Abstract:
A memory cell stores data permanently in a memory material that can assume a first, high-resistance state and a second, low-resistance state, that is in a phase-changeable or ovonic material. A heating device is disposed to heat the memory material at different rates to a programming temperature. The memory material either has a high resistance or a low resistance after cooling, depending on the heating rate. The heating device has a switching device and a heating element in immediate vicinity to the memory material. The switching device has a field-effect transistor and a drain region of the field-effect transistor forms the heating element. Alternatively, the heating element includes a diode or a diode chain.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to non-volatile memory cells for the permanent storage of data. The invention relates, in particular, to memory cells in which the storage of data is carried out by an “ovonic” memory material, in particular an ovonic solid-state memory. 
     The “ovonic” memory material, also referred to a “phase-changeable memory material,” can assume a high-resistance state and a low-resistance state. The memory material is usually an alloy which may be present in two phase state forms: in a low-resistance polycrystalline structure and in a high-resistance amorphous structure. In order to bring the memory material into one of the two states, it has to be melted and then cooled again, so that it solidifies in one of the two state forms. 
     If the memory material is heated rapidly and with a high supply of energy until it melts, the crystal structure of the memory material is destroyed, and it solidifies into the amorphous state on cooling. If the memory material is melted more slowly in a longer heating step with a lower supply of energy, then it assumes a polycrystalline state upon cooling. The memory material has a high resistance in the amorphous state and a lower electrical resistance in the polycrystalline state. 
     “Reborn Memory May Put Flash in Shade”, IEEE spectrum, March 2002, pages 20 to 21, discloses the construction of a memory cell with the aid of such an ovonic memory material. The memory cell shown therein has a heating device with a bipolar transistor and a heating resistor. The heating resistor is provided in direct proximity to the memory material. The memory material and the heating resistor are connected in series with the emitter of the bipolar transistor. Depending on whether reading or writing is to be effected, a specific voltage is present between that terminal of the memory material which is not connected to the heating resistor and the collector of the bipolar transistor. The current through the memory material and the heating resistor can be controlled via the base input of the bipolar transistor. 
     When the memory cell thus formed is written to, a write voltage, lying above the read-out voltage required for the read-out, is applied as the specific voltage. Through activation of the bipolar transistor with the aid of a control signal applied to the base input, current flows through the memory element, the heating resistor and the bipolar transistor. The current heats the heating resistor and thus the memory material provided in direct proximity to the heating resistor. The heating is controlled by the control signal at the base input and the memory material is heated in this way slowly or rapidly until it melts. After cooling or resolidification, the memory material assumes a high-resistance amorphous or low-resistance polycrystalline state. 
     Through the activation of the memory cell during read-out with the aid of a control signal supplied via a word line to the base input of the bipolar transistor, a different voltage drop across the entire memory cell can be measured depending on the state of the memory material. 
     In the case of the integrated construction of such a memory cell, the heating resistor is arranged very near to the bipolar transistor, so that the bipolar transistor is likewise heated to a great extent. Since the melting point of memory materials that are customarily to be used, such as e.g. an alloy comprising germanium, antimony tellurium, is approximately 600° C., it is necessary also to heat the heating resistor up to this temperature range. However, since the functionality of a conventional bipolar transistor is ensured only up to a maximum of 150 to 200° C., overheating of the bipolar transistor and the failure thereof may occur. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a memory cell configuration which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a memory cell of the above-mentioned type in which the disadvantage of overheating and destruction of the transistor can be avoided. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a memory cell for permanently storing data, comprising: 
     a memory material capable of assuming a first, high-resistance state and a second, low-resistance state; 
     a heating device configured to heat the memory material at different heating rates to a programming temperature, the memory material having a relatively high resistance or a relatively low resistance after cooling, depending on the heating rate; 
     the heating device having a switching device and a heating element in direct proximity with the memory material, and the switching device having a field-effect transistor with a drain region formed as a heating region. 
     With the above and other objects in view there is also provided, in accordance with the invention, a non-volatile memory cell in which the heating element includes a diode or a diode chain. 
     A first aspect of the present invention provides a memory cell for the permanent storage of data with an “ovonic” memory material. The memory material can assume a first high-resistance state and a second low-resistance state. A heating device is provided in order to heat the memory material at different rates to a programming temperature, the memory material having high resistance or low resistance after cooling, depending on the heating rate. The heating device has a switching device and a heating element in direct proximity to the memory material. The switching device has a field-effect transistor, a drain region of the field-effect transistor being provided as heating element. 
     Such a memory cell has the advantage that the heating region is essentially better separated from the active region of the field-effect transistor than in the case of a memory cell according to the prior art. Furthermore, it is not necessary to provide a separate heating element in the form of e.g. a heating resistor, so that the production of such a memory cell can be simplified. Since virtually the entire voltage drop in a field-effect transistor takes place in the drain region, the drain region can simultaneously be used as a heating resistor. 
     It may furthermore be provided that the drain region comprises a highly doped contact-making region for making contact with the memory material. 
     It may furthermore be provided that the field-effect transistor is constructed vertically in a substrate and is surrounded by an insulation material having low thermal conductivity. In this way, it is possible to arrange the memory cell according to the invention in integrated form in an array arrangement as well. The insulation material ensures that the heat produced in the drain region of the field-effect transistor is not emitted, or is emitted only to a reduced extent, into the memory material of adjacent cells. Said insulation material may have a silicon compound, in particular silicon dioxide or silicon nitride. 
     A further aspect of the present invention provides a memory cell for the permanent storage of data with a memory material. A heating device is provided in order to heat the memory material at different rates to a programming temperature, the memory material having high resistance or low resistance after cooling depending on the heating rate. In this case, the heating device has a switching device and a heating element in direct proximity to the memory material. The heating element is designed as a diode or diode chain. In particular, the diode may be designed as a Zener diode operated in the reverse direction. 
     The Zener diode or the diode chain has the advantage that the heating region is even further away from the active region of the switching device. The Zener diode or the diode chain is chosen such that the largest voltage drop is present across the Zener diode or the diode chain rather than across the switching device, e.g. a transistor, so that the majority of the electrical power in the diode is converted into heat. Consequently, the diode serves as a heating element for the memory material. 
     Preferably, the diode is formed by a semiconductor material which exhibits functionality at the programming temperature. In particular, the Zener diode may be formed by silicon carbide. A diode made of silicon carbide has the advantage that it is functional even at high temperatures, i.e. at temperatures higher than 600° C. Consequently, the diode can be used as a heating element in a suitable manner since the diode can be dimensioned such that a large part of the voltage is dropped across it, so that the power consumption is greatest across the Zener diode. 
     It may be provided that the switching device has a field-effect transistor and the diode or the diode chain is formed by a layer sequence on the drain region of the field-effect transistor. The memory cell according to the invention can be produced in a simple manner in this way. Preferably, the drain region of the field-effect transistor and the diode or the diodes are separated by a highly conductive semiconductor layer, in particular a highly doped semiconductor layer, in order to form a thermal resistor between the field-effect transistor and the diode as heating element. In this way, it is possible to prevent the heating element, i.e. the diode, from overheating the active region of the transistor and thus to avoid the destruction or impairment of said transistor. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a non-volatile memory cells, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagrammatic perspective view of the construction of a non-volatile memory cell in accordance with the prior art; 
         FIG. 1B  is a circuit diagram of the memory cell according to the prior art in accordance with  FIG. 1A ; 
         FIG. 2  is a circuit diagram of an embodiment of a memory cell according to the invention; 
         FIG. 3  is a cross section through the construction of the memory cell according to the invention as shown in  FIG. 2 ; 
         FIG. 4A  is a circuit diagram of a further embodiment of a memory cell according to the invention; 
         FIG. 4B  is a circuit diagram of a further embodiment of a memory cell according to the invention; and 
         FIG. 5  is a cross section through the further embodiment of the memory cell according to the invention as shown in FIG.  4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a section of a prior art non-volatile memory cell. The non-volatile memory cell has a selection transistor  1  formed from a emitter region  2 , a base region  3 , and a collector region  4 . A resistance element  5  is connected to the emitter region  2 . The element  5  is operated as a heating resistor and it is surrounded by an insulation layer  6 . Situated at that end of the resistance element  5  which is remote from the emitter  2  of the bipolar transistor  1  is a layer of an ovonic memory material  7 , in which the information is to be stored. A contact-making layer  8  is connected to the memory material  7  in order to make contact with the memory cell. 
     The “ovonic” memory material  7  is a material from the group of chalcogenides, which are usually formed from alloys. Typical chalcogenides have materials such as, for instance, germanium, antimony, tellurium, sulfur, etc. In particular the ternary alloy germanium-antimony-tellurium represents a suitable material for the construction of this memory cell. 
     The information is stored in the memory material  7  in that the memory material  7  can be transferred into two different state forms. The memory material  7  can be present in polycrystalline and amorphous form. The memory material  7  essentially has low resistance in its polycrystalline form and essentially has high resistance in the amorphous state. The difference in resistance is so considerable that it can be used for information storage purposes. 
     The different states of the “ovonic” memory material  7  are achieved by a procedure in which the memory material  7  is momentarily melted and, upon cooling and resolidification, assumes either the polycrystalline or the amorphous form. The fact of whether the polycrystalline or the amorphous form is assumed essentially results from the nature of the heating or melting operation. If the memory material is heated slowly up to the melting point, or to the writing temperature, then the memory material solidifies in an amorphous, i.e. high-resistance, state. By contrast, if the memory material is heated very rapidly to the melting point, then the memory material solidifies in a polycrystalline form and accordingly has a lower resistance than in the amorphous form. 
       FIG. 1B  illustrates the circuit diagram of a memory cell constructed in this way. The memory cell is addressed via a word line WL and a bit line BL. The word line WL is connected to the base region of the bipolar transistor  1 , a pnp transistor. The collector region  4  of the bipolar transistor  1  is connected to a ground terminal and the emitter region  2  is connected to a first terminal of the heating resistor  5 . A second terminal of the heating resistor  5  is connected to a first terminal pad of the memory material  7 , so that the heating resistor  5  has a low thermal resistance with respect to the memory material  7 . A second terminal pad of the memory material  7  is connected to the bit line BL. 
     For writing to the memory cell, a write voltage is applied to the bit line. The write voltage is large enough to provide a sufficient power supply for the heating resistor in order to reach the melting point of the memory material  7 . Afterward, a control signal is applied to the base region  3  of the bipolar transistor  1  via the word line WL. The control signal determines what information is stored in the memory material  7 . The control signal is chosen, for storing a first item of information, such that it brings about a low and relatively lengthy current flow through the memory material and the heating resistor  5 . In this case, however, the control signal is chosen such that the current flow is large enough to momentarily bring the memory material to or above its melting point. 
     In order to store an inverse item of information with respect thereto a control signal is applied to the base input  3  of the bipolar transistor  1  via the word line WL, said control signal effecting a larger current flow through the heating resistor  5  and the memory material  7 , so that the memory material  7  is heated more rapidly to the melting point. As a result, the memory material  7  solidifies into an amorphous form upon cooling and thereby acquires high resistance. 
     The memory cell can then be read by a procedure in which firstly the word line WL is activated and the bipolar transistor  1  is essentially completely turned on. In this way, a current which is dependent on the information stored in the memory material  7  flows via the bit line BL, the memory material  7 , the heating resistor  5  and the bipolar transistor  1 . Consequently, the read-out of the information can be performed by means of the current flowing via the bit line BL. 
     A first embodiment of the invention now provides for the heating resistor  5  to be replaced by an enlarged drain region of a field-effect transistor. The circuit diagram according to  FIG. 2  illustrates the circuit of such a memory cell.  FIG. 3  shows a cross section through the construction of such a memory cell according to the invention. 
     The switching transistor is formed by a field-effect transistor  10  comprising silicon. The field-effect transistor is an n-channel transistor and an n-doped source region  11 , a p-doped gate layer  12  and an n-doped drain region  13  are formed. The field-effect transistor  10  is constructed vertically in a substrate and is electrically and thermally insulated from its surroundings by an oxide layer  14 , preferably a silicon oxide layer. In the oxide layer  14 , the gate  15  is arranged in such a way that it can bring about a separation of the charge carriers in the gate region  12 . The gate  15  is preferably formed from polysilicon. 
     Source region  11  and drain region  13  are preferably n-doped, and the gate region  12  is preferably p-doped. The drain region  13  is enlarged relative to conventional field-effect transistors, thus resulting in an LDD field-effect transistor (largely doped drain field-effect transistor). In an LDD field-effect transistor, a large part of the voltage is dropped essentially in the drain region during operation on account of the electrical resistance, so that, in the event of a current flow, the electrical power is converted into heat in the drain region. This heat is used for heating the memory material  7 . 
     Thus, according to the invention, it is no longer necessary to provide a separate heating resistor  5 , but instead an enlarged drain region  13  across which a large part of the write voltage is dropped during the process of writing to the memory cell. In order to be able to make better contact between the memory material  7  and the silicon, a highly doped n + -type contact-making layer  16  is provided between the drain region  13  and the memory material  7 . 
     It will be understood that a p-channel field-effect transistor can also be used instead of the n-channel field-effect transistor. 
     Designs using SiO technology or with semiconductor materials other than silicon also lie within the scope of this invention. 
       FIGS. 4A and 5  and also  FIG. 4B  illustrate further embodiments of the present invention.  FIG. 4A  shows a circuit diagram in which the heating element is designed in the form of a Zener diode  20  which can be operated in the reverse direction. A Zener diode  20  has the effect that it has an essentially current-independent voltage drop across its terminals in reverse-biased operation. In this way, when the field-effect transistor  10  is activated, the large part of the voltage is present across the Zener diode  20 . 
     The Zener diode  20  is arranged in direct proximity to the memory material  7  since the Zener diode  20  takes up virtually the entire power of this current path. As a result, the Zener diode  20  is heated to the greatest extent and can be used for heating up the memory material  7 . To ensure that the Zener diode  20  as far as possible does not heat up the field-effect transistor  10  and thus render the latter non-functional, a thermal resistor  21  in the form of a readily conductive, highly doped semiconductor material is provided between the Zener diode  20  and the field-effect transistor  10 . The thermal resistor  21  has the task of as far as possible not passing on, or passing on only to a slight extent, the high temperature of the Zener diode  20  to the field-effect transistor  10 . Consequently, the thermal resistor  21  should be made of a material which has very good electrical conductivity, in order that it is heated as little as possible on account of the current flow, and have a low thermal resistance. Very highly n + -doped silicon is very highly suitable as thermal resistor. 
     The Zener diode  20  is preferably produced from a semiconductor material with which the Zener diode  20  remains functional even at very high temperatures. The Zener diode  20  should preferably be able to withstand temperatures at which the memory material  7  can be written to without the Zener diode  20  forfeiting its functionality. Silicon carbide, for example, is conceivable as a suitable material. A Zener diode  20  which has been produced from the material silicon carbide withstands temperatures above 600° C. and is thus able to serve as a heating element for the memory material  7 . The Zener diode  20  may equally have materials such as diamond, gallium nitride or semiconductor materials having a large band gap in order still to function as semiconductors at high temperatures of above 400° C. to 600° C. 
       FIG. 5  illustrates the construction of such a memory cell in cross section. The field-effect transistor  20  is essentially similar to the field-effect transistor in accordance with the embodiment as shown in  FIG. 3. A  thermal resistor  21  is arranged above the drain region  13  of the field-effect transistor  10 . The thermal resistor  21  is formed from a highly doped silicon material, preferably an n-doped silicon material. The size of the region of the thermal resistor  21  is chosen such that this region has a lowest possible electrical resistance in order to keep down the voltage drop in this region and, on the other hand, in order to form a sufficient thermal isolation between the field-effect transistor  10  and the Zener diode  20 . 
     The Zener diode  20  is formed from a p-doped silicon carbide layer  22  and an n-doped silicon carbide layer  23  arranged above the latter. The Zener diode formed by the layers  22 ,  23  is arranged in direct proximity to the memory material  7 . 
     During operation, i.e. when writing to the memory cell, a large part of the voltage, then, is dropped across the Zener diode  20  and thus heats it. A rapid or slow heating of the memory material  7  can thus be effected, controlled via the gate  15 . 
       FIG. 4B  shows a further embodiment of the invention, in which a plurality of conventional diodes  24  are provided as heating element. These diodes  24  are connected in the forward direction and have the voltage drop typical of these diodes, which depends on the semiconductor material, the doping thereof or the band gap thereof. The diodes  24  are connected in series and arranged as compactly as possible in proximity to the memory material  7 . The diodes  24  are likewise arranged as pn layers above the thermal resistor region  21  and can be constructed in a similar manner to the Zener diode in FIG.  5 .