SEMICONDUCTOR DEVICE WITH A NON-VOLATILE MEMORY

Abstract: The invention relates to a semiconductor device with a semiconductor body which is provided at a surface with a programmable and electrically erasable non-volatile memory comprising a matrix of memory cells which each comprise a field effect transistor with floating gate. A device according to the invention is characterized in that each memory cell comprises a select transistor T2 which is connected in series with the floating gate transistor T1, in that the memory cells form a matrix of the NOR type, and in that the select transistor is connected to the source of the floating-gate transistor, while both writing and erasing are carried out on the basis of the Fowler-Nordheim tunneling mechanism.

Description: Semiconductor device with a non-volatile memory The invention relates to a semiconductor device with a semiconductor body which is provided at a surface with a programmable and electrically erasable non-volatile memory comprising a matrix of memory cells which each comprise a field effect transistor with floating gate. A major embodiment of such a device is a CMOS circuit manufactured in a standard CMOS process and provided with an embedded memory. Such semiconductor devices are generally known. A high reliability, short access time, and a low power during writing and erasing are often required from embedded non-volatile memories. Technologies used for making so-called stand-alone memories usually do not fully comply with the requirements imposed on embedded memories. Thus, for example, Flash memories, in which the cells are arranged in a NOR architecture and in which writing and erasing take place by means of CHEI (channel hot electron injection) and FN tunneling (Fowler-Nordheim tunneling mechanism) often suffer from the problem of overerasure. In addition, writing (programming) in general requires much current. Memories with a N AND architecture, in which both writing and erasing take place by means of Fowler-Nordheim tunneling, require high voltages for erasing and writing, which in its turn may have important consequences for the technology. The invention has for its object inter alia to provide a non-volatile memory which does not have the above drawbacks and which accordingly is particularly suitable as an embedded memory. A semiconductor device of the kind described in the opening paragraph, according to the invention, is characterized in that each memory cell also comprises a select transistor which is connected in series with the floating-gate transistor, in that the memory cells form a matrix of the NOR type, and in that the select transistor is connected to the source of the floating-gate transistor, while both writing and erasing of the memory cells can be carried out on the basis of the Fowler-Nordheim tunneling mechanism. The use of the NOR architecture, in which the cells are not in series, renders possible a short access time. The problem of overerasure can be solved by the select transistor. The use of Fowler-Nordheim for both writing and erasing renders it possible to limit the current (power) for writing and erasing. It is possible to use the entire surface area of the channel for FN tunneling in that furthermore the select transistor is placed at the source side of the floating gate transistor. The FN tunneling mechanism as a result has a high efficiency, so that lower voltages can suffice. A preferred embodiment is characterized in that the transistors of each cell are of the n-channel type, while the semiconductor body comprises a p-type surface region adjoining the surface, and the transistors are provided in a p-type well which adjoins the surface and which is insulated from the p-type surface region by an interposed n-type well. The use of an insulated p-type well renders it possible to use voltages of both polarities, thus halving the maximum voltage (in absolute value), which is of major importance inter alia for the total number of write/erase cycles which can be carried out. These and other aspects of the invention will be explained in more detail below with reference to an embodiment. In the drawings: Fig. 1 is an equivalent circuit diagram of a non-volatile memory according to the invention; and Fig. 2 is a cross-sectional view of a memory cell of the device of Fig. 1. Fig. 1 represents a diagram of a non- volatile, programmable, electrically erasable memory according to the invention. The device comprises a matrix of memory cells, arranged in m rows and n columns. The cells in a row are identified with Mil, Mi2, ..., Min, i being the number of the row. The cells in a column j are identified with Mlj, M2j, ..., Mnj. Each memory cell comprises a floating gate transistor Tl in which data can be stored on a floating gate, as is generally known. Each memory cell further comprises a second transistor T2 which is connected in series with Tl and which forms a select transistor connected to the source of the floating gate transistor Tl . The sources of the select transistors T2 are connected to a common junction point 1. The drains of the transistors Tl in one column are connected to a bit line BLi, i being the number of the column. The bit lines BL are connected to means 2 for applying the desired voltages to the selected bit lines. Each floating gate transistor Tl is provided with a control gate which is connected to word line Cgi, i being the number of the row. Similarly, the gate- of the select transistors^-! are connected to a word line Sgi. The lines Sg and Cg are connected to means 3 by which the suitable voltage can be applied to the selected lines. The arrangement of the memory cells described here is referred to in the literature as the NOR architecture. Since the read current between the bit line BL and the junction point 1 runs through the selected cell only, comparatively low voltages on the word lines can suffice, in contrast to circuits of, for example, the NAND type, in which the cells of a column are connected in series. Fig. 2 shows a cross-section of a single memory cell. Obviously, the device comprises peripheral electronics, which are not shown, apart from the memory cell shown here. In addition, the device may also comprise a logic portion manufactured in a standard CMOS process, also not shown, in embedded applications. The silicon semiconductor body comprises a surface region 5 of the p-type which adjoins the surface 4. The surface region may cover the entire semiconductor body, but this is not necessarily the case. A deep n-type well 6 is provided in the surface region 5 and is provided with a less deep p-type well in which the n- channel transistors Tl and T2 are provided. The n-well 6 insulates the p-type well 7 from the p-type substrate 5, so that different voltages, for example positive voltages, can be applied to the p-type well 7 compared with the voltages applied to the substrate 5, and/or negative voltages may be applied to the bit line. The transistor T2 comprises an n-type source 8, an n- type drain 9, and a gate 10 which is separated from the channel between the source and the drain by gate oxide. The source is connected to the junction point 1 , as is indicated diagrammatically, and the gate to a word line Sg. The transistor Tl comprises a source formed by the zone 9 and an n-type drain 11 connected to a bit line BL. The floating gate 12 is provided above the channel, electrically insulated from the latter. The control gate 13 is provided above the floating gate, electrically insulated therefrom, and is connected to a select line Cg. In the embodiment of Fig. 2, the control gate 13 is provided so as to overlap the floating gate 12, whereby a large capacitive coupling between the gates is obtained. Obviously, the gates may alternatively be arranged as a stack, so that the capacitance between the gates is somewhat smaller, but the cell can also be made smaller then. Reference is made to Table 1 below for the operation of the memory. Table 1 Writing (programming). A low (negative) voltage Vnn (for example -5 V) is applied to all word lines Sg, so that the select transistors are not conducting. The low voltage Vnn is also applied to the selected bit line, so that the relevant drain can temporarily act as a source. A positive voltage Vpp (for example 5 V) is applied to the selected word line Cg, so that an inversion channel is formed in the transistor Tl. Since the select transistor is not conducting, no current flows through the cell, so that no or substantially no power is dissipated. The maximum voltage is present between the channel and the control gate, which voltage is chosen (depending on, for example, oxide thicknesses and other process parameters) such that electrons are stored on the floating owing to Fowler-Nordheim tunneling. Since the charge transport takes place over the entire channel, a high efficiency is obtained, so that the voltages used may be comparatively low. As a result of this, the field strength across the tunnel oxide is also comparatively small, so that damage to the oxide can remain limited, which is important inter alia for the number of write/erase cycles which can be carried out. 0 V is applied to the non-selected bit lines, so that the voltage across the oxide becomes so small that no Fowler-Nordheim tunneling occurs in the non-selected cells. The low voltage Vnn is applied to the p-type well 7 during programming so as to prevent the pn junctions belonging to the selected bit line from becoming forward biased. Erasing The positive voltage Vpp is applied to the p-type well 7 and also to the n-type well 6 so as to prevent the pn junction between the n-type well and the p-type well from becoming forward biased. The low voltage Vnn is applied to the selected word line Cg, and 0 V to the other word lines. The voltage across the gate oxide of the selected cell is sufficiently high again now for Fowler-Nordheim tunneling, so that electrons will tunnel from the floating gate to the substrate 5. The potential of the floating gate rises and the threshold voltage of the transistor becomes low. As during writing, tunneling takes place during erasing over the entire channel surface, so that comparatively low voltages can be used also during erasing. The dissipation is very small also during erasing thanks to the use of the tunneling mechanism. Since each cell comprises a select transistor, moreover, there are absolutely no objections against erasing to the point where the threshold voltage becomes very low, even lower than 0 V, which has important advantages inter alia for reading. Reading For reading a certain cell, a voltage is applied to the selected word line Cg which lies between the threshold voltage of a programmed cell (high threshold voltage) and the threshold voltage of a non-programmed cell (low threshold voltage of an erased cell), and it is ascertained whether the floating gate transistor is or is not conducting. The highest available voltage Vdd is applied to the word line Sg of the cell, so that the select transistor is conducting. A low read voltage of 0.5 V is applied to the selected line and 0 V is applied to the non-selected bit lines, so that Vds = 0 V in these cells and no current can flow in these cells. Since the threshold voltage of the non-programmed cells is low, and may even be lower than 0 V as explained above, a comparatively low voltage can be used on the selected word line Cg, a voltage of 1 V in the example according to the Table. It will be obvious that the invention is not limited to the example described here, but that many more variations are possible to those skilled in the art.

Claims: CLAIMS 1. A semiconductor device with a semiconductor body which is provided at a surface with a programmable and electrically erasable non-volatile memory comprising a matrix of memory cells which each comprise a field effect transistor with floating gate, characterized in that each memory cell also comprises a select transistor which is connected in series with the floating-gate transistor, in that the memory cells form a matrix of the NOR type, and in that the select transistor is connected to the source of the floating-gate transistor, while both writing and erasing of the memory cells can be carried out on the basis of the Fowler-Nordheim tunneling mechanism. 2. A semiconductor device as claimed in claim 1 , characterized in that the transistors of each cell are of the n-channel type, while the semiconductor body comprises a p- type surface region adjoining the surface, and the transistors are provided in a p-type well which adjoins the surface and which is insulated from the p-type surface region by an interposed n-type well.

Inventors: CUPPENS ROGER, DE GRAAF CAROLINE, DORMANS GUIDO J M, TAO GUOQIAO, VERHAAR ROBERTUS D J, CUPPENS, ROGER, DE GRAAF, CAROLINE, DORMANS, GUIDO, J., M., TAO, GUOQIAO, VERHAAR, ROBERTUS, D., J.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H01L27
IPC codes: G11C16, H01L21, H01L27, H01L29