Digital Electronics: Principles, Devices and Applications

336 Digital ElectronicsLogic CellSRAMLogic CellSRAMSRAMLogic CellLogic CellFigure 9.32SRAM-controlled interconnect.use CMOS technology, which is one of the main reasons for their wide use in PLDs, FPGAs inparticular. A typical antifuse consists of an insulating layer sandwiched between two conductinglayers. In the unprogrammed state, the insulating layer isolates the top and bottom conducting layers.When programmed, the insulating layer is transformed into a low-resistance link. Typically, metalis used for conductors and amorphous silicon for the insulator. The application of high voltageacross amorphous silicon permanently transforms it into a polycrystalline silicon–metal alloy havinga low resistance. There are other antifuse structures too, such as that used in the Actel antifuse.This antifuse, known as PLICE, uses polysilicon and n+ diffusion as conductors and ONO asinsulator. Figure 9.33(a) shows the construction. This type of antifuse is usually triggered by a smallcurrent of the order of a few milliamperes. The high current density produced in the thin insulatinglayer produces heat, thus melting the insulating layer and creating an irreversible resistive siliconlink.Antifuses are widely used as programmable interconnects in PLDs [Fig. 9.33(b)]. Antifuse PLDsare one-time programmable, in contrast to SRAM-controlled interconnect-based PLDs, which arereprogrammable. It may be mentioned here that the reprogrammable feature helps the designers fixlogic bugs or add new functions. Antifuse PLDs have advantages of nonvolatility and usually higherspeeds. Antifuses may also be used in PROMs. In that case, each bit contains both a fuse and anantifuse. The device is programmed by triggering one of the two.