1. Technical Field
The present disclosure relates to the field of integrated circuits. The present disclosure relates more particularly to memories in integrated circuits.
2. Description of the Related Art
Integrated circuits are used in a large variety of applications. Individual integrated circuits can take the form of voltage amplifiers, voltage regulators, digital signal processors, microcontrollers, wireless adapters, microprocessors, smart cards, and many other devices. Some integrated circuits have circuitry that performs many of these functions on a single chip.
Many integrated circuits are provided with memory cells that store executable instructions for the proper functioning of the integrated circuit. For example, microcontrollers often include an array of memory cells that store data for the operation of the microcontroller. Often this memory is one-time programmable and is pre-programmed for specific applications by the manufacturer. In some instances the array of memory cells is programmable by the user.
Common types of memory used in microcontrollers and other integrated circuits are EPROM, EEPROM, and Flash. An EPROM memory cell includes a transistor with a floating gate. The floating gate can be programmed by storing charge thereon and is erasable by irradiating the transistor with ultra-violet (UV) light. Like EPROM, a traditional EEPROM memory cell includes a transistor with a floating gate. But unlike EPROM, an EEPROM memory cell can be erased electrically without irradiation by UV light. A flash memory cell also includes a transistor with a floating gate and differs from traditional EEPROM in that cells are not singly erasable and must be erased in large blocks.
The above-described types of memory cells suffer from the drawback that they all include transistors formed on the semiconductor substrate. As integrated circuits become more and more complex, more area of the semiconductor substrate is used to form control circuitry. A large array of memory cells that each includes one or more transistors thus takes up precious area of the semiconductor substrate that could be used to increase the complexity of control circuitry or to produce more integrated circuits on a single wafer.