JPS6010396B2 - semiconductor storage device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device using a P-n junction (hereinafter abbreviated as polysilicon diode) formed in a polycrystalline silicon thin film.

Various types of memory elements are involved in semiconductor memory devices.
In particular, programmable read-only memory (P-R)
OM) uses a NiCr fuse,
Bipolar transistors are known in which the P-n junction of a bipolar transistor is subjected to secondary breakdown.

However, these P-ROM devices have drawbacks such as complicated manufacturing methods and high energy requirements for writing. Furthermore, the memory cell area also increases, making it unsuitable for large-capacity memory devices. The object of the present invention is to provide a P-ROM copy that eliminates the above-mentioned drawbacks.

The semiconductor memory device of the present invention uses a polysilicon diode as a memory cell, and the principle of the present invention will be explained below.

Many crystal grain boundaries exist in polysilicon, and many trap levels exist in these grain boundaries.

Therefore, the current flowing through the P-n junction formed in polysilicon is dominated by the recombination current caught in this trap level rather than the diffusion current. Therefore, the current component is E
The portion proportional to XP [qv/2kT] is dominant.
On the other hand, due to potential barriers existing at crystal grain boundaries, the electrical resistance of PoIJ silicon is higher than that of single crystal silicon. This backward trend becomes more pronounced as the impurity concentration becomes lower. A Volisilli diode formed by low-concentration diffusion has a large resistance component and deviates greatly from the relationship of EXP [qv/2kT]. The forward characteristics of these Policy J diodes manufactured under typical manufacturing conditions are shown in FIG. 1, and the reverse characteristics are shown in FIG. In the figure, A shows the characteristics of a diode with a high impurity concentration, and B shows the characteristics of a diode with a low impurity concentration. By the way, when the diode B undergoes primary breakdown, its characteristics easily change to those shown by C in FIGS. 1 and 2.

A diode with characteristic A is stable against primary breakdown, whereas a diode with characteristic B changes to characteristic C with a reverse current of several hundred A to several milliamps. × with the curve in Figure 2 B
The mark indicates the point at which it changes to C, and the characteristics are stable and irreversible even after the change has been made once. By making the initial characteristics of the diode B and the characteristics of C changed by the primary breakdown correspond to the two states "0" and "1" of binary logic, it can be used as a memory element.

Since the change from B to C is irreversible, there is a restriction that it can only be used as a read-only memory circuit (ROM), but it has the advantage of being able to be programmed arbitrarily from the outside. A first embodiment of the present invention will be described below.

FIG. 3 is a diagram explaining the first embodiment.

Polysilicon diode 11, 12,……ln, 21,
22,・・・・・・・・・・・・2n is multi-emitter np
One each is connected to the emitters of n-transistors T1 and T2. This polysilicon diode has the characteristics of B in FIGS. 1 and 2, and the transistor T1 is placed in a biased state such that no current can flow through the diode unless an external pulse is applied to cause the diode to surrender. Assume that T2 is placed. In Figure 3, terminals 1, 2,...n are bit lines,
Terminals 101, 102, . . . represent word lines.

Therefore, each transistor T1, T2,...
... can each store n bits of information. For convenience of explanation, a state in which current flows out from the emitter of a transistor is defined as "1", and a state in which current is cut off is defined as "0". Now, consider the case where information "1" is stored between terminal 101 and terminal 1. The operation for this is to set the terminal 101 under bias conditions that make the npn transistor TI conductive, and set the remaining terminals 102, . . . , under bias conditions that make the corresponding transistors disconnected. put it on. By doing this, a current flows between terminal 1 and Vcc,
It is possible to change the characteristic from B to C by breaking down only the polysilicon diode 11. The required current value at this time varies depending on the manufacturing conditions of the diode. Generally, the higher the impurity concentration of the diode, the higher the current value required for breakdown, and the lower the impurity concentration, the higher the current value required for breakdown.
The current value becomes lower. In the case of a diode having characteristics B in FIG. 2, a write current value of 10 mA is sufficient. In this way, only diode 11 can go into the conductive state and a "1" is stored. By repeating the same procedure thereafter, it is possible to store arbitrary information. Once written, information is permanently stored and can be read when needed. For reading, select the word lines 101, 102, . . . . . . . . . . . . . . . . . ” or “0” can be read. During reading, the transistor must be operated at a low voltage in order to prevent breakdown of unwritten diodes and to obtain a sufficient current ratio between "1" and "0". For example, in the case of a diode with characteristics B in Figure 2, the potential difference between Vcc and bit lines 1.2,......n is 3V.
The following is desirable. FIG. 4 shows a second embodiment of the present invention.

This is a method in which a diode is connected to the collector of a multi-collector NPN transistor, and is a convenient method when combined with 12L (MTL). The principle is similar to the first embodiment, but the second embodiment differs in that the forward characteristics of a polysilicon diode are used. Therefore, the change in the forward characteristic of the diode with characteristic B is utilized. In other words, the diode before breakdown has a large series resistance component and a high forward voltage, so even if transistors T1, T2, etc. become conductive in the connection shown in Figure 4, sufficient current will not flow to the collector. Can't flow. However, once the diode is broken down to the state of characteristic C, the forward voltage becomes smaller.
This time, sufficient current will flow through the collector. In this way, if the state where collector current flows is "1" and the state where almost no collector current flows is "0", then the circuit in Figure 4 will be "1", "
0" signal can be stored. The method of writing information is slightly different from the first embodiment. For example, a "1" signal can be stored in the diode 11.
”, the terminal 101 is grounded and the other terminals 102, . . . are left open.

Then, a negative voltage is applied to the bit line 1, and a current necessary to change the diode characteristics is caused to flow through the diode 11, thereby changing the diode characteristics from B to C. The current value required at this time is the same as in the first embodiment, but in order to make the bottle direction voltage sufficiently low, a somewhat larger current is required than to change the reverse direction characteristics. Alternatively, for the same current value, it takes more time than changing the reverse characteristics. When reading data, the base of the target transistor (ie, the word line) is normally biased, and it is determined whether it is "1" or "0" depending on whether a current is flowing through the bit line.

The area of the circuits of FIGS. 3 and 4 can be minimized when the structure shown in FIG. 5 is adopted.

This structure is common to FIGS. 3 and 4. In other words, the only difference is whether the npn transistor is used in a normal connection or with the emitter and collector reversed. In FIG. 5, a polysilicon diode is formed in a polysilicon film grown over the n+ diffusion.

Since a polysilicon diode can be formed in the contact portion of the secondary transistor, the cell area of the memory circuit does not become larger than that of one transistor. In FIG. 5, 1001 indicates an n+ buried region.

In the circuit of FIG. 4, 1001 may be an n+ substrate, but in the circuit of FIG. 3, since each transistor must be insulated and separated, it must be an n+ buried layer formed on a p-type substrate. 1002 is an n-type epitaxial region, 10
03 is the P type base region, and the base contact is 10
It is 08.

The n10 layer 1004 means an emitter in the circuit shown in FIG. 3, and a collector in the circuit shown in FIG. A polysilicon film 1006 is doped with a p-type impurity, and a p-type impurity is formed in the polysilicon film 1006 between the region 1004 and the n-type impurity.

1007 is a metal layer for wiring.

Since the portion 1008 contains only p-type impurities, an oscic connection is established.

As described above, according to the present invention, it is possible to realize a PROM using bipolar transistors with high integration density.

[Brief explanation of the drawing]

Figures 1 and 2 show the voltage and current characteristics of a polysilicon diode, Figure 3 shows the first embodiment of the present invention, Figure 4 shows the second embodiment, and Figure 5 shows how to minimize the cell area. FIG. In the figure, 1, 2, ......n are bit lines, 101
, 102 are word lines, T1 and T2 are npn transistors, 1 1, 12, . . . ln and 2
1, 22, ……2n are polysilicon diodes, 100
1 is an n+ buried layer or an n+ substrate, 1002 is an n-type epilayer,
1003 is a P type base region, 1004 is an n+ emitter or collector, 1005 is Si02, 1006 is a polysilicon film, 1007 is a metal wiring region, and 1008 is a base contact. Rare figure Rare Z figure Tsutomu 3 figure 4 cure figure

Claims (1)

[Claims] 1. Utilizing the fact that the forward or reverse characteristics of a P-n junction formed in polycrystalline silicon irreversibly change depending on the reverse bias condition applied to the junction, A semiconductor memory device characterized in that characteristics before and after a change correspond to respective values of a binary logic signal. 2. A polycrystalline silicon diode is connected to each emitter or collector of a transistor having a multi-emitter or multi-collector structure, one end of the diode is connected to a bit line, and the base of the transistor is connected to a word line. semiconductor storage device.