Abstract:
The anti-fuse circuit includes three sub-blocks: a multiplexer having inputs of control signals and addresses and yielding the activation of a programming signal and program addresses; a programming voltage generator consisting of an oscillator and a charge pump; and an anti-fuse unit circuits for the program/read of anti-fuse states. For an anti-fuse program at the special test mode, a program address generation circuit having inputs of control signals and addresses activates the programming voltage generator and makes a special or program address for selection of anti-fuse. In the normal mode, the program address generation circuit and an internal power generator remain at an inactive state. In anti-fuse unit circuit, the program address and the programming voltage signal from the programming voltage generator serve to switch the terminal of the anti-fuse up to a programming voltage level when the anti-fuse is selected for programming of anti-fuse elements.

Description:
The application claim benefit the provisional application 60/115,377 Jan. 11, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an electrically programmable integrated circuit and an associated device structure for implementing a nonvolatile memory; and, more particularly, to anti-fuse circuitry, which is capable of effectively providing a post-package repair with electrically programmable anti-fuses. 
     DESCRIPTION OF THE PRIOR ART 
     A laser trimmed poly-silicon fuse structure is widely used in a programmable repair of a dynamic random access memory (DRAM) chip which is typically performed at a wafer level, typically, before a burn-in test. In a laser trimmed repair method employing the laser trimmed poly-silicon fuse structure, defective memory cells are identified by a wafer probe testing process. The poly-silicon fuse structure is then programmed by using a laser trimming technique to activate an address decoding of redundant memory cells in order to repair the DRAM chip. While the laser trimmed fuse structure is compact and reliable, the laser trimmed repair method can only be efficiently performed at the wafer level. As a result, it may preclude the ability for repairing any defective memory cell, which is found after a packaging process and generally occurs during the burn-in test. 
     On the other hand, with an electrically programmable nonvolatile memory, many types of post package failures can be repaired resulting in a significant yield improvement of a high density DRAM. 
     An anti-fuse structure and associated circuitry suitable for use in integrated circuits are generally incorporated in a nonvolatile memory device. It is expected to be particularly useful for an electrically programmable repair technique of a DRAM using a redundant memory capacity. Specifically, through the addition of special test modes, it is possible to implement this functionality without any alteration of the existing product pin-out specifications. 
     On the other hand, in addition to its uses in DRAM manufacture, it may be envisioned that this functionality can also be effectively utilized in the field or by end users as a part of a test-and-repair procedure. Similarly, it is also possible to program other useful and unique nonvolatile data into the DRAM component such as encryption keys, serial numbers, manufacture dates and other quality tracking identification. 
     A basic anti-fuse element is generally a resistive fuse component which has a very high resistance (&gt;100 Mohm) in its initial unprogrammed state and, after an appropriate programming operation, will have a significantly lower resistance (&lt;10 Kohm). The anti-fuse element is typically composed of a very thin dielectric material such as silicon dioxide, silicon nitride, tantalum oxide or a sandwich combination of dielectrics such as ONO (silicon dioxide-silicon nitride-silicon dioxide) between two conductors. The anti-fuse is programmed by applying an appropriate programming voltage under sufficient current flow through terminals of the anti-fuse for a sufficient time to cause the resistance of the anti-fuse to permanently change from high to low. 
     The programming voltage is typically larger in magnitude than a normal operating voltage so that the programming voltage may cause damage and reduce the reliability of associated neighboring devices and peripheral circuitry, which are improperly isolated. In particular, the peripheral circuitry for providing the programming voltage and for reading an anti-fuse resistance will typically be directly attached to the anti-fuse element to thereby be subjected to potential damage. 
     The integrity of the anti-fuse in both of its initial unprogrammed and programmed states may be adversely affected by several factors. For example, an extended exposure at elevated temperatures or application of a continuous current or voltage bias across the anti-fuse element may alter the properties of the thin dielectric resulting in an increase or decrease in the anti-fuse resistivity and potentially causing an error or degraded performance thereof. When programming a single anti-fuse element, an internally or externally generated programming voltage (or current) signal, Vhv (or Ihv), is applied across terminals of the anti-fuse element for a sufficient time. However, when a plurality of anti-fuse elements is used such as in a multiplexed array, non-selected anti-fuse elements may be subjected to unintentional programming signals resulting in an accidental change of conductivity of the thin dielectrics. 
     The reliable programming and reading of the anti-fuses requires several important key components. 
     First, an appropriate programming voltage or current signal must be generated internally or supplied externally. Specifically, an internal high voltage for the anti-fuse programming requires a careful isolation and biasing of device structures such as PN junctions and gate dielectrics, in order to insure that they are not subjected to large voltage differences. The large voltage differences may cause a premature breakdown, a reduced reliability, an excessive leakage current, a field oxide inversion, latch-up or failure. Similarly, if the programming voltage is provided externally, there must be a method of supplying this voltage without interference from normal electro-static discharge(ESD) circuitry typically used on the integrated circuit output pads and/or pins. 
     Second, there should be a method to address-select and program individual anti-fuses which also requires further manipulation of a programming signal. 
     Third, an appropriate method for sensing or reading the state of the anti-fuse is required. The anti-fuse state is typically read upon device activation or immediately after powered up. In order to reduce a risk of an anti-fuse failure due to a continuous read operation and to improve a read access speed to the anti-fuse information, a volatile memory should be provided with appropriate circuitry which can effectively provide a proper sense/latch operation of the anti-fuse state in a wide range of operating conditions. 
     However, up to date, there is no circuitry for effectively implementing the above three requirements. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an anti-fuse circuit, for use in an anti-fuse based random access memory (RAM), which is capable of proving improved yield, reliability and functionality of a RAM, specifically a synchronous DRAM (SDRAM). 
     A schematic block of this invention is simply illustrated as shown in FIG.  1 . The present invention includes three sub-blocks: a multiplexer having inputs of control signals and addresses and yielding the activation of a programming signal and program addresses (sub-block  10 ); a programming voltage generator consisting of an oscillator and a charge pump (sub-block  20 ); and an anti-fuse unit circuit for the program/read of anti-fuse states (sub-block  30 ). 
     First, for an anti-fuse program at a special test mode, the sub-block  10  having inputs of control signals and addresses activates the programming voltage generator and makes a special or program address for the selection of an anti-fuse unit circuit. In a normal mode, the sub-block  10  and the sub-block  20  remain at an inactive state. In the sub-block  30 , the program address and the programming voltage signal from the programming voltage generator serve to switch the terminal of the anti-fuse up to a programming voltage level when the anti-fuse is selected for programming of anti-fuse elements. 
     In accordance with one embodiment of the present invention, in the sub-block, an internal voltage generator comprises several specific-devised elements for enduring a negative voltage or a high voltage. As shown in FIG. 2, two types of voltage generators are illustrated with diodes and capacitors used for a special purpose. The capacitors (C 2 ˜C 6 ) coupled to a high voltage generator and each diode are made of poly and metal layers, which are formed as layer-by-layer stacked arrays (named as “finger-shaped stacked-array capacitor”). They have a bigger capacitance (6˜7 times) than that of a planar metal capacitor at the same area and a higher voltage endurance (&lt;20 V) than that of an ONO or gate capacitor. The diodes shown in FIG. 2 are made of a triple well, which has good characteristics such as preventing a leakage current and isolating other devices from negative or high voltages of the programming mode. 
     In another embodiment of the present invention, the multiplexer used in the sub-block  10  prevents undesirable voltages from being applied to the anti-fuse in the sub-block  30  while other anti-fuse is programmed. Then, anti-fuse unit circuits in the sub-block  30  have a variety of schemes such as uni-polar and bipolar according to programming voltages or types of transistors used for a programming power transfer. For a reading mode, some circuits are devised to improve the reliability of anti-fuses in certain types of systems by substantially eliminating the undesirable continuous supply of voltage signals to the anti-fuse terminals and improving the sensing speed of the programmed anti-fuses for a failed-bit repair. The evaluation of the state of the anti-fuse, such as “programmed” or “unprogrammed”, is operated with special circuits during a power-up period so that an extra time for the evaluation of the anti-fuses does not need. The signal describing the state of the anti-fuse is then stored in a latch through a buffer, which effectively improves the immunity of a variation of the impedance of the programmed anti-fuse or unexpected noises. If the anti-fuse is unprogrammed, then its latch signal is in a high voltage level and the NMOS transistor replacing the poly fuses or metal fuses for a laser repair scheme is normally in an on-state. In this way, it is not necessary to accomplish a direct read operation of anti-fuse state only except when the power is on, and a read current does not flow in the anti-fuses in response to the standard chip operation mode. Thus, continuous voltages across the two terminals of the anti-fuse are avoided and an unprogrammed anti-fuse is not easily converted back to its high impedance state (i.e. “unprogrammed”). The circuit further includes a memory system capable of being coupled and de-coupled to the anti-fuse by a latch circuit through an evaluation buffer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a schematic diagram of a programmable anti-fuse circuit used for a DRAM redundancy scheme in accordance with one embodiment of the present invention; 
     FIGS. 2A to  2 F illustrate circuit diagrams for sources of programming potentials and their device elements such as diodes devised with a triple-well structure and a finger-shaped stacked-array capacitors devised with poly and metal layers; 
     FIGS. 3A and 3B illustrate a circuit diagram of an anti-fuse unit circuit for improving the readability and reliability of anti-fuses based on a programming voltage of −4 V and 4 V between two terminals of anti-fuse; and a timing chart depicting the operation of the anti-fuse unit circuit in accordance with another embodiment of the present invention; 
     FIG. 4 illustrates a circuit diagram of an anti-fuse unit circuit for improving the readability and reliability of anti-fuses based on a programming voltage of 8 V and 0 V between two terminals of anti-fuse in accordance with another embodiment of the present invention; and 
     FIGS. 5A and 5B illustrate a circuit diagram of an anti-fuse unit circuit for improving the readability and reliability of anti-fuses based on modified transistors and a programming voltage of 8 V and 0 V between two terminals of anti-fuse; and a diagram showing the structure of a hybrid transistor employed in the anti-fuse unit circuit in accordance with further another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention includes any function or device, which is required for nonvolatile memory capability, such as but not limited to: 1) an electrical redundancy programming of memory devices; 2) serial number or identification programming of integrated circuits; 3) security and encryption key programming of the integrated circuits; 4) programming of function options of the integrated circuits; and 5) replacement for a read only memory (ROM) or an erasable programmable read only memory (EPROM). 
     The functions in accordance with the present invention can be simply described as: (1) special or program address generation for anti-fuses selected for programming by a selection circuit and other address circuitry such as the special address multiplexer in the special test mode; (2) internal voltage generation for programming: and (3) programming during special test mode and reading during power-up. During programming, a programming voltage is selectively and sequentially applied across each anti-fuse designated for programming. Non-designated anti-fuses are protected from programming voltages to prevent unintentional programming or unprogramming. Typically this scheme can be mixed for an internal and external power for programming. That is, the internal power generator can be coupled to an external power pin on an integrated circuit or a DRAM chip being programmed, if needed. If the external power pin is used, the ESD protection circuit will be included for the external power pad. 
     In accordance with the present invention, the state of the anti-fuse is preferably stored in a latch since the pre-latched buffer serves to effectively sense the anti-fuse state during the power-up period although the impedance of the programmed anti-fuse is to be widely varied. Due to the relatively reliable reading operation, the period of the programming time, the magnitude of the programming voltage, and the amount of programming current can be also decreased. 
     Referring to FIG. 1, there is illustrated a programmable anti-fuse circuit in accordance with a preferred embodiment of the present invention. The programmable anti-fuse circuit includes a program address generation circuit  10 , an internal power generator  20  and a plurality of anti-fuse unit circuits  30 . 
     Each of the anti-fuse unit circuit  30  is coupled to a switching transistor G50 which is connected to a dummy cell G51 and serves to activate the dummy cell G51. The program address generation circuit  10  has a test decoder  11  and an address decoder  12  which are activated by receiving a control signal representing a special test mode to generate a program address for the anti-fuse unit circuits  30 . The special test mode signal can be generated by, for example, a user activation. When the special test mode is activated by an external control signal, the program address generation circuit  10  selects one of the anti-fuse unit circuits  30  by using the program address for an anti-fuse programming and provides an internal control signal to the internal power generator  20 . A programming voltage signal generated from the internal power generator  20  which is also responsive to the internal control signal, is then applied to the selected anti-fuse unit circuit  30 . During the programming procedure, the anti-fuse unit circuits  30  can be sequentially selected by the program address circuit  10 . 
     The internal power generator  20  includes two parts: an oscillator  21  and a charge pump circuit  22 . In a uni-polar voltage system in accordance with one embodiment of the present invention, as shown in FIGS. 4 and 5A, a high voltage such as 8 V, Vhv, generated from the internal power generator  20  is coupled to the selected anti-fuse unit circuit  30  during the programming procedure. However, in a bipolar voltage system in accordance with another embodiment of the present invention as shown in FIG. 3A, a negative voltage, Vnv, can be coupled to the selected anti-fuse unit circuit  30  during the programming procedure. When the programming procedure is complete and the external control signal representing the special test mode is changed into a disable state, all of the anti-fuse unit circuits  30  are deselected and the internal power generator  20  is also changed to a disable state. 
     When a power-up procedure is initiated, a reading or evaluation for anti-fuses states is carried out in the anti-fuse unit circuits  30 . Each anti-fuse unit circuit  30  receives a power-up signal PWRUP and a state of an anti-fuse contained in each anti-fuse unit circuit  30  is transmitted as a latched signal to the switching transistor G 50 . That is, each anti-fuse unit circuit  30  generates the latched signal as a low voltage level signal representing an anti-fuse programmed state or a high voltage level signal denoting an anti-fuse unprogrammed state. In addition, an external voltage source Vcc can be changed from 4 V for the programming procedure to 3.3 V for a reading operation. 
     Referring to FIGS. 2A and 2B, two embodiments of the charge pump circuit contained in the internal power generator  20  shown in FIG. 1 are illustrated. The control signal is activated and applied to the oscillator  21  and the charge pump circuit  22  shown in FIG.  1 . During the programming procedure at the special test mode, the control signal PGM generated from the program address generation circuit  10  shown in FIG. 1 become to a high voltage state for both negative and positive voltages. FIG. 2A illustrates a charge pump circuit, which contains three parts: a high voltage generator  24 , a high voltage driver  26  and a pre-charge voltage generator  28  for applying the external source voltage Vcc. An NMOS transistor D 1  is operated as a diode when a PGM signal represents a high voltage state and applies a voltage signal Vcc-Vtn, wherein Vtn is a threshold voltage of D 1 , into node N 1 . Diodes D 2 ˜C 7  are newly designed with a PN diode connected with a P-well to an N-well junction as shown in FIG.  2 C. 
     As shown, the PN diode structure is substantially formed with a triple well structure where a P-well is inserted into a N-well formed on a P-substrate. In such a diode, the P-substrate is connected to a ground Gnd and the N-well serves to isolate the P-well from the P-substrate in order to prevent a current flow from the P-well to the P-substrate. Another merit of the PN diode structures is that a breakdown voltage between the N-well and the P-well is higher than a programming voltage. Capacitors C 1 ˜C 3  are used for a charge pumping effect and capacitors C 4 ˜C 6  are used as loading capacitors reducing the oscillation amplitude of output voltage signal Vhv, which are determined to have relatively a small capacitance than that of the charge pumping capacitors C 1 ˜C 3 . 
     As shown in FIG. 2E, when the PGM signal becomes a high voltage level for N 1  node pre-charge operation and in-phase and out-phase clocks OSC 1  and OSC 2  generated from the oscillator  21  are continuously provided, a high voltage signal generated from the high voltage generator  24  is supplied to the selected anti-fuse unit circuit  30 . Initially, the voltage of the node N 1  becomes a voltage Vcc-Vtn when the PGM signal is enabled as a high voltage level. When the clock OSC 1  changes from a ground voltage level to an external voltage level Vcc, charges supplied by the OSC 1  are fed to the node N 1  whose voltage level becomes 2Vcc-Vtn. The voltage level is again transferred to a node N 2  whose voltage level becomes 2Vcc-2Vtn. 
     Thereafter, when the clock OSC 2  changes from the ground voltage level to the external voltage level Vcc, a voltage level of the node N 2  is charged to 3Vcc-2Vtn and a voltage level of the node N 3  changes to 3Vcc-3Vtn. Next if the clock OSC 1  changes from the ground voltage level to the external voltage level Vcc, the voltage level of the node N 3  is charged to  4 Vcc- 3 Vtn. Finally, the voltage levels of the nodes N 1 , N 2 , N 3  and Vhv are changed to 2Vcc-Vtn, 3Vcc-2Vtn, 4Vcc-3Vtn and 4Vcc-4Vtn, respectively. A high voltage output signal Vhv can be generated by supplying the above clocks. The high voltage output signal Vhv is then coupled to the anti-fuse unit circuit  30  as a programming voltage signal. 
     The high voltage driver  26  includes two diodes D 5 ˜D 6  which serve to provide two different output voltage levels such as Vhv 2  and Vhv 3  of 4Vcc-4Vtn and 3Vcc-3Vtn, respectively. When a reading mode, the pre-charging voltage generator  28  changes a voltage level of the output node Vhv into a pre-charging voltage level Vcc-Vtn. The pre-charging voltage level can be used for the evaluation of an anti-fuse state. 
     As shown in FIG. 2D, the capacitors C 2 ˜C 6  used in a high voltage are formed with poly and metal layers M 1  to M 2  and P 1  to P 2 . The high voltage capacitors C 2 ˜C 6  can provide a larger capacitance by using added mutual capacitances Cjj, Cji and Cjk due to a finger-stacked type which is called a finger-shaped stacked-array capacitor. 
     Referring back to FIG. 2B, a charge pump circuit  22  in accordance with another embodiment of the present invention is illustrated. The charge pump circuit  22  serves as a negative voltage generator for used in a bipolar voltage scheme of the anti-fuse programming procedure. Diodes D 12 ˜D 14  shown in FIG. 2B are similar to those of FIG. 2A, wherein a structure of each diode is also described in FIG.  2 F. Capacitors C 11  and C 12  are formed by using PMOS transistors and used in a charge pumping operation, whereas a capacitor C 13  is a loading capacitor. 
     When the clock OSC 1  changes from the ground voltage level to the external voltage level Vcc, a voltage level of a node N 5  is charged to the external voltage level Vcc. The external voltage level Vcc is then transferred into a node N 4  until the voltage level of the node N 5  reaches a threshold voltage level Vtn of the diode D 12 . The node N 4  is tied to the ground voltage through a transistor D 11 . When the OSC 1  and the OSC 2  clocks concurrently change to the ground voltage level and the external voltage level Vcc, respectively, the node N 5  and a node N 6  are changed to voltage levels Vtn-Vcc and Vcc, respectively. The voltage level of the node N 6  remains at a voltage level 2Vtn-Vcc after its charges are discharged through the diodes D 11 , D 12  and D 13 . As a result, an output voltage level of an output node Vnv becomes 3Vtn-Vcc. When the OSC 1  and OSC 3  clocks change to the external voltage level Vcc and the ground voltage level, respectively, the nodes N 5  and N 6  change to Vtn and 2Vtn-2Vcc, respectively. Therefore, a voltage level of the output node Vnv becomes 3Vtn-2Vcc. Finally, the nodes N 4 , N 5 , N 6  and the output node Vnv converge to the ground voltage level, Vtn-Vcc, 2Vtn-2Vcc, 3Vtn-2Vcc, respectively, after repeating clock operations. The output voltage level Vnv is then coupled to the anti-fuse unit circuit  30  as the programming voltage signal. 
     Referring to FIG. 3A, an anti-fuse unit circuit  30  employing a bipolar voltage programming scheme in accordance with one embodiment of the present invention is illustrated. The anti-fuse unit circuit  30  includes an anti-fuse selection circuit  331 , an anti-fuse element  332 , an anti-fuse state evaluation circuit  333 , and a latch circuit  334 . 
     The external voltage level Vcc, e.g., +4 V is applied via two PMOS transistors P 0  and P 2  and an NMOS transistor N 1  to one terminal of the anti-fuse element  332  and the programming voltage signal Vnv, e.g., −4 V is applied to the other terminal thereof during the programming procedure. The program address is applied to the gate of the PMOS P 0  which is turned on to thereby transmit the external voltage level to the PMOS transistor P 2 . The PMOS transistor P 2  is serially connected to the PMOS transistor P 0  and is used as a pass-transistor, wherein a power-up signal includes a first power-up signal PWRUP, a second power-up signal PWRUPB and a third .power-up signal PWRUP_D. The NMOS transistor N 1  is connected to a node A 01  and maintains a turn-off state in response to the third power-up signal PWRUP_D during the anti-fuse programming procedure, wherein the third power-up signal PWRUP_D initializes the node A 01  to ground voltage level within about 5 nsec. The PMOS transistor P 2  serves to protect junctions of the PMOS transistor P 0  and a PMOS transistor P 5  from the programming voltage signal Vnv, e.g., −4 V. The programming voltage signal Vnv is connected between the terminal of the anti-fuse element  332  and an NMOS transistor N 4  which serves as a diode having a common gate, a substrate and a drain. 
     In the read mode, as shown in FIG. 3B, the state evaluation circuit  333  is operated during a power-up period shown in FIG.  3 B. When the anti-fuse element  332  is programmed, the terminal A 02  of the programmed anti-fuse element  332  is brought to a low voltage level. That is, the voltage level of the terminal A 02  is typically a threshold voltage level Vtn of the NMOS transistor N 4  since the charge pump circuit  20  in FIG. 1 is not operated and the programming voltage signal Vnv is floated. The first power-up signal PWRUP maintains a low voltage level state until the power is fully stabilized and the second power-up signal PWRUPB is increasing proportionally to the external voltage level Vcc. The external voltage signal Vcc is transmitted to a node A 02  through the PMOS transistor P 2  and the PMOS transistor P 5  during the power-up period. If the anti-fuse element  332  is programmed, the voltage of the node A 02  decreases gradually and changes to a low voltage level state (near 1 V). However, if the anti-fuse element  332  is unprogrammed, the voltage of the node A 02  maintains a high voltage level state (near Vcc). In addition, When the external voltage signal Vcc is transmitted to the node A 03  through the PMOS transistor P 6  and the second power-up signal PWRUPB maintains a high voltage level state, an NMOS transistor N 7  becomes a turn-on state. On or off state of an NMOS transistor N 8  is determined depending on the voltage of the node A 02  so that the voltage level of the node A 03  is then determined based on the state of the NMOS transistor N 8 . When the anti-fuse element  332  is programmed, the node A 02  maintains a low voltage level state and the node A 03  remains a high voltage level state. If unprogrammed, the node A 02  is a high voltage level state and the node A 03  becomes a low voltage level state. 
     The latch circuit  334  maintains information read from the anti-fuse element  332  as a latched signal through the state evaluation circuit  333 . Such an initial latch of the anti-fuse state improves a sensing speed of a repair cell reading mode and a programming accuracy over wide range of programming bias and current. Further, since the number of an anti-fuse element accesses can be reduced, the reliability of the anti-fuse element also can be improved. 
     Referring to FIG. 4, there is shown an anti-fuse unit circuit  30  employing a uni-polar high voltage scheme which includes an anti-fuse selection circuit  341 , an anti-fuse element  342 , an anti-fuse state evaluation circuit  343 , and a latch circuit  344 . The external voltage signal Vcc for the programming procedure is coupled to the anti-fuse element  342  through a PMOS transistor P 0 . A programming voltage signal Vhv is coupled through D 1  to one terminal of the anti-fuse element  342  and the ground voltage is fed to the other terminal of the anti-fuse element  342  via two NMOS transistors N 2  and N 4  during the programming procedure. The diode D 1  prevents a current flow from the external voltage source Vcc to a power lead for the programming voltage signal Vhv floated during the read operation. 
     A power-up signal PWRUP_D initializes a node A 01  within about 5 nsec after the power stabilization operation so that the node A 01  is pre-charged near Vcc through a PMOS transistor P 3  and an NMOS transistor N 2 . The high voltage signal for a program address is applied to an NMOS transistor N 4  to discharge the pre-charged voltage signal Vcc and connect the ground voltage to the anti-fuse element  342 . Therefore, the voltage level of the node A 02  is changed to the ground voltage level, turning on the NMOS transistor N 2  and the NMOS transistor N 4 , simultaneously. Consequently, the voltage difference between the two terminals of the selected anti-fuse element  342  becomes a relatively high voltage, Vhv, during the programming procedure. In case of the unselected anti-fuse unit circuit  30 , the low voltage signal of the program address is unchanged and the voltage difference between the two terminals of the unselected anti-fuse element  342  maintains near Vhv-Vcc. The NMOS transistor N 2  protects junctions and gates of a PMOS transistor P 3 , the NMOS transistor N 4 , an NMOS transistor N 5 , and an NMOS transistor N 7  from the programming voltage signal Vhv to thereby prevent junction breakdowns or gate breakdowns. 
     In the read mode, the anti-fuse state evaluation circuit  343  operates during the power-up period shown in FIG.  3 B. The first power-up signal PWRUP maintains a low voltage state until the power is fully stabilized and the second power-up signal PWRUPB is increased proportionally to the external voltage signal Vcc. The terminal of the anti-fuse element  342  is brought to a high voltage, typically Vcc-Vtp through the PMOS transistor P 0  since the charge pump circuit  20  in FIG. 1 is not operated so that the lead for the programming voltage signal Vhv is floated. If the anti-fuse element  342  is programmed, the voltage of the node A 02  maintains a high voltage state (near Vcc-Vtn). In case that the anti-fuse element  342  is unprogrammed, the voltage at the node A 02  remains at the ground voltage since the NMOS transistor N 5  remains turned-on by the high voltage level of the second power-up signal PWRUPB. Therefore, the NMOS transistor N 5  is small in size so that the voltage of the node A 02  is not seriously reduced by a turned-on state of the NMOS transistor N 5 . 
     When the first power-up signal PWRUP maintains a low voltage state, the external voltage Vcc is transmitted to the PMOS transistors P 7  and P 6 . An NMOS transistor N 8  is then turned on by the high first power-up signal PWRUP so that the PMOS transistor P 7  turns on according to the level of the node A 02 . When programmed, the node A 02  maintains a high voltage state so that the PMOS transistor P 7  is at weakly turned-on state. Consequently, the node A 03  serially connected to the NMOS transistor N 8  is a low voltage state. If unprogrammed, the node A 02  is a low voltage state and the PMOS P 7  becomes a fully turned-on state. The node A 03  by the turned-on states of the PMOS transistors P 6  and P 7  is transmitted to a high voltage state. The size of the NMOS transistor N 8  is small enough for a smaller current flow through the NMOS transistor N 8  than that through the PMOS transistor P 7 . The low or high voltage state of the node A 04  is dependent on the voltage of the node A 03  since the first power-up signal PWRUP maintains a low voltage state. The variation of the voltage difference between the programmed and the unprogrammed states is not large enough for controlling the PMOS transistor P 7  so that a buffer is needed. The controlled buffer consisting of a pull-up PMOS transistor P 9 , a PMOS transistor P 10  and a pull-down NMOS transistor N 11  connected between the node A 03  and the node A 04  can be easily evaluated on the state of the anti-fuse element  342 . The latch circuit  344  can easily latch the state of the anti-fuse element  342  by the amplified signal of the node A 04 . 
     The latch circuit  344  maintains information read from the anti-fuse element  342  as the latched signal. 
     Referring to FIG. 5A, there is demonstrated an anti-fuse unit circuit  30  employing a uni-polar high voltage scheme in accordance with another embodiment of the present invention, which has an anti-fuse selection circuit  351 , an anti-fuse state evaluation circuit  353 , and a latch circuit  354 . 
     The anti-fuse selection circuit  351  includes same circuit elements of the anti-fuse selection circuit shown in FIG. 4 except the PMOS transistor P 3  and the NMOS transistor N 2  shown in FIG.  4 . The anti-fuse state evaluation circuit  353  has same circuit elements of the anti-fuse state evaluation circuit  343  shown in FIG. 4 except an NMOS transistor connected between nodes A 01  and A 02  shown in FIG.  5 A. In the programming procedure, the NMOS transistor serves to substantially isolate the anti-fuse selection circuit  351  from the anti-fuse evaluation circuit  353 . 
     The external voltage Vcc is coupled to one terminal of the anti-fuse element  352  through a PMOS transistor P 0 . The programming voltage Vhv is coupled through a diode D 1  to the one terminal of the anti-fuse element  352  and the ground voltage is coupled to the other terminal of the anti-fuse element  352  during the programming procedure. The diode D 1  prevents a current flow from the external voltage source Vcc to a power lead for the programming voltage signal Vhv floated during a read operation. The high voltage signal of a program address is applied to the NMOS transistor N 2  to discharge the pre-charged voltage by using the capacitance coupling of the anti-fuse element  352  and connect a source of the ground voltage on the anti-fuse element  352 . Consequently, the voltage difference between the two terminals of the selected anti-fuse element  352  becomes a relatively high voltage Vhv, during the programming procedure. 
     In case that the anti-fuse unit circuit  30  is unselected, the low voltage signal of the program address is applied to the NMOS transistor N 2 . The voltage difference between the two terminals of the unselected anti-fuse element  352  is determined by the ratio of leakage currents through the anti-fuse element  352  and the NMOS transistors N 2  and N 3  junctions. 
     As shown in FIG. 5B, the NMOS transistors N 2  and N 3  are prepared by using a hybrid transistor. The source of the hybrid transistor contains a heavily doped N+ region  515  and two lightly doped N− regions  520  and  521  one of which is overlapped over the lower portion of the gate region of the hybrid transistor. The drain of the hybrid transistor is formed only with an N− region  518 . A drain electrode DRAIN is connected through a buffered poly  523  to the N− region  518 , wherein the upper portion of the buffered poly is extended over the upper portion of the gate. A smaller N+ region is formed on the lower portion of the buffered poly to form an ohmic contact between the N+ region and the lower portion of the buffered poly. The gate region includes a poly oxide layer  517 , a poly layer  516  and spacers  512  formed on side portions thereof. Therefore, if a high voltage, e.g., 8 V is applied to the hybrid transistor, a depletion region  519  is formed on the lower portion of the drain region  518  so that an electric field is distributed over the depletion region  519 . The depleted region  519  is overlapped on the lower portion of the gate region to thereby effectively protect the gate breakdown due to the high voltage. 
     Referring back to FIG. 5A, in the read mode, the state evaluation circuit  353  operates during the power-up period. 
     In case that the anti-fuse element  352  is programmed, a voltage of the node A 02  maintains a high voltage state (near Vcc-Vtn). In case that the anti-fuse element  352  is unprogrammed, the voltage of the node A 02  still remains at the ground voltage since the NMOS transistor N 4  remains a turned-on state by the high voltage state of the first power-up signal PWRUP. The operation of the anti-fuse evaluation circuit  353  is similar to that of the anti-fuse circuit  343  shown in FIG.  4 . 
     Consequently, the latch circuit  354  effectively maintains information read from the anti-fuse element  352  as the latched signal. 
     While the present invention has been described with respect to certain preferred embodiments only, other modifications and variations may be made without departing from the spirit and scope of the present invention as set forth in the following claims.