Patent Application
20060133172
This application claims foreign priority benefits under 35 U.S.C. § 119 to co-pending German patent application number DE 10 2004 055 674.1, filed 18 Nov. 2004. This related patent application is herein incorporated by reference in its entirety.
1. Field of the Invention
The invention relates to an apparatus and a method for writing to and/or reading from a memory cell in a semiconductor memory.
2. Description of the Related Art
Dynamic memory cells in semiconductor memories, such as DRAMs, store information by storing charge in a storage capacitor which is selected and connected by a selection transistor via a word line. In this case, a flow of charge when writing the information travels from a driver transistor, which is also used as a sense amplifier, via a connecting line (bit line) and the selection transistor to the storage capacitor.
A critical characteristic in a specification for the semiconductor memory includes a time period tWR which defines a time period between a write operation (WRITE) and deactivation of the word line. An extent for the time period tWR is stipulated in the memory cell by virtue of, inter alia, a particular minimum of electrical charge needing to be written to the storage capacitor before the word line which actuates the selection transistor is deactivated. Deactivating the word line terminates the write operation. The minimum of electrical charge is prescribed by, inter alia, a sensitivity of the sense amplifier during the read operation (READ). What is also crucial for a time used to write a particular quantity of charge to the storage capacitor is an RC constant for the signal path described above.
It is the object of the present invention to provide an apparatus for improved writing to a memory cell in a semiconductor memory.
The inventive apparatus is provided for writing to and/or reading from a memory cell in a semiconductor memory having a selection transistor and a storage capacitor, where the apparatus has a device which is used to influence a threshold voltage Vt for the selection transistor contrary to the influence of an ambient temperature.
This advantageously means that the inventive device is used to compensate for a temperature response in the threshold voltage Vt, which means that a time constant for a signal path which is used to write electrical charge into the storage capacitor of the memory cell is essentially independent of the temperature. This means that, by way of example, a specification parameter tWR for the dynamic semiconductor memory is advantageously minimized for high and low temperatures and needs to be observed to an equally uncritical degree.
In one preferred development of the inventive apparatus, the device comprises an electrical voltage generator, with an electrical voltage generated by the voltage generator being applied to a substrate well in the selection transistor. The electrical voltage generated by the voltage generator represents an offset voltage which influences the threshold voltage Vt contrary to the influence of an ambient temperature via the substrate well in the selection transistor, advantageously in a direct mechanism of action.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
In this case, a significant influencing variable is a turned-on resistance Ron of the selection transistor 2 in the on state. This resistance is determined by the threshold voltage Vt, the threshold voltage Vt being proportional to the turned-on resistance Ron. A higher threshold voltage Vt signifies a higher turned-on resistance Ron, and vice versa. The threshold voltage Vt, like the turned-on resistance Ron, rises as temperature falls. This increases the RC constant of the signal path described above, which means that a specified time tWR, indicating the time in which sufficient electrical charge needs to be written to the storage capacitor 3, needs to be observed more critically at low temperatures than at high temperatures.
The problems outlined are intensified even more by the fact that today's usual small feature sizes of semiconductor memories mean that a contact resistance between the selection transistor 2 and the storage capacitor 3 may be very high. This contact resistance is likewise part of the signal path for writing to/reading from the dynamic memory cell 1, which means that the entire RC constant of the signal path may conventionally be very high, which is a drawback. A time requirement for a write or read operation in the dynamic memory cell 1 may therefore be disadvantageously very high at low temperatures.
To compensate for any temperature dependency of the turned-on resistance Ron of the selection transistor 2, the invention proposes influencing the threshold voltage Vt of the selection transistor 2. This is achieved by virtue of an output of an electrical voltage generator 5 being connected to a substrate well 4 in the selection transistor 2, with the output outputting an offset voltage Voff to the substrate well 4. A temperature sensor 6 is used to ascertain an ambient temperature T for the dynamic memory cell 1, and to regulate the electrical voltage generator 5 on the basis of the ascertained ambient temperature. By way of example, the temperature sensor 6 may be arranged within a memory device (not shown) having a large number of dynamic memory cells 1. In this way, the threshold voltage Vt is influenced by the voltage generator 5 contrary to the influence of the ambient temperature. This makes it possible to compensate for a temperature response in the threshold voltage Vt of the selection transistor 2.
The fact that the threshold voltage Vt is proportional to the turned-on resistance Ron of the selection transistor 2 means that this also compensates for any temperature dependency of the turned-on resistance Ron of the selection transistor 2. This makes a write operation to the dynamic memory cell 1 uniform for low and for high temperatures to the extent that an RC constant for a signal path via which the charge is written to the storage capacitor 3 is essentially independent of temperature.
The temperature independency of the signal path means that besides the write operation it is also possible to perform a read operation from the storage capacitor 3 in the dynamic memory cell 1 in improved fashion. This can be substantiated in that a time specification parameter for the selection transistor 2 is easier to observe for the read operation for the essentially temperature-independent and hence constant turned-on resistance Ron of the selection transistor 2.
The inventive device having the voltage generator 5 and the temperature sensor 6 can thus advantageously be used to utilize or dimension time specification parameters for the dynamic memory cell 1 in the semiconductor memory in improved fashion. Advantageously, the read/write response of the dynamic memory cell 1 is therefore essentially independent of the ambient temperature. Memory devices having a large number of semiconductor memories with the inventive memory cells 1 thus allow accelerated timing properties and can therefore be of improved design.
Using suitable semiconductor implantations and dopings in the substrate well 4 in the selection transistor 2, it is possible to shape the threshold voltage Vt in a production process for the selection transistor 2. This is done through suitable selection of implantation sequences or implantation angles and depths in the substrate well 4. In addition, the threshold voltage Vt can be influenced through additional doping of a region between the source and drain (“channel implantation”) of the selection transistor 2. The channel implantation enhances or compensates for the doping of the substrate.
The methods outlined can be used to lower or raise an absolute value of the threshold voltage Vt and hence of the turned-on resistance Ron of the selection transistor 2 in advantageous fashion. Using the semiconductor implantations, it is thus possible to set the absolute value of the threshold voltage Vt to be as low as necessary and possible, so that the selection transistor 2 has well-defined switching properties both at low and at high ambient temperatures T.
Owing to the semiconductor implantations, the inventive compensation for the temperature response in the threshold voltage Vt results in a reduced threshold voltage Vt and hence in a further improvement in the read/write timing in the selection transistor 2 in the dynamic memory cell 1.
A curve 2A shows, in principle, a conventional curve for the threshold voltage Vt as a function of the ambient temperature T. It can be seen that the threshold voltage Vt falls as ambient temperature T rises, which means that the turned-on resistance Ron of the selection transistor 2 is high at low ambient temperatures and the result of this is a disadvantageously high RC constant for the read/write signal path of the dynamic memory cell 1. The inventive device having the electrical voltage generator 5 and the temperature sensor 6 is used to achieve a curve 2B for the threshold voltage Vt.
Although the invention has been described by way of example with reference to an n-FET, it goes without saying that the invention can also be applied to p-FETs. In this case, the inventive influencing of the threshold voltage Vt requires appropriate selection of a polarity for the offset voltage Voff of the electrical voltage generator 5. It is also regarded as advantageous that the invention can also be implemented with static memory cells.
The inventive apparatus may advantageously thus be used to speed up read and write operations for dynamic memory cells in semiconductor memories, or to make them more uniform, by a virtue of a resistance component in the form of the turned-on resistance Ron of the signal path for writing to/reading from the selection transistor 2 being at a minimum.
The aspects of the invention which are disclosed in the description, claims and drawings can be fundamental to the invention both individually and in combination.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 055 674.1 | Nov 2004 | DE | national |
