G. C. Vezzoli

Threshold switching and the on-state in non-crystalline chalcogenide semiconductors: An interpretation of threshold-switching research

Abstract Experimental research on chalcogenide glasses is reviewed and an interpretation concerning the cause of threshold switching and the basis of the low-impedance on-regime is presented. Observed behavior is consolidated in terms of a single conductivity mechanism and two conducting species governing the off- and on-state electrical characteristics. Phenomenological equations and their consequences are given which are shown to be capable of successfully describing both in functional form and accurate magnitude a very wide variety of experimental data including current — voltage transient characteristics, the Haberland condition, delay time, maximum allowable interruption times, rise and decay characteristics and negative capacitance. Recent research is presented concerning two types of non-thermal radiative emission during the on-regime of a threshold switching event. Various switching models are described and critically discussed in terms of the experimental data and phenomenological equations.

Equations governing threshold switching in amorphous semiconductors

Extensive data are collected and presented to test the coupled‐carrier equations and attending density‐switching criterion proposed to describe nonthermal threshold switching in amorphous semiconductors. The equations and switching criterion predict in magnitude and functional form a wide range of phenomena found in the off‐state, during the switching‐on, in the on‐state, and during the switching‐off of amorphous chalcogenide films. The critical temperature noted in a variety of experiments is identified as the elevated temperature at which the switching criterion is fulfilled thermally in the presence of a small field.

Radiation emission during the on state in a noncrystalline chalcogenide threshold switch

Two types of radiation emission have been detected during the on state of threshold switching events in noncrystalline chalcogenide thin films having a transparent SnO2 (NESA) bottom electrode and a heat conducting top electrode. The common form of the radiation is an emission at about 1.2–1.4 μm which (i) is propagated through a wide solid angle, (ii) is relatively independent of time during the on‐state pulse (iii) has an integrated intensity which is approximately a linear function of on‐state power and a superlinear function of on‐state current, and (iv) requires a minimum on‐state current for initiation. The second type of emission is between 0.75 and 3 μm and is detected rather rarely. This latter emission has a characteristic intensity which is a steeply peaked function of on‐state time and appears to be possibly highly collimated. The peaked emission has been detected in pulsed modes using single set pulses and has also been detected continuously using low‐frequency repetitive threshold set pulses...

Optical and electrical effects during polymerization and depolymerization in liquid sulfur: indications for the nonuniformity model for covalent liquids

The polymerization and depolymerization of liquid sulfur refers, respectively, to the processes of chain growth and chain scission promoted by changing thermodynamic conditions. These structural transitions cause major optical and electrical consequences, including: (1) color change from yellow to orange to red, (2) peaked high angle and low angle scattering and minimum in transmission of He-Ne laser light at the structural changes, (3) a decrease in the current induced by a pulsed ruby laser above the polymerization temperature T(p) compared with below T(p), (4) two changes in the sign of the temperature coefficient of resistance in the vicinity of T(p) and the depolymerization temperature T(d), and (5) superlinear current-voltage characteristics above T(d). The data are interpreted as being suggestive of the structure of a liquid being better described by a nonuniformity rather than random network model.

Amorphous semiconductor threshold on−state properties as functions of decay time, ambient temperature, and polarity

The on−state conductance and radiative emission properties of an amorphous semiconductor threshold switch have been investigated. The transient on−state conductance GT and differential conductance GT are found to be constant for time intervals τ of up to 120 nsec during which the voltage is below the low−frequency holding voltage but above the knee voltage. The knee voltage is the voltage at which the transient on−state current−voltage curve coalesces into the transient off−state curve in the down−voltage direction. For τ≳120 nsec, GT and GT vary asymptotically with time. As ambient temperature is increased from 300 to 400 °K, GT is observed to remain constant or increase slightly as would be expected in dGT/dT for a metal. If the on−state is assumed to have a band gap, conductance vs temperature measurements indicate that the gap would have a value less than 1.2×10−4 eV, and thus for practical purposes the gap could be considered to be essentially zero. The intensity of the nonthermal radiative emissi...

Maintenance of threshold on‐state of an amorphous semiconductor by cw bias: Effects of temperature, frequency, and bias interruption

Threshold chalcogenide switching devices have been maintained in an on‐state using cw functions in conjunction with the original set pulse. The on‐state is distinct from a memory state in that the off‐state can be recovered by lowering the signal amplitude (or frequency) without the need of a high current reset pulse. Once set on, the maintained or preserved on‐state shows no switching discontinuity. Essentially zero current is observed (in either polarity) until a critical voltage level is reached, defined as the barrier voltage, beyond which the current rises almost vertically. The I‐V curve of the preserved on‐state displays its most pronounced temperature dependence at frequencies just above the value at which the device turns off but is relatively temperature independent at significantly higher frequencies (1 MHz and above). The temperature dependence has been investigated near the turn‐off frequency from 500 down to about 20 °K, and consistently shows a maximum in barrier voltage vs ambient temperat...

Preservation of threshold on‐regime in amorphous semiconductor threshold switch

Chalcogenide amorphous threshold switching devices have been pulse switched and studied in the on‐regime by applying cw functions. A 1‐MHz square, sine, or triangular test wave of maximum amplitude ±7 V applied to a device which is then pulse switched (65, 30, or 10 V threshold), will cause the device to remain in the on‐regime after the set pulse turns off. Lowering the cw frequency or lowering the wave amplitude beneath critical values causes the specimen to turn off. At frequencies significantly above the turn‐off value, the positive and negative high‐current segments of the V‐I on‐regime curve are connected by a very high‐resistance essentially linear region passing through zero. The on‐regime can be preserved in this manner at ambient temperatures varying from at least −196 to + 150 °C; however, increasing ambient temperature causes a decrease in the turn‐off frequency. Decreasing the wave amplitude causes an increase in the turn‐off frequency.

Transformation from Ohmic to offset behavior for the on state of an amorphous semiconductor threshold switch for interruptions greater than 12 ns

In measuring the on‐state I‐V curve of an amorphous semiconductor threshold switch it is essential that the addressing voltage pulse does not allow a subholding voltage interval which is longer than a ’’distribution free carrier lifetime’’ in order that only on‐state properties be detected. If the voltage is not allowed to fall beneath the holding level for longer than about 12 ns, the well‐known offset barrier of the transient on characteristics (TONC) does not develop, and, instead, the on‐state curve is linear and largely temperature independent. The time interval τ1≈12 ns is interpreted as the distribution free carrier lifetime, or the average time interval of subholding voltage during which a sufficient number of carriers can either redistribute across the device, or can recombine into traps, to initiate the development of the offset barrier voltage in the TONC. The offset voltage, or what is called the blocked on state or the transient off state, is thus not a true element of the metal‐like on state...

Variation in optical transmission and electrical resistance in a single sulfur liquid droplet during polymerization

Electrical resistance and optical transmission anomalies are observed in a single droplet of liquid sulfur during polymerization and depolymerization from 155 degrees C to 190 degrees C. Correspondence is found with some of the effects observed in electrooptic studies on bulk samples. Unlike the bulk studies, however, the single droplet shows an optical transmission of laser light that appears to follow the viscosity curve in thermal dependence. This divergence between the two studies is interpreted in terms of scattering effects at the droplet surface.