What are the structure, characteristics and classification of semiconductor read-only memory?

Introduction

Read-only memory (ROM for short) stores the data, generally written in advance before loading the whole machine. During the operation of the whole machine, only the data stored in advance can be read from the read-only memory, instead of being rewritten quickly and conveniently like the random memory. Because the data stored in the ROM is relatively stable and not easy to change, the stored data will not change even after the power is turned off; secondly, its structure is relatively simple, and it is more convenient to read, so it is often used to store various fixed programs and data.

Structure

The memory cell in a semiconductor read-only memory is a semiconductor device, such as a diode, bipolar transistor, or MOS transistor, which is located at the intersection of a word line and a bit line. Taking the enhanced N-channel MOS transistor as an example, the gate lead-out line is connected to the word line, the drain lead-out line is connected to the bit line, and the source lead-out line is grounded. When the word line is high, the transistor turns on, and the bit line outputs low (logic "0"). If no transistor is connected at the intersection, the bit line is pulled high by the load transistor (logic "1"). All other unselected word lines are at a low level, and all the transistors hanging on the word line are non-conductive, so the output level of the bit line is not affected. In this way, whether the data stored at this point (memory unit) is "0" or "1" is determined by whether a transistor is connected to the intersection of the word line and the bit line.

The function of the data stored in the read-only memory is determined by the mask used in the manufacturing process, so it is also called the mask read-only memory. In practice, except for a few varieties of read-only memory (such as character generators, etc.) that can be used universally, the content of read-only memory required by different users is different. For the convenience of users, it is suitable for industrial mass production. Later, programmable read-only memory appeared. The circuit design is to connect a fuse in series to each memory cell (such as Schottky diode). Under normal working conditions, the fuse acts as a wire; when the working bias is increased on the word line and bit line connected to it, the fuse is blown. In this way, users can write and store the required data by themselves.

Programmable read-only memory can only be written once in any unit, which is still very inconvenient. To solve this problem, erasable programmable read-only memory has emerged. The most commonly used in this type of memory is the floating gate avalanche injection into MOS cells. When a sufficiently high voltage is applied to the source lead or drain lead of the selected cell to cause an avalanche breakdown of the device, high-energy hot electrons are injected into the floating gate through the gate oxide layer to charge the floating gate, thereby changing the channel The on state achieves the purpose of writing: erasing is done by ultraviolet light irradiation. The irradiation of ultraviolet light enables the electrons on the floating gate to get enough energy to penetrate the barrier of the gate oxide layer, thereby eliminating the charged state of the floating gate.

Features

Electrically rewritable read-only memory is a new type of read-only memory. Its principle is to inject electrons onto the floating gate through the tunnel effect under the action of a strong electric field, or vice versa to pull electrons away from the floating gate. In this way, the read-only memory can be easily erased and written by electrical methods.

The write speed of erasable programmable read-only memory is relatively slow, each write speed takes about tens to hundreds of milliseconds, and writing to a complete piece of memory takes tens to hundreds of seconds: the erase speed is slower, in general It takes tens of minutes under ultraviolet light. The erasable and write speed of the electrically rewriteable read-only memory is several orders of magnitude slower than the read speed.

Classification

Semiconductor read-only memory can be divided into the following four types: mask ROM (MROM), one-time programmable ROM (PROM), erasable programmable ROM (EPROM), flash memory.

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