ELECTRICALLY PROGRAMMABLE MEMORY CELL

Abstract: An EPROM cell comprises an MOS device including a floating gate electrode (44, 64) overlying, and ohmically insulated from, the channel region (26) of the MOS device, and a separate diode (18) including a p-n junction (54) having a substrate surface intercept (30). A floating gate electrode (64) overlies the diode p-n junction intercept (54) and is ohmically isolated therefrom by an intervening insulating layer. Writing of data into the floating gate electrode of the MOS device is achieved by causing a voltage breakdown across the diode p-n junction (54) and the flow of high energy electrons across the junction. A voltage is simultaneously applied to the diode gate electrode (64) thereby attracting some of the high energy electrons through the overlying insulating layer into the diode floating gate electrode. The diode gate electrode (64) is ohmically connected to the MOS floating gate electrode (44) on which some of the electrons are stored for affecting the turn-on, turn-off, characteristics of the MOS device.

Description: Electrically Programmable Memory Cell Background of the Invention This invention relates to electrically programmable memory (EPROM) cells, and particularly to a new arrangement for writing and reading data into 5 and from such cells. EPROM cells comprise a metal-oxide-semiconductor (MOS) device including a control gate electrode overlying a channel region of the device and an ohmically isolated floating gate electrode disposed between the control gate electrode and the channel region. Because of the ohmic isolation of the floating 10 gate electrode, electrons reaching it remain stored thereon and provide a voltage bias which affects d e turn on characteristics of the MOS device. For example, with an NMOS device (N type source and drain regions separated by a P type channel region) having a sufficient quantity of electrons stored on the floating gate, the device is biased in its off state, and requires the 15 application of a positive voltage to the control gate electrode to turn it on. The presence or absence of electrons stored on the floating gate electrode corresponds to either die storage of a logic 1 or 0 in memory. Reading of the memory comprises applying a voltage to die control gate electrode for changing, or not changing, die on-off state of die MOS device 20 depending upon die presence or absence of the stored electrons. The writing of data into such cells comprises die collection of electrons onto the floating gate electrode dirough the insulating material disposed between it and the channel region. This is done by applying a relatively large positive voltage to die control gate electrode and the drain (widi respect to die 25 source) which generates a stream of high energy "hot electrons" through die channel region. Owing to die high positive voltage on die control gate electrode, which is capacitively coupled to die floating gate electrode, some of die high energy channel electrons have sufficient energy to overcome die insulating barrier layer, and are attracted to the floating gate electrode. When * •** • 30 die data writing voltages are turned off, die electrons which have reached die floating gate electrode remain trapped thereon. During subsequent reading of die cell, the voltages used to determine die on-off state of die MOS device are sufficiently low to prevent further tunneling of electrons to or from the floating gate electrode, diereby providing a relatively permanent storage of me written data. One condition necessary for inducing the tunneling mechanism is that the channel region of the MOS devices be relatively short, e.g., approximately one micron or shorter in lengtii, in order tiiat sufficiently high electric fields are present to generate the necessary high energy electrons. MOS devices having such short channels are available, but require high resolution photolithographic manufacturing processes . Thus, in semiconductor devices which are conventionally manufactured widi lower resolution processes, it has not been possible to use EPROM cells of the type described herein. This limitation is overcome in accordance widi tiiis invention. Summary of the Invention An EPROM cell comprises an MOS device including a floating gate electrode and a separate diode having a p-n junction extending to a surface of a semiconductor substrate. A floating gate electrode is disposed over me surface intercept of die diode p-n junction and is spaced merefrom by a layer of insulating material. The diode gate electrode is ohmically connected to the MOS device floating gate electrode. For writing data into me MOS cell, die diode p-n junction is sufficiently highly reverse biased to cause voltage breakdown of me diode and d e flow of high energy electrons merethrough. A high positive voltage is applied to the source and drain regions of die MOS device to capacitively couple die positive voltage to d e MOS device floating gate electrode and to the diode floating gate electrode connected thereto. Some of me high energy electrons flowing across the p-n junction of me diode are attracted through die intervening insulating layer by me positive voltage on me MOS floating electrode and are stored mereon when die writing voltages are removed. Reading of the data stored in me MOS device is done by applying a voltage to me anode and camode of die diode for capacitively coupling the voltage to the diode floating gate electrode, hence to d e MOS floating gate electrode. Widi appropriate voltages applied to die MOS device, die MOS device will be on or off depending upon me presence or absence of charge stored on its floating gate electrode. Alternatively, and in addition to die structure above-described, a ςontrol gate electrode can be provided capacitively coupled to the MOS device floating gate electrode for use in die reading process. Description of the Drawing Fig. 1 is cross section of an EPROM cell made in accordance with this invention; and Fig. 2 is a plan view of me cell shown in Fig. 1. Description of Preferred Embodiments of the Invention Wim reference to Figs. 1 and 2, one example of a single EPROM cell according to tiiis invention comprises a substrate 10 of semiconductor material, e.g., silicon, of P type conductivity, and including two spaced apart "wells" 12 and 14 of N type conductivity. The two wells 12 and 14 include die separate devices of die EPROM cell of the invention. Normally, a plurality of EPROM cells are included in an array of memory cells contained in a substrate. Neitiier me array of EPROM cells nor me various connections merebetween is shown. Arrays of EPROM cells are known. It is also known to use spaced apart wells for electrically isolating devices contained therein. The use of the wells shown herein is but one example of how EPROM cells according to this invention can be made. The well 12 contains an MOS device 16 and the well 14 contains a diode 18. Both me MOS device 16 and die diode 18 can be of generally known type and can be fabricated using known processing techniques. The MOS device 16 includes two p-type regions 22 and 24 spaced apart by an n-type channel region 26. The surface 30 of me substrate 12 is covered by an insulating material layer 32, e.g., silicon dioxide, which is common to both wells 12 and 14. Conductive electrodes 34 and 36 (shown schematically) extend d rough die layer 32 and into ohmic contact with the regions 22 and 24, respectively. The electrodes 34 and 36 provide means for applying voltages to me regions 22 and 24. Similarly, for applying a voltage to the well 12 itself, referred to as die substrate of d e MOS device, a region 40 of n+ conductivity is provided widiin the well 12 at the surface thereof to which an ohmic connection is made via an electrode 42. Overlying die channel region 26, but ohmically insulated therefrom by a reduced tiiickness portion of the layer 32 is a floating gate electrode 44. The diode 18 includes a highly doped p+ region 50 adjoining a highly doped n+ region 52 and forming a p-n junction 54 dierewidi. The p-n junction 54 extends to d e surface 30 of the well 14 which is covered widi the insulating material layer 32. Electrodes 56 and 58 extend dirough the layer 32 and into 105 ohmic contact widi the regions 50 and 52, respectively. Overlying me surface intercept of die diode p-n junction 54, but ohmically insulated from it by a portion of me layer 32 is a diode floating gate electrode 64, which, in mis embodiment of the invention, is at an extension (Fig. 2) of the floating gate electrode 44 of me MOS device 16. Any ohmic 110 connection between me two electrodes 44 and 64 will suffice. The various electrodes can be, for example, doped polysilicon. In this embodiment of the invention, and in contrast to the previously described prior art EPROM cells, a separate control gate electrode is not used. However, die MOS device 16 of die EPROM cell disclosed herein can be 115 similar to the MOS devices described in an article by Yoshikowa, et al entitled, "EPROM Cell Structure for EPLD's Compatible with Single Poly-Si Gate Process", IEEE Transactions on ED, Vol. 37, No. 3, March, 1990. The EPROM cells in tiiis article, however, do not include separate diodes of die type shown herein. 120 For writing data into die inventive EPROMs, hot electrons are caused to pass dirough the insulating layer 32 overlying the diode junction 54 and into me diode floating gate electrode 64, hence into die MOS device floating electrode 44. This is accomplished by reverse biasing die p-n junction 54 of the diode 18 to a sufficiently high voltage to cause electrical breakdown of the 125 semiconductor material at die junction. The electrical breakdown characteristics of various semiconductor materials are well known. In the breakdown process, high electric fields are created across the p-n junction, and me breakdown currents include high energy electrons. To attract me high energy electrons through the insulating layer 32, a 130 positive voltage is applied to d e diode floating electrode 64. This is accomplished by applying a relatively high (e.g., around 8 volts) positive voltage to me electrodes 34, 36 and 42 of die MOS device 16 for capacitively coupling die positive voltage to the floating electrode 44 and tiience to me floating electrode 64. 135 When die data writing process is completed, die writing voltages are removed, and the electrons which have passed through the diode insulating layer 32 to die gate electrode 44 remain stored thereon. The stored charge on die MOS device floating gate electrode 44 functions similarly to the prior art EPROM cells for affecting the turn on 140 characteristics of die MOS device 16. For reading the EPROM cell 10, a voltage is applied between the two regions 22 and 24, and a voltage is capacitively coupled dirough die layer 32 to the MOS gate electrode 44 by applying a bias voltage to the electrodes 56 and 58 of the diode 18. Widi the PMOS device 16 shown, me stored charge, if 145 present, acts as a negative bias due to die fixed electron charge on the floating gate 44 which is sufficient to bias the PMOS device in its on state. The amplitude of me electrode 56 and 58 bias voltage is selected so tiiat, even when added to die electron derived negative bias, the floating gate 44 remains negatively biased widi respect to die source (22 or 24), and therefore in its on 150 state, and a current will flow between die regions 22 and 24. Conversely, if an electron derived negative bias is not present due to no stored charge, me floating gate voltage is high enough to bias die PMOS device in its off state and no current will flow. The amplitude of die gate electrode voltage used during die reading 155 process is a function of die parameters of the MOS device, and d e selection of me various voltages for use widi die disclosed EPROM cells is within die skill of workers in tiiis technology. As above-described, me means for applying the reading voltage to the MOS gate electrode 44 comprises applying a voltage to both electrodes 56 and 160 58 of die diode 18 for capacitively coupling a voltage via dielectric layer 32 to die diode electrode 64 and hence to the MOS floating electrode 44. Alternatively, a separate control gate electrode 70, e.g., a buried highly doped region, can be provided within the substrate 10 extending beneatii die MOS floating gate electrode 44 at some point along its lengtii and capacitively 165 coupled to it via me intervening layer 32 of dielectric material. Thus, a voltage -* for reading die data stored in the MOS device 16 can be applied to die MOS floating gate electrode 44 via me capacitively coupled control gate electrode 70. This arrangement for reading die data is similar to that shown in die afore-cited article. 170 As previously noted, a problem associated widi die use of prior known EPROM cells is tiiat very short length channel MOS devices are required for purposes of writing data. In die herein disclosed EPROMS, however, die writing of data is accomplished by means of a separate diode, and die prior art dimensional requirements for die MOS devices are not present or relevant. 175 Also, die dimensions of die diodes used are not critical or process limiting.

Claims: What is claimed is: 1. An electrically programmable memory cell comprising an MOS device including source and drain regions and a channel region, a floating gate electrode overlying said channel region and ohmically insulated 180 therefrom, a diode disposed in a body of semiconductor material and including a p-n junction having a portion extending towards a surface of said body, and a diode gate electrode overlying said junction portion and ohmically insulated dierefrom, said diode gate electrode being ohmically connected to said floating gate electrode. 185 2. A memory cell according to claim 1 wherein said MOS device is disposed widiin said body in spaced apart relation with said diode, a layer of insulating material on said surface between said MOS device and said diode, said floating gate electrode and said diode gate electrode comprising a continuous layer of electrically conductive 190 material extending between said MOS device and said diode and overlying said layer of insulating material. 3. A memory cell according to claim 1 including a control electrode overlapping said floating gate electrode and ohmically insulated dierefrom by an intervening layer of dielectric material. 195 4. A method of writing data into an EPROM cell comprising a MOS device including a floating gate electrode overlying and ohmically insulated from a channel region of said device, and a diode having a p-n junction and a gate electrode overlying a portion of said junction and ohmically insulated dierefrom, said MOS and diode electrodes being ohmically 200 connected to one another, the method comprising reverse biasing the diode to cause voltage breakdown and flow of current across said p-n junction, and providing a voltage on said diode electrode for causing transfer of electrons from said diode current to said diode gate electrode. 205 5. A method of writing data according to claim 4 including, for reading data stored in said MOS device, applying a voltage to said diode for capacitive coupling to said diode gate electrode, and using said voltage for biasing the gate electrode of said MOS device for determining the data stored therein.

Inventors: PRESLAR DONALD RAY, PRESLAR, DONALD, RAY
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1993
Main classification: G11C11
IPC codes: G11C16, G11C17, H01L21, H01L27, H01L29