Abstract:
Loadless 4 transistor SRAM cell operation can be substantially improved yielding area saving and more stable operation by use of optical-light load. Parasitic photocurrents in PMOS anodes-substrate junctions act as load currents. Light can be introduced by either ambient light through transparent window on top of the chip or by cheap LED diode attached to chip surface.

Description:
TECHNICAL FIELD OF INVENTION  
       [0001]    Invention relates to static random access memory devices. In particular it is related to loadless 4 transistor SRAMs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Loadless static RAM memories implemented in CMOS technology, consisting of two NMOS driver transistors and two PMOS pass transistors are well known. PMOS pass transistors supply load current as well However, the main drawback is difficulty to accurately control the nonselected word-line voltage to maintain the load current. If load current would be supplied by other source than PMOS&#39;s drain currents, it would make possible wide-voltage margin of the cell. This would enable simple standard CMOS process and design of peripheral circuits, as well as area saving Another type of static RAM cell is developed recently where NMOS pass transistors and PMOS drive transistors are used. PMOS drive transistors are connected between the storage nodes and a supply voltage. 
         [0003]    Both aforementioned types of SRAM cells use only 4 transistors and enable area saving up to 46% compared to standard 6T SRAM cell However, they suffer from problems related to complex control circuitry yielding area penalty and narrow-voltage margin of cells. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0004]    Aforementioned problems can be completely avoided if load current is supplied from source different than NMOS (PMOS) adjustable weak drain currents. Constant current source, independent of temperature and voltage, can be used in a form of photocurrents generated in parasitic PMOS transistors&#39; PN photodiodes (N substrate—P drain and N substrate—P source). Light can be supplied from ambient through transparent window on top of SRAM chip, similarly as in UV EPROMs. Red part of the visible spectrum penetrate only few micrometers in the semiconductor causing useful photocurrent. Another option is attachment of ultra cheap (mass produced) red LED diode on top of chip. The price of the required miniature low power LED is in the order of 1% of the SRAM chip price. 
       DESCRIPTION OF THE EMBODYMENT 
       [0005]      FIG. 3  shows 4 transistor SRAM cell with PMOS pass transistor.  FIG. 4  shows NMOS and PMOS transistors (chip cross section) in P-well CMOS technology. In the first case, see  FIG. 3 , PMOS first drain/source terminal ( 1 ) is connected to storage node. Parasitic photodiode (N substrate on Vdd-PMOS drain/source terminal) provides optical-photocurrent load. In the second case, see  FIG. 5 , pass transistors are NMOS and PMOS drivers are connected between storage nodes and Vdd. First PMOS drain/source terminals ( 1 ) in both cases, see  FIGS. 3 and 5 , are connected to storage nodes providing photocurrent load. Second drain/source terminals ( 2 ) of PMOS transistors in both cases generate non useful photocurrents. In the first case it flows to bit line and in the second case to Vdd. Smaller part of load photocurrent is generated from direct absorption in P diffusion (drain/source terminal), see  FIG. 4 . Most of the light is generated between transistors and near the cell. Light generates electron-hole pairs which diffuse to either P diffusions, where they are separated in PN junctions&#39; depletion layers under electrical field, generating load photocurrent or to grounded P wells. Grounded P wells thus act as a guard to unwanted photocurrents in N diffusions (NMOS drain/source terminals see  FIG. 3  and  FIG. 4 ) which tends to discharge storage node. Photocurrent flows allways from positive cathode (N type) to grounded anode (P type).  FIGS. 1 and 2  show light absorption/penetration in silicone and typical photodiodes characteristics with conductive (negative voltage) and photovoltaic (positive voltage) mode. Ambient light can be a small fraction (10%) of mobile phone display or keyboard backlight. 1 pA/um2 photocurrent may be generated which is sufficient for cell&#39;s static operation. The same method of optical loads can be used in 4T SRAMs made in N-well CMOS technology. Useful photocurrent is generated in N-well (Vdd)—P diffusion junctions. Area around N diffusions can be covered by dummy poly layer during poly layer deposition and patterning. This will make area around N diffusions opaque and thus poly layer acts as a guard to unwanted-discharging photocurrents. 
         [0006]    Different modifications of the aforementioned invention can be derived by those knowledgeable in the art. Standard 6 transistor SRAM cell can be optically refreshed in the same way as loadless 4 transistor cell as described herein. One of the PMOS load transistors can be ommited resulting in 17% area saving. Remaining PMOS transistor&#39;s drain/source N-well (or drain/source N substrate) PN junctions function as parasitic photodiodes which generate load photocurrents. 
         [0007]    In case when LED diode light source is used pulsed operation is also possible in which case cell act as a periodically refreshable DRAM cell instead of SRAM cell Refreshing does not interfere with read/write like in classical electronically refreshable DRAM cells.