The basic characteristics of a PAC based SM from Savransky's work are shown in Figure 1. The characteristics are for a single two terminal device with two threshold voltages. The conducting state, after threshold switching, is characterized by two regions of different slope connected at a transition point marked “T” in Figure 1. The two conducting regions are for the different polyamorphic states.
In operation, starting with the device in the low threshold voltage state (Vth1), it is switched along a load line to its conducting state. The current is then increased to bring the device through the transition point to the higher current region. This moves the material into its second polyamorphic state, the current is then rapidly reduced to zero, leaving the device in its high threshold voltage state. For switching from the high to the low threshold voltage state, the process is reversed. The read operation is to apply a low threshold voltage pulse and detect if the device switches.
Savransky describes a transition step that allows the device in the high threshold voltage state to be switched along a load line directly into the low threshold switch conducting state. I had concerns about this as a possibility, based on the characteristics shown in Figure 1.
If, as is shown in Figure 2, the two threshold switching states are considered as separate devices in the same package--Figures 2(a) and 2(b)--it is difficult at first sight to see how a device in the low threshold voltage state can be directly switched into the conducting state of the high threshold voltage device. (This is illustrated in the figure as the load lines ending in a question mark.) To aid the explanation of the write process for the article, an alternative picture of the write characteristics was developed. It invokes the existence of a quasi-stable dynamic condition for the post switching conducting state for a SM device in both the high and low threshold voltage conditions, as is illustrated in the combined characteristics of Figure 2(c) by the dotted extrapolations. The author of  concurs  with the inclusion of a time dependent quasi-stable extension to the conducting states of the SM device as an aid to understanding.
It appears to me there is another anomaly associated with the switching characteristics of Figure 1 and in the individual characteristics of Figure 2(b), which deserves further brief consideration here. The post switching characteristics of a threshold switch, as illustrated in Figure 3, are usually considered as the sum of two component parts, a constant voltage (Vh), constant current density region (the characteristics of an expanding and contracting filament) in series with an Ohmic resistor.
The value of Vh is determined as the voltage where the back extrapolation of the combined characteristics intercepts the voltage axis. The problem with the characteristics as described by Savransky and Figure 2(b) is the slope of the resistive component of the second state extrapolates to a holding voltage with a negative value. The explanation of this may be as follows and relates to the fact that all of the active material of the SM device is now involved in its conducting state; it has become a bulk device. Some concept of the characteristics of the conducting region when it cannot expand and is constrained by the sidewalls of the structure might perhaps be determined by the experiment illustrated in Figure 4.
It shows the characteristics of the conducting region of a threshold switch, ignoring for the moment the series resistance element. If while in the conducting state, at a fixed current, the filament is subjected to current pulses, shorter in rise time and duration than the time constant of filament expansion, there are three options as the temperature of the material is raised. These options are illustrated for material with a positive, zero or negative temperature coefficients of resistance.
Moving back to the SM characteristics, if the switched material of a PAC SM is constrained by sidewalls of the device and if the material in the conducting state has a positive temperature coefficient of resistance, the resulting dynamic characteristics would extrapolate to a negative voltage axis intercept, as illustrated in Figure 4. Without the actual electrical conductivity as a function of temperature for the PAC materials used in SMs it is difficult to comment further.
Tg range is 140-270 deg. C, it can be expanded but
lower Tg glasses are not very technological and higher Tg will probably not be power efficient.
I did not observed crystallization in PAC in calorimetry and long-term storage. Based on calorimeter min heating speed estimate for crystallization energy is above 5 eV.
Semyon, thanks for your reply. I was only referring to Figure 1. I accept that it would not be an issue in other cases where the current does not jump so high after exceeding Vth.
I couldn't get to attend MRS so I couldn't get to ask you other questions, such as the Tg range and also the crystallization temperature range.
I agree that threshold voltage Vth drift is possible in switching memory (SM). There are few methods to deal with drift in phase-change memory (PCM) on the active alloy, cell structure, programming pulses and array periphery levels, some of these solutions are applicable to SM. I think one of the most attractive is opportunity to use SM as DRAM with long refreshment times (about 1000 sec) there drift would not be so important.
I disagree with your assumption about destructive read. I was able indeed read low Vth few times without destruction but more studies are needed to claim fully non-destructive read, investigate drift, recovery, temperature coefficients for Vth, etc…..
The PAC films fabrication is not more difficult than fabrication of films for PCM or for threshold switches for nano-devices that have been done by Intel, Samsung and other companies.
Thank you for the review that really extends my MRS presentation.
Let me clarify some points:.
PCM as well as some of types of RRAM have low efficiency (only few percents) because of huge entropy penalties related with operation between ordered and disordered states, while switching memory SM supposes to be more total energy efficient and operates at lower currents.
Ge-Si-As-Te glass was used by Stanford Ovshinsky in stable threshold switches but to best of my knowledge it is NOT polyamorphic.
Although initial current filamentation occurs in SM devices they are NOT necessary should have pore structure, actually I used devices with structure similar to a planar capacitor. I not sure that PAC material must be deposited in the low threshold voltage state for nano-size SM devices but no studies have been performed. Initial values of threshold voltage were masked by imperfections of technology used to produce micro-size devices.
I observed a threshold voltage range of the factor slightly above2 only.
Because no detailed studies of PAC were conducted I do not have data about threshold voltage temperature coefficients.
When I made PCM presentation on behalf of Intel Corporation, managers always asked to use arbitrary units and mirror some relations to protect the company proprietary information and IP, so I learned the lesson and applied it to this MRS presentation.
Volatile Memory: I think I commented in my conclusion on the use of arbitrary units as a problem. Rather than personal attacks I think the first order of business must be for an independent third party to try and reproduce the results reported by Savransky.
Resiston: I agree threshold voltage drift, or post switching recovery, will be an important consideration for PAC based SMs.
If the post switching recovery in threshold switches is the same as in PCM memory devices after reset, then that may be an important clue to the conditions, especially temperature, that exist in the conducting filament of a threshold switch. Be it PAC based or any other type for that matter.