Abstract:
A one-time programmable circuit uses forced BJT h FE  degradation to permanently store digital information as a logic zero or logic one state. The forced degradation is accomplished by applying a voltage or current to the BJT for a specific time to the reversed biased base-emitter junction, allowing a significant degradation of the junction without destroying it.

Description:
BACKGROUND OF THE INVENTION 
   One-time programmable (“OTP”) circuits are widely used in many types of integrated circuits such as analog, digital, or mixed signal integrated circuits. They are used to precisely trim bias circuits such as bandgap reference circuits, generic internal voltage circuits or current references, as well as controlling other generic circuit parameters such as offset, gain, delay, and the like. 
   Typical OTP circuits include polysilicon fuse based circuits used with or without a separate pad for blowing the fuse using external components, metal fuse based circuits, circuits that use the complete breakdown of the base-emitter junction of an NPN bipolar transistor (bipolar junction transistor or “BJT”), circuits that use the breakdown of a thin-oxide layer, as well as EEPROM fuse cell based circuits. 
   The disadvantages of these circuits include large integrated circuit layout area as well as high fuse blowing currents (polysilicon and metal fuse based circuits) or high fuse blowing voltages (thin oxide and EEPROM based circuits). Additional disadvantages include one or two additional mask layers that are not normally used, resulting in additional manufacturing cost. 
   Polysilicon based circuits therefore need high currents for fuse blowing, and need a dedicated pad or a large MOS switch to blow the fuse. The fuse must be blown rapidly (within 10 to 100 ns) or it may easily fail to blow or recombine. Polysilicon based circuits need redundancy because of possible recombination, and may need many additional circuits for testing the overall quality of the fuse blowing. 
   Metal fuse based circuits also need high currents for fuse blowing, and also need a dedicated pad or large MOS switch for blowing the fuse. The fuse blowing must also be done rapidly or the fuse may fail to blow or will recombine. Redundant fuses are usually used with metal fuse based circuits. Additionally, metal fuses are not preferred because these fuses may destroy the oxide layer above the fuse by opening a hole in the oxide layer. 
   NPN bipolar transistor based circuits need a medium to high current for blowing the fuse, which is dependent upon the process used. However, a considerably high voltage is needed to blow the fuse, which is also process dependent. These types of circuits are usually preferred over polysilicon and metal based fuse blowing circuits. 
   EEPROM based circuits need a high voltage in order to trap charge on an MOS gate. One or two additional mask layers are needed, which adds unnecessary expense. 
   Thin oxide breakdown based circuits need high voltages to blow the fuse and usually take a relatively large integrated circuit die area, which also adds unnecessary expense. 
   What is desired, therefore, is an OTP circuit that is smaller than the prior art circuits mentioned above, that uses small fuse blowing currents and voltages, and is available in a standard BiCMOS process technology. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a plot of the current and voltage used to reverse bias and degrade the h FE  of an NPN bipolar transistor to be used as the fuse element in an OTP circuit according to an embodiment of the present invention; 
       FIG. 2  is a current-voltage plot of collector and base currents of an NPN bipolar transistor before and after h FE  degradation according to the present invention; 
       FIG. 3  is a plot of h FE  versus emitter voltage with the base grounded before and after h FE  degradation according to the present invention; 
       FIG. 4  is a plot of h FE  versus current before and after h FE  degradation according to the present invention; 
       FIG. 5  is a plot of h FE  degradation versus reverse bias emitter-base voltage according to the present invention; 
       FIG. 6  is a plot of h FE  degradation versus reverse bias emitter current according to the present invention; 
       FIG. 7  is a plot of h FE  versus current for press-stress h FE , post-stress h FE , and post-stress h FE  after 168 hours of baking time according to the present invention; 
       FIG. 8  is a circuit diagram of an OTP circuit using a single fuse according to an embodiment of the present invention; and 
       FIG. 9  is a circuit diagram of an OTP circuit using multiple fuses according to an embodiment of the present invention. 
   

   SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, an OTP circuit uses h FE  degradation in an NPN bipolar transistor. The degradation is only partial, and does not completely destroy the base-emitter junction of the transistor or harm the surrounding integrated circuit die area as in prior art OTP or fuse blowing circuits. The OTP circuit of the present invention is very small compared to prior art circuits, uses a small fuse blowing current on the order of 20 μA and a small fuse blowing voltage on the order of four volts or less. The circuit of the present invention can be designed in a 0.35 μm CMOS, SiGe bipolar technology, although other types of process technologies including an NPN bipolar transistor can be used. The desirable small fuse cell area is provided without using additional masks. 
   In a multiple fuse embodiment, the fuse blowing is done in parallel for all cells. There is no need for a special pad or special high voltage component or a large MOS switch to effectuate the fuse blowing. There is no damage done to the integrated circuit die area surrounding the fuse since no physical damage is done to the NPN transistor; there is only a permanent degradation of the h FE  of the transistor. Additionally, there is no possibility of recombination or recovery as in the prior art circuits mentioned above. 
   The OTP functionality of the circuit can be used to provide useful wafer information such as lot number, wafer number, and chip position in the wafer using only 20 to 30 bits of information, giving extremely useful information regarding traceability; this is particularly useful for bare die products. 
   DETAILED DESCRIPTION 
   The conditions for fuse blowing are established in the present invention by reverse biasing the base-emitter junction of an NPN bipolar transistor under specific voltage, current, and time constraints. Reverse biasing the base-emitter junction activates traps at the interface between silicon and silicon dioxide between the base and emitter contacts. 
   Referring now to  FIG. 1 , a plot of the current and voltage used to reverse bias and degrade the h FE  of an NPN bipolar transistor to be used as the fuse element in an OTP circuit is shown according to an embodiment of the present invention. In  FIG. 1  the collector, base, and substrate voltages are all set to ground potential. In  FIG. 1 , trace  100  is a plot of the reverse bias current versus voltage for an NPN bipolar transistor having an emitter size of 0.4×0.8 μm 2 . Trace  102  is a plot of the reverse bias current versus voltage for an NPN bipolar transistor having an emitter size of 0.4×12.8 μm 2 . 
   Referring now to  FIG. 2 , a current-voltage plot of collector and base currents of an NPN bipolar transistor before and after h FE  degradation according to the present invention are shown.  FIG. 2  is a Gummel plot of the collector and base currents of an NPN bipolar transistor having an emitter area of 0.4×0.8 μm 2 . Note that base current is strongly affected by the stress of reverse biasing the emitter-base junction. Trace  200  is the base current curve before stress, and trace  202  is the base current curve after stress. The collector current is substantially unaffected, and so the pre-stress and post-stress collector curves are represented by a single trace  204 . 
   Degradation of h FE  has been performed on an NPN bipolar transistor having an emitter size of 0.4×0.8 μm 2 . The stress condition used to reverse bias the base-emitter junction is VBE of −4 volts, an emitter current of 20 nA, and a duration for the stress conditions of 30 milliseconds. The reading conditions for detecting the h FE  degradation were a VBE of 669 millivolts and a collector current of 100 nA. Under the conditions set forth above an 81.3% degradation of h FE  was observed. A very high level of degradation as set forth above is desirable for operation of the sensing circuits surrounding the fuse element. The h FE  degradation established above should be stable in time. It has been demonstrated that the h FE  degradation does not recover in time. This is explained further below with respect to  FIG. 7 . 
   Referring now to  FIG. 3 , a plot of h FE  versus voltage before and after h FE  degradation is shown according to the present invention. In  FIG. 3 , trace  300  is the h FE  curve versus voltage before the reverse-bias stress has been applied. Note that the peak of the h FE  curve occurs at about −0.64 volts as highlighted by vertical line  304 . Trace  302  is the h FE  curve versus voltage after the reverse-bias stress has been applied. Note that the peak is significantly lower at −0.64 volts. 
   Referring now to  FIG. 4 , a plot of h FE  versus current before and after h FE  degradation is shown according to the present invention. In  FIG. 4 , trace  400  is the h FE  curve versus current before the reverse-bias stress has been applied. Note that the peak of the h FE  curve occurs at about 0.1 μA as highlighted by vertical line  404 . Trace  402  is the h FE  curve versus current after the reverse-bias stress has been applied. Note that the peak is significantly lower at 0.1 μA. 
   Referring now to  FIG. 5 , a plot  500  of h FE  degradation versus base-emitter voltage (V Eb ) is shown. A negative V EB  is a positive V BE , which means an NPN transistor is in the normal mode of operation, and not reverse biased. At low V BE , h FE  is significantly degraded (approaching or at 100%). Above 0.65 volts, h FE  shows less degradation. The NPN fuse should be used at voltages on the left side of vertical line  502  where h FE  degradation is very high. 
   Referring now to  FIG. 6 , a plot  600  of h FE  degradation versus reverse bias emitter current is shown. At low collector currents, h FE  is significantly degraded (approaching or at 100%). Above 0.1 μA, h FE  shows less degradation. The NPN fuse should be used at currents on the left side of vertical line  602  where h FE  degradation is very high. 
   Referring now to  FIG. 7 , a plot of h FE  versus collector current for press-stress h FE , post-stress h FE , and post-stress h FE  after 168 hours of baking time is shown. Trace  700  shows the pre-stress h FE . Trace  702  shows the post-stress h FE . Note that the post-stress h FE  curve  704  is significantly smaller than the pre-stress h FE  curve over a wide range of current values. Trace  702  shows the post-stress h FE  curve after 168 hours of baking at 250° C. Note that the h FE  curve rebounds only slightly, but is still significantly smaller than the pre-stress h FE  curve  700  over a wide range of current values. 
   Referring now to  FIG. 8  a circuit diagram of an OTP circuit  800  using a single fuse (transistor Q 3 ) according to an embodiment of the present invention is shown. Before h FE  degradation, NPN bipolar transistors Q 1  and Q 3  have the same nominal h FE  value. Since transistors Q 1  and Q 3  are driven by the same base current through current mirror  804 , and since current mirror  802  is unbalanced, the output voltage at the OUT node is a logic zero. To program the output at the OUT node to a permanent logic one value, the BLOW node (“blow pin”) is set to a logic one value (such as 3.3 volts), which ground the base of transistor Q 3 . Simultaneously, the emitter of transistor Q 3  (VGND_FUSE) is raised to 3.5-4 volts for 20-30 milliseconds. Transistor Q 3  has a reverse-biased emitter-base junction of about 3.5-4 volts and its h FE  degrades from a nominal value of about 110 as shown in  FIG. 4  to 0.15 times the nominal value to about 17 as is also shown in  FIG. 4 . 
   Circuit  800  shown in  FIG. 8  is but one of many such circuits that can be designed for a single fuse case as is known by those skilled in the art. For a single fuse case all that is required is a node for reverse biasing the emitter of an NPN bipolar transistor, and a sensing circuit to sense the value of h FE  before and after degradation, and an ability to provide a logic value corresponding to whether or not the h FE  value has been degraded. 
   Referring now to  FIG. 9 , a circuit diagram of an OTP circuit  900  using multiple fuses according to an embodiment of the present invention is shown. While two fuses (transistors Q 7  and Q 9 ) are shown, the concepts embodied in circuit  900  can be extended to any number of fuses desired. In circuit  900  the base current of transistor Q 8  is copied once using current mirror  904 . Only one NPN bipolar transistor branch can be selected at a time. For this reason, the various outputs of the fuses (OUT 1 , OUT 2 ) are latched by flip-flops  906  and  908  once they are selected through control signals SEL_FUSE_ 1  and SEL_FUSE_ 2 . The actual digital control circuitry for the control signals is not shown in  FIG. 9 . 
   In circuit  900 , before any h FE  degradation, all of the NPN transistors Q 9 , Q 7 , and Q 8  have the same nominal value of h FE . Since these transistors are driven by the same base current through current mirror  904 , and since current mirror  902  is unbalanced, the various outputs OUT 1  and OUT 2  are set to a logic zero value. Note that transistors M 3  and M 2  are only 0.6 the size of transistor M 1  and thus deliver less current than provided by the current reference I 2 . 
   To program an output to a logic one, either BLOW 1  or BLOW 2  is set to a logical one value to ground the base of the selected transistor, and the common VGND_FUSE pin is set to 3.5-4 volts for 20-30 milliseconds. Since this is a common pin, all of the transistor emitters will rise to this voltage level. However, it is important to note that only the selected transistor having a grounded base through the action of the blow pin will have a degraded value of h FE . The h FE  values change in a similar manner as described above with respect to single fuse circuit  800 . 
   Circuit  900  shown in  FIG. 9  is but one of many such circuits that can be designed for a multiple fuse case as is known by those skilled in the art. For a multiple fuse case all that is required is a node for applying a voltage to the emitter of the NPN bipolar transistors, control signals for grounding the bases of selected transistors, a sensing circuit to sense the value of h FE  before and after degradation, and an ability to provide a logic value corresponding to whether or not the h FE  value has been degraded. 
   Those skilled in the art will understand that the present invention works with a wide range of different high speed, low voltage integrated circuit process technologies that include a bipolar NPN transistor. The exact voltage/current/time profile for degrading the h FE  of the fuse transistor may have to be slightly adjusted to provide an optimum amount of degradation as required by the exact process and circuit implementation used. The minimum allowed emitter size for the fuse transistor will be ordinarily used to minimize layout and therefore integrated circuit die area, although larger emitter sizes can used if desired. The four volt case described above is particularly valid for a proprietary BiCMOS process technology using Si—Ge transistors used by the assignee of the present invention. This voltage value may change for another high speed, low voltage process technology but not dramatically so. Reverse bias voltages are likely to be in the range between two and five volts. What is important for determining the exact voltage is that inside the structure of the NPN fuse transistor, by reversing the emitter-base junction according to the present invention, traps are activated at the silicon/silicon dioxide interface between the base and the emitter contacts that degrade the h FE  of the transistor. 
   While there have been described above the principles of the present invention in conjunction with specific memory architectures and methods of operation, it is to be clearly understood that the foregoing description is made only by way of example and not as a limitation to the scope of the invention. Particularly, it is recognized that the teachings of the foregoing disclosure will suggest other modifications to those persons skilled in the relevant art. Such modifications may involve other features which are already known per se and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure herein also includes any novel feature or any novel combination of features disclosed either explicitly or implicitly or any generalization or modification thereof which would be apparent to persons skilled in the relevant art, whether or not such relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as confronted by the present invention. The applicants hereby reserve the right to formulate new claims to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.