopant, and growth substrate. As a result, we can modulate the properties of FLGs, which provides materials that could be used in FETs, OFETs, and NEM devices. Despite remarkable advances in this field, further exploration is required to produce consistent, homogeneous graphene samples with single layer, single crystal, and large area for graphene-based electronics.

Electrical Contact Properties between the Accumulation Layer and Metal Electrodes in Ultrathin Poly(3-hexylthiophene)(P3HT) Field Effect Transistors

Probing contact properties between an ultrathin conjugated polymer film and metal electrodes in field effect transistors (FETs) is crucial not only to understanding charge transport properties in the accumulation layer but also in building organic sensors with high sensitivity. We investigated the contact properties between gold electrodes and poly(3-hexylthiophene) (P3HT) as a function of film thickness using gated four-point sheet resistance measurements. In an FET with a 2 nm thick P3HT film, a large voltage drop of 1.9 V (V(D) = -3 V) corresponding to a contact resistance of 2.3 × 10(8) Ω was observed. An effective FET mobility of 1.4 × 10(-3) cm(2)/(V s) was calculated when the voltage drop at the contacts was factored out, which is approximately a factor of 3 greater than the two-contact FET mobility of 5.5 × 10(-4) cm(2)/(V s). A sharp decrease in the ratio of the contact resistance to the channel resistance was observed with increasing film thickness up to a thickness of approximately 6 nm, separating a contact limited regime from a charge transport limited regime. The origin of the large contact resistance observed in the device prepared with an ultrathin P3HT film is discussed in light of results from X-ray diffraction (XRD) and atomic force microscopy (AFM) studies.

Acid–Base Property of Ethylammonium Nitrate Ionic Liquid Directly Obtained Using Ion-selective Field Effect Transistor Electrode

Abstract Ethylammonium nitrate (EAN) is a protic room-temperature ionic liquid and classified into an amphoteric solvent. The equilibrium constant KS = [EtNH2][HNO3] has directly been obtained for the first time by using an ion-selective field effect transistor (ISFET) electrode. The obtained KS value is of 10−10.0 mol2 dm−6, indicating that the neutral EAN involves HNO3 and EtNH2 molecules as much as 1.0 × 10−5 mol dm−3, or the pH (= −log[HNO3]) is 5 at 298 K.

Self-aligned and stray-field-free electrodes for spintronics: An application to a spin field effect transistor

We present a ringlike design for spin field effect transistor electrodes. This configuration solves the local Hall effect drawback as these electrodes do not generate any stray magnetic fields. The shape and size of the electrodes are adjusted in order to match the physical constraints. The gate configuration and channel length are discussed for the [110] growth direction; the GaInAs channel length for the spins to fully switch is calculated to be of the order of 0.15μm for a gate electric field of 150–300kV∕cm.

Synthesis and Self-Organization of Core-Extended Perylene Tetracarboxdiimides with Branched Alkyl Substituents

The influence of the core extension of perylene tetracarboxdiimides on the thermotropic behavior has been investigated. A homologous series of alkyl substituted tetracarboxdiimides, namely, perylene diimide, terrylene diimide, quaterrylene diimide, and coronene diimide, was synthesized. These compounds display absorption maxima in the region of 430−760 nm with high extinction coefficients and show a high thermal stability up to 450 °C. Structural evaluation revealed an identical columnar self-organization for the derivatives below their isotropization temperature. An intracolumnar packing of the disks with a lateral rotation of 45° to each other resulted in a helical pitch containing four molecules. The phase transition to the isotropic phase is shifted to higher temperatures for larger aromatic cores within this series of compounds. On the other hand, differences in the self-assembly during crystallization from the isotropic phase were observed. While perylene tetracarboxdiimide and terrylene tetracarboxdiimide formed large and highly ordered domains with arranged edge-on molecules, the coronene tetracarboxdiimide disks self-organized face-on leading to a homeotropic phase. The different molecular orientation on surfaces was correlated with diversified substitution patterns of the aromatic cores. The manipulation of the molecular architecture opens thus the opportunity to control the spontaneous self-alignment. This improvement of the macroscopic organization ensures an undisturbed percolation pathway for charge carriers between electrodes in field-effect transistors or in photovoltaic cells.

Structural, Mechanical and Tribological Properties of TiN and CrN Films Deposited by Reactive Pulsed Laser Deposition

ABSTRACTNitride thin films have potential applications in different areas of silicon device technology, namely as diffusion barrier in metallization schemes, rectifying and ohmic contacts, and gate electrodes in field effect transistors. In the present investigation, TiN and CrN films have been deposited by reactive pulsed laser deposition technique using Ti and Cr targets at 10mTorr background pressure of N2. Si (100) and AISI 440C steel substrates were used for the present study. Films were deposited at different temperatures in the range of 200°C to 600°C. The deposited films exhibited densely packed grain, with smooth and uniform structures. X-ray Photoelectron Spectroscopy (XPS) analysis of the films showed and 50% Ti and 40% of N in TiN films, 45% of Cr and 45% of N in CrN films deposited on Si (111), with the balance mostly oxygen, indicating near stoichiometric composition of the deposited nitride thin films. Hardness of the films changed from 22 GPa at 200°C to 30 GPa at 600°C for TiN, whereas for CrN we obtained 26 GPa at 200°C to 31 GPa at 600°C. The residual stress in the films showed a change from compressive stress at 200°C to tensile stress at 600°C in both the cases. Friction coefficient of the films were measured by pin-on-disk technique for all films, up to the tested limit of 10, 000 cycles at 1 N load and found to be very high (≥ 1) in both cases.

Acid diffusion through extracellular polysaccharides produced by various mutants of Streptococcus mutans

Mutants of Streptococcus mutans V403, constructed by allelic exchange and altered in their capacity to produce enzymes involved in the production of extracellular polysaccharides from sucrose, were used to study the role of glucans and fructans in the diffusion of ions through cell concentrates. A 4.0 mm diameter, 0.75 mm deep diffusion chamber with an ion-sensitive field-effect transistor electrode positioned at the base was used to monitor the diffusion of hydronium ions from a sodium lactate buffer using cell concentrates prepared from bacteria grown in various concentrations of sucrose and glucose. The wild-type strain V403 produced at least seven times as much water-insoluble glucan (ISG) from sucrose as mutants deficient in various combinations of glucosyltransferase B (GTF B), GTF C, GTF D and fructosyltransferase. The fastest diffusion of hydronium ions occurred with sucrose-grown cell concentrates of strain V403, and the time of diffusion to the bottom of the chamber was approximately 2.3 times longer when this strain was grown in glucose. The speed of diffusion with glucose-grown V403 was similar to that obtained with each of the mutants. When cells of strain V403 grown in sucrose and glucose were mixed, increases in diffusion speed were found to be directly related to the proportion of sucrose-grown cells. The mixing of ISG with several strains of S. mutans revealed that increases in diffusion speed were directly related to the quantity of ISG added.