Biasing device for memory cell integrated structure

Abstract: The invention relates to an integrated structure (1) for memory cells formed over a semiconductor substrate (2) doped with a first dopant type and including at least one memory cell (CM) in turn formed in a conductive well (3) provided in said semiconductor substrate (2) and doped with a second dopant type, said conductive well (3) having an additional well (4) formed therein which is doped with the first dopant type and comprises active areas (5,6) of the memory cell (CM). According to the invention, a substrate bias terminal (8), formed in the additional well (4), is further associated with the memory cell (CM) to receive a suitable bias voltage (Vpol) to lower the threshold voltage (Vth) of the memory cell (CM) by body effect. The invention also relates to a biasing device (13) for a memory cell (CM) which has at least one substrate bias terminal (8) associated therewith, the biasing device comprising at least a first sub-threshold circuitry block (A) adapted to supply an appropriate current during the device standby phase through a restore transistor (M1) connected between a supply voltage reference (Vcc) and the substrate bias terminal (8) of the memory cell (CM), and having a control terminal connected to a bias circuit (14), in turn connected between the supply voltage reference (Vcc) and a ground voltage reference (GND) to drive the restore transistor (M1) with a current of limited value. The device according to the invention further comprises a second feedback block (B) for fast charging the substrate bias terminal (8), being connected between the supply voltage reference (Vcc) and the ground voltage reference (GND) and comprising a first bias transistor (M2) having a control terminal connected to the ground voltage reference (GND) via a stabilization transistor (M3), having in turn a control terminal connected to an output node (OC), and to the control terminal of a first regulation transistor (M4) connected between the supply voltage reference (Vcc) and the ground voltage reference (GND), the stabilization transistor (M3) and first regulation transistor (M4) providing feedback for the bias transistor (M2), thereby to restrict the voltage range of the output node (OC).

Description: Field of the Invention The invention relates to a device for biasing a memory cell as disclosed in the appended claims. Prior Art As is well known, a non-volatile memory cell is read in by means of a current-to-voltage conversion, to suitably bias the cell for it to draw a current when the stored information has a first logic value of "1". In particular, for a proper memory cell reading operation, the current taken up by the cell should be greater than a preset value, termed the reference value Iref, which is usually a function of the supply voltage Vcc. On the other hand, where the information has a second logic value of "0", the current taken up should be less than the reference value Iref. Latest generation memory devices use values of the supply voltage Vcc ever so much smaller that a proper reading of memory cells becomes increasingly more of a problem. In fact, discriminating between an erased cell (first logic value "1") and a written cell (second logic value "0") is only possible if the erased cell is able to draw a larger current than the reference value Iref, and this can only be if the voltage applied to the gate terminal of the cell in the reading phase, which is normally the same as the supply voltage Vcc, is higher than the cell threshold voltage. For proper performance of the reading operation, there are available a number of circuit modifications which are primarily concerned with distributing the values of the threshold voltages Vth of the erased memory cells. The highest value of a threshold voltage Vth for an erased cell currently sets in the 2.5V range. The US Patent No. 5,243,559 to Murai relates to a semiconductor memory device integrate in a triple-well structure, wherein a positive voltage is applied to a P-type substrate in order to lower the threshold voltage Vth of the memory cells integrated therein. The underlying technical problem of this invention is to provide an integrated structure and a biasing device for memory cells, which have such constructional and functional features as to overcome the constraints from the distribution of the threshold voltage values which still beset prior art memory devices. Summary of the Invention This invention is based on the concept of only modifying the value of the threshold voltage of a memory cell during the reading phase, by body effect. In particular, this concept can be applied by a biasing device according to claim 1. The features and advantages of the integrated structure and the biasing device for memory cells according to the invention can be better understood by having reference to the following description of embodiments thereof, to be read as non-limitative examples in conjunction with the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows schematically, to an enlarged scale, a portion of a semiconductor substrate wherein an integrated structure for memory cells is provided; Figure 2 is a graphical representation of an internal potential signal to the structure of Figure 1, in a first operational condition; Figure 3 is a graphical representation of an internal potential signal to the structure of Figure 1, in a second operational condition; Figure 4 shows in further detail the integrated structure for memory cells of Figure 1; Figure 5 shows a biasing device for a memory cell according to the invention, which has been formed using the integrated structure of Figure 1. Detailed Description Referring to the drawing figures, generally and schematically shown at 1 is an integrated structure for memory cells. The integrated structure 1 for memory cells comprises a semiconductor substrate 2 doped with dopant of a first type. Where a memory cell CM is manufactured by a triple well process, in the semiconductor substrate 2 it is formed a conductive well 3, which is doped with dopant of a second type and has an additional well 4 doped with the first-type dopant formed therein to accommodate the active areas 5 and 6 of the memory cell CM. In particular, the semiconductor substrate 2 and additional well 4 are p-doped, while the conductive well 3 is n-doped, in the example of Figure 1. Also, the memory cell CM of Figure 1 has a floating gate terminal 7 formed above the active areas 5 and 6, and a first substrate bias terminal 8 connected to the additional well 4. The integrated structure 1 for memory cells in Figure 1 further includes a select transistor TR which has active areas 9 and 10 formed in the semiconductor substrate 2. The select transistor TR also has a gate terminal 11 formed above the active areas 9 and 10, and a second substrate bias terminal 12 connected to the semiconductor substrate 2. Advantageously the active areas 5 and 6 of the memory cell CM are formed in the additional well 4, which is physically separated from the semiconductor substrate 2 where the active areas of the external circuitry of the memory cell CM are formed. Thus, the integrated structure 1 for memory cells has first 8 and second 12 substrate bias terminals which are separated from each other. By applying a bias voltage Vpol to the first substrate bias terminal 8, and hence to the additional well 4 of the memory cell CM, the cell threshold voltage Vth can be lowered -- by means of the body effect -- and a larger current can be taken up by the cells for the same supply voltage Vcc being applied. By virtue of this increased current request, the reliability of the cell reading process can be improved in a memory cell CM according to the invention, even at low values of the supply voltage Vcc. Shown in Figure 2 are characteristic curves for memory cells at different threshold voltages values; in particular, the curves in Figure 2 illustrate variations occurring in the current request as the supply voltage Vcc varies. Figure 3 shows characteristic curves for the same cells, as a 0.5V voltage is applied to the first substrate bias terminal 8 and, hence, to the additional well 4, illustrating the shifts in value of the current request and, therefore, of the cell threshold voltage, which threshold voltage is largely dependent on the amount of charge stored on the floating gate terminal. In the use of a memory cell CM as described above, as suitably biased through its first substrate bias terminal 8, some limitations on circuitry incorporating such a memory cell CM should be considered. Firstly, the value of the bias voltage Vpol can and should not exceed the conduction threshold of parasitic diodes present in the integrated structure 1 for memory cells. In particular, a parasitic diode D1 is formed by the semiconductor substrate 2 and the active area 6, that is an n-junction providing the source terminal of the memory cell CM, as shown in Figure 4, where the active area 6 of the memory cell CM has been connected to a ground voltage reference GND. This parasitic diode D1 prevents the first substrate bias terminal 8 from attaining the value of the supply voltage Vcc. Secondly, during the power-on phase of a biasing device which includes a memory cell CM as described above, the parasitic capacitances formed of the semiconductor substrate 2 and the n-type active areas 5 and 6 must be charged at the fastest possible rate, such that the triggering of the device operative phase can take place within a short time. These parasitic capacitances can have indeed large values which are tied to the dimensions of the n-type conductive well 3. Finally, during the standby phase, when the biasing device including the memory cell CM as described above is on but "off-like" as regards its functions, consumption should be kept quite low. Thus, a major problem is created by the leakage current which is bound to appear at any junctions or doped areas. This leakage current tends to discharge previously charged parasitic capacitances, triggering the biasing device operative phase. It is therefore necessary to hold these parasitic capacitances constantly latched to the supply voltage reference Vcc. It should be noted, however, that the junctions of the memory cell CM as described above are brought, rather than to the value of the supply voltage Vcc, to that of a stable intermediate voltage which is to be generated by voltage division; all this increases consumption. A circuit diagram of a biasing device 13 including a memory cell CM according to the invention is shown in Figure 5. In Figure 5, the same reference numerals are used for the elements of the memory cell CM as in Figures 1 and 4. Figure 5 also shows some of the main parasitic capacitances involved. The biasing device 13 comprises a first block A which includes sub-threshold circuitry effective to provide the required current for restoring the leakage current during the standby phase through a restore transistor M1. The first block A includes a bias circuit 14 connected between the supply voltage reference Vcc and the ground voltage reference GND, and connected to the gate terminal of the restore transistor M1, in turn connected between the supply voltage reference Vcc and the first substrate bias terminal 8 of the memory cell CM. The bias circuit 14 allows the restore transistor M1 to be driven by a current of limited value. It should be noted that the sub-threshold circuitry in block A is operative in all conditions, being no associated to an enable signal. The biasing device 13 further comprises a second block B which includes a feedback circuit for fast-rate charging the first substrate bias terminal 8. The second block B is connected between the supply voltage reference Vcc and the ground voltage reference GND and includes a first bias transistor M2 which is in turn connected between the supply voltage reference Vcc and the first substrate bias terminal 8 of the memory cell CM. The bias transistor M2 has its gate terminal connected to the ground voltage reference GND through a stabilization transistor M3 which has its gate terminal connected to an external enable terminal 15 and to the gate terminal of a first regulation transistor M4. The first regulation transistor M4 is connected to the supply voltage reference Vcc and a second regulation transistor M5, in turn connected to the ground voltage reference GND via a third regulation transistor M6. Furthermore, the second M5 and third M6 regulation transistors have their gate terminals connected together and to the first substrate bias terminal 8 of the memory cell CM. Advantageously in this invention, the second M5 and third M6 regulation transistors are selected suitably unbalanced. The stabilization transistor M3 and regulation transistors M4, M5, M6 provide feedback for the bias transistor M2 which is delivering current. The second block B also includes an output node OC whose voltage range is limited by the value of the threshold voltage of the stabilization transistor M3. Advantageously in this invention, the bias transistor M2 is a large-size transistor, so that its charging phase can be fast. On this account, the second block B consumption is high, and the block must be held 'off' in the standby condition. The biasing device 13 then comprises a third block C connected between the first substrate bias terminal 8 and the ground voltage reference GND, and connected to the output node OC of the second block B. The third block C includes a series of a first, diode-connected enable transistor M7 and a second enable transistor M8. In the embodiment shown in Figure 5, the restore transistor M1 and regulation transistors M4, M5 are PMOS transistors, illustratively having a threshold voltage value of -1V. Moreover, the first bias transistor M2, stabilization transistor M3, and second enable transistor M8 are NMOS transistors, illustratively having a threshold voltage value of 1V. Finally, the third regulation transistor M6 and first enable transistor M7 are NMOS transistors with a reduced threshold, illustratively down to 0.5V. An enable signal, such as the chip enable signal used in conventional memory devices, applied to the external enable terminal 15, is a logic 'low' value in normal operation, thereby turning off the second enable transistor M8, and is pulled to a logic 'high' value in the standby condition, thereby turning off the second block B, which is a source of consumption, and to hand over to the first block A the task of maintaining the voltage value at the first substrate bias terminal 8 of the memory cell CM. To avoid possible problems in the standby condition, such as a rise in the voltage at the first substrate bias terminal 8, the second enable transistor M8 in the block C is enabled to provide a discharge path for the first substrate bias terminal 8 through the first enable transistor M7. In this way, overtaking a preset alert threshold can be positively prevented. In fact, with the first block A held always 'on' in the standby condition, the third block C will act as a switch to stop the voltage at the first substrate bias terminal 8 from rising and, accordingly, reduce the leakage voltages of the parasitic capacitances in the memory cell CM. In particular, the first block A could be used to regulate the voltage being applied to the first substrate bias terminal 8 until an optimum condition is reached for triggering the third block C.

Claims: A biasing device (13) for a memory cell (CM) formed in a conductive well (3) provided in a semiconductor substrate (2) of a first dopant type, said conductive well (3) being doped with a second dopant type opposite to the first dopant type and having an additional well (4) formed therein which is doped with said first dopant type and comprises active areas (5,6) of said memory cell (CM), said additional well (4) being connected to a substrate bias terminal (8), said biasing device comprising: a sub-threshold circuitry block (A) adapted to supply an appropriate current during a device standby phase, said sub-threshold circuitry block (A) comprising a restore transistor (M1) connected between a supply voltage reference (Vcc) and said substrate bias terminal (8) of said memory cell (CM), and a bias circuit (14), connected between said supply voltage reference (Vcc) and a ground voltage reference (GND) and connected to a control terminal of said restore transistor (M1) of said sub-threshold circuitry block (A) to drive said restore transistor (M1) with a current of limited value, characterized in that it further comprises: a feedback block (B) connected between said supply voltage reference (Vcc) and said ground voltage reference (GND) and connected to said substrate bias terminal (8) for fast charging it, said feedback block (B) in turn comprising: a first bias transistor (M2) connected between said supply voltage reference (Vcc) and said substrate bias terminal (8); a stabilization transistor (M3) connected between a control terminal of said first bias transistor (M2) and said ground voltage reference (GND) and having in turn a control terminal connected to an output node (OC) of said feedback block (B); a first regulation transistor (M4) connected between said supply voltage reference (Vcc) and said ground voltage reference (GND) and having a control terminal connected to said output node (OC) of said feedback block (B), said stabilization transistor (M3) and first regulation transistor (M4) providing feedback for said bias transistor (M2), thereby to restrict the voltage range of said output node (OC). A biasing device (13) according to Claim 1, characterized in that said second block (B) further comprises second and third regulation transistors (M5,M6) connected in series with each other between said first regulation transistor (M4) and said ground voltage reference (GND) and having respective control terminals connected together and to said substrate bias terminal (8) of said memory cell (CM). A biasing device (13) according to Claim 2, characterized in that said second and third regulation transistors (M5,M6) are selected suitably unbalanced. A biasing device (13) according to Claim 1, characterized in that said first bias transistor (M2) is a large-size transistor. A biasing device (13) according to Claim 1, characterized in that said stabilization transistor (M3) has its control terminal further connected to an external device enable terminal (15). A biasing device (13) according to Claim 1, characterized in that it further comprises a switching block (C) connected between said first substrate bias terminal (8) and said ground voltage reference (GND) and connected to said output node (OC) of said feedback block (B) so as to switch off said feedback block (B) when in the standby condition and stop said substrate bias terminal (8) voltage from rising. A biasing device (13) according to Claim 6, characterized in that said switching block (C) comprises a diode-connected first enable transistor (M7) and a second enable transistor (M8) connected in series with each other between said substrate bias terminal (8) and said ground voltage reference (GND).

Inventors: BRANCHETTI MAURIZIO, CAMPARDO GIOVANNI, GHEZZI STEFANO, ZANARDI STEFANO, BRANCHETTI, MAURIZIO, CAMPARDO, GIOVANNI, GHEZZI, STEFANO, ZANARDI, STEFANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: H01L27
IPC codes: G05F3, H01L27