CIRCUITS

FIELD OF THE INVENTION

The invention relates to a semiconductor memory device comprising at least a single-ended sense amplifier, and more particularly to the provision of at least a reference circuit from which a regulation signal is generated to be applied to said sense amplifier and said reference circuit in order to compensate for the effects of variation of operating conditions (e.g. temperature, supply voltage) or process (e.g. ratio of NMOS/PMOS current drive capability etc.).

BACKGROUND OF THE INVENTION

Memory read operation in all kinds of sense amplifiers is achieved by comparing input data with a reference signal to determine the logic state of the input data. Conventional differential sense amplifiers, which are used in memory devices such as standard Dynamic Random Access Memory (DRAM) devices, are connected to two bit lines, one bit line providing the data to be read while the other bit line is used for reference voltage generation.

In contrast to that, single-ended sense amplifiers, which are used in memory devices such as embedded or special Dynamic Random Access Memory (DRAM) devices, are coupled to a single bit line only.

A particular style of single-ended sense amplifier employs a sense amplifier with two input/output nodes, one of which is coupled to an input data line (bit line), and the other is used to feed in a reference signal required for the read operation process. Fig. 1 shows an example of such a single-ended sense amplifier which has been proposed by the Applicant and previously described in publication EP 2 365 487 A2. It comprises two branches, each having a PMOS transistor and a NMOS transistor connected in series, arranged in a way that they form cross-coupled inverters. The transistors are double gate transistors having a first control gate and a second control gate that can be independently operated. The first branch comprises PMOS transistor T ₄ in series with NMOS transistor T ₃ in between power supplies V ₄ and V ₃ , while the second branch comprises PMOS transistor Ti in series with NMOS transistor T ₂ in between power supplies Vi and V ₂ . The first gates of transistors Ti and T ₂ are connected together (input of the inverter formed by the second branch) and connected to the middle node between transistors T ₃ and T ₄ (output of the inverter formed by the first branch), thereby forming a first input/output node Nl of the sense amplifier which is used to feed in a reference signal REF. The first gates of transistors T ₄ and T ₃ are connected together (input of the inverter formed by the first branch) and to the middle node between transistors Ti and T ₂ (output of the inverter formed by the second branch), thereby forming a second input/output node N ₂ of the sense amplifier which is coupled to a bit line BL. The second gates of transistors T _{l s} T ₂ , T ₃ and T ₄ are denoted BGi, BG ₂ , BG ₃ and BG ₄ and are herein referred to as back control gates. Such back control gates are used to modulate the first-gate-related threshold voltages of their respective transistors.

A problem arises from the fact that such a circuit is not necessarily symmetrical or, depending on the particular operation principle, is by purpose designed asymmetrically. Indeed both branches may have different designs concerning dimensions of the respective transistors (Ti being compared to T ₄ , and T ₂ being compared to T ₃ ). Moreover, the first and second nodes Ni, N ₂ may be operated in a different manner and with different loads (with nodes Ni and N ₂ being either both inputs or both outputs), and the operating points of all transistors may differ from each other during the sensing process (once again Ti is compared to T ₄ , and T ₂ to T ₃ ). Note that these asymmetries usually apply to the operation of a single-ended sense amplifier. Thus for ensuring proper operation of the circuit, widths and lengths of the transistors must be thoroughly chosen (and Ti and T ₄ , respectively T ₂ and T ₃ , may intentionally differ). Reference, bias, and supply voltage must also be well defined and well controlled, too.

In practice however, all these parameters and the transistor properties (current drive capability, threshold voltage, transconductance, drain conductance, etc.) are prone to variations (PVT: process, voltage, temperature). As the single-ended sense amplifier is usually designed and operated as a non-symmetrical circuit, PVT variations do not compensate for each other in the two branches and malfunction of the circuit may result.

Generally speaking, this problem occurs in the case of any sense amplifier that does not rely on a fully-symmetrical differential circuit structure, and more specifically for sense amplifier circuits sensing a bitline against a reference voltage, i.e. against a non in-situ generated reference, which is not the case for sense amplifiers that generate their own references through shorting first and second bit lines in a precharge/equalizing phase or at least by precharging them to same potentials before a sense process is started.

SUMMARY OF THE INVENTION

The invention aims to provide a single-ended sense amplifier together with supporting circuitry and an operation principle, so that the single-ended sense amplifier is not prone to PVT variations.

In this respect, the invention relates, according to its first aspect, to a semiconductor memory device comprising:

- at least one sense amplifier circuit for reading data sensed from selected memory cells in a memory array,

- at least one reference circuit, each said reference circuit being a replica of said sense amplifier circuit and having an output through which said reference circuit delivers an output physical quantity,

- a regulation network providing a regulation signal to each said sense amplifier circuit and each said reference circuit, wherein said regulation signal is derived from an averaging over time and/or space of said output physical quantity,

wherein the regulation network comprises a control unit configured to sum up the physical quantities of each output of said reference circuit and a target mean value, wherein said physical quantities of said output of said reference circuit and said target mean value are summed up with opposite signs, said control unit delivering a regulation signal based on said sum, said regulation signal being fed in to each said regular sense amplifier circuit and to each said reference circuit. Other preferred, although non limitative, aspects of this memory device are as follows:

- the target mean value is a voltage of VL+(VH-VL)/2, with VL and VH respectively being low and high voltage levels of the operating voltages of the sense amplifier circuits and reference circuits, defining the respective low and high logic levels;

- each of said reference circuit outputs is connected to a capacitor connected to a DC voltage source; - the regulation signal is an average over time of the difference between the sum of said output physical quantities of each said reference circuit and said target mean value;

- the regulation signal is a voltage used as a regulation voltage by each said sense amplifier circuit and by each said reference circuit;.

- the control unit comprises an operational amplifier with an inverting input and a non- inverting input, the target mean value being applied to said non-inverting input while said inverting input is connected to each said reference circuit output through at least one switch and/or resistor and/or capacitor;

- each said reference circuit output is connected to the control unit through at least one switch that allows decoupling the control unit from said output prior and during sensing operations of said at least one sense amplifier circuit and of said at least one reference circuit;

- the control unit comprises an integrator;

- the integrator comprises resistors connected to each reference circuit output and to a summing node, and a common integration capacitor;

- the integrator comprises a switched capacitor configuration associated with each reference circuit output;

- the control unit comprises

- a digital up/down counter, each reference circuit output being connected to an input of said digital up-down counter,

- a Digital-to-Analog Converter for converting the digital output signal of said digital up/down counter into the regulation signal to be fed in to each sense amplifier circuit and to each reference circuit.

The invention further relates to a semiconductor memory incorporating a semiconductor memory device according to the invention, as well as to a process of operating a semiconductor memory device according to the invention.

The invention also relates to a process of operating a semiconductor memory device according to the invention, comprising the steps of

- operating similarly each sense amplifier circuit and each reference circuits, - generating a regulation signal in the regulation network from the physical quantities of each output of said reference circuits and applying said regulation signal to the sense amplifier circuit and to the reference circuits. BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects and advantages of the present invention will become better apparent upon reading the following detailed description of preferred embodiments thereof, given as a non-limiting example, and made with reference to the appended drawings wherein:

Figure 1 already discussed above shows a single-ended sense amplifier as introduced in publication EP 2 365 487 A2;

Figure 2 is a schematic diagram that shows an embodiment of a semiconductor memory device according to the invention comprising a series of sense amplifier circuits associated with a series of reference circuits connected to a control unit for compensating the effect of PVT induced variations;

Figure 3 is a schematic diagram of a possible embodiment of the control unit comprising an operational amplifier operated as an integrator, whose inverting input is connected through resistors and switches to the outputs of the series of reference circuits;

Figure 4a and 4b shows two possible embodiments for the switches;

Figure 5 is a schematic diagram of a possible embodiment of the control unit comprising an operational amplifier operated as an integrator, whose inverting input is connected through switches and switched capacitors to the outputs of the series of reference circuits, ;

Figure 6 is a schematic diagram of a possible embodiment of the control unit comprising a digital up-down counter and a digital-to-analog converter.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a semiconductor memory device comprising at least one sense amplifier circuit for reading data sensed from selected memory cells in a memory array and at least one reference circuit. Each one of said reference circuits is a replica of said sense amplifier circuits and is coupled to a control unit. Stated briefly, the control unit is designed and operated so as to generate a regulation signal fed to the sense amplifier circuits to compensate for all kinds of PVT variation induced drifts and to maintain correct operation of the sense amplifier circuits independent of PVT variations.

It shall be noted that the semiconductor memory device according to the invention may comprise only one sense amplifier circuit and/or reference circuit. However, for the sake of comprehensiveness, the following description will be made in reference to an embodiment comprising a plurality of sense amplifier circuits and reference circuits. For ease of representation, the plurality of sense amplifier circuits and reference circuits are discriminated by a subscript number from 1 to n with index "i" corresponding to an integer number between 1 and n, and 1 to m with index "j" corresponding to an integer number between 1 and m, respectively.

Fig. 2 schematically illustrates the principles of the invention. A series of sense amplifier circuits SA, i = 1 ... n, are provided for reading data sensed from selected memory cells in a memory array. For example, the sense amplifier circuits SAi may be similar to the one of Fig. 1, but they may be of any type of sense amplifier circuits that sense a bitline BL against an external reference voltage, i.e. against a non-in-situ generated reference voltage. All sense amplifier circuits SAi are identical.

The reference circuits RSA _j , j = 1 ... m, are replicas of the sense amplifier circuits, their designs are identical. In particular, the transistors of a reference circuit RSA _j , j = 1 ... m, are designed to exhibit the same properties as the transistors of a sense amplifier circuit SAi, for example with respect to their type, channel widths and lengths, or doping levels. All reference circuits RSA _j are identical to the sense amplifier circuits SAi. Accordingly, as an example, a reference circuit RSA _j can be the circuit of Fig. 1 if the sense amplifier circuit SAi is also the circuit of Fig. 1.

Each sense amplifier circuit SAi and each reference circuit RSA _j have: - a bitline referred input/ouput terminal I/O-BL to sense data from and write data to a bitline , for example corresponding to the second input/output node N ₂ connected to the bitline BL for the circuit of Fig.1 ,

- a reference signal referred input terminal I/O-R that may optionally also serve as a data output terminal, for example corresponding to the first input/output node Ni for the circuit of Fig. 1 ,

- means to evaluate the data read from a connected bitline,

- means to write back or refresh data to the connected bitline memory cell , and

- if required, means to precharge the bitline BL or any other load connected to the I/O- BL terminal.

Each reference circuit RSA _j has an output OUT _j connected to an input of a control unit CU, and connected to an output load OL. The output load OL can be realized by means of simple capacitors connected to a DC voltage Vo, as illustrated in Fig. 2. The DC voltage Vo is chosen to be an appropriate operating point for the capacitors and thus preventing dielectric breakdown and leakage. Preferably, the DC voltage Vo is equal to VL+(VH-VL)/2, with VL and VH being the low and high supply voltage levels of the operating voltages of the sense amplifier circuits SAi and reference circuits RSA _j , respectively, defining the logic low and high voltage levels, respectively.

Once a sensing operation is carried out as known by a skilled person, the voltages of the outputs OUT _j of the reference circuits RSA _j are obtained. The voltages of the outputs OUT _j represent the output physical quantity from which the regulation signal REG is derived through an averaging over time and/or space. The regulation network generating the regulation signal REG provides said regulation signal REG to each sense amplifier SAi and to each reference circuit RSA _j .

The regulation network comprises a control unit CU configured to sum up the physical quantities of each output OUT _j of said reference circuit RSA _j and a target mean value. Said physical quantities of said output OUT _j of said reference circuit RSA _j and said target mean value are summed up with opposite signs.

Each reference circuits RSA _j is operated as a usual sense amplifier circuit SA. The reading result of a reference circuit RSA _j results from a random process involving the PVT variations and the regulation signal REG supplied to said reference circuit RSA _j . The regulation signal REG supplied to a reference circuit RSA _j controls the mean value, over space and time, of the physical quantity delivered on the output OUT _j of said reference circuit RSA _j . Accordingly, PVT variations are compensated by means of the averaging of the output physical quantity performed by the regulation network and the dynamically operated reference circuits RSA _j .

In the exemplary embodiments depicted in Figs. 2, 3, and 5, the control unit CU sums up the voltages of the outputs OUT; of the reference circuits RSA _j with a target mean voltage acting as the target mean value. The target mean voltage equals the mean value of the operating voltages, i.e. VL+(VH-VL)/2. The voltages of the reference circuit outputs OUT _j are summed up with a negative sign whereas the target mean voltage VL+(VH-VL)/2 is summed up with a positive sign. In such embodiments with several reference circuits RSA _j , the contributions of the outputs OUT _j of the reference circuits RSA _j are summed up and averaged in the control unit CU. In the embodiments shown in Figs. 3 and 5 this is done at the inverting input of the operational amplifier, highlighted as node SUM in these figures.

The control unit CU then delivers the regulation signal REG based on said sum, said regulation signal REG being fed in to each said regular sense amplifier circuit SAi and to each said reference sense amplifier RSA _j .

The resulting weighted difference between the voltages of the reference circuit outputs OUT _j and the target mean voltage is used to generate a regulation signal REG that is fed to the reference signal referred input terminals I/O-R of each said sense amplifier circuit S and of each said reference circuit RSA _j . Accordingly, the output of the control unit CU is connected to each said sense amplifier circuit SAi and each reference circuit RSA _j .

The average over time of the summed output voltages of the reference circuit outputs OUT _j equals the target mean voltage VL+(VH-VL)/2, which is chosen to represent the expected value of sense amplifier circuit output voltages over time in the case of randomly distributed data.

Since the reference circuits RSA _j and the sense amplifier circuits SAi are identical, the same PVT changes affect similarly the sense amplifier circuits SAi and the reference circuits RSA _j . PVT changes lead to shifts of the required input voltages at reference signal referred input terminals I/O-R for proper operation of the reference circuit output OUT _j , and in turn the entire circuit acts as a balanced regulation loop providing the regulation signal REG generated by the control unit CU.

The regulated regulation signal REG compensates for all kinds of PVT variation induced drifts and maintains proper operation of all sense amplifier circuits SAi within a larger circuit such as a memory array, independently of PVT variations. It shall be noted that the regulation signal REG is described as a voltage in the described examples (Figs. 3, 5, 6), but it may also be a current.

For example, the regulation signal REG may be used as the reference signal REF in the sense amplifier circuit of Fig. 1 as a reference for sensing operations. Alternatively, the regulation signal REG may be a voltage used to control the second control gate (i.e. back control gate) of one or more transistors of each said sense amplifier circuit SAi and each reference circuit RSA _j . The reference signal REF may then be provided externally to the sense amplifier SAi and the regulation signal REG may be applied to the back control gate BG ₂ of the transistor T ₂ , or to the back control gate BG ₃ of transistor T ₃ . The regulation signal REG generated by the control unit CU may be extended by further control signals set externally. In the above-mentioned example, the back control gate BG ₂ of the transistor T ₂ may be controlled by the regulation signal REG while the back control gate BG ₃ of transistor T ₃ is controlled by an external voltage, or conversely, the back control gate BG ₃ of the transistor T ₃ may be controlled by the regulation signal REG while the back control gate BG ₂ of transistor T ₂ is controlled by an external voltage.

Further, instead of using FDSOI (Fully-Depleted Silicon-On-Insulator) devices with a front gate and a back control gate, any kind of double gate transistors can be used in the structure displayed in Fig. 1 , which is a non- limiting example. Moreover it should be noted, that principle and embodiments described in Figs. 2 - 6 do not require a single- ended sense amplifier realized in FDSOI or any other kind of double-gate technology, but can be applied to single-ended sense amplifiers realized in standard bulk CMOS technologies as well.

Fig. 3 shows a possible structure for the control unit CU. Each of said reference circuit outputs OUT _j is connected to the control unit CU through at least one switch SW _j that allows decoupling the control unit CU from said reference circuit outputs OUT _j prior and during sensing operations of the sense amplifier circuits SAi and of the reference circuits RSA _j . The switches ensure that the outputs OUT _j of the reference circuits RSA _j are connected to the control unit CU only during the time frames after a decision has been made in the sensing operation and prior to the next operation step.

The signals of the reference circuit outputs OUT _j are summed up at a common node SUM in Figs. 3 In this embodiment, the control unit CU comprises an operational amplifier OA with an inverting input and a non-inverting input, the target mean voltage being applied to said non-inverting input while said inverting input represents the summing node SUM.

The control unit CU further comprises an integrator or a low pass filter achieved through the depicted configuration, wherein resistors R _j are arranged between each reference circuit output OUT _j and the inverting input of the operational amplifier OA, with a common capacitor Ci _nt . In more detail, the resistors R _j are arranged between the switches SW _j and the summing node SUM, while the common capacitor Cint is arranged between said summing node SUM and the output of the operational amplifier OA.

Figures 4a and 4b show two examples of switches that may be used. The switch illustrated in Fig. 4a is a single transistor operated at sufficiently high overdrive voltage to ensure an adequate transmission of the output voltages of the reference circuits RSA _j . The transistor can be any FET, i.e. any field effect transistor.

The switch illustrated in Fig.4b is a transmission gate, wherein a P-type transistor 41 is associated in parallel with an N-type transistor 42. With this configuration, the two transistors are operated in parallel in order to achieve an overall lower on-resitance and decreased dependence of the resistance on the voltage to be switched. The transistors can be any FET transistors.

Fig. 5 shows another possible structure for the control unit CU, which is similar to the structure illustrated in Fig. 3, but the resistors R _j are replaced by switched capacitors C _j . One of the terminals of each capacitor Q is connected between a first switch SW _j and a second switch SWc _j , and the second terminal of the capacitor Q is grounded or connected to any other DC voltage. The switches SW _j and SW _cj connect the capacitors C _j alternatively to the outputs OUT _j of the reference circuit RSA _j , to the common summing node SUM, or also disconnect them from both nodes, respectively. The first switch SW _j serves also as previously for decoupling the control unit CU from the reference circuit outputs OUT _j prior and during sensing operations of the reference circuits RSA _j . For example, the switches SW _j and SW _cj can be those illustrated in Fig. 4. Fig. 6 shows an alternative embodiment wherein the summation and integration are realized in the digital domain and the regulation signal REG is generated through a digital-to-analog converter DAC. A digital up/down counter CNT is used to calculate the average of the summed output data of the reference circuits RSA _j . An up/down counter is a counter that can increment or decrement its value depending on the logic input data.

Each reference circuit output OUT _j is connected to an input of said digital up- down counter CNT, from which data are read into the counter CNT, controlled by a trigger signal TRG, after decision-making by the reference circuits RSA _j .

The counter output is fed to the digital-to-analog converter DAC, which provides the regulation signal REG in the analog domain for the reference circuits RSA _j as well as for the sense amplifier circuits SA. It is optionally possible to design the up/down counter CNT in a manner that it only makes a step if the absolute value of the summed output voltages of the reference circuits RSA is superior to a given integer number d, e.g. d = 2, 3, . ... This allows a decrease in switching activity, and a lower power consumption. The regulation signal REG is generated through averaging the voltages of the reference circuit outputs OUT _j , which represent the binary input data of this fully digital implementation of the circuit configuration.

Averaging may be made in the time-domain, as described in the discussed embodiments, but may also be made only in the space-domain, by using enough reference circuits RSA _j for an acceptable averaging. Mixed time- and space domain averaging is possible as well.

Hence, the integrator of the embodiments depicted in Fig. 3 and 5 can be omitted and replaced by another integrating function, for instance a simpler integrating function in case of a sufficiently high number of reference circuits RSA _j for compensating for the lack of the integrator mentioned above. The same reasoning applies for the up/down counter CNT of the embodiment of Fig. 6. The control unit CU can then be implemented on a more limited area.

Generally speaking, in each configuration, using a larger number of reference circuits RSA _j leads to faster settling of the regulation signal REG.