Technical Field

.This invention relates to read-only memories of the kind including addressing means, a plurality of field effect transistors arranged to represent predeter- mined information, and output means.

Background Art

A read-only memory of the kind specified is known from the publication "A User's Handbook of Semicon- ductor Memories" by E. R. Hnatek (John Wiley and Sons, New York, 1977) pages 194-196. In this known memory a thin oxide layer over the channel area of an MOS trans¬ istor separates the channel from the gate electrode; if the layer is thick the gate will not work. Thus stored logic "l"s and logic "0"s in the memory cells of the memory are established by thin and thick regions in the oxide.

To facilitate the description herein, rather than refer to the "presence" or "absence" of a fieϊd - effect transistor (FET), it will be considered that every memory cell contains a FET which either has an output or, for example, has no channel or no effective channel and, thus no output.

The aforementioned ^' known read-only memory has the disadvantage that the information storage density which can be achieved is limited.

It is an object of the present invention to provide a read-only memory of the kind specified wherein the aforementioned disadvantage is alleviated.

Disclosure of the Invention

According to the present invention there is provided a read-only memory of the kind -speci ied char¬ acterized in that the channels of said field effect transistors have predetermined conductance values

selected from at least three possible conductance values, whereby in response to address signals applied to said addressing means the addressed transistor or transistors provide a sense signal determined by the con- ductance value or values associated with the addressed transistor or transistors, said sense signal being applied in operation to said output means.

It will be appreciated that a read-only memory according to the invention has the advantage that a high information storage density can be achieved since each field effect transistor has the capability of storing more information than a single binary bit of inform¬ ation.

According to a preferred embodiment of the invention, the predetermined conductance values are selected from 2 ⁿ possible conductance values (n>l) . This has the advantage that each field effect transistor has the capability of storing n binary bits of inform¬ ation, as will be explained hereinafter. The channel cross-sectional area can be varied to select the channel conductance. Conveniently, the cross-sectional area may be determined by the effective channel width, which in turn may be determined by the width of the channel-defining source and drain diffus- ions.

Brief Description of the Drawings

One embodiment of the invention will now be described by way of example with reference to the ac¬ companying drawings, in which: Fig. 1 is a schematic representation of the layered components of a ROM memory which stores two bits of information per cell;

Figs. 2 and 3 are cross-sectional views taken along lines II-II and III-III of Fig. 1, showing the varied channel geometry which provides the electrical outputs characteristic of the possible combinations

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of two bits;

Fig. 4 is a cross-sectional view taken along line IV-IV of Fig. 1 parallel to the channel width;

Fig. 5 is a block diagram of a ROM array which utilizes FET memory cells embodying the principles of the present invention;

Fig. 6 is a partial, schematic representation of Fig. 5 showing the arrangement for applying read voltages to a single FET memory cell; and Fig. 7 is a chart showing the relative magni¬ tudes of the reference voltages and FET output voltages for the arrangements of Table I and Figs. 5 and 6.

Best Mode for Carrying out the Invention

The principles utilized in the embodiment de- scribed herein for binary information storage, as an ex¬ ample, will first be ^" discussed. There are 2 different permutations or arrangements of bits taken n at a time. Tables I and II show the possible arrangements for n = 2 and n = 3. The channel width of the FET memory cell is conveniently set to one of 2 predetermined widths with corresponding FET output values which represent the pos¬ sible arrangements. As shown, an ordered sequence of these output values is chosen to arbitrarily represent the sequence of 1, 2... 2 ⁿ possible arrangements of bits taken n at a time. Note that the output values, not the physical channel dimensions per se, represent the binary sequence. Thus, rather than merely using, e.g., w.. = 0; w ₂ = 1/3 w. ; w_. = 2/3 w. in Table I, the physical channel widths are those which provide an output sequence such as V, = 0; V« = 1/3 V.; V., = 2/3

V V In Table II, the physical channel widths are those necessary to provide outputs such as V, = 0; V ₂ = 1/7 V ₈ ; ₃ = 2/7 Vg, V ₄ = 3/7 Vg; V ₅ = 4/7 Vg; ₆ = 5/7 V _g ; V _? = 6/7 V _g ; Vg. As described below, an array of 2 ⁿ -l sense amplifiers can be used to read the

stored binary data from any number of such FET memory cells, limited only by the operational characteristics, such as the sensitivity of the sense amplifiers, and by the circuit design.

Table I (n = 2)

FET

Combination Bits Channel FET Sense Amplifier Output No. B 0,y B1 Width Output 12 ² 1 3_4

1 0 0 w. 0 0 0 2 ^' 0 1 w. •1 0 0 3 1 0 W- V- 1 1 0

4(2 ⁿ ) 1 1 w, V, 1 1 1

Table II (n = 3)

T

Combination Bits FE

Channel FET Sense Amplifier Out :pu No. ^B 0 ^B l ^B 2 Width Output 12 23 34 45 56 67 78

1 0 0 0 ^w l ^V l 0 0 0 0 0- 0 0

2 1 0 0 ^W 2 ^V 2 1 0 0 0 0 0 0

3 0 1 0 ^W 3 ^V 3 1 1 0 0 0 0 0

4 1 1 0 ^W 4 ^V 4 1 1 1 0 0 0 0

5 0 0 1 ^W 5 ^V 5 1 1 1 1 0 0 0

6 1 0 1 ^W 6 ^V 6 1 1 1 1 1 0 0

7 0 1 1 ^W 7 ^V 7 1 1 1 1 1 1 0 8(2 ⁿ ) 1 1 1 ^W Q _σ 1 1 1 1 1 1 1

Fig. 1 is a schematic plan view of a portion of a ROM 35 which contains an m x k, row by column matrix of FET cells. The cells of the matrix are designated 51- 55 and 61-65, where the first number of each numerical designation is the row and the second is the column. The figure identifies the stored binary information for each cell location.

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The cell design of ROM 35 is conventional, except for the varied widths of the diffusions which define the FET sources and drains. Because the channel conductance and the resulting channel current or voltage output are determined by the effective channel width provided -by these diffusions, the binary state of each FET can be determined from the magnitude of the channel current or voltage. Thus, FET 51, which does not have a source-forming diffusion or a drain-forming diffusion, has no channel and represents binary 00. (Unless speci¬ fied otherwise, "diffused" and "diffusion" include doped regions formed by ion implantation techniques as well as by. iffusion techniques). For FETs 52-55, pro¬ jections 56-59 from diffusion stripe 41 form the drains, and the sources are formed by the upper ends 76, 77, 79, 81 of diffusion areas 72-75. The widths w~; _? ; w- and w. of the channel-defining diffusions 56 and 76; 57 and 77; 58 and 79; and 59 and 81 are tailored as described previously so that the outputs of FETs 52-55 represent binary 01; 01; 10; and 11 in accordance with Table I. Although row 6 will not be described in detail, the channel widths provided by drain-forming ends of dif¬ fusions 71-75 and source-forming projections of dif¬ fusion stripe 42 provide binary 01, 10, 00, 10, and 00 for FETs 61-65, respectively, in accordance with Table I.

Control voltages can be applied to the drains of cell row 5 via electrical contact (not shown) which can be made at one end of the diffusion stripe 41. Control voltages can be applied to the source diffusions via metal column lines 85-89. Polysilicon line 43 forms ^" the gate electrode for each of the transistors in . row 5. Electrical contact is made at one end of this polysilicon line for providing control signals to the gates. Other electrical connections, such as to the ROM circuit substrate (not shown) of course can be provided as necessary. The gate voltage controls the

conductive state of the channel between the source and drain, i.e., turns the FET on or off, while voltages can be applied to the source, drain, and substrate to control, e.g., the threshold voltage and the magnitude of the drain-to-source current.

The construction of the FETs of Fig. 1 is further clarified in Figs. 2-4. Fig. 2 is a cross- sectional view of FET 51, ^" which has no channel. Fig. 3 is a cross-sectional view of transistor 52, and also is representative of all the row 5 FETs 52-55, which have an effective channel width greater than 0 (i.e. w-, w ₃ or w ). Fig. 4 is a cross-sectional view of FET 52 taken within the channel looking perpendicular to the channel width. It should be noted that the memory array of Fig. 1 is constructed using conventional, well-known process techniques. The method of construc¬ tion does not form a part of this invention and will not be explained here.

Referring further to Fig. 2, the absence of source and drain-defining projections associated with the diffusions 41 and 71 in substrate 6 precludes the formation of a channel beneath the section of the poly¬ silicon gate stripe 43 associated with FET 51. Also, the gate electrode 43 is surrounded by thick silicon oxide layer 66, which precludes the formation of a chan¬ nel under normal operating conditions.

Referring now to figs. 3 and 4, FET 52 com¬ prises source 56 and drain 76 which extend from the diffused regions 41 and 72, respectively. A gate di- electric layer 68, typically of silicon dioxide, extends over the channel region between the source and drain and has the polysilicon gate electrode 43 formed thereon. A thick silicon oxide layer 69 iso¬ lates the gate electrode 43 from the metal column line 86, which contacts the source-forming diffused region 72 at 82.

Fig. 5 is a block diagram of the organization of an m x k ROM array 90 which utilizes the ROM cell

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. design of Fig. 1. The rectangular array 90 stores two bits (n=2) per memory cell location. This is done using .one of four FET channel widths (2 =4) in each memory cell location and sensing the FET output voltage using three (2 -1=3) differential sense amplifiers. The FET memory -cells are designated 111, 112, 113, ... k. The X lines correspond to polysilicon gate electrode lines such as line 43 of Fig. 1. The P and Y lines correspond respectively, to metal contact lines for the source, such as 85-89, and to drain diffusion stripes, such as 41 and 42, all of Fig. 1.

The array 90 is prepared for reading the stored binary digital values by first precharging all the Y lines and sense amplifiers. Current source FET - 106 can be on at all times. Referring to Figs. 5 and

6, voltage V is driven high to turn on FET 91 and precharge the individual Y T. lines (j=l,2,...k) to Vs_s_.

Then Vpre g^oes low to turn off FET 91 and X decode 92 selects a row line X. (i=l,...m) and drives the selected row line to a "1". That is, Vro ^• w is ap ^c p ^c lied to the selected row X. so that all the transistors therein are turned on (unless they are binary 00) and drive into the Y loads 93. The transistors generate an output voltage on each Y. line which is determined by the threshold voltage at I of each memory FET. Stated cs differently, the output voltage is determined by the voltage division between the memory FET and current (Ics) source device 106. Each output voltage is one of the four possible output voltages V -V shown in Table I. To decode the output, a Y. line is selected by the Y decode 94 and drives through the Y decode 94 and into a first input terminal 95, 96, 97 of each sense amplifier. The differential sense amplifiers compare the output voltage against three reference voltages V. _, V__, and V,., which are applied, respec¬ tively, to a second input terminal 98, 99, 100 of each sense amplifier. The reference voltages are between

and, typically, are an average of the three pairs of adjacent FET output voltages. (For example, V ₁₂ =(V ₁ +V ₂ )/2.) A sense amplifier provides an output signal if the voltage at the first input terminal ex- ceeds the reference voltage. Referring to Figs. 5 and

7, the memory cell output voltage V,, V,, V ₃ , or V ₄ thu turns on none of the sense amplifiers (V,) , sense ampli fier SA12 (V ₂ ), two sense amplifiers SA12 and SA23 (V ₃ ) or all ^' three sense amplifiers SA12, SA23 and SA34 (V.). The outputs at terminal pairs 101 and 101; 102 and 102; 103 and 103 of the three sense amplifiers are the in¬ verse of one another and are provided as inputs to a logic array 104. The collective sense amplifier output control the operation of logic circuit 104, which gener ates the two bits of data associated with the particula channel width of the selected FET. See Table I. (The on and off states of the sense amplifiers are indicated respectively by "1" and "0" in the tables.) These two bits of data can be used as parallel outputs or can be applied to a Z decode 105 which would then output a selected one of the data bits depending upon the value of the Z control signal.

The sense amplifiers, logic array and X, Y an Z decodes are of conventional design and need not be de scribed in detail.

As an example of decoding a particular FET memory cell output, refer to Fig. 5 and assume that the channel width of FET 111 in array 90 is w_. That is, the width of the channel of FET 111 is selected to provide an output V_ = 2/3 V., which corresponds to bit combination 10, Table I. Up ^c on ap ^c p ^c lication of Vrow to row X as described above, this output V-. is generated on line Y, and is applied via Y decode 94 to one of the input terminals 95, 96, 97 of each sense amplifier. Referring further to Fig. 5 and also to Fig. 7, ₃ is greater than the reference voltages V, ₂ and V~ ₃ which are applied to sense amplifiers SA12 and SA23, respec¬ tively, but smaller than the reference voltage V

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applied to sense amplifier SA34. Accordingly, SA12 and SA23, but not SA34, are turned on by V ₃ and their col¬ lective outputs drive logic array 104 to generate the two bits 10. As another example, if the channel width of

^■• - FET ^" 111 had' been . (w ₂ is tailored"to-provide output ₂ = 1/3 V.), only sense amplifier SA12 would be turned on by the output, and this single output would drive logic array 104 to generate the bits 01 associated with output

An NCR standard PDISG (p-channel dielectri- cally insulated silicon gate) ROM cell utilizes a cell size of 15 microns x 23 microns (345 square microns) and provides a storage density of about 290,000 bits per 5 square centimeter, excluding control circuitry. For this standard cell.size, a two bit per cell, PDISG ROM array 90 provides storage of about 580,000 bits per square centimeter, excluding control circuitry. A maximum physical channel width w. = 7.5 microns is used. o This dimension is an example only, for greater or lesser widths are available at the option of the circuit de¬ signer.

Although in the described embodiment the chan¬ nel conductance is determined by the channel width, it 5 should be understood that in modified embodiments the channel conductance can be determined alternatively by channel length, a combination of channel length and channel width, or channel implantation levels.