Famos Transistor
The FAMOS transistor — floating-gate avalanche-injection MOS — is the specific device structure at the heart of the first EPROM, a MOS transistor with an electrically isolated floating gate that stores charge injected into it by an avalanche process. It was the invention of Dov Frohman at Intel that made the Intel 1702 EPROM possible, and its name encodes exactly how it works: a floating gate for storage, and avalanche injection as the mechanism for getting charge onto that gate. Understanding the FAMOS device is understanding the physical basis of charge-storage non-volatile memory, the principle that runs forward to flash.
The defining structural feature is a gate completely surrounded by insulator, with no electrical connection to anything, which is why it is called floating. In a conventional MOS transistor the gate is driven by an external voltage to control conduction. In the FAMOS device there is a gate buried in insulating oxide with no wire to it at all, so any charge placed on it is trapped: there is no low-resistance path for it to leak away. That trapped charge alters the transistor's threshold — the voltage at which it turns on — and the presence or absence of stored charge is read as the stored bit. Because the gate is isolated, the data persists for years without power, giving the device its non-volatility.
The avalanche-injection part of the name describes the original writing mechanism: using a high voltage to create energetic carriers that can jump onto the isolated gate. Getting charge onto a gate that is surrounded by insulator is not trivial precisely because the insulator is meant to keep charge from moving. The FAMOS device accomplished it with avalanche breakdown — applying a high enough voltage to generate high-energy carriers in the silicon, some of which gain enough energy to cross the insulating barrier and land on the floating gate, where they then remain trapped. This high-voltage injection to write, contrasted with UV light to erase, is the operating cycle of the original EPROM cell.
The FAMOS device cleanly separates the two hard problems of non-volatile memory — getting charge in and getting charge out — which is the right way to see why later technologies evolved as they did. The original device wrote by avalanche injection and erased by ultraviolet light, two quite different and somewhat inconvenient mechanisms. Later floating-gate memories kept the trapped-charge storage idea but changed the injection and removal mechanisms, for example using tunneling to move charge both ways electrically, which enabled electrical erasure in EEPROM and flash. Seeing the FAMOS device as "storage plus two charge-transport mechanisms" explains the trajectory of the whole field.
The FAMOS transistor's importance is that its floating-gate storage concept proved durable far beyond the specific device, becoming the basis of modern non-volatile memory. While the original avalanche-injection, UV-erased device was superseded, the central idea — store data as charge trapped on an isolated gate — was retained and refined through EEPROM and flash, which fill today's phones, drives, and memory cards. The FAMOS device is therefore studied not as an obsolete curiosity but as the structural origin of a storage principle still in use at enormous scale.
Read the FAMOS transistor as the physical cell that made EPROM possible: a MOS device with a gate sealed in insulator, written by avalanche-injecting charge onto it and erased by ultraviolet light, whose trapped-charge storage principle, refined with better charge-transport mechanisms, became the foundation of EEPROM and flash.