Non-Volatile Semiconductor Memory

The present invention relates to a floating gate-type non-volatile semiconductor memory of MOS structure, and more particularly, to a non-volatile semiconductor memory capable of writing electric charges in a floating gate electrode with a low voltage and a high injection efficiency.

Conventionally, a writing (injecting electric charges into a floating gate electrode) voltage of a floating gate-type non-volatile semiconductor memory using a channel injection method is generally no less than 15 V. And even in an improved memory, a writing voltage of no less than 7 V is required. In accordance with the unification of an operation voltage of a circuit system to 5 V, the requirement for a non-volatile semiconductor memory capable of being written and read at 5 V has been on the rise.

FIG. 1 shows a sectional view of a conventional floating gate-type non-volatile semiconductor memory capable of being written by channel injection at around 7 V. An n.sup.+ source region 2 and an n.sup.+ drain region 3 are formed on a P-type silicon semiconductor substrate 1 (a P-well formed on a n-type substrate can also be used), and the source and drain regions are connected to the outside by electrodes 8 and 9. The source region 2 and drain region 3 define therebetween a channel region having a first channel region portion l.sub.1 and a second channel region portion l.sub.2, the two channel region portions l.sub.1 and l.sub.2 contacting one another in between the source and drain regions. On the drain region 3 and on the channel region portion l.sub.2 contiguous to the drain region 3 is formed a floating gate electrode 6 made of poly-crystal silicon through a thin (e.g. 100 .ANG.-200 .ANG.) gate oxide film 5 and electrically isolated by an oxide film 10. On the channel region portion e.sub.1 which is in contact with the source region 2 is formed a selective gate electrode 7 through a gate oxide film 4.

The electric potential V.sub.F of the floating gate electrode 6 is controlled by the drain voltage V.sub.D applied to the drain region 3 by electrostatic capacitance coupling between the drain region 3 and the floating gate electrode 6. In case no electrons are fed to the floating gate electrode 6, for example, the electric potential V.sub.F of the floating gate region 6 becomes at around 7 V by applying V.sub.D =7 V to the drain region 3. Accordingly the surface electric potential .phi.SF of the channel region under the floating gate electrode 6 approaches the electric potential of the drain region 3. On the other hand, when a voltage almost equivalent to the threshold voltage of the selective gate electrode 7 is applied, the surface electric potential .phi.SS of the channel region under the selective gate electrode 7 becomes almost equivalent to the electric potential of the source region 2. The surface electric potential .phi.S steeply changes from .phi.SS to .phi.SF near the channel region under the border of the selective gate electrode 7 and the floating gate electrode 6. At this time, electrons are accelerated in the electric field to become hot electrons and jump into the floating gate electrode 6. The electrons can be injected into the floating gate electrode 6 only when the surface electric potential difference .DELTA..phi.S is larger than the electric potential barrier 3.2 V between the silicon substrate 1 and the silicon oxide film 5 and the surface electric potential .phi.S steeply changes.

FIG. 2 shows an example of the distribution of the surface electric potential .phi.S of the memory shown in FIG. 1 when in the writing mode. Regions I, II, III and IV corresponds respectively to the source region 2, the channel region portion l.sub.1 under the selective gate electrode 7, the channel region portion l.sub.2 under the floating gate electrode 6, and the drain region 3. The solid line indicates the distribution of the surface electric potential .phi.S in the case where the impurity density of the P-type substrate 1 is high. It shows that there exists a drop of the surface electric potential .phi.S in the region III near the region IV. The drop of the surface electric potential .phi.S is caused by the electric potential of the floating gate electrode 6 being incapable of sufficiently inverting the surface thereunder if the impurity density of the substrate 1 is high. The surface electric potential difference or drop .DELTA..phi.S at the border of the region II and the region III becomes smaller by a reduction of the surface electric potential .phi.S, which results in disturbance of a reduction in the writing voltage. The broken line shows the distribution of the surface electric potential .phi.S in the case where the impurity density of the P-type substrate 1 is low. No drop of the surface electric potential .phi.S in the region III is found and the variation of the potential .phi.S in the region II is relaxed, whereby the probability of generating hot electrons with high energy becomes smaller.

As illustrated, a reduction in the writing voltage is difficult in the conventional non-volatile semiconductor memory.

Accordingly, it is an object of the invention to provide a non-volatile semiconductor memory having a comparatively low writing voltage.

FIG. 1 is a sectional view showing a conventional non-volatile semiconductor memory,

FIG. 2 shows the distribution of the electric potential on the surface of the non-volatile semiconductor memory in FIG. 1 in the writing,

FIG. 3 is a sectional view showing an embodiment of a non-volatile semiconductor memory according to the present invention,

FIG. 4 is the distribution of the electric potential on the surface of the non-volatile semiconductor memory in FIG. 3 in the writing mode, and

FIG. 5 is a sectional view showing another embodiment of the present invention.

A non-volatile semiconductor memory according to the present invention will be in accordance with FIGS. 3-5.

FIG. 3 is a sectional view showing an embodiment of a non-volatile semiconductor memory according to the present invention, in which a P-type high density impurity region 11 is formed under the selective gate electrode 7 to define the channel region portion l.sub.1.

FIG. 4 shows the distribution of the surface electric potential .phi.S in applying a voltage near the threshold voltage to the selective gate electrode 7 and applying a voltage necessary for writing to the drain region 3 in the memory shown in FIG. 3. The regions I, II, III and IV correspond respectively to the source region 2, the channel region portion l.sub.1 under the selective gate electrode 7, the channel region portion l.sub.2 under the floating gate electrode 6, and the drain region in FIG. 3. By lowering the impurity density of the P-type substrate 1, the surface electric potential .phi.S of the region III can be lowered to approach that of the surface electric potential of the region IV (drain region 3). The surface electric potential .phi.S of the channel region portions l.sub.1 under the selective gate electrode 7 is kept substantially constant over the entire region II excepting the vicinity of the border of the region II and the region III where the channel region portions l.sub.1 and l.sub.2 contact each other owing to the high density impurity region 11, and the surface electric potential changes largely and steeply near the border. Accordingly, the energy of the hot electrons becomes higher, and electric charges escaping from the source region 2 are accelerated and injected into the floating gate and thus can be written at a low drain voltage (e.g. 5 V). Moreover, since the efficiency of injecting the channel electrons into the floating gate is higher, the electric charges can be written with a low current consumption.

On the other hand, reading of the memory is enabled by changing the drain voltage in accordance with the amount of electrons in the floating gate electrode 6 by applying a voltage to the selective gate electrode 7 in order to sufficiently invert the channel region thereunder and further by flowing a reading current I.sub.R in the drain region 3. This is because, in the writing mode in which a number of electrons are injected into the floating gate electrode 6, the threshold voltage is high, whereas in the non-writing mode in which no electrons are injected, the threshold voltage is low. In case electric charges are read at the same voltage condition as in the writing mode for the memory, unexpected injection of electrons may occur. This can be prevented if a voltage sufficiently higher than the threshold voltage of the selective gate electrode 7 is applied to the selective gate electrode 7, whereby a comparatively large inclination of the surface electric potential .phi.S over the whole area of the region II and the region III is produced, and the drop of the surface electric potential .DELTA..phi.S at the border of the region II and the region III can be reduced.

FIG. 5 is a sectional view showing another embodiment of the present invention. The drain voltage V.sub.DD controls the electric potential of the floating gate electrode 6 in FIG. 3, while a gate electrode 12 for exclusively controlling the electric potential of the floating gate electrode 6 is formed thereon in FIG. 5. Although the operation of the embodiment in FIG. 5 as a non-volatile semiconductor memory is the same as that of FIG. 3, the dimension of the device is further reduced in the embodiment of FIG. 5.

As described, the present invention realizes a non-volatile semiconductor memory whose current consumption in the writing mode is reduced.

Although the present invention has been illustrated with respect to an N-type memory cell using a P-type silicon substrate by way of example, it is to be noted that a P-type memory cell using an N-type silicon substrate can be formed in a similar way. Further, although the present invention has been illustrated with respect to a memory in which a high density impurity region is formed under a selective gate electrode, it is readily apparent from the above description that a similar effect can be attained by a memory in which a substrate includes a high density impurity and a low density impurity region is formed under a floating gate electrode.