Abstract:
A memory device is provided comprising a transistor having a floating body positioned between source and drain regions, the floating body being sandwiched between first and second insulated gates each comprising a gate electrode. A control circuit is arranged to program the state of said floating body to have an accumulation or depletion of majority carriers by applying one of first and second voltage levels between the first gate and at least one of the source and drain regions, and to retain the programmed state of said floating body by applying a third voltage level to the second gate. The voltages are switched over a time duration shorter than 100 ns.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the national stage application under 35 U.S.C. §371 of the International Application No. PCT/EP2009/050031, filed Jan. 5, 2009, and claims the benefit of U.S. Provisional Application No. 61/006,280, filed Jan. 4, 2008, the entire disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a memory device comprising two gates and a floating body, to a method of controlling such a memory device and to a memory comprising an array of such memory devices. 
     BACKGROUND OF THE INVENTION 
     DRAMs (dynamic random access memories) comprise an array of memory cells, each memory cell generally comprising a transistor and a capacitor. One bit of data is memorized by each cell by storing, via the transistor, a level of electrical charge on the capacitor. The stored data is refreshed periodically. 
     A problem with such DRAM devices is that the capacitor is bulky and therefore it is very difficult to reduce the size of each memory cell and thus reduce the overall size of the memory or increase its capacity. 
     To solve this problem, alternative memory structures have been proposed, in which a memory cell comprises just a single transistor having a floating body. Thus rather than storing charge on a capacitor, charge is stored in the floating body to store data. However, the solutions that have been proposed until now are generally inadequate in terms of the retention time of each memory cell, the supply voltage requirements for writing, holding or reading data, and/or the ability of the memory cells to be used in an array. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a memory device comprising a transistor having a floating body positioned between source and drain regions, the floating body being sandwiched between first and second insulated gates each comprising a gate electrode; and control circuitry arranged to program the state of said floating body to have an accumulation or depletion of majority carriers by applying one of first and second voltage level between the first gate and at least one of the source and drain regions, and to retain the programmed state of said floating body by applying a third voltage level to the second gate, said voltages being switched in a time duration shorter than 100 ns, and preferentially shorter than 10 ns. 
     In an embodiment of the present invention, the gate electrode of the second gate is spaced by a first distance from said floating body, and by a second distance greater than the first distance from the source and drain regions. 
     In an embodiment of the present invention, the first and second distances are determined by the thickness of the insulating layer of the second gate, which is thicker between each of the source and drain regions and the gate electrode of the second gate than between the floating body and the gate electrode of the second gate. 
     In an embodiment of the present invention, the source and drain regions are recessed with respect to the floating body on the side of the second gate. 
     In an embodiment of the present invention, the second gate is formed to be narrower than the spacing between the source and the drain, and wherein regions of said body between the source and drain regions are lightly doped. 
     In an embodiment of the present invention, said first gate is arranged to overlap the source and drain regions by a greater distance than the overlap of the source and drain regions by said second gate. 
     In an embodiment of the present invention, said source and drain regions are formed to be more conductive on the side of said first gate than on the side of said second gate. 
     An embodiment of the present invention also provides a memory comprising an array of memory cells, each memory cell comprising the above memory device, wherein the gate electrodes of the second gates of the memory devices are coupled in rows by first word lines and the drain regions are coupled in rows by second word lines, the first and second word lines being coupled to row control circuitry arranged to select rows of said memory cells by applying voltages to the first and second word lines; and the gate electrodes of the first gates of the memory devices are coupled in columns by first bit lines and the source regions are coupled in columns by second bit lines, the first and second bit lines being coupled to column control circuitry for programming or reading said memory cells. 
     An embodiment of the present invention also provides an electronic device comprising the above memory and a memory interface circuit for controlling access to said memory. 
     An embodiment of the present invention also provides a method of manufacturing a memory device comprising: forming, in a thin silicon film, a floating body positioned between source and drain regions; and forming first and second gates on opposite sides of the thin silicon film, each gate comprising a gate electrode and an insulating layer positioned between the floating body and the gate electrode; wherein the first and second gates are formed such that the spacing between the source and drain regions and the gate electrode of the second gate is greater than the spacing between the source and drain regions and the gate electrode of the first gate. 
     In an embodiment of the present invention, the method comprises increasing the spacing between the source and drain regions and the second gate electrode by performing, after formation of the insulating layer in oxide and of the gate electrode of the first gate, an oxidization step such the oxide thickness at ends of the insulating layer in the areas of the source and drain regions is increased. 
     An embodiment of the present invention also provides a method of operating a memory device comprising a transistor having a floating body positioned between source and drain regions, the floating body being sandwiched between first and second gates each comprising a gate electrode and an insulating layer positioned between the floating body and the gate electrode, the method comprising applying one of first and second voltage level between the gate electrode of the first gate and at least one of the source and drain regions to program the state of said floating body to have an accumulation or depletion of majority carriers; and applying a third voltage level to the gate electrode of the second gate to retain the programmed state of said floating body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
         FIG. 1  schematically illustrates in cross-section a memory device according to an embodiment of the present invention; 
         FIGS. 2A to 2D  schematically illustrate in cross-section the memory device of  FIG. 1  at various stages of operation according to embodiments of the present invention; 
         FIG. 3A  schematically illustrates in cross-section a memory device according to an alternative embodiment of the present invention; 
         FIG. 3B  schematically illustrates in cross-section part of the memory device of  FIG. 3A  in more detail according to an embodiment of the present invention; 
         FIGS. 4A to 4C  schematically illustrate steps in forming the memory device of  FIG. 3  according to embodiments of the present invention; 
         FIGS. 5 to 9  schematically illustrate in cross-section memory devices according to alternative embodiments of the present invention; 
         FIG. 10  schematically illustrates a memory device according to an embodiment of the present invention; 
         FIG. 11  schematically illustrates a memory array of memory devices according to an embodiment of the present invention; and 
         FIG. 12  schematically illustrates an electronic device according to an embodiment of the present invention. 
     
    
    
     As is usual in the representation of semiconductor devices, the various cross-sections shown in the figures are not drawn to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically illustrates in cross-section a memory device  100 , comprising a thin silicon film  101  sandwiched between two opposing gates. The thin silicon film  101  comprises a P-type floating body  102  positioned between N-type source and drain regions  104  and  106  respectively. 
     The top gate in  FIG. 1  is formed on the top side of the thin silicon film  101  and comprises an insulating layer  108  separating the body  102  from a gate electrode  110 , which is for example formed of polysilicon or another conducting material. The bottom gate in  FIG. 1  is formed on the opposite side of the thin silicon film  101  and comprises an insulating layer  112  separating the body from a gate electrode  114 . 
     Gate contacts  116  and  118  make electrical contact with the gate electrodes  110  and  114  respectively. A source contact  120  makes contact with the source region  104 , while a drain contact  122  makes contact with the drain region  106 . 
     The thin silicon film  101  for example has a thickness of between 30 and 150 nm and is formed based on SOI (silicon on insulator) technology. The effective channel length is for example approximately 50 nm, and the channel width is for example approximately 0.1 μm. The insulating layer  112  (injection gate) is a thin dielectric layer, for example between 3 and 10 nm in thickness. The insulating layer  108  (retention gate) is for example formed of an oxide layer between 6 and 30 nm in thickness. 
     In alternative embodiments, rather than being based on SOI technology, the memory device  100  could be formed based on other planar DG (double gate) technologies such as by bonding. Alternatively, the memory device  100  could be formed based on FinFET technology, for example semi-vertical FinFET technology. 
     Control circuitry  124  is provided for generating voltages that are applied to the contacts  116 - 122  of the memory device  100  to program the body  102  to store data and allow the data to be read or refreshed. The data is stored by programming the body  102  of the device to be in one of two states. 
     In a first state, the body  102  has an accumulation of majority carriers, in this example holes. The control voltages are selected so that the majority carriers are for example generated based on B2B (band to band) tunneling. In particular, B2B tunneling is an effect whereby electrons tend to tunnel from the valence band on the body side of the junction into the conduction band, where they readily move to the drain terminal. At the same time, holes on the drain side are naturally generated and are transported into the floating body. 
     In a second state there is no accumulation of majority carriers in the body  102 . 
     The accumulation of majority carriers and the potential level in the body  102  determine the drain current of the device. Thus, during a read operation, the state of the memory device can be determined by detecting the drain current. 
     For simplicity, throughout the following description it will be assumed that the first state in which the body contains an accumulation of majority carriers encodes a “1” bit value, while the second state in which the body contains no accumulation of majority carriers encodes a “0” bit value. Obviously, in alternative embodiments, the opposite encoding could be used. 
     A voltage level V G1  applied to the gate electrode  114  is chosen to cause injection of majority carriers in the body  102  during the writing operation of a “1”, and to create a conductive channel during the read operation of a “1”. This gate will be referred to herein as the injection gate. A voltage level V G2  applied to the gate electrode  110  is chosen to attract majority carriers, and retains the carriers in the body close to the insulating layer  108 . This gate will be referred to herein as the retention gate. 
     Write and read operations of the memory device  100  will now be described in more detail with reference to  FIGS. 2A to 2D . 
     According to an aspect of the invention, from one state to another one, the voltages are switched very sharply, the voltage transitions lasting less than 100 ns, preferentially less than 10 ns, for example between 1 and 3 ns. When the gates are switched, dynamic effects occur and the body potential variations are maximized, whereby the differences between the states 0 and 1 are maximized. In the following when the words “rapid” or “rapidly” are used, they apply to these ranges of switching times. 
     In the following, particular voltage values are indicated for the write and read voltages. These values are only exemplary and correspond to the above mentioned dimensional features. Those skilled in the art will easily adjust these voltage values as a function of specific dimensional features of a memory device for obtaining the disclosed results. 
       FIGS. 2A and 2B  illustrate the memory device  100  during the write operation of a “0” bit (WR 0 ) and a “1” bit (WR 1 ) respectively. In both cases, one starts from a read or hold state that will be disclosed later and the source contact  120  is coupled to ground. 
     As shown in  FIG. 2A , to store a “0” bit, the drain contact  122  is coupled to a 2 V supply and the retention gate electrode  110  is coupled via gate contact  116  to 0 V while the injection gate electrode  114  is switched via gate contact  118  to 0.2 V. This results in a current flow of electrons represented by solid circles in  FIG. 2A . All the majority carriers, in this case holes, leave the body towards the source region, as represented by hollow circles. Thanks to the rapid voltage switching, the body potential attains a value of for example 0.5 V, which is higher than the source potential. 
     As shown in  FIG. 2B , to store a “1” bit, the drain contact  122  is coupled to a 2 V supply and the retention gate electrode  110  is coupled via gate contact  116  to 0 V while the injection gate electrode  114  is coupled via gate contact  118  to a −1.5 V supply. This results in an injection of majority carriers into the body from the drain region, in this case of holes represented by hollow circles. Injection for example results from B2B tunneling triggered by the voltage difference of 3.5 V between the injection gate electrode  110  and the drain region  106 . The injected majority carriers are attracted to the injection gate electrode  114 . When the injection gate is rapidly switched to −1.5 V, the body potential attains a value of for example −0.7 V at the interface with the injection gate  114  and of +0.1 V at the interface with the retention gate  110 . 
       FIGS. 2C and 2D  illustrate the memory device  100  during the reading of a “0” bit (RD 0 ) and a “1” bit (RD 1 ) respectively. During both read operations the source contact  120  is coupled to ground, the drain contact  122  is coupled to a 0.1 V supply, the retention gate electrode  110  is coupled via gate contact  116  to a −2 V supply, and the injection gate electrode  114  is coupled via gate contact  118  to a 0.2 V supply. 
     As shown in  FIG. 2C , when there is no majority carrier accumulation in the body  102 , no channel is formed in the body, and there is no drain current. In view of the dynamic process used, the current is then very low and practically null. 
     As shown in  FIG. 2D  on the other hand, when there is an accumulation of majority carriers in the body, these carriers are attracted to the retention gate electrode  110 , a channel is formed in the body  102  on the side of the injection gate electrode  114 , resulting in a drain current flowing. According to the invention, the current is much higher in this 1-state than in the 0 state: at least 10 times higher. For proper operation, the retention gate voltage  116  must switch (for example, from 0 V to −2 V) before the injection gate voltage  118  switches (for example, from −1.5 V to 0.2 V). At least, if the two gate voltages  116  and  118  signals are triggered at the same time (i.e. begins to switch at the same time), the retention gate voltage  116  must become sufficiently negative before the injection gate voltage  118  reach the reading voltage (for example 0.2 V) in order to keep the holes inside the body  102 . 
     In periods between write or read operations, the states stored by the body are for example held by applying the same voltage as for a read operation. Alternatively, the drain contact  122  could be grounded, and the injection gate electrode  114  also coupled to a negative voltage. For the storage of a “1” bit, the majority carriers are thereby retained in the body, and the present inventors have found that this state can be conserved for relatively long periods (longer than 10 minutes). For the storage of a “0” bit, the present inventors have found that there may be a slow build-up of majority carriers in the body  102 . Thus, voltages can be applied periodically to refresh the data values. 
     The memory device  100  of  FIG. 1  is for example symmetrical, in other words having gates formed having substantially the same dimensions, and substantially uniform source and drain. However, to improve the retention time of the memory device  100 , and in particular the “0” bit retention time, dissymmetry may be introduced to favour the injection of majority carriers into the body  102  by the injection gate electrode  114  during a “1” write operation and to impede the injection of majority carriers into the body  102  at the retention gate electrode  110  during a “0” hold or read, as will now be explained in more detail with reference to  FIGS. 3A to 9 . 
       FIG. 3A  illustrates the memory device  100 , which is the same as the memory device  100  of  FIG. 1 , except that the insulating layer  108  is formed so as to be thicker close the edges of the gate electrode  110 . This is for example achieved during an oxidization step, which results in features sometimes referred to as “birds beaks”. 
       FIG. 3B  illustrates a portion of the memory device  100  of  FIG. 3A  in a region close to the right-hand edge of the gate electrode  110 , and shows the “birds beak” in more detail. 
     The thickness represented by arrow  302  of the thin silicon film  101  in the body  102  is for example between 30 and 100 nm, and in this example 80 nm. Thickness represented by arrow  304  of the thin silicon film  101  in the outer part of the drain region  304  is for example between 10 and 40 nm lower than the thickness  302 , and in this example is 60 nm thick. The insulating layer  108  has a thickness  305  of between 4 and 10 nm, and in this example of 6 nm. The thickness  306  of the insulating layer at a point level with the right-hand edge of the gate electrode  110  is for example between 10 and 40 nm thicker than the thickness  305  under the gate electrode  110 , and in this example is 30 nm in thickness. The distance  308  of overlap between the right-hand edge of the gate electrode  110  and the metallurgical junction  310  between the drain region  106  and the body  102  is for example between 10 and 100 nm, and in this example is 100 nm. This distance  308  arises from the drain doping (for example arsenic) diffusion produced during the N +  high temperature activation. It can be reduced to 10 percent of the gate length. 
     The present inventors have found that the injection of majority carriers into the body of the memory device is particularly dependent on the electrical field generated at the edges of the gate electrodes in the vicinity of the source and drain regions, and more particularly in the vicinity of the drain. The advantage of the “birds beak” feature is that the increased oxide thickness reduces the strength of the electrical field at the edges of the retention gate electrode  110 , thereby reducing the B2B tunneling and impact ionization and hence impeding the accumulation of majority carriers in the 0-state. 
       FIGS. 4A to 4C  illustrate one example of steps in a method for forming the structure of  FIG. 3A , and in particular the feature of the “birds beak”. 
     As shown in  FIG. 4A , the gate oxide  108  is formed over the thin silicon film  101  by thermal oxidization, for example having a thickness of 6 nm. A polysilicon gate electrode  110 , for example, is deposited over the oxide layer  108 , and lithography and etching steps are performed to form the gate. 
     As shown in  FIG. 4B , an oxidization step is then performed, for example at using oxygen at a rate of around 1.5 l/min, at approximately 950° C., and for approximately 40 minutes. This results in an oxide of approximately 30 nm in thickness formed on the surface of the thin silicon film  101  on each side of the gate electrode  110 . Furthermore, the ends of the original oxide layer  108  formed under the gate electrode  110  are thickened. Then a ion implantation is performed, for example of arsenic followed by a diffusion step, at approximately 850° C. and for around 200 minutes to form the source region  104  and drain region  106 . 
     As shown in  FIG. 4C , a further etching step is for example performed to remove the oxide covering the source and drain regions. 
       FIGS. 5 and 6A  and  6 B illustrate the upper portion of the memory device  100  and show alternative embodiments in which the electrical field strength between the edges of the retention gate electrode  110  and the source and drain regions is reduced, by increasing the spacing between the gate electrode  110  and the source and drain regions. 
     In  FIG. 5 , the length of the retention gate  108 ,  110  is reduced with respect to the length of the body  102  and an LDD (lightly doped drain) technique is used in which lightly doped N-type regions  502  and  504  are formed in the body  102 . 
     In  FIG. 6A , the source and drain regions  104  and  106  have been recessed with respect to silicon of the body  102 , for example by mechanical or chemical treatment, thereby increasing the spacing between the edges of the retention gate electrode  110  and the source and drain regions  104 ,  106 . The source and drain regions  104 ,  106  are for example recessed with respect to the body by a spacing, which is labeled  602 , of between 5 and 20 nm. 
     In  FIG. 6B , the length of the retention gate  108 ,  110  is reduced with respect to the length of the body  102  and an appropriate element (for example caesium) is implanted into an SiO 2  layer  604  covering the device around the gate  108 , which reduces the electric field and hence B2B tunneling at the retention gate. 
       FIGS. 7 ,  8  and  9  illustrate alternative structures of the memory device  100  in which, rather than or in addition to reducing the electric field at the edges of the retention gate electrode  110  to reduce majority carrier injection (during the 0 read or hold state), the electric field at the edges of the injection gate electrode  114  is increased to improve majority carrier injection for writing 1-bits. 
     In the embodiment of  FIG. 7 , the conductivity of the injection gate side of the source and drain regions  104 ,  106  is increased by partial silicidation, for example to form a germanium silicide (SiGe). This is for example achieved by depositing a layer of germanium over the underside of the source and drain regions  104 ,  106 , and diffusing the germanium into the source and drain regions  104 ,  106  by heating. The silicide for example extends to a depth of between 10 and 40 nm into the injection gate side of the source and drain regions  104 ,  106 . 
     In the embodiment of  FIG. 8 , the conductivity of the injection gate side of the source and drain regions  104 ,  106  is increased by forming heavily doped N-type regions  802 ,  804 , for example by providing an inhomogeneous doping profile, for example by plasma doping. 
     In the embodiment of  FIG. 9 , injection gate  112 ,  114  is formed to be wider than the retention gate  108 ,  110 . In particular, the injection gate  112 ,  114  overlaps the source and drain regions  104 ,  106  whereas the retention gate  108 ,  110  underlaps the source and drain  104 ,  106 . Furthermore, additionally the thickness of the insulating layer  108  of the retention gate is chosen to be greater than the thickness of the insulating layer  112  of the injection gate. 
     Alternatively or additionally, the differentiation between the retention and injection gates may be performed by non-uniform implantation of the channel. For example, techniques such as tilt implantation, or asymmetric halos can be used. 
       FIG. 10  schematically illustrates the memory device  100 , which is for example the device of any of  FIGS. 1 ,  3 A or  5  to  9 . The memory device  100  of  FIG. 10  has its injection gate electrode coupled via contact  118  to a bit line BL 1 , its retention gate electrode coupled via contact  116  to a word line WL 1 , its source region coupled via source contact  120  to a bit line BL 2 , and its drain region coupled via contact  122  to a word line WL 2 . As will be explained with reference to  FIG. 11 , when coupled in an array, the word lines WL 1  and WL 2  are used to select a row of memory cells, while the bit line BL 1  is used to program the memory cell, and the bit line BL 2  is used to read the memory cell. This is an advantage for optimizing the speed performance of the read-out circuitry in RAM. 
       FIG. 11  illustrates an array  1100  of memory cells  100 , each coupled according to the example of  FIG. 10 . Three rows and three columns of memory cells  100  are illustrated, although in practise the array may comprise hundreds, thousands or millions of cells. Common bit lines BL 1  and BL 2  are provided for each column of memory cells. Each of the bit lines BL 1  is coupled to the injection gates of each memory cell in a column, while each of the bit lines BL 2  is coupled to the source contacts of each memory cell in a column. Common word lines WL 1  and WL 2  are provided for each row of memory cells. Each word line WL 1  is coupled to the retention gate of each memory cell in a row, while each word line WL 2  is coupled to the drain contacts of each memory cell in a row. 
     Each of the word lines WL 1  and WL 2  is coupled to row control circuitry  1102 , which applies voltages on these word lines for selecting rows of memory cells for read or write operations. Each of the bit lines BL 1  and BL 2  is coupled to column control circuitry  1104 , which applies voltages on these bit lines for reading or programming the memory cells during read or write operations. 
     In particular, to select a row for a write operation, a low voltage of for example 0 V is applied to word line WL 1  of the row to be selected, and high voltage of for example 2 V or higher is applied to word line WL 2  of the row to be selected. The word lines WL 1  of the other rows are for example coupled to a lower voltage than the selected row, for example −2 V (=read and hold voltage), and the word lines WL 2  of the other rows are for example coupled to around the reference voltage, for example at 0 V. At the same time, the bit lines BL 2  are coupled to a reference voltage, for example 0 V. The bit lines BL 1  are coupled to a relatively negative voltage, for example between −0.5 and −2V to store a “1” value, or to a voltage corresponding to a reading level of for example around 0.2 V to store a “0” value. 
     To select a row for a read operation, a relatively high negative value for example of −2 V is applied to the word line WL 1  of the row to be selected, and a read voltage for example of around 0.1 V is applied to the word line WL 2  of the row to be selected. The word lines WL 1  of the adjacent rows also receive a voltage of for example −2 V, but the word lines WL 2  are kept equal to approximately the reference voltage, for example 0 V. At the same time, the bit lines BL 1  are set to a low positive voltage, for example of around 0.2 V, while the bit lines BL 2  are coupled to the reference voltage, for example 0 V, via sense amplifiers or compared to reference dummy cells. 
       FIG. 12  illustrates an electronic device  1200  comprising a processing unit  1202  coupled to a memory interface or DMA (direct memory access) unit  1204 , which provides an interface with a memory  1206 , which for example comprises the memory array  1100  of  FIG. 11 , or similar memory array comprising memory devices  100 . Processing unit  1202  accesses memory array  1100  via the memory interface  1204 , for example for writing or reading data to/from the memory  1206 . 
     The electronic device  1200  is for example a portable electronics device such as a laptop computer, media player, mobile telephone, camera, games console, or other type of electronic device such as a personal computer, set-top box etc. 
     An advantage of the memory device described herein is that, by using a first gate for injecting majority carriers into the body, and a second gate for retaining the majority carriers, the retention times of the memory device may be improved. For example retention times of as much as 10 seconds may be achieved. 
     Furthermore, the operation voltages of the device are relatively low, for example between around −2 and 2 V, which correspond to standard supply voltage ranges often seen in integrated circuits. 
     A further advantage is that the device consumes low power, particularly in the state (0-state) with no majority carrier accumulation in the body, given that no current flows during the read operation. A further advantage is that there is a high on/off current ratio. For example, when an accumulation of majority carriers is present, the current is for example in the range of 1 μA/μm to 100 μA/μm, whereas when no accumulation of majority carriers is present, the current is for example in the range of 1 nA/μm to 10 nA/μm. Furthermore, the memory device may be used in an array in which groups of devices can be individually selected for reading or writing. 
     A further advantage of the memory device described herein is that the gates may be optimized based on their purpose. In particular, the injection gate can be structured to encourage injection of majority carriers into the body, by increasing the electric field strength close to the source and drain regions. Furthermore, the retention gate can be structured to impede the injection of majority carriers into the body. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. 
     While the embodiments described herein relate to an N-channel device having a P-type body and N-type source and drain regions, the principles described herein apply equally to a P-channel device having an N-type body and P-type source and drain regions, in which the majority carriers are electrons rather than holes. Obviously the operation voltages will be adjusted accordingly, majority carrier injection for example being achieved by a positive voltage applied to the injection gate. 
     Furthermore, it will be apparent to those skilled in the art that the various features described herein may be combined in any combination. For example, any of the retention gate structures described in relation to  FIGS. 1 ,  3 A,  3 B,  5 ,  6  and  9  may be combined with any of the injection gate or source/drain structures described in relation to  FIGS. 7 ,  8  and  9 . 
     Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.