the following detailed description is made with reference to the figures . preferred embodiments are described to illustrate the present invention , not to limit its scope , which is defined by the claims . those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows . a chalcogenide alloy contains one or more elements from column six of the periodic table of elements . many chalcogenide phase - change alloys have been described in technical literature , including alloys of : ga / sb , in / sb , in / se , sb / te , ge / te , ge / sb / te , in / sb / te , ga / se / te , sn / sb / te , in / sb / ge , ag / in / sb / te , ge / sn / sb / te , ge / sb / se / te and te / ge / sb / s . in the family of ge / sb / te alloys , a wide range of alloy compositions may be workable . the compositions can be characterized as te a ge b sb 100 −( a + b ) . one researcher has described the most useful alloys as having an average concentration of te in the deposited materials well below 70 %, typically below about 60 % and ranged in general from as low as about 23 % up to about 58 % te and most preferably about 48 % to 58 % te . concentrations of ge were above about 5 % and ranged from a low of about 8 % to about 30 % average in the material , remaining generally below 50 %. most preferably , concentrations of ge ranged from about 8 % to about 40 %. the remainder of the principal constituent elements in this composition was sb . these percentages are atomic percentages that total 100 % of the atoms of the constituent elements . ovshinsky &# 39 ; 112 patent , cols 10 – 11 . particular alloys evaluated by another researcher include ge 2 sb 2 te 5 , gesb 2 te 4 and gesb 4 te 7 . noboru yamada , “ potential of ge — sb — te phase - change optical disks for high - data - rate recording ”, spie v . 3109 , pp . 28 – 37 ( 1997 ). more generally , a transition metal such as cr , fe , ni , nb , pd , pt and mixtures or alloys thereof may be combined with ge / sb / te to form a phase - change alloy that has programmable resistive properties . specific examples of memory materials that may be useful are given in ovshinsky &# 39 ; 112 cols . 11 – 13 , which examples are hereby incorporated by reference . phase - change alloys are capable being switched between a first structural state in which the material is generally amorphous and a second structural state in which the material is generally crystalline in its local order . these alloys are at least bistable . the term amorphous is used to refer to a relatively less ordered structure , more disordered than a single crystal , which has the detectable characteristics such as high electrical resistivity . the term crystalline is used to refer to a relatively more ordered structure , more ordered in an amorphous structure , which has detectable characteristics such as lower electrical resistivity than the amorphous state . typically , phase - change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states . other material characteristics affected by the change between amorphous and crystalline phases include atomic order , free electron density and activation energy . the material may be switched either into different solid phases or into mixtures of two or more solid phases , providing a gray scale between completely amorphous and completely crystalline states . the electrical properties in the material may vary accordingly . phase - change alloys can be changed from one phase state to another by application of electrical pulses . it has been observed that a shorter , higher amplitude pulse tends to change the phase - change material to a generally amorphous state . a longer , lower amplitude pulse tends to change the phase - change material to a generally crystalline state . the energy in a shorter , higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state . appropriate profiles for pulses can be determined , without undue experimentation , specifically adapted to a particular phase - change alloy . the physical phase - change process has motivated research into structures that use a small amount of programmable resistive material . fig1 c and 2 are block diagrams of novel cross - sections of programmable resistive material . fig1 depicts a multilayer structure , having a first electrode 103 , an insulating layer 102 , over the first electrode , in the second electrode 101 , over the insulating layer . the first electrode preferably is tialn . the first electrode preferably has a thickness of 10 to 30 nm , which is less than the minimum lithographic feature size of current lithographic processes . the inter - electrode insulating layer may be silicon oxide , silicon nitride , al 2 o 3 or an ono or sono multi - layer structure . alternatively , the inter - electrode insulating layer may comprise one or more elements selected from the group consisting of si , ti , al , ta , n , o , and c . the inter - electrode thickness may be 100 to 300 nm . the second electrode may be tiw . it may have a thickness of 200 to 400 nm . alternatively , the electrodes may be tin or tan , or may comprise one or more elements selected from the group consisting of ti , w , mo , al , ta , cu , pt , ir , la , ni , and o . a multilayer structure has sidewalls illustrated in fig1 the right and left sides of the multilayer structure . the multilayer structures are constructed over a non - conductive or insulating base layer 104 , which may be a substrate or a layer on top of other layers . in some embodiments , the contact via and plug 105 may be defined through the insulating layer 104 to make contact with the first electrode 103 . the programmable resistive material in this structure forms a spacer 111 ″ along the sidewalls of the multilayer structure . the spacer structure is formed using conventional methods of forming a spacer . initial spacer deposition layer thickness may be 100 to 300 nm or less . after etching , the space width may be reduced to 10 to 20 nm , which is less than the minimum lithographic feature size of current lithographic processes . in this figure , the active region of phase change is preferably at the interface between the first electrode and spacer . a low first electrode thickness improves device performance by reducing the interface area in which phase change takes place . alternatively , the current flow , electrode materials and thicknesses could be reversed and the phase change area could appear at the interface to the second electrode . useful characteristics of the programmable resistive material include the material having at least two solid phases that can be reversibly induced by electrical current . these at least two phases include an amorphous phase and a crystalline phase . however , in operation , the programmable resistive material may not be fully converted to either an amorphous or crystalline phase . intermediate phases or mixtures of phases may have a detectable difference in material characteristics . the two solid phases should generally be bistable and have different electrical properties . the programmable resistive material may be a chalcogenide alloy . a chalcogenide alloy may include ge 2 sb 2 te 5 . alternatively , it may be one of the other phase - change materials identified above . fig2 depicts an alternate multilayer structure , having a first electrode 203 formed as a buried diffusion in semiconductor base layer 204 . the base layer may be a substrate or a layer over other layers . in this multilayer structure , the insulating layer 202 , is over the first electrode , and the second electrode 201 , is over the insulating layer . the programmable resistive material 211 ″ forms a spacer along the sidewalls of the multilayer structure , generally corresponding to the sides of the insulating layer 202 and the second electrode 201 . the interface between the spacer and the second electrode defines a phase change region , as a high resistance material is more easily used for a second electrode than for the buried diffusion . this figure illustrates a buried diffusion without need for contact via . in alternate embodiments , a contact via could connect the buried diffusion through the additional insulating layer to layers below . fig3 depicts an additional structure that may be combined with a spacer memory structure . in particular , an isolation transistor is illustrated . one pole 321 of the transistor is electrically connected to the contact 105 . a gate 322 controls the flow of current from the other pole 323 to the contact . use of an isolation transistor may be useful , as the electrical resistance of a programmable resistive material is unlikely ever to be so great as to block current leakage from the first electrode to the second electrode . alternatively , an isolation junction or an isolation diode may be incorporated in the structure . a conventional sequence for forming a spacer is generally illustrated in fig1 a – 1c . fig1 a illustrates a multilayer structure formed using conventional method . in fig1 b , a programmable resistive material 111 is the deposited over the multilayer structure . techniques for depositing such a film include sputtering and chemical vapor deposition . a film deposited by such methods generally conforms to the structure presented , with some filling at low spots . an anisotropic etch is used to be moved most of the programmable resistive material , leaving spacers 111 ″ along the sidewalls of the multilayer structure . the anisotropic etch may be a plasma etch , a reactive ion etch , or any other etch compatible with the materials used . while the present invention is disclosed by reference to the preferred embodiments and examples detailed above , it is understood that these examples are intended in an illustrative rather than in a limiting sense . it is contemplated that modifications and combinations will readily occur to those skilled in the art , which modifications and combinations will be within the spirit of the invention and the scope of the following claims .