JPS6015894A - Semiconductor memory - Google Patents

Abstract

PURPOSE:To attain the setting of an initial value efficiently in high speed by providing a difference to channel length of a transistor (TR) in the initial state after application of power supply to the initial setting by a program or resetting. CONSTITUTION:TRs 1-4 are formed respectively at a part where a diffusion region and polycrystalline silicon parts 4, 5 are crossed together. The channel length of the TRs 2 and 4 are shown respectively in L2 and L4. In the manufacture of an RAM cell, the channel lengths are formed differently so as to make threshold voltages Vth2 and Vth4 of the TRs 2 and 4 different from each other thereby setting the initial state of the RAM cell.

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION The present invention relates to memories, particularly semiconductor memories with a determined initial state.

) Technology Background and Problems For memories such as RAM (random access memory) whose initial values are undefined when the power is turned on, it is necessary to reset the memory after the power is turned on, or to initialize the memory. Initial settings are generally performed by a program. However, in cases such as microcomputers, the memory is not connected to the peripheral circuits until the power is turned on, but is often already connected to the peripheral circuits before the power is turned on. In such a case, it takes time for the power to be turned on and initialized by a program or reset, so an undefined output is generated during this time, and the peripheral circuitry may be undesired while the above reset is being performed. Being affected occurs. For this reason, when controlling peripheral circuits using such a RAM, a situation may occur in which accurate control cannot be expected.

Therefore, in memories such as RAM and registers used in microcomputers, it is necessary to guarantee the period from when the power is turned on until the initial settings are made by a program or reset.

(Objective and Structure of the Invention The object of the present invention is to provide a memory that meets the above-mentioned requirements, and for this reason, the present invention provides a method for determining element parameters of a pair of cross-connected transistors constituting a memory cell. are different from each other, and the initial state of the memory cell when the power is turned on is determined to be a specific state.

[F]) Examples of the invention Hereinafter, the present invention will be explained based on the drawings.

FIG. 1 shows the basic flip-flop of a static RAM cell representing one embodiment of the memory of the present invention. This flip-flop consists of MOS transistors T1 and T2
and an inverter 2 consisting of MOS transistors T3 and T4.

In this clip-flop, when the power is turned on, that is, when the power supply voltage VCC rises, transistors T1 and T2 are on, so transistors T2 and T
This flip depends on which of the four turns on first.
It is determined what initial value the flop will generate. Normally, there is no difference between the channel lengths of transistors T2 and T4, that is, they are not unbalanced, so what initial value is generated?
Determined by channel length variations or random factors.

However, according to the present invention, at the stage of manufacturing the RAM, a difference is made between the channel lengths of the transistors T2 and T4, and the threshold voltage of each transistor is made different, so that a certain transistor is always turned on. I have to.

For example, if the transistor T2 is manufactured so that the channel length L2 is larger than the channel length L4 of the transistor T4, the threshold voltage Vth2 of the transistor T2 becomes larger than the threshold voltage Vth4 of the transistor T4, and the transistor T2 becomes larger than the threshold voltage Vth4 of the transistor T4. The state is such that it is difficult to turn on, and the transistor T4 turns on first. When transistor T4 turns on, inverter 2 made up of transistors T3 and T4 outputs a low level, and positive feedback is generated to inverter 1 made up of transistors T1 and T2, making the on state of transistor T4 more and more stable. I will do it. In this way, the flip-flop can have a predetermined initial value. Moreover, by making this initial value match the initial value determined later by a program or reset, malfunctions due to undefined conditions can be avoided.

FIG. 2 shows an actual pattern of static RAM cells as described above. This figure shows only a portion corresponding to one bit, and to simplify the drawing, bit lines, word lines, contacts, electrodes, etc. are omitted. A portion 3 surrounded by a dotted line represents a diffusion region, and portions 4 and 5 surrounded by a solid line represent polycrystalline silicon portions. Transistors TI, T2, T3, T
4 are formed respectively. These transistors
It corresponds to the transistor shown in the figure. In the figure, the channel lengths of transistors T2 and T4 are respectively L2. L4
Indicated by When manufacturing the RAM cell, the initial state of the RAM cell can be set by providing a difference in channel length and making the threshold voltages Vth 2 and Vth4 of each transistor T2b and T4 different. In the explanation of 0 or more that is possible, the channel length of the transistors is made different, but other element parameters of the transistors constituting the memory cell are made different from each other, and flip
It is also possible to make the flop unbalanced. Furthermore, C and R in FIG. 3 may be made different by changing the mask pattern when forming the transistor.

■ Effects of the Invention As described above, according to the memory of the present invention, the initial state from the time the power is turned on until the initial setting by programming or resetting is established by creating a difference in the channel length of the transistors. can be carried out efficiently and at high speed.

In addition, since the initial state set due to channel length imbalance matches the initial state set by the program or reset, there is no instability due to the old state, and therefore memory such as RAM and registers can be used effectively. becomes possible.

In particular, the memory according to the present invention is effective as a register in a microcomputer, and if it is used as a memory to store a program used only when the system is started, the memory where the program was stored will be used after the system is started. Since it can be rewritten and used, memory can be used effectively.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an R'AM cell according to an embodiment of the present invention, and FIG. 2 is a diagram showing a pattern of the RAM cell shown in FIG. FIG. 3 shows another embodiment. In the figure, 1 and 2 are inverters, 3 is a diffusion region, 4 and 5 are polycrystalline silicon portions, TI and T2. T3 and T4 represent MOS transistors, respectively. Patent applicant Fujitsu Ltd. Representative Patent Attorney Mori 1) Hiroshi (1 other person) Figure 1 Figure 3

Claims (1)

[Claims] A semiconductor memory characterized in that a pair of cross-connected transistors constituting a memory cell have different element parameters so that the initial state of the memory cell when power is turned on is determined to be a specific state.