Negative Voltage Pulses Research Articles

Effect of positive pulse voltage on color value and corrosion property of magnesium alloy black micro-arc oxidation ceramic coating

To improve the corrosion resistance of the magnesium alloy and to extend its applications in light absorption field, black anti-corrosion and absorption integrated ceramic coatings were prepared on AZ31 magnesium alloy by asymmetric bipolar pulse micro-arc oxidation (MAO) technique. The effect of applied positive pulse voltage on the color value and corrosion properties of the coatings were investigated. The results showed that the Cu2+ ions were dissolves into MgO lattice to form the substitutional solid solution (Mg1-XCuXO) during MAO of magnesium alloys. The color value was mainly determined by the effect of the content of Cu2+ ions on the band gap of MgO. On the premise of constant negative pulse voltage, with the increase of positive pulse voltage, the solid solubility of Cu2+ ions in ceramic coating increased first and then remained unchanged, while the color value decreased first and then kept constant. The average absorption rate of black ceramic coating to visible light could be up to 95%. Moreover, the rising positive pulse voltage increased the thickness and decreased the density of the black ceramic coating, resulting the anti-corrosion property increases first and then decreases.

The Characteristics of Transparent Non-Volatile Memory Devices Employing Si-Rich SiOX as a Charge Trapping Layer and Indium-Tin-Zinc-Oxide

We fabricated the transparent non-volatile memory (NVM) of a bottom gate thin film transistor (TFT) for the integrated logic devices of display applications. The NVM TFT utilized indium–tin–zinc–oxide (ITZO) as an active channel layer and multi-oxide structure of SiO2 (blocking layer)/Si-rich SiOX (charge trapping layer)/SiOXNY (tunneling layer) as a gate insulator. The insulators were deposited using inductive coupled plasma chemical vapor deposition, and during the deposition, the trap states of the Si-rich SiOx charge trapping layer could be controlled to widen the memory window with the gas ratio (GR) of SiH4:N2O, which was confirmed by fourier transform infrared spectroscopy (FT-IR). We fabricated the metal–insulator–silicon (MIS) capacitors of the insulator structures on n-type Si substrate and demonstrated that the hysteresis capacitive curves of the MIS capacitors were a function of sweep voltage and trap density (or GR). At the GR6 (SiH4:N2O = 30:5), the MIS capacitor exhibited the widest memory window; the flat band voltage (ΔVFB) shifts of 4.45 V was obtained at the sweep voltage of ±11 V for 10 s, and it was expected to maintain ~71% of the initial value after 10 years. Using the Si-rich SiOX charge trapping layer deposited at the GR6 condition, we fabricated a bottom gate ITZO NVM TFT showing excellent drain current to gate voltage transfer characteristics. The field-effect mobility of 27.2 cm2/Vs, threshold voltage of 0.15 V, subthreshold swing of 0.17 V/dec, and on/off current ratio of 7.57 × 107 were obtained at the initial sweep of the devices. As an NVM, ΔVFB was shifted by 2.08 V in the programing mode with a positive gate voltage pulse of 11 V and 1 μs. The ΔVFB was returned to the pristine condition with a negative voltage pulse of −1 V and 1 μs under a 400–700 nm light illumination of ~10 mWcm−2 in erasing mode, when the light excites the electrons to escape from the charge trapping layer. Using this operation condition, ~90% (1.87 V) of initial ΔVFB (2.08 V) was expected to be retained over 10 years. The developed transparent NVM using Si-rich SiOx and ITZO can be a promising candidate for future display devices integrating logic devices on panels.

Artificial Neuron Based on TiO2 Cbram for Neuromorphic Computing

Recently, CMOS-based artificial neurons have successfully implemented neuron functions such as integrate and fire (IF) or leaky integrate and fire (LIF). In general, CMOS-based artificial neurons use capacitors to implement integration function of neuron. In this case, required RC time constant is about few milliseconds. And it requires large area of capacitor more than 1000F2. Therefore, it is necessary to develop capacitor-less artificial neuron to achieve high neuronal density. In this works, we implemented the integration function of neurons using CBRAM instead of capacitors. For the neuron device, CuTe/TiO2/TiN CBRAM cell was fabricated as shown in figure 1a. The neuron device has W bottom electrode with ~113 nm2 pattern size and 10 nm-thick TiO2 layer, and 200 nm-thick CuTe top electrode with 60 μm2 size. The TiO2-based CBRAM showed a typical bi-stable I-V curve as shown in figure 1b. The CBRAM cells showed negative differential resistance (NDR) during reset process. And it showed integrate characteristic when consecutive negative voltage pulse was induced to top electrode of the CBRAM cell. Figure 1c shows integrate characteristic of TiO2-based CBRAM cell. As shown in figure 1c, as the voltage pulse is applied, current gradually decreases. Also as amplitude of voltage pulse increased, the change of current became greater. Because the area of the CBRAM cell is only 4F2, it is expected that area of artificial neuron can be significantly reduced by replacing conventional capacitor based neuron. We will present how the CBRAM cell can be used to implement artificial neuron. Figure 1

Generators of Atmospheric Pressure Diffuse Discharge Plasma and Their Use for Surface Modification

Studies of the properties of runaway electron preionized diffuse discharges (REP DDs) and their possible use have been carried out for more than 15 years. Three experimental setups generating a low-temperature atmospheric-pressure plasma and differing in the geometry of a discharge gap were developed. They allow the treatment of surfaces of different materials with an area of several tens of square centimeters. A diffuse discharge plasma was formed in the pulse–periodic mode by applying negative voltage pulses with an amplitude of several tens of kilovolts and a duration of 4 ns to a discharge gap with sharply non-uniform electric field strength distribution. This paper presents experimental results of the study of the surface layer microstructure of copper and steel specimens of different sizes after treatment with the REP DD plasma in nitrogen flow mode and nitrogen circulation mode in the discharge chamber. It was shown that after 105 discharge pulses, the carbon concentration decreases and a disoriented surface layer with a depth of up to 200 nm is formed. Moreover, the results of X-ray phase analysis did not reveal changes in the phase composition of the surface of copper specimens. However, as a result of surface treatment with the REP DD plasma, the copper lattice becomes larger and the microstress increases.

Deposition and Thermal Stability of DLC/Si-N Composite Films Synthesized Using a Sputtering-PBII Hybrid System

Diamond-like carbon (DLC) is a metastable amorphous material that exhibits unique properties. However, there are many limitations regarding the use of this material due to factors such as its tribological characteristics at high temperature and limited thermal stability. In this study, the thermal stability and tribological properties of DLC/silicon-nitrogen (DLC/Si-N) composite films were investigated and compared to those of pure DLC films. All the films were synthesized using a combination of radio frequency (RF) magnetron sputtering and plasma-based ion implantation (PBII) (a so-called sputtering-PBII hybrid system) which is newly developed by us. A high purity silicon nitride (99.9%) disk was used as the target, applying an RF power in the range of 500 - 700 W and a negative pulsed bias voltage of 5 kV to the substrate. An Ar-CH4 mixture was used as the reactive gas. The CH4 partial pressure was varied between 0 and 0.15 Pa, while the total gas pressure and total gas flow were fixed at 0.30 Pa and 30 sccm, respectively. The structures of the resulting films were characterized using Raman spectroscopy, while the thermal stabilities were assessed using thermogravimetric-differential thermal analysis (TG-DTA) and friction coefficients were obtained via ball-on-disk friction tests. The results indicate that the DLC/Si-N composite films produced in this work exhibit improved thermal stability relative to that of pure DLC owing to the presence of thermally stable atomic-scale Si-N compound in the carbon main flame networks. A DLC/Si-N film containing approximately 11 at.%Si and 18.5 at.%N shows good thermal stability in air over 800°C up to 1100°C, together with excellent tribological performance at 500°C in air. Overall, the data demonstrate that DLC/Si-N composite films offer improved thermal stability and superior tribological performance at high temperatures.

The Surface Treatment of Niobium Superconducting Reentrant Cavities by Means of High Temperature Nitrogen Plasma Based Ion Implantation

High Temperature Nitrogen Plasma Based Ion Implantation (HT-NPBII) has been used to treat the surface of niobium superconducting reentrant cavities, which are part of parametric transducers in a resonant-mass gravitational wave detector. The aim is to enhance the corresponding electrical quality factors (Q-factors) which are closely related with the increase of the sensitivity of the system. In this experiment, the cavities are immersed in plasma and bombarded by energetic nitrogen ions, which are implanted into the surfaces of these heated substrates. The heating temperature of the cavities is controlled during the treatment and its level directly affects the N implantation depth profile due to the diffusion process. Additional tailoring of the nitrogen doping can be performed by the adjustment of the intensity and the duty cycle of the high negative voltage pulses used for ion implantation. For implantations performed at 5 kV /20 µs /300 Hz/ 700 °C, nitrogen atoms occupy interstitial spaces in the crystal lattice of niobium. The treatment of niobium superconducting cavities under these parameters caused the enhancement of two orders of magnitude of respective Q-factors. A set of characterization techniques was performed herein in order to help with the understanding of the underlying mechanism behind this phenomenon.

Synaptic Learning and Forgetting Behavior in Ag/AlN/Al Memristor With ${\text{O}_{2}}$ Annealing Effect

The learning and forgetting behavior of AlN thin film with O 2 annealing effect memristor had been investigated in this study. Such synaptic behavior could be observed after AlN thin film executed O 2 annealing process. The oxygen-induced AlN thin film exhibited memristive effect in resistance with positive and negative current-voltage (I-V) measurement loops which had never been observed in AlN thin film without annealing effect. The O 2 annealing process could decrease the number of oxygen vacancies (V+o ) in interface between top electrode and active layer. A typical I-V measurement is shown in this study. If continuous positive voltage pulses applied, the device conductance would increase gradually which is called learning behavior of device. If negative voltage pulse applied, the device conductance would decrease gradually which is called forgetting behavior of device. The memristor conductance was thought to be closely associated with synaptic weights, the strength of synaptic connections. The learning and forgetting behavior of memristor is corresponding to LTP and LTD process in synaptic system. The ion diffusion played an important role in memristive properties which could form one or more local conductive filaments (CFs) in active layer. If the paths were gradually formed or broke, this would cause the memristor conductance changed gradually with measured voltage. The retention time and memristor conductance should depend on stimuli pulse shape, number and interval time.

Plasma based nitrogen ion implantation to hydrogenated diamond-like carbon films

Abstract Surface modification of hydrogenated diamond-like carbon films prepared by C7H8 plasma is carried out by the implantation of nitrogen ions in pure nitrogen plasmas, changing the modification time from 0 to 30 min. A negative high voltage pulse with a value of 10 kV, a width of 10 µs, and a repetition rate of 2 kHz is applied to the substrate. The electrical resistivity decreases from 2.1 Ωcm to 0.08 Ωcm with increasing the modification time. The film hardness also decreases from about 11 GPa to about 7.5 GPa. The near surface region in the films modified by the nitrogen implantation is expected to contain the relative nitrogen content of about 10%, based on the results of X-ray photoelectron spectroscopy (XPS). The XPS measurements in the depth direction show that the nitrogen content depends on the modification time and the depth of nitrogen implantation is approximately 20 nm or more at the modification time longer than 18 min. The relative content of carbon increases from 80% on the surface to 99% except for the content of hydrogen and sp2 C bond is always dominant in the XPS. The estimated resistivity of the nitrogen ion implanted layer decreases to 0.008–0.013 Ωcm with increasing the modification time.

Overcoming sheaths overlapping in a small diameter metallic tube with one end closed and using a high density plasma from a high power pulsed hollow cathode discharge

High voltage sheaths are formed when plasmas are produced by application of high negative voltage pulses to conductive supports or components, as in Plasma Immersion Ion Implantation (PIII) treatments of materials surfaces. For parts with concave shape, as inside metal tubes, these sheaths behave quite differently according to the tube configuration and size, as well as, PIII treatment pressure of operation and pulsing parameters. In this work, an SS304 tube of 1.1 cm internal diameter and 20 cm length was pulsed typically at -0.5 to -2.6 kV, 20 μs pulse length, 500 Hz repetition rate, nitrogen pressure of 5x10-2 mbar and with one side closed configuration. Different currents (between 10 and 30 A) were used to produce plasmas with sheaths that overlapped or not, depending on the currents used. To study these sheath behaviors, a simple plasma diagnostic technique based on a bi-dimensional mapping of the deposition of sputtered materials and by etching via the plasma on a Si wafer target surface, both coming out from the tube, was used. This mapping showed clearly the border line situation between overlapping and non-overlapping sheaths in that small tube which allowed to estimate the plasma density to be around 1011 cm-3 at such a sheath condition, as previously anticipated by Sheridan. Above that border condition, nitrogen PIII was successfully obtained in such a small tube of SS304, producing TiN and Ti2N in samples of Ti6Al4V placed inside the tube, when temperatures higher than 800°C were reached there. Below the border, no significant uptake of nitrogen was possible. Using this type of experimental set-up, it is now possible to explore different hollow cathode behaviors, efficient or high temperature (above 800°C) PIII conditions and also new utilizations of the plasma ejected from the tube.

The resistive switching characteristics of Ni-doped HfOx film and its application as a synapse

The effect of Ni doping concentration on resistive switching characteristics of Au/HfOx:Ni/Pt structure was investigated, and the 3.1% Ni-doped HfOx film was used to imitate the function of biological synapse. From the X-ray absorption near edge structure analysis, the forms and the local electronic structure of Ni atoms in the HfOx films with different Ni doping concentrations were investigated. According to the X-ray photoelectron spectroscopy analysis, the Ni doping introduced more oxygen vacancies in HfOx films. Gradual Reset process and concentrated distribution of resistive switching parameters were achieved for 3.1% Ni-doped HfOx films, which could be used to emulate the “learning” and “forgetting” function of the biological synapse. Multiple resistance levels can be observed in the continuously pulse number under identical pulse width of positive or negative voltage pulses. Moreover, the application of pulse-train operation scheme is an effective method to control analog synaptic devices during the Reset process, which can contribute to understand the nature of the conductive nano-filament evolution.

Characterization of flashover plasma across a large-scale ceramic vacuum interface initiated by explosive electron emission

Flashover plasma characteristics across a large-scale ceramic vacuum interface initiated by explosive electron emission (EEE) are investigated with the experimental and simulation methods. Driven by a negative high voltage pulse (-300 kV, 150 ns), flashover plasma luminescence processes were observed by a high-speed framing camera and the accompanied spectrum characteristics were measured by a spectrograph. Compared with flashover initiated by field electron emission (FEE), a faster light expansion velocity and higher electron temperature of flashover initiated by EEE were obtained which were 200 cm/μs and 4.57 eV, respectively. Radial and angular components characteristics of expansion velocity were analyzed as well and their maximum values were 200 cm/μs and 50 cm/μs. Furthermore, comparisons of flashover initiated by EEE and FEE were conducted by particle-in-cell methods and the results agreed with the experimental observations. From these results it can be concluded that due to higher primary electron energy and stronger secondary electron avalanche and gas ionization, flashover along the ceramic interface initiated by EEE has an easier and faster develop process compared with that initiated by FEE. This work can give a reference for evaluation and design of ceramic vacuum interfaces for high-current applications.

Streamers at the Subnanosecond Breakdown of Argon and Nitrogen in Nonuniform Electric Field at Both Polarities

An ICCD camera was used to study plasma glow at the stage of the streamer (ionization wave) formation in the tip–plane gap with a length of 3 mm filled with argon or nitrogen at a pressure of 12.5–400 kPa. Positive and negative nanosecond voltage pulses were applied across the gap. Images of streamer were obtained at different time at its propagation along the gap. A streak-camera equipped with a spectrometer was used to measure time evolution of the radiation intensity of nitrogen molecules at a wavelength of 337.1 nm in several regions along the gap at the negative polarity. Average streamer velocity (1.8 cm/ns) was estimated from experimental data at atmospheric pressure of nitrogen. Amplitude–time characteristics of voltage, discharge current and the current of runaway electron beam behind the aluminum-foil anode with a thickness of 10 μm were measured. Reasons for a diffuse discharge under the given experimental conditions were discussed.

A Negative Low-Voltage Power Supply Integrated With High-Voltage Devices

A negative low-voltage power supply with the highest positive potential as reference is proposed in this paper. The proposed power supply is integrated with high-voltage devices. When the high-voltage devices are applied as a switch, negative low-voltage pulses are first produced from the positive high voltage applied on devices by using the designed junction termination of the devices. Then, a stable negative power voltage is finally produced from the negative low-voltage pulses by using a designed low-voltage circuit (LVC) that consists of pMOS and capacitors. The mechanism of the proposed power supply is verified using an insulated-gate bipolar transistor (IGBT) device, whose junction termination is realized by the optimum variation lateral doping technique [breakdown voltage (BV) = 2100 V]. The simulation results show that the power supply can achieve a stable value of −6.2 V whenever the IGBT is operated at 500 or 1500 V. Moreover, an experimental sample of LVC is fabricated using a 0.5- $\mu \text{m}$ BiCMOS process. The transient experiments confirm that LVC can convert negative voltage pulses with different periods and amplitudes to a stable negative low voltage. The steady-state experiments show that the input BV of LVC is 57 V and its output voltages can be adjusted from −2 to −14.5 V.

Formation of 1.4 MeV runaway electron flows in air using a solid-state generator with 10 MV/ns voltage rise rate

Fulfillment of the condition that the voltage rise time across an air gap is comparable with the time of electron acceleration from a cathode to an anode allows a flow of runaway electrons (REs) to be formed with relativistic energies approaching that determined by the amplitude of the voltage pulse. In the experiment described here, an RE energy of 1.4 MeV was observed by applying a negative travelling voltage pulse of 860-kV with a maximum rise rate of 10 MV/ns and a rise time of 100-ps. The voltage pulse amplitude was doubled at the cathode of the 2-cm-long air gap due to the delay of conventional pulsed breakdown. The above-mentioned record-breaking voltage pulse of ∼120 ps duration with a peak power of 15 GW was produced by an all-solid-state pulsed power source utilising pulse compression/sharpening in a multistage gyromagnetic nonlinear transmission line.

Preparation of nickel-containing conductive amorphous carbon films by magnetron sputtering with negative high-voltage pulsed substrate bias

Nickel-containing amorphous carbon (a-C-Ni) films were deposited by magnetron sputtering of graphite and nickel. Graphite magnetron sputtering power was 1500 W. Nickel magnetron sputtering power changed from 0 W to 1000 W, allowing control of the Ni concentration in the films from 0 to 58 at.%. Growth rates of a-C-Ni films were 1.4–2.5 μm/h. High-voltage negative pulsed bias voltage of ‐3 kV amplitude, frequency of 1 kHz, and pulse duration of 50–250 μs was applied to a substrate. The chemical composition of the films was determined using Auger electron spectroscopy. The surface morphology was observed by atomic force microscopy (AFM). The film structure was characterized by Raman spectroscopy and X-ray diffractometry (XRD). The resistivity of the films was measured by a four-point probe method. Film properties that are changed by the added metal, such as structure, electrical resistivity, and hardness, were evaluated and compared with those of pure a-C films as well as with literature values for a-C-Ni films. It is shown that the resistivity of a-C films depends on the duration of the bias voltage pulses and varies from 3.4 to 7.6·10−2 Ohm·cm at pulse duration 50 and 250 μs, respectively. In the case of Ni doping, the a-C-Ni film resistivity can be reduced to 6.7·10−4 Ohm·cm. Where Ni is included in the film in the form of Ni3C carbide, which has hexagonal (hcp) modification, the crystallite size averages 12 nm. The film has a columnar structure and carbon is also present as a graphite-like interlayer 3 nm thick between the Ni3C crystallites. At sputtering power of 600 W of nickel magnetron, a-C-Ni film with resistivity of 5.3·10−3 Ohm·cm is obtained, which can be used as a barrier layer in thermoelements based on bismuth telluride.

Synthesis of Diamond‐Like Carbon Films Deposited by the Ionization Vapor Method and Development of its Semiconductive Devices Fabricated by the Focused Ga Ion Beam Implantation

SUMMARYIn this study, we have attempted to synthesize the Gallium (Ga)‐implanted diamond‐like carbon (DLC) film for new functional devices as substituting Si‐based materials. Intrinsic‐DLC (i‐DLC) films (energy gap: 1.45 eV) were deposited by the ionization vapor method with applying the negative pulsed bias voltage (frequency: 2 kHz, duty ratio: 30%, peak voltage: 500 V) to SiO2 substrates. Ga atoms were implanted to i‐DLC films as accepters utilizing by focused ion beam irradiation system. The Raman scattering spectra of i‐DLC and Ga‐DLC films showed typical DLC characteristics which consisted of I(D) and I(G) peaks. In order to evaluate working function of the Ga‐DLC film, several electrode materials (Au, Pt, Cu, Al, and Sn) were deposited on the films. Current–voltage characteristics of Au and Pt electrodes on Ga‐DLC films showed Ohmic contacts, and Cu, Al, and Sn electrodes were Schottky contacts. These results suggested that a work function of the Ga‐DLC film was in the range of 4.47 eV to 4.58 eV. To apply these contact properties to DLC semiconductive devices, we produced the DLC Schottky diode using Al and Pt electrodes deposited on the Ga‐DLC film. A current–voltage characteristic of DLC Schottky diode showed diode property in which amount of forward and backward voltage were 7.0 V and 17.0 V, respectively. The ideality factor n of produced diode was 11.3.

Resistive switching of the HfO x /HfO2 bilayer heterostructure and its transmission characteristics as a synapse.

In this work, HfOx/HfO2 homo-bilayer structure based resistive random access memory devices were fabricated, and the resistive switching characteristics of the devices were investigated. The samples with an Ar/O2 ratio of 12 : 2 exhibited improved switching performance including better uniformity, endurance and retention, which was selected to imitate the “learning” and “forgetting” function of biological synapses. The multilevel conductance of the HfOx/HfO2 homo-bilayer structure under the model of pulse voltage suggests its potential to emulate the nonlinear transmission characteristics of the synapse, and a model of multilevel conductance of the HfOx/HfO2 homo-bilayer structure was proposed. The device conductance continuously increases (decreases) in accordance with the number of positive (negative) voltage pulses during the potentiation (depression) process, which can emulate the change of synaptic weight in a biological synapse.

A Microbeam Resonator With Partial Electrodes for Logic and Memory Elements

We demonstrate logic and memory elements based on an in-plane clamped–clamped microbeam resonator. The microresonator is electrostatically actuated through a drive electrode and the motional signal is capacitively sensed at a sense electrode, while the resonance characteristics are modulated by dc voltage pulses provided at two separate partial electrodes, independent of the drive/sense electrodes. For the logic applications, we use two separate electrodes to provide dc voltages defined as the logic inputs. The high (low) motional signal at on-resonance (off-resonance) state is defined as the logic output state “1” (“0”). For the memory operation, two stable vibrational states, high and low, within the hysteretic regime are defined as the memory states, “1” and “0,” respectively. We take advantage of the split electrode configuration to provide positive and negative dc voltage pulses selectively to set/reset the memory states (“1”/“0”) without affecting the driving and sensing terminals. Excluding the energy cost for supporting electronics, these devices consume energy in tens of picojoules per logic/memory operation. Furthermore, the devices are fabricated using silicon-on-insulator wafers, have the potential for on-chip integration, and operate at moderate pressure (~1 Torr) and room temperature.

A Magnetic Coupling Based Gate Driver for Crosstalk Suppression of SiC MOSFETs

Silicon carbide (SiC) devices have attracted widespread attention because of their superior characteristics. However, not only the higher slew rate of drain–source voltage but also the higher slew rate of reverse recovery current can result in a more serious crosstalk problem than Si-based devices in a half-bridge application. Crosstalk suppression should be integrated into the gate driver to ensure the safe operation of SiC devices. Therefore, a specific mathematical analysis is done in this paper to figure out the crosstalk phenomenon. The limitations of the existing suppression methods are illustrated. Thus, a gate driver based on the magnetic coupling is proposed to ensure the gate–source voltage within the safe range, even when the positive and negative spurious pulse voltages appear. The proposed gate driver uses three ring transformers to insulate the control signal and driver power supply. So, it is feasible to drive a half-bridge circuit in the medium or high power applications. By saving optical couplers and isolated drive power supplies, the gate driver can realize fully galvanic isolation and generate the positive and negative gate–source driving voltage simply. The results derived from the proposed mathematical analysis and the effectiveness of the proposed driver in suppressing the spurious pulse voltage are verified by the experiments.

Formation of ball streamers at a subnanosecond breakdown of gases at a high pressure in a nonuniform electric field

The formation of a diffuse discharge plasma at a subnanosecond breakdown of a “cone–plane” gap filled with air, nitrogen, methane, hydrogen, argon, neon, and helium at various pressures has been studied. Nanosecond negative and positive voltage pulses have been applied to the conical electrode. The experimental data on the dynamics of plasma glow at the stage of formation and propagation of a streamer have been obtained with intensified charge-coupled device and streak cameras. It has been found that the formation of ball streamers is observed in all gases and at both polarities. A supershort avalanche electron beam has been detected behind the flat foil electrode in a wide range of pressures in the case of a negatively charged conical electrode. A mechanism of the formation of streamers at breakdown of various gases at high overvoltages has been discussed.