S-type Negative Differential Conductivity Research Articles

Threshold and Filament Current Densities in Chalcogenide-Based Switches and Phase-Change-Memory Devices

Threshold current density of switching was found to grow several decades at constant threshold electric field with reducing interelectrode distance. Data are compared with predictions of various switching theories. Experimental estimate of current density in an electronic filament is reported based on a programming "dead-space" observation in a ring contact device with one subcritical bottom contact dimension. Significance of the shape of S-type negative differential conductivity curve and, namely, the slope of the second part with positive differential conductivity for the programming of phase-change memory is discussed.

S-type negative differential conductivity and voltage switching due to the avalanche in semiconductor heterostructures

A large S-type negative differential conductivity effect in reversed-biased heterostructure hot-electron diodes was found experimentally and explained theoretically. It was demonstrated that this effect originates both from an enhanced band-to-band impact ionization in the lightly doped -GaAs cladding region and from an accompanying electric field redistribution throughout the device.

Voltage switching and oscillations in a single barrier heterostructure hot-electron diode

S-type negative differential conductivity and relaxation oscillations in a single-barrier n−-GaAs/Al0.34Ga0.66As/n+-GaAs heterostructure hot-electron diode are studied experimentally and by ensemble Monte Carlo simulations at 300 and 77 K lattice temperature. The influence of interband impact ionization on the instability is investigated. The frequency is found to be limited by a slow electron dispersal from the accumulation layer adjacent to the n−-GaAs/Al0.34Ga0.66As interface to less than 10 GHz. The maximum frequency of the oscillations observed experimentally was about 0.5 GHz on account of the parasitic capacitance of the investigated device structure.

Bistability and hysteresis in the characteristics of segmented-anode lateral IGBTs

The voltage and current controlled bistability of segmented anode lateral insulated gate bipolar transistors (SA-LIGBTs) containing more than one p-anode segment has been investigated experimentally by means of monitoring the infrared recombination radiation, as well as theoretically by two-dimensional (2-D) device simulation. We have found out that the instabilities are caused by the switching of single p-anode segments from the MOS-mode into the conductivity modulated state and vice-versa. The switching current and voltage show pronounced memory effects, if the single cell structure exhibits S-type negative differential conductivity (SNDC) because of conductivity modulation. The simulation of a simplified vertical hybrid diode/pnp-transistor structure shows very good qualitative agreement with measured characteristics and carrier distributions in the SA-LIGBT. Generally, such hysteresis effects may occur in all electronic devices containing SNDC-elements switched in parallel.

Interband and impurity breakdown in a semiconductor with an impurity band in a high electric field

Hot electron kinetics and electric breakdown in a semiconductor with a band of delocalized impurity states are investigated theoretically for the first time. We consider the case when the impurity band is separated from two allowed bands of the crystal by gaps of forbidden states. Contributions to the kinetics from all three groups of carriers in the conduction, valence and impurity bands are taken into account including changes in carrier concentrations due to impact ionization in a high field. It is shown that a new combined overheating mechanism of S-type negative differential conductivity can take place in the system which is connected with simultaneous changes in the carrier concentrations and mobilities. It is predicted that in some cases the current-voltage characteristics of the system can have two S-shaped parts. The process of breakdown in the system combines features of impurity and interband breakdown.

Numerical simulation of the hysteresis in the sidegating effect in GaAs MESFETs-the effect of Schottky contacts

Two-dimensional simulation of the sidegating effect in GaAs MESFETs has been performed. The result confirms that Schottky contacts on semi-insulating substrate cause serious high substrate leakage current and drain current reduction in GaAs MESFETs. The competition between the currents or biases of the contacts is found to be the cause of the S-type negative differential conductivity (S-NDC) or hysteresis observed when measuring the sidegating threshold. >

Inclusion of impact ionization in the backgating of GaAs FETs

It is shown that the familiar threshold behavior of the backgate current of GaAs MESFETs has hysteresis. This is associated with an S-type negative differential conductivity (S-NDC) of the semi-insulating substrate. It is difficult to account for this hysteresis using conventional trap-fill-limited (TFL) theory, and it is attributed to the impact ionization of traps in the substrate. A simple model of this ionization, involving two trap levels, is used to incorporate its effect into an existing analytical model of GaAs FETs. The result is a qualitative interpretation of the backgating characteristics of GaAs MESFETs. The calculations show that a simple combination of two ohmic elements to represent parasitic resistances, and a nonohmic one to represent impact ionization in the substrate, can imitate the observed backgating behavior.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Impact ionization of deep traps in semi-insulating GaAs substrates

The behavior of the leakage current in the semi-insulating substrate of a GaAs device is more complicated than previously recognized. The voltage dependence of this current seen in a conventional voltage-controlled experiment has hysteresis, which arises from an S-type negative differential conductivity (S-NDC). This is incompatible with the conventional trap-fill-limited model, and we propose an alternative explanation based on the impact ionization of deep-level traps. We show how this simple model can account qualitatively for the S-NDC and the associated current instability, and how it can be extended when the deep traps are photoexcitable.

Current-controlled negative differential conductivity in semi-insulating GaAs

The leakage current in the semi-insulating substrate of a GaAs device measured in a conventional voltage-controlled experiment shows hysteresis. Experiments in constant-current conditions show that this arises from an S-type negative differential conductivity. These features are incompatible with the conventional trap-fill-limited model, and we outline an alternative explanation based on the impact ionization of multiple deep level traps.