Programming
A typical PROM comes with all bits reading as "1". Burning a fuse bit during programming causes the bit to read as "0". The memory can be programmed just once after manufacturing by "blowing" the fuses, which is an irreversible process. Blowing a fuse opens a connection while programming an antifuse closes a connection (hence the name). While it is impossible to "unblow" the fuses, it is often possible to change the contents of the memory after initial programming by blowing additional fuses, changing some remaining "1" bits in the memory to "0"s. (Once all of the bits are "0", no further programming change is possible.)
The bit cell is programmed by applying a high-voltage pulse not encountered during normal operation across the gate and substrate of the thin oxide transistor (around 6V for a 2 nm thick oxide, or 30MV/cm) to break down the oxide between gate and substrate. The positive voltage on the transistor’s gate forms an inversion channel in the substrate below the gate, causing a tunneling current to flow through the oxide. The current produces additional traps in the oxide, increasing the current through the oxide and ultimately melting the oxide and forming a conductive channel from gate to substrate. The current required to form the conductive channel is around 100µA/100nm2 and the breakdown occurs in approximately 100µs or less.
Read more about this topic: Programmable Read-only Memory
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