Field of the Invention

The present invention to relates to applying bias voltages in an adaptive manner to the access transistors in the cells in dynamic random access memory (hereinafter will be referred to as DRAM) structures depending on predetermined temperature values.

Background of the Invention

Today, DRAM manufacturers set pre-determined refresh times for DRAMs during production at the factory level resulting with retention times according to the DRAM characteristics. Refresh time can be expressed as a predetermined time during which the capacitor used to retain data in the cells is recharged periodically before being fully discharged, and retention time can be expressed as the amount of time that a cell can retain data without being refreshed.

Some of the cells in a DRAM are weaker than other cells. A cell being weak means that the said cell loses the data it retains in a shorter period of time than the other cells, in other words, the retention time is shorter. Some cells can retain the data therein for a longer period of time. These changes in the retention time are caused by production-related differences. Even if the proportion of the weak cells is low, the frequency of refresh for all DRAM cells is determined by the manufacturers according to the retention time value of these weak cells. In this case, unnecessary refreshing is performed for many cells (even for the non-weak cells) since the refresh operation is carried out for the weakest cell. For DRAMS, leakage currents increase as the temperature increases and the bit cells lose the data they store more rapidly as the temperature increases. As the temperature increases, mobility of the atoms increases, they move more rapidly and the transistors leak more. This causes the data stored in the DRAM to be lost more rapidly. In short, as the temperature increases more frequent refresh operations are required for the DRAM. As a solution to this problem, in the current DRAMS, refresh frequency is increased when the temperature value rises above a certain limit. As the temperature increases the retention time decreases and the leakage of currents increases. Due to this problem, the predetermined refresh time (rate) is updated to be more frequent. For example, the refresh time value determined as 64 ms (milliseconds) for all DRAM cells is determined as 32 ms (milliseconds) and is doubled when the temperature rises above 85°C. In this case, while more refresh operations are performed than it should be due to the weak cells, the number of refresh operations increases much more due to the said increase in the temperature, and these refresh operations causes unnecessary refreshing for the DRAM structure where the majority of the cells are comprised of strong cells. The said situation causes too much performance loss at high temperatures.

In addition to the power consumption required for the refresh operation, the requests for these cells have to be held as read and write operations cannot be performed during the refresh operation. Therefore, a decrease in the total number of refresh operations or in the refresh frequency is needed in order to reduce power consumption as wells as enhancing performance. With the present patent application, instead of increasing the refresh time at high temperatures, the threshold value is increased by applying bias voltage to the cells in the DRAM at high temperatures, and thus the refresh frequency does not need to be increased at the said high temperatures. Thus, a different solution is provided at high temperatures without changing the refresh time value set to 64 ms (milliseconds) to 32 ms (milliseconds) for DRAM cells at high temperatures as indicated in the above given example.

In the United States patent document no. US2003067824 Al, an application known in the art, use of variable bias voltage generators is recommended for performing refresh operations at different times in self -refresh mode. Body bias voltage means applying difference for body voltage. The said document describes generating a difference in voltage for the clock generator. In the patent application of the present invention, when the DRAM temperature reaches high temperature values, bias voltage is applied to the cells (to the transistors within the cells) and the increase of leakage current is adjusted by means of the bias voltage.

United States patent document no. US2010332943 (Al), an application in the state of the art, discloses a data refreshing mechanism. The said mechanism comprises a controller, a threshold adjuster and a refresh module. The system sorts out the recently processed and controlled regions and adjusts the threshold values according to their process errors and refresh times to exclude them from the processing. In the said document, a threshold value is determined for the refresh time, this threshold value can be adjusted, and thus refresh operation at different times is made possible. In the patent application of the present invention, when the DRAM temperature reaches high temperature values, bias voltage is applied to the cells (to the transistors within the cells) and the increase of leakage current is adjusted by means of the bias voltage.

Summary and Objects of the Invention

The object of the present invention is to provide a DRAM which requires a lower refresh frequency (varies according to the applied bias voltage) at high temperatures than the basic-design DRAMs in the state of the art, and thus wherein power consumption and the likelihood of read/write operations clashing with refresh operation are reduced. Another object of the present invention is to provide a DRAM, which, instead of increasing the refresh time as the leakage currents increase due to temperature increase, enables to adjust the increase of leakage current by means of the bias voltage by applying bias voltage to the transistors within the cells.

A further object of the present invention is to provide a DRAM, which prevents increase of the leakages and performance loss by enabling to increase the threshold value of the cells with respect to a predetermined temperature increase.

Detailed Description of the Invention

A DRAM structure having an adaptive body bias voltage developed to fulfill the objects of the present invention is illustrated in the accompanying figures, in which:

Figure 1 is a schematic view of the DRAM structure.

Figure 2 is a schematic view of a cell.

The components shown in the figures are each given reference numbers as follows:

1. DRAM structure with adaptive body bias voltage

2. Basic DRAM

21. Cell

211. Access Transistor

212. Bias line

3. Control unit

4. Multiplexer

5. Bias driver G. Ground

B. Bias voltage The DRAM structure with adaptive body bias voltage (1) with respect to the temperature limit which enables to apply bias voltages in an adaptive manner to the access transistors (211) in the cells (21) in the dynamic random access memory structures depending on its temperature essentially comprises the following elements:

• at least one basic DRAM (2) including at least one row consisting of a plurality of cells (21),

• an access transistor (211) which forms each cell (21) together with a capacitor,

• a bias line (212) to which the body terminals of each of the access transistors (211) in the cells (21) forming a row are connected,

• at least one multiplexer (4) for controlling whether a predetermined value of bias voltage (B) can be supplied to the bias line (212),

• at least one control unit (3), which is connected to the selection input of the multiplexer (4) and controls operation of the multiplexer (4), and which is adapted to generate an output signal depending on DRAM temperature and send it to the multiplexer (4) and to enable a value of bias voltage (B) predetermined according to temperature to be transmitted to the cells (21) via the multiplexer (4),

• at least one bias driver (5) through which the bias voltage (B), which, according to the data coming from the control unit (3) to the multiplexer (4), will be received from at least one of the data inputs of the multiplexer and will be transmitted to the bias line (212), passes.

In the inventive DRAM structure with adaptive body bias voltage (1) depending on temperature limit, there is provided a basic DRAM (2) for retaining the data, program code and similar information required for performing the functions of the processor in the electronic device in which it is used. There are cells (21) in the basic DRAM (2). Each cell (21) comprises a capacitor and an access transistor (211). The cells (21), which are comprised of a capacitor and an access transistor (211), are lined up to form rows and the rows build up one under the other to form the structure of the basic DRAM (2).

The threshold voltage of an access transistor (211) in a cell (21) can be adjusted by changing the difference between the source voltage and the body voltage of that access transistor (211). As the threshold voltage of a transistor (211) increases, that transistor (211) starts to leak less, and hence, the cell (21) wherein the said transistor (211) is located can retain the data stored therein for a longer period of time, that is to say, the retention time increases. By applying different threshold voltage to the transistor (211), the retention time of the transistor (211) can be increased thereby reducing leakage.

In the DRAM structure (1) of the present invention, the control unit (3) decides whether the bias voltage (B) will be applied to a row and thus to each access transistor (211) in the cells (21) forming the row and this decision is applied by means of the multiplexer (4). In the preferred embodiment of the present invention, temperature of the DRAM structure is controlled when making the said decision. As the temperature of the DRAM structure (1) and the temperature of the cells (21) in the DRAM structure (1) increase the leakage current increases and the retention time of the transistors decreases. The DRAM structure (1) directly affects the retention time.

The retention time data of a row is determined and labeled as follows: The retention time for a row is determined according to the weakest cell (21) in that row. The row is labelled by the interval to which the retention time of the row corresponds among the predetermined preferred number of retention time intervals. As an exemplary embodiment for this process that is called classification, when a time of 128 milliseconds (ms) is desired to be divided into two intervals, two intervals are determined as 0-64 ms and 64-128 ms. Taking into account the retention times that vary due to production-related differences, this means that the cells (21) can be refreshed every 64 ms or every 128 ms. For example, since a cell (21) having a retention time of 64 ms to 128 ms (e.g. 75 ms) should be refreshed before expiration of the said time, it is labelled in the 0-64 ms interval among the determined retention time intervals. A cell (21) having a retention time greater than 128 ms is labelled in the 64-128 ms interval. However, even if a row contains cells (21) labeled in the higher retention time interval, it receives the same label with its cell (21) labeled in the shortest retention time interval. In the currently used DRAMs, the leakage current increases as the temperature increases, and the refresh time frequency determined for the DRAM is increased. For example, for a DRAM whose retention time is set as 128 milliseconds with respect to a temperature of 70°C, the retention time is changed as 64 milliseconds when the temperature rises to 80°C. If the temperature value exceeds 80°C, then the retention time will be changed as 32 milliseconds. Change of the DRAM’s temperature influences the retention time substantially, and since the retention time decreases with the increase in the temperature the refresh rate increases likewise. While the refresh operation is performed every 128 milliseconds when the temperature is 70°C, when the temperature rises to 80°C the refresh operation is performed every 64 seconds and when it exceeds 80°C the refresh operation is performed every 32 seconds. The said situation causes too much performance and energy loss at high temperatures. By means of the invention of the present patent application, the refresh rate, in other words the refresh frequency, does not need to be increased. By means of the present patent application, the threshold value is increased by applying bias voltage to the cells (21) in case of high temperatures and/or temperature increases and the increase in the leakage currents is controlled via the bias voltage (B).

The control unit (3) in the DRAM structure (1) of the present invention is adapted to provide the required voltage to the rows through the selection input. The control unit (3) can provide the required voltage from the selection input according to the label data of the rows. The control unit (3) also controls the temperature of the DRAM structure (1). Temperature of the DRAM structure (1) is measured via temperature sensor(s). The temperature sensor may be arranged in or out of the DRAM structure (1). Preferably, temperature of the DRAM structure (1) is measured by means of the temperature sensor and the temperature value is transmitted to the control unit (3). The control unit (3) monitors the temperature of the DRAM structure (1) instantly. The leakage current value changes according to the temperature value of the DRMA structure (1), and the control unit (3) contains each temperature value and leakage current value. As the temperature value of the DRAM structure (1) increases, the leakage current also increases; e.g. a significant increase occurs in the leakage current at high temperature values such as 70°C, 80°C, and 85°C. The control unit (3) is adapted to enable supply of bias voltage (B) to the cells (21) according to the temperature value of the DRAM structure (1). Thus, the refresh frequency does not need to be increased as the temperature of the DRAM structure (1) increases.

The control unit (3) enables bias voltage (B) to be transmitted to the cells (21) when the pre-determined temperature limit is exceeded or according to the temperature increase. In one embodiment of the invention, the control unit (3) checks the temperature value to determine for example whether the temperature is above 70°C, 80°C, 85°C, in other words, if the temperature limit has been exceeded, and transmits bias voltage (B ) to the cells (21) when the temperature is above the determined limit. In this embodiment, the control unit (3) decides whether to apply the bias voltage (B) according to the determined temperature limit values and enables the bias voltage (B) to be applied to the cells (21). In this embodiment, the control unit (3) can ensure application of different bias voltages (B) according to the temperature value.

In another embodiment of the invention, the control unit (3) checks whether the temperature value is within the specified limit range. For example, the control unit (3) checks whether the temperature is lower than 50°C, between 50°C and 70°C, between 70°C and 80°C, or higher than 80°C, and enables bias voltage (B) to be applied to the cells (21) according to the range into which the temperature value falls. For example, the control unit (3) applies a bias voltage (B) of 100 millivolts to the cells if the temperature is between 50°C and 80°C, and a bias voltage (B) of 200 millivolts if the temperature is between 80°C and 90°C. In this embodiment, the control unit (3) ensures that the bias voltage (B) value determined according to the specified temperature range is applied to the cells (21).

In another embodiment of the invention, the control unit (3) does not apply bias voltage (B) to the cells (21) if the temperature value is below the determined limit value, and it applies bias voltage (B) if the temperature is above the predetermined value. For example, if the temperature of the DRAM structure (1) is below 80°C, the control unit (3) does not apply bias voltage (B) to the cells (21), and if the temperature is above 80°C, it applies bias voltage (B) to the cells (21).

The control unit (3) also controls operation of the multiplexer (4). It decides whether the bias voltage (B) connected to the selection input of the multiplexer (4) will be transmitted to the cells (21). The control unit (3) generates output signal depending on the temperature of the DRAM structure (1). If the temperature of the DRAM structure (1) exceeds the predetermined temperature value or temperature limit range or temperature limit, the control unit (3) transmits a signal to the multiplexer (4) to enable a predetermined value of bias voltage (B) to be transmitted to the cells.

When the temperature of the DRAM structure (1) is evaluated by the control unit (3) as above the limit value, the control unit (3) generates the selection input required to transmit bias voltage (B) from the output of the multiplexer (4) to the bias line (212). When the temperature of the DRAM structure (1) is evaluated by the control unit (3) as below the limit value, the control unit (3) generates a selection input such that bias voltage (B) will not be transmitted from the output of the multiplexer (4) to the bias line (212).

For a row evaluated to have a high DRAM structure (1) temperature, the bias voltage (B) is received from at least one of the data inputs of the multiplexer (4) and transmitted to the bias line (212) via the bias driver (5) according to the voltage coming from the selection input of the control unit (3). The bias voltage (B) coming from the bias line (212) is applied to the body terminal of the access transistor (211) of each cell (21) in the said row. When bias voltage (B) is applied to the access transistor (211), the threshold voltage of that transistor (211) increases. With the increase of the threshold voltage, the leakage of the transistor (211) decreases, therefore the retention time of the cell (21), in which the said transistor (211) is located, increases.

When the bias voltage (B) is applied to the cells (21), leakage of all the cells (21) in a row decreases and the retention time increases. Thus, despite the increase in temperature, the refresh time can be selected as longer for that row and thus the refresh frequency decreases.

In one embodiment of the invention, the access transistors (211) in each row are preferably connected to a single bias line (212). The access transistors (211) in each row are connected to a same and single bias line (212). The bias voltage (B) is applied to the access transistors (211) in each row through a single and same bias line (212). The bias voltage (B) coming from the bias line (212) is applied to the body terminal of the access transistor (211) of each cell (21) in the said row. When bias voltage (B) is applied to the access transistor (211), the threshold voltage of that transistor (211) increases.

In a preferred embodiment of the invention, the control unit (3) is adapted to use the temperature value of the cells (21) or the temperature value of the DRAM structure (1) to decide whether to apply bias voltage (B) to a row.