JPH0782755B2 - Semiconductor memory device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device that records information by changing a threshold voltage.

2. Description of the Related Art Conventionally, as a semiconductor memory device that uses a change in threshold voltage, EPROM (Erasable and Programmable Read Only Memo) is used.
There is a so-called floating gate type MOSFET having a floating gate 3 between a control gate 1 and a substrate 2 as shown in FIG. These devices record information by changing the threshold voltage by injecting and trapping charges in this charge storage layer by hot electron injection or tunnel injection. Therefore, the written information is replaced with the magnitude of the threshold voltage or the presence or absence of the threshold voltage fluctuation and stored.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described above, in the conventional semiconductor memory device utilizing the fluctuation of the threshold voltage, one bit per element is used to store information by replacing the recording of information with the presence or absence of the threshold voltage fluctuation. There was a problem that the amount of information could not be stored.

The present invention has been made to solve the above problems.
It is an object of the present invention to realize a large-capacity, high-density storage device in which the amount of information recorded per element is a plurality of bits or more.

Means for Solving the Problems In order to solve the above problems, the present invention separately controls a charge injection amount into a charge storage layer by separately dividing a charge injection element and a recording element to control a threshold voltage of a storage element. Alternatively, a circuit for setting the drain current to an arbitrary value and a circuit for reading the fluctuation amount of the threshold voltage or the drain current are provided, and the threshold voltage or the drain current is changed instead of the conventional storage method simply by the presence or absence of the change of the threshold voltage. By recording information as the amount, it is possible to realize a large-capacity high-density storage device in which the recording amount of information per element is a plurality of bits or more.

Action The present invention has the above-described configuration to control the amount of charge injection into the charge storage layer, change the threshold voltage or drain current of the MOSFET to an arbitrary value, and record information according to the value and the amount of change. For this reason, the amount of information that can be recorded per element is significantly increased as compared with the conventional recording based on the presence or absence of fluctuations in threshold voltage. For example, if the injection amount control circuit and the reading circuit are designed so that the threshold voltage fluctuation can be recognized every 100 mV, one element can store 4 bits with a fluctuation width of 1.6V.

Embodiment An embodiment according to the present invention will be described with reference to FIG.

Reference numeral 7 is a storage element, that is, a floating gate MOSFET for storage, which is a charge injection element, that is, a floating gate for hot carrier injection.
It is connected to the MOSFET 10 by a source and a floating gate 20.
11 is a current-voltage converter, 12 is an operational amplifier, and 13 is a comparator.

A drain voltage (for example, 21V) capable of injecting hot electrons is applied to the hot carrier injection element 10 through the transfer gate switch 9. In this state, the drain current flowing through the channel is accelerated in the high electric field region near the drain junction into hot electrons in the floating gate of the floating gate type MOSFET 10 to become hot electrons and injected through the gate oxide film. As a result, the IV characteristics of the floating gate type MOSFET for storage in which the floating gates are electrically connected to each other move in parallel in the gate voltage axis direction as shown in FIG. 2 (that is, the threshold voltage changes). ). Since a voltage controlled by the operational amplifier 12 is applied to the gate terminal 15 so that a constant drain current always flows through the MOSFET 7, this voltage also changes as much as the amount of charges injected into the floating gate. The hot electron injection is continued while monitoring by the comparator 13 until the gate voltage becomes a voltage (voltage applied to the terminal 18) corresponding to the information to be stored. Comparator 13
Detected by, transfer gate 9 is turned off and floating gate type M
When the application to the gate and drain terminals of OSFET 10 is completed, a MOSFET having a threshold voltage corresponding to the information to be stored can be obtained.

When reading, a voltage is applied to the gate so that the drain current has a predetermined value, and the gate voltage at that time is set as the threshold voltage for reading.

The embodiments according to claim 1 have been described above.

Next, an embodiment according to claim 2 will be described with reference to FIG. In the above-described embodiment, the information is recorded by changing the threshold voltage value. However, in the present embodiment, the information is recorded by the drain current amount flowing when the voltage applied to the gate electrode 15 is applied. Is to do. Writing and reading are performed with a constant drain current in the above-described embodiment, whereas this embodiment is different in that a drain current amount under a constant gate voltage is used.
The circuit configuration is simpler than that of the above-described embodiment.

Also, in reading, the element used as a storage element when obtaining the variation of the threshold voltage or drain current is used as a comparison element, and the variation of each is obtained by the difference with the characteristics of this element. A memory device that is not easily affected by temperature changes and the like can be provided.

EFFECTS OF THE INVENTION As described above, according to the present invention, since the information is recorded instead of the variation of the threshold voltage of the MOSFET or the variation of the drain current, the analog information is recorded as it is or 1 Multiple bits can be recorded per element. By dividing the charge injection element and the storage element, the same drain voltage can be used during writing and reading. Further, since the charge injection element and the storage element are different, the deterioration of the recording element characteristics due to the injection (for example, the deterioration of gm due to the generation of the interface state) can be ignored. Therefore, the threshold voltage and the drain current can be controlled more accurately than in the case where the same element is used for the injection / recording.

[Brief description of drawings]

FIG. 1 is a block diagram of a semiconductor memory device according to an embodiment of the present invention,
FIG. 2 is a diagram showing characteristic fluctuations of a floating gate type MOSFET due to hot electron injection, FIG. 3 is a configuration diagram of a semiconductor memory device for showing another embodiment, and FIG. 4 is a semiconductor for explaining a conventional example. It is a partial schematic sectional drawing of a memory | storage device. 7,10 Floating gate MOSFET, 9 Transfer gate, 11 Current / voltage converter, 12 Operational amplifier, 13
... comparator.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H01L 29/792

Claims (2)

[Claims] 1. A semiconductor memory device for storing information by changing a threshold voltage by injecting / accumulating charges into a floating gate of a floating gate type MOSFET, the floating gate having floating gates, wherein each floating gate is electrically connected to each other. A storage element and a charge injection element connected to the storage element, and a comparison means for reading a threshold voltage of the storage element and comparing the storage element with a predetermined voltage. A semiconductor memory device, wherein charges are injected into the storage element until the predetermined voltage is reached, and the injection of charges is completed when the threshold voltage of the storage element reaches a predetermined voltage. 2. A semiconductor memory device for storing information by changing a threshold voltage by injecting / accumulating charges into a floating gate of a floating gate type MOSFET, the floating gate having floating gates, and each floating gate being electrically connected to each other. A storage element and a charge injection element connected to the storage element, and a comparing means for reading a drain current of the storage element and comparing the drain current with a predetermined current, the drain current of the storage element being given in advance. A semiconductor memory device, wherein charges are injected into the storage element until a predetermined current is reached, and the injection of charges is completed when the drain current of the storage element reaches a predetermined current.