Current-controlled Negative Differential Resistance Research Articles

Current-controlled negative differential resistance in InAs/AlxGa1−xSb tunnel structures

We report the design and demonstration of a novel resonant tunneling device which displays bistability in operating current over a range of applied voltages. The device is based on an InAs/AlxGa1−xSb double barrier heterostructure. Although similar in design to conventional resonant tunneling structures, the type-II InAs/AlxGa1−xSb heterostructure permits the simultaneous accumulation of electrons and holes in the quantum well/barrier region under an applied bias. The steady-state free-charge distribution, and hence the device current, is strongly dependent upon the bias history of the device. This effect is manifested as a current-controlled (S-shaped) negative differential resistance under conditions in which the device current is specified while the device voltage is measured; a bistable (hysteretic) characteristic is observed under conditions in which the voltage is specified and the current is measured. We have observed current bistability for AlxGa1−xSb barrier compositions in the range 0.4<x<0.6, with high/low state separation in the 105 A/cm2 range. Under current-controlled measurement conditions, peak-to-valley voltage ratios as high as 1.5 have been observed.

Electrical characterisation of ESD degradation in GaAs devices

This paper presents a study of electrical waveforms which characterise the behaviour of the gate Schottky diode of a GaAs MESFET (MEtal Schottky FET) under electrostatic discharge (ESD) conditions. It is demonstrated that the ESD breakdown threshold is highly dependent on the external circuit elements of the ESD pulsing apparatus and pulse transmission medium and therefore dependent on the circuit model used for ESD threshold assessment. It is also established that a GaAs MES device under forward bias subthreshold/threshold ESD stress will exhibit a region of current controlled negative differential resistance (NDR) which is not observed in the DC characteristics. Beyond this NDR region a region of positive differential resistance (PDR) exists and it is whilst operating within this region that thermal breakdown can occur. In contrast, the reverse bias threshold is highly voltage dependent and the DC avalanche voltage must be attained during an ESD pulse for impact ionisation to occur. Continued operation in this avalanche condition, which is possible with ESD pulses greater than ≈-100 V, allows localised heating sufficient to initialise thermal runaway.

Negative differential resistance in proton-beam modified silicon

The study of current-controlled (S-type) negative differential resistance (NDR) in the reverse current-voltage ( I- V) characteristics of p +-n diodes made on the base of proton-bombarded and subsequently annealed silicon wafers has been carried out. After hydrogen implantation of 1.2–12.5 × 10 13 cm −2 with energies of 400, 300, 200 and 100 keV, and annealing during 1 h at temperatures between 500 and 600°C, the profiles of shallow donors and deep impurities and defects were measured by C- V and DLTS techniques. A reduction of the shallow impurity concentration from 8 × 10 14 cm −3 (intrinsic value) to 10 14 cm −3 in the near-surface region with 4.5 μm thickness of implanted and annealed silicon was observed. The five deep levels in the near-surface region were measured as E c-0.15, E c-0.19, E c-0.21, E c-0.30 and E c-0.35 eV. Depending on the temperature (77–300 K), above a reverse bias of 5–10 V we observed six or more regions of current-controlled NDR in the I- V) curves, electrical instabilities, and hysteresis in the reverse I– V characteristics of the diodes.

Characterization of a GaAs current-controlled bipolar-unipolar transition negative differential resistance transistor

GaAs current-controlled bipolar-unipolar transition negative differential resistance (NDR) transistors using an n+-i-p+-i-n+ homojunction structure prepared by molecular beam epitaxy are demonstrated. For a base thickness of 200 Å and using a highly doped sheet concentration of 1013 cm−2, a NDR region is revealed for IB<100 μA. The peak-to-valley current ratios are about 8 at room temperature. This is proposed to be due to the bipolar–unipolar transition reaction. When IB>=100 μA, the proposed device operates as a conventional bipolar transistor. Additionally, the effects of base thickness on current-voltage characteristics are also investigated.

A novel GaAs current-controlled bipolar-unipolar transition negative differential resistance transistor prepared by molecular-beam epitaxy

Abstract GaAs current-controlled bipolar-unipolar transition negative differential resistance (NDR) transistors using n+-i-p+-i-n+ structure prepared by molecular-beam epitaxy (MBE) are demonstrated. Using a base thickness of 200 Å and a highly doped sheet concentration of 1013 cm−2, a NDR phenomenon is revealed at base low injection level. The peak-to-valley current ratios are about eight at room temperature. This is proposed to be due to the bipolar-unipolar transition reaction. When the base is under a high injection level, the proposed device operates just like a conventional bipolar transistor. A hypothetical model is used and confirmed by experiments.

Ionisation waves in solids

It is shown that ionisation waves, familiar in connection with the positive column of a gaseous plasma, can occur in solids. This is demonstrated with a simple model in which a single type of carrier (electrons) impact ionise a deep-level trap in an insulator. Impact ionisation is described by lucky-drift theory taking into account the non-local nature of the process. An analytic theory is presented for linear waves and conditions for the existence and stability of ionisation waves are derived. It is shown that stationary, forward-travelling and backward-travelling waves are all possible, and they occur as a consequence of the non-local nature of the impact ionisation process. It is also shown that the presence of stationary waves in a finite length of sample leads to a current-controlled negative differential resistance (NDR). The end point of the growth of stationary waves in time is shown to be saw tooth waves of unchanged wavelength with an amplitude determined by the occupation of the trap at electrical neutrality; being zero for completely full or completely empty traps and being a maximum when the traps are half filled. The theory is applicable to semiconductors as well as insulators.

Nascent states of current filamentation in semiconductors governed by negative differential resistance

We report the first two-dimensional imaging of just nucleating filamentary current flow during low-temperature impact ionization breakdown of n-type GaAs. Various nascent stages of transient spatial structures leading to the formation of a stationary minimal-size filament are resolved in the highly nonlinear breakdown regime of current-controlled negative differential resistance.

Current-controlled hot-electron instability in the four-terminal heterostructure device

We demonstrate new effects of the current-controlled (S-shaped) negative differential resistance (NDR) in different circuit configurations of four-terminal modulation-doped AIGaAs/GaAs heterostructure devices, with the magnitude of the voltage drop varied by external electrodes. We observed two S-shaped instabilities having different physical origins. One effect arises due to an avalanche in the top AIGaAs layer underneath the reverse-based gate region. Another phenomenon relies on the creation, during the avalanche, of nonequilibrium hot electrons at the conducting channel (quantum well). This dramatically enhances electronic flow over the heterostructure barriers and gives rise to a strongly pronounced current-controlled NDR with a peak-to-valley ratio of ∼ 10.

Current-controlled negative differential resistance in four-terminal heterostructure devices

We demonstrate new effects of the current-controlled (S-shaped) negative differential resistance (NDR) in different circuit configurations of four-terminal modulation-doped AlGaAs/GaAs heterostructure devices, with the magnitude of the voltage drop varied by external electrodes. We observe two S-shaped instabilities having different physical origins. One effect arises due to an avalanche in the top AlGaAs layer underneath the reverse-biased gate region. Another phenomenon relies on the creation, during the avalanche, of nonequilibrium hot electrons at the conducting channel (quantum well). This dramatically enhances electron flow over the heterostructure barriers and gives rise to a strongly pronounced current-controlled NDR with a peak-to-valley ratio of ∼10.

On the electrical conduction and current-controlled negative differential resistance with memory in bulk samples of glassy semiconductor Cu0.05As0.50Te0.45: NTC thermistor-type behaviour

The essential characteristics of NTC thermistor-type behaviour, besides the transition from a high resistance state to a permanent low resistance or memory state, have been analysed in the bulk modified glassy chalcogenide semiconductor Cu0.05As0.50Te0.45. Experimental results have shown a strong decrease in electrical resistivity (which ranges from 104 to 105 Omega cm in AsTe glassy alloys and its value for this alloy is 7.9*103 Omega cm at room temperature) and in the semiconductors electron mobility gap (which ranges from 0.4 to 0.5 eV for AsTe glassy semiconductors and is 0.37 eV for this alloy), due to the presence of the metallic element copper. Also, it has been shown that there are no marked non-ohmic effects in the range of voltages under study (up to 75 V for an interelectrode distance of 2.3 mm). Furthermore, the current-controlled negative differential resistance phenomenon has been justified in terms of a thermal model and confirmed by turnover voltage measurements (28.4 V for 24 degrees C and 16.9 V for 52 degrees C). Finally, several features of this glassy alloy, such as an easy synthesis, processing and low electrical resistivity values, make it appropriate for its use in the manufacture of NTC thermistors.

Dc electroluminescence in copper-free Zns:Mn thin films. I. Local destructive breakdown and its dependence on preparation and test conditions

The proportion of random localized destructive breakdown to stable electroluminescence has been varied continuously by adjusting (1) the recipe used to fabricate dc thin-film electroluminescent devices and (2) the conditions under which they have been tested. It has been shown that neither simple geometrical artifacts nor migration effects can be responsible for localized destructive breakdown. Rather, it has been established that the experiments require an explanation based on a fundamental tendency for dc thin-film electroluminescence to be associated with a current-controlled negative differential resistance which gives rise to filamentary and sometimes destructive current concentration. It is concluded that the existence and origins of such current-controlled negative differential resistance should be tested by theoretical and materials science studies.

Observation of electrical breakdown and negative resistance in an epitaxial GaAs film of a field effect structure

We have observed electrical breakdown and current-controlled negative differential resistance (NDR) in an epitaxial n-GaAs film at room temperature by using a field effect structure. The breakdown is attributed to the impact excitation of electrons trapped in 0.9-eV centers of concentration ∼1016 cm−3. The origin of NDR is explained by the theory which was originally developed for the breakdown phenomenon in Ge at low temperatures.

Current controlled negative differential resistance in a-CdAs 2 thin film sandwich structures

Thin film current controlled negative differential resistance (CNDR) devices using amorphous (a)-CdAs 2 as the active material have been fabricated and studied in detail. Comparison of the data on temperature dependence of the turnover voltage V T with data on low field conductivity has shown that the turnover mechanism arises from Joule heating, with bulk space charge limited current effects setting in for thin (<1 μ m) film devices operated below room temperature. Measurements made as a function of device area and a-CdAs 2 film thickness lend further support to this conclusion. Additional data on fabrication yield and uniformity show that large area arrays of a-CdAs 2 CNDR devices are feasible.

Thermal and nonthermal switching in a-CdAs2 thin-film sandwich structures: The effect of bias rate

Thin-film devices incorporating a-CdAs2 which display current-controlled negative differential resistance (CNDR) under quasistatic bias conditions are described. By biasing devices with short rapid rise-time pulses, threshold switching is observed to set in at much higher voltages than needed for quasistatic CNDR. The transition region between these extremes has been studied by biasing the devices with an adjustable power ramp whose slope was varied over many orders of magnitude. Under ramp bias, the devices show CNDR with a turnover voltage approaching the quasistatic value for bias times to turnover of 0.1 sec or longer. More rapid bias rates result in CNDR with a strongly increasing turnover voltage with increasing bias rate. Analysis of the temperature of the device during ramp bias indicates that the turnover mechanism is Joule-heat induced for bias times of 0.5 sec or longer, ’’electronic’’ for bias times of 10−3 sec or less, and of mixed origin in between.

Observation of current-controlled negative differential resistance in films of borosilicate glass

Observation of current-controlled negative differential resistance in films of borosilicate glass

The influence of device geometry on instabilities in current-controlled negative resistors

Experiments have shown that either current-controlled negative differential resistance (CNDR) or threshold switching (TS) devices can be fabricated from a given chalcogenide glass. Device behaviour (CNDR versus TS) is controlled by the geometry of the thin film device, and is interpreted as arising from differences in the thermal boundary conditions between the various geometries. When lateral thermal instabilities are precluded, CNDR behaviour is obtained, and when such instabilities are allowed, TS behaviour is obtained. These results are predicted by consideration of the rate of entropy production in the device, thus confirming that it is device geometry and not choice of the material that governs whether CNDR or TS behaviour will be displayed.

Enhanced optical frequency detection with negative differential resistance in metal-barrier-metal point-contact diodes

Enhanced optically induced voltages have been observed across the junction of metal-barrier-metal point-contact diodes which exhibit a current-controlled negative differential resistance. Responsivities in excess of 5 V/W have been demonstrated when the incident optical beam is focused to ∼1 W/cm2 at the junction. This enhancement is in agreement with the behavior expected from the rectification of the optical field.

Analytic solution of a simple model for negative differential resistance

Current-controlled negative differential resistance under the constraint of uniform current flow has been shown previously to result from the incorporation of the thermal conductance of the substrate in solving for the steady-state electrical and thermal behaviour of a Joule heated semiconducting thin film mounted on a substrate. Herein are derived expressions which yield the current density-voltage characteristic in the commonly applicable limit of small temperature variation in the film. An analytic expression for the voltage at the onset of negative differential resistance is induced from the expressions in the above limit and in the isothermal limit, and is demonstrated to be an approximate solution of the model for all values of substrate thermal conductance. The predictions of this model are discussed, and are shown to be consistent with experimental measurements of the threshold voltage as a function of film thickness and of ambient temperature.

Substrate Effects in the Theory of Negative Differential Resistance

Current-controlled negative differential resistance under the constraint of uniform current flow is shown to result from the incorporation of realistic boundary conditions in solving for the steady-state electrical and thermal behavior of a Joule heated semiconducting thin film mounted on a substrate. The thermal conductance of the substrate, which is a necessary feature of the experimental situation, is shown to influence significantly the measured properties of the film-substrate combination. An analytic expression is induced for the voltage at the onset of negative differential resistance (the turnover voltage), and is demonstrated to be an approximate solution of the model for all values of substrate thermal conductance. The predictions of this model are shown to be consistent with experimental measurements of the threshold voltage as a function of film thickness and of ambient temperature.

The significance of substrate parameters in the theory of negative differential resistance

The incorporation of realistic boundary conditions in solving for the electrical and thermal properties of a Joule heated semiconducting thin film mounted on a substrate is shown to result in current-controlled negative differential resistance. Calculations of the current density-electric field characteristic for representative values of the film and substrate parameters demonstrate the importance of the thermal properties of the substrate, and correlate well with the experimental and theoretical results of others.