FIG. 1 shows a prior art static read/write memory cell 10 such as is typically used in high-density static random access memories (SRAMs). A static memory cell is characterized by operation in one of two mutually-exclusive and self-maintaining operating states. Each operating state defines one of the two possible binary bit values, zero or one. A static memory cell typically has an output which reflects the operating state of the memory cell. Such an output produces a "high" voltage to indicate a "set" operating state. The memory cell output produces a "low" voltage to indicate a "reset" operating state. A low or reset output voltage usually represents a binary value of zero, while a high or set output voltage represents a binary value of one.
Static memory cell 10 generally comprises first and second inverters 12 and 14 which are cross-coupled to form a bistable flip-flop. Inverters 12 and 14 are formed by n-channel driver transistors 16 and 17, and p-channel load transistors 18 and 19. Driver transistors 16 and 17 are typically metal oxide silicon field effect transistors (MOSFETs) formed in an underlying silicon semiconductor substrate. P-channel transistors 18 and 19 are typically thin film transistors formed above the driver transistors.
The source regions of driver transistors 16 and 17 are tied to a low reference or circuit supply voltage, labelled V.sub.SS and typically referred to as "ground." Load transistors 18 and 19 are connected in series between a high reference or circuit supply voltage, labelled V.sub.CC, and the drains of the s corresponding driver transistors 16 and 17. The gates of load transistors 18 and 19 are connected to the gates of the corresponding driver transistors 16 and 17.
Inverter 12 has an inverter output 20 formed by the drain of driver transistor 16. Similarly, inverter 14 has an inverter output 22 formed by the drain of driver transistor 17. Inverter 12 has an inverter input 24 formed by the gate of driver transistor 16. Inverter 14 has an inverter input 26 formed by the gate of driver transistor 17.
The inputs and outputs of inverters 12 and 14 are cross-coupled to form a flip-flop having a pair of complementary two-state outputs. Specifically, inverter output 20 is cross-coupled to inverter input 26, and inverter output 22 is cross-coupled to inverter input 24. In this configuration, inverter outputs 20 and 22 form the complementary two-state outputs of the flip-flop.
A memory flip-flop such as that described typically forms one memory element of an integrated array of static memory elements. A plurality of access transistors, such as access transistors 30 and 32, are used to selectively address and access individual memory elements within the array. Access transistor 30 has one active terminal connected to cross-coupled inverter output 20. Access transistor 32 has one active terminal connected to cross-coupled inverter output 22. A plurality of complementary column line pairs, such as the single pair of complementary column lines 34 and 36 shown, are connected to the remaining active terminals of access transistors 30 and 32, respectively. A row line 38 is connected to the gates of access transistors 30 and 32.
Reading static memory cell 10 requires activating row line 38 to connect inverter outputs 20 and 22 to column lines 34 and 36. Writing to static memory cell 10 requires first placing selected complementary logic voltages on column lines 34 and 36, and then activating row line 38 to connect those logic voltages to inverter outputs 20 and 22. This forces the outputs to the selected logic voltages, which will be maintained as long as power is supplied to the memory cell, or until the memory cell is reprogrammed.
FIG. 2 shows an alternative prior art static read/write memory cell 50 such as is typically used in high-density static random access memories. Static memory cell 50 comprises n-channel pulldown (driver) transistors 80 and 82 having drains respectively connected to load elements or resistors 84 and 86. Transistors 80 and 82 are typically metal oxide silicon field effect transistors (MOSFETs) formed in an underlying silicon semiconductor substrate.
The source regions of transistors 80 and 82 are tied to a low reference or circuit supply voltage, labelled V.sub.SS and typically referred to as "ground." Resistors 84 and 86 are respectively connected in series between a high reference or circuit supply voltage, labelled V.sub.CC, and the drains of the corresponding transistors 80 and 82. The drain of transistor 82 is connected to the gate of transistor 80 by line 76, and the drain of transistor 80 is connected to the gate of transistor 82 by line 74 to form a flip-flop having a pair of complementary two-state outputs.
A memory flip-flop, such as that described above in connection with FIG. 2, typically forms one memory element of an integrated array of static memory elements. A plurality of access transistors, such as access transistors 90 and 92, are used to selectively address and access individual memory elements within the array. Access transistor 90 has one active terminal connected to the drain of transistor 80. Access transistor 92 has one active terminal connected to the drain of transistor 82. A plurality of complementary column line pairs, such as the single pair of complementary column lines 52 and 54 shown, are connected to the remaining active terminals of access transistors 90 and 92, respectively. A row line 56 is connected to the gates of access transistors 90 and 92.
Reading static memory cell 50 requires activating row line 56 to connect outputs 68 and 72 to column lines 52 and 54. Writing to static memory cell 10 requires first placing selected complementary logic voltages on column lines 52 and 54, and then activating row line 56 to connect those logic voltages to outputs 68 and 72. This forces the outputs to the selected logic voltages, which will be maintained as long as power is supplied to the memory cell, or until the memory cell is reprogrammed.
A static memory cell is said to be bistable because it has two stable or self-maintaining operating states, corresponding to two different output voltages. Without external stimuli, a static memory cell will operate continuously in a single one of its two operating states. It has internal feedback to maintain a stable output voltage, corresponding to the operating state of the memory cell, as long as the memory cell receives power.
The two possible output voltages produced by a static memory cell correspond generally to upper and lower circuit supply voltages. Intermediate output voltages, between the upper and lower circuit supply voltages, generally do not occur except for during brief periods of memory cell power-up and during transitions from one operating state to the other operating state.
The operation of a static memory cell is in contrast to other types of memory cells such as dynamic cells which do not have stable operating states. A dynamic memory cell can be programmed to store a voltage which represents one of two binary values, but requires periodic reprogramming or "refreshing" to maintain this voltage for more than very short time periods.
A dynamic memory cell has no internal feedback to maintain a stable output voltage. Without refreshing, the output of a dynamic memory cell will drift toward intermediate or indeterminate voltages, resulting in loss of data. Dynamic memory cells are used in spite of this limitation because of the significantly greater packaging densities which can be attained. For instance, a dynamic memory cell can be fabricated with a single MOSFET transistor, rather than the. six transistors typically required in a static memory cell. Because of the significantly different architectural arrangements and functional requirements of static and dynamic memory cells and circuits, static memory design has developed along generally different paths than has the design of dynamic memories.
Tunnel diodes are known in the art. A tunnel diode is a diode having a p-n junction, with both sides of the junction highly doped (p+/n+). In other words, both sides of the diode are doped into degeneracy. Because of the high doping levels, tunnelling of electrons is permitted in the forward direction across the junction (i.e., when a positive voltage is applied to the p+ region). FIG. 3 is an energy band diagram of a tunnel diode. As applied positive bias increases, the height of the potential barrier at the junction decreases, and the width increases. As positive bias increases, the tunnel diode exhibits a characteristic with a negative differential resistance portion, as tunnel effect contributes progressively less towards conductance of the diode (FIG. 4). As positive bias further increases, current reaches a valley point 96 where tunnel effect ceases. For voltages above the valley point 96, the tunnel diode behaves like an ordinary p-n junction diode. The dashed line in FIG. 4 represents the characteristic of an ordinary p-n junction diode. Tunnelling also occurs in the reverse direction in a manner similar to that for a Zener diode, except that breakdown voltage occurs at a small positive value of voltage, at peak point 98. Tunnel diodes are described in greater detail in U.S. Pat. No. 4,198,644 to Esaki, which is incorporated by reference.
Attention is also directed to applicants' application Ser. No. 08/492,774, filed Jun. 21, 1995, titled "NOVEL STATIC MEMORY CELL AND METHOD OF MANUFACTURING A STATIC MEMORY CELL."
Resonant tunnel diodes (or "resonant tunneling diodes," or "double-barrier electron resonators") are also known in the art. A resonant tunnel diode has many different on and off states. Texas Instruments has constructed a resonant tunnel diode for an integrated circuit. They started with a crystalline silicon substrate. Then, using ultra-high vacuum techniques, they deposited three ultra-thin layers. First, they deposited a layer of silicon dioxide (SiO.sub.2), then a layer of silicon, then another layer of silicon dioxide. The ultra-thin layers were each about 10-20 atoms thick, or about 5 billionths of a meter thick. They then attached electrical contacts to the top and bottom.
Electrons trying to pass through the resonant tunnel diode, in the direction from the bottom contact to the top contact, see the silicon dioxide layers as barriers. Each silicon dioxide layer is called a "tunnel barrier." The slicon layer is called a "quantum well." When a small voltage is applied to the resonant tunnel diode, very few electrons are able to tunnel through both of the silicon dioxide layers. Current flow is therefore negligible, and the device is in an "off" state. As voltage increases, energy of electrons increases and the wavelength of the electrons shortens. Eventually, a whole number of electron wavelengths fits inside the silicon layer, and a resonance condition is created. Electrons that are able to tunnel through the first silicon dioxide layer reflect back and forth many times, and some are eventually able to tunnel through the second silicon dioxide layer. Therefore, many electrons flow through the structure and an "on" condition is established. As voltage is further increased, there are no longer electrons with the correct wavelength to pass through, and another "off" condition is established. As voltage further increases, the resonance condition can change again to allow large numbers of electrons to pass through. The number of distinct on or off states that can occur is determined by materials and sizes. FIG. 5 is a current-voltage characteristic of a resonant tunnel diode having five double barriers, and showing the multiple possible on or off states.
A resonant tunnel diode can switch on and off quickly because electrons don't have to travel very far (e.g., they only have to travel about 100 Angstroms or 0.01 microns).
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