Abstract:
Both a device-identification feature and a temperature-sensor feature are combined on a single integrated circuit. In various embodiments, both features are not operative simultaneously.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to, and the benefits of, U.S. Ser. Nos. 13/238,003 (the “&#39;003 application”) and 13/238,010 (the “&#39;010 application”), both entitled “Device Identification and temperature sensor circuit” and filed on Sep. 21, 2011. The present application is a continuation-in-part of the &#39;003 application, and the entire disclosures of the &#39;003 application and the &#39;010 application are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally identification of a device such as an integrated circuit, and measuring the temperature of the device. 
       BACKGROUND 
       [0003]    Performance of an integrated circuit, such as an image sensor, can depend on the temperature. For example, the “dark current” inside an image sensor—i.e., the unwanted current produced by the sensor even during periods when it is not actively exposed to light—is highly temperature-dependent. The dark current will increase with increasing temperature, and higher dark current levels degrade the performance of the image sensor. In particular, as the dark current rises, the dynamic range of the image sensor diminishes and the dark reference level wanders or becomes uncertain, since current flows regardless of the ambient darkness. As a result, various defects may appear in captured images, and if the temperature becomes too high the sensor may sustain permanent damage. Accordingly, the ability to monitor the temperature of an image sensor may be crucial not only to detect and compensate for temperature-induced anomalies, but to protect the sensor from damage. 
         [0004]    One conventional technique for measuring the temperature of an image sensor is to mount a thermal couple on the package of the image sensor, either at the front side or at the back side of the package, depending on how the sensor is arranged on the circuit board. The thermal couple can occupy a significant area, however, increasing the size and cost of the sensor and complicating its integration into an image-capture device, such as a camera. Also, over time, the epoxy used to affix the thermal couple to the package can age and loosen, in which case the temperature typically cannot be measured until the loose epoxy is repaired. 
         [0005]    Furthermore, different devices may exhibit very different temperature sensitivities. Accordingly, the actions taken in response to a particular temperature reading will be device-specific. Knowing the device temperature, in other words, is insufficient to determine the optimal action to be taken without knowledge of the device and its response to, and tolerance of, temperature variations. 
       SUMMARY 
       [0006]    In various embodiments, the present invention implements both a device-identification feature and a temperature-sensor feature on a single integrated circuit without introducing an extra bond pad or package pin. Manufacturers of products that currently use either of the features can now utilize both with minimum modifications to the current electronics. Integrated circuits, such as image sensors, having unique features can use the device-identification feature to automatically identify the integrated circuit and/or to implement or optimize settings, programs, and operating conditions based on the device&#39;s specification. The device-identification feature allows manufacturers to design a single system that may be deployed on different integrated circuits or other devices. Additionally, the temperature-sensor feature can be used to periodically or continuously monitor the temperature of the integrated circuit and prevent device failures due to overheating. It can also perform or facilitate some image-improvement algorithms such as dark current subtraction at different temperatures. To prevent or limit dark current from the temperature sensor, an opaque layer may cover the temperature sensor (and, in various implementations, the region surrounding it). 
         [0007]    In one aspect, the invention pertains to an integrated circuit including thereon a device-identification circuit for identifying the integrated circuit, a temperature sensor, and an opaque layer covering at least the temperature sensor to prevent photocurrent generation therein. The device-identification circuit and the temperature sensor are not simultaneously operable. In some embodiments, the device-identification circuit comprises a resistor connected to a diode-connected transistor; the temperature sensor may be a diode connected in parallel with the device-identification circuit. The integrated circuit may be or comprise an image sensor. 
         [0008]    In another aspect, the invention relates to an integrated circuit including thereon, in various embodiments, a device-identification circuit comprising a resistor connected between a diode-connected transistor and a first reference voltage, where the diode-connected transistor has a first polarity applied relative to the first reference voltage; a temperature sensor diode connected in parallel with the device-identification circuit and having a second polarity applied to a second reference voltage; and an opaque layer covering at least the temperature sensor to prevent photocurrent generation therein. Once again, the integrated circuit may be or comprise an image sensor. 
         [0009]    A further aspect of the invention pertains to a system for identifying a device and to measure its temperature, in which the system has a power supply connected to a common node. In various embodiments, the inventive system comprises a device-identification circuit connected between the common node and a first reference voltage; a temperature sensor connected (i) in parallel with the device-identification circuit and (ii) between the common node and a second reference voltage; and an opaque layer covering at least the temperature sensor to prevent photocurrent generation therein. 
         [0010]    In some embodiments, the device-identification circuit includes a diode-connected transistor connected to a resistor; the diode-connected transistor has a first polarity and is connected to the common node, and the resistor is connected between the diode-connected transistor and the first reference voltage. The temperature sensor may be a diode having a second polarity. A second resistor may be connected between the power supply and the common node. In a representative implementation, the power supply and the second resistor are disposed in an image-capture device; for example, the device-identification circuit and the temperature sensor diode may be disposed in an image sensor. 
         [0011]    The system may, in various embodiments, further include a processor connected to the integrated circuit; a memory connected to the processor; and a driver circuit connected to the processor and to the integrated circuit. The memory may contain a look-up table relating different resistance values to associated devices, with the processor being configured to identify the device based on a substantial match between a value in the look-up table and a value of the identification-circuit resistor. In some embodiments, the memory contains a look-up table relating different values of an electrical parameter of the temperature sensor to associated temperatures, and the processor is configured to identify a device temperature based on a substantial match between a present value of the electrical parameter and a value in the look-up table. In some implementations, the sensor comprises a diode and the electrical parameter is current through the diode or voltage across the diode. 
         [0012]    Still another aspect of the invention pertains to a method for identifying an integrated circuit and determining its temperature. In various embodiments, the method comprises using a device-borne device-identification circuit to identify the device; and using a device-borne temperature sensor to report the temperature of the device. The device-identification circuit and the temperature sensor may be connected in parallel and not operated simultaneously, and light is preferably blocked from at least the temperature sensor to prevent photocurrent generation therein. The device-identification circuit may comprise a resistor connected to a diode-connected transistor, and the temperature sensor may comprise a diode connected in parallel with the device-identification circuit. 
         [0013]    The integrated circuit can be identified by determining the value of the resistor included in the device-identification circuit, and identifying the integrated circuit based on the determined resistance value. For example, known resistance values for different integrated circuits can be maintained in a look-up table. The temperature of the integrated circuit can be determined by measuring an electrical parameter of the temperature sensor (e.g., diode), comparing the measured parameter against a plurality of parameter values associated with known temperatures, and determining the temperature of the integrated circuit based on the comparison. In one embodiment, the electrical parameter is the level of current through the diode. In another embodiment, the electrical parameter is the voltage across the diode. 
         [0014]    In one configuration, the device-identification circuit and the temperature-sensor diode are connected to a common node, and the method further comprises applying a first voltage to the common node to place (i) the device-identification circuit in an ON state to identify the integrated circuit and (ii) the temperature-sensor diode in an OFF state. The method may also include applying a second voltage to the common node to place (i) the device-identification circuit in an OFF state and (ii) the temperature-sensor diode in an ON state to determine a temperature of the integrated circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Embodiments of the invention are better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other. 
           [0016]      FIG. 1  is a schematic diagram of a device identification and temperature sensor circuit in an embodiment in accordance with the invention; 
           [0017]      FIG. 2  illustrates examples of I-V curves of a PN junction diode at different temperatures in an embodiment in accordance with the invention; 
           [0018]      FIG. 3  is a cross-sectional view of a portion of a first integrated circuit that includes device identification and temperature sensor circuit  100  in an embodiment in accordance with the invention; 
           [0019]      FIGS. 4 and 5  are schematic diagrams depicting one example of an external circuit connected to device identification and temperature sensor circuit  100  shown in  FIG. 1 ; 
           [0020]      FIG. 6  is a simulated I-V curves based on the circuit shown in  FIGS. 4 and 5  in an embodiment in accordance with the invention; 
           [0021]      FIG. 7  depicts a relationship between diode current and temperature along line A-A in  FIG. 6  in an embodiment in accordance with the invention; 
           [0022]      FIG. 8  depicts a relationship between diode current and temperature for different voltages at different temperatures obtained along line B-B in an embodiment in accordance with the invention; 
           [0023]      FIG. 9  depicts the relationship between the diode current and the diode temperature when the current is at the particular current represented by line B-B in  FIG. 8 ; 
           [0024]      FIG. 10  is a cross-sectional view of a portion of a second integrated circuit that includes device identification and temperature sensor circuit  100  in an embodiment in accordance with the invention; and 
           [0025]      FIG. 11  is a simplified block diagram of an image-capture device in an embodiment in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Throughout the specification and claims the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” The term “connected” means either a direct electrical connection between the items connected or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means either a single component or a multiplicity of components, either active or passive, that are connected together to provide a desired function. The term “signal” means at least one current, voltage, or data signal. 
         [0027]    Additionally, directional terms such as “on,” “over,” “top,” and “bottom” are used with reference to the orientation of the figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting. When used in conjunction with layers of an integrated circuit wafer or corresponding integrated circuit, the directional terminology is intended to be construed broadly, and therefore should not be interpreted to preclude the presence of one or more intervening layers or other intervening integrated circuit features or elements. Thus, a given layer that is described herein as being formed on or formed over another layer may be separated from the latter layer by one or more additional layers. 
         [0028]    The term “substrate” is to be understood as a semiconductor-based material including, but not limited to, silicon, silicon-on-insulator (SOI) technology, doped and un-doped semiconductors, epitaxial layers formed on a semiconductor substrate, and other semiconductor structures. The terms “substantially” and “approximately” mean±10% and, in some embodiments, ±5%. In the drawings, like numbers indicate like parts throughout the views. 
         [0029]    Refer first to  FIG. 1 , which illustrates a representative device identification and temperature sensor circuit implemented in accordance with the invention. The device-identification circuit  101  is connected in parallel with a temperature-sensor diode  106 . Device-identification circuit  101  and temperature-sensor diode  106  are connected to a common node  108  and also to a pair of reference voltages  109 ,  111 . The reference voltages can be one common voltage, such as ground, or two different voltages. 
         [0030]    Device-identification circuit  101  includes diode-connected transistor  102  connected in series with resistor  104 . Temperature-sensor diode  106  is implemented as a PN junction diode and diode-connected transistor  102  as a diode-connected metal-oxide-semiconductor field-effect transistor (MOSFET) in an embodiment in accordance with the invention. The impedance of diode-connected transistor  102  may be smaller than the resistance value of resistor  104 . 
         [0031]    The anode of temperature-sensor diode  106  is connected to the reference voltage  109 , which can be ground as shown or another other reference voltage. The cathode is connected to common node  108 , which is itself connected to a bond pad  110 . The forward current across diode  106  depends on temperature. The Shockley diode equation relates the diode current I of a PN junction diode to the diode voltage V. This relationship is known as the diode I-V characteristic, which can be characterized by the equation, 
         [0000]    
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       I 
                       S 
                     
                      
                     
                       ( 
                       
                         
                            
                           
                             qV 
                             nkT 
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    where I is the forward current through the diode, I s  is the reverse bias saturation current, V is the voltage across the diode, T is temperature of the PN junction in Kelvins, and n is a junction constant (typically around 2 for diode). The parameters q and k are constants, where k is Boltzmann&#39;s constant (1.38×10 −23  joule/K) and q is the magnitude of charge on an electron (1.6×10 −19  coulomb). 
         [0032]    The reverse saturation current can be defined by the equation, 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     S 
                   
                   = 
                   
                     
                       I 
                       C 
                     
                      
                     
                        
                       
                         - 
                         
                           
                             qE 
                             g 
                           
                           nkT 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    where I c  is a current constant and E g  is the diode material bandgap (1.12 eV for silicon). From Equations 1 and 2, diode I-V curves versus temperature can be calculated and plotted, as shown in  FIG. 2 . Three I-V curves for temperatures of 0° C., 50° C., and 100° C. are illustrated in  FIG. 2 . The three I-V curves show the forward current through a diode increases with temperature. 
         [0033]    Refer now to  FIG. 3 , which illustrates a representative integrated device including the device-identification and temperature-sensor circuit  100 . A well  302  is disposed in substrate  304 . Well  302  is a p-type well and substrate  304  an n-type substrate in an embodiment of the invention. A temperature-sensor diode  106  is formed between p-type well  302  and an n-type well  306  disposed in p-type well  302 . The anode of the diode  106  is connected to a ground through a contact (not shown) in the p-select region  308 . The cathode of diode  106  is connected to bond pad  110  through a contact in the n-select region  310 . N-select region  310  is disposed in n-type well  306 . 
         [0034]    Diode-connected transistor  102  is also built in p-type well  302  with the source region  312  and the drain region  314  of transistor  102  disposed in p-type well  302 . Source region  312  and drain region  314  are n-type regions in an embodiment in accordance with the invention. The gate  316  of diode-connected transistor  102  is tied to drain region  314 , and both the gate  316  and drain region  314  are connected to bond pad  110 . Source region  312  is connected to one end of resistor  104 . The other end of resistor  104  is connected to the same ground that is connected to temperature-sensor diode  106  through a contact in p-select region  308 . Resistor  104  may be made of polysilicon material. 
         [0035]    In the illustrated embodiment, one or more additional circuits or components  318 ,  320  are constructed in or on well  322 . In one embodiment, well  322  is a p-type well. A current flows through p-type well  302  into n-type well  306  when diode  106  is forward-biased. The size (i.e., current-carrying capacity) of the temperature diode  106  is much larger than that of the transistor  102  to minimize the impact of the current flow from the transistor  102  when a negative voltage is applied to the bond pad. The p-type wells  302  and  322  can be formed separately to prevent the diode current from affecting the performance of the one or more additional circuits or components  318 ,  320 . In addition, diode  106  and resistor  104  can both be connected to a reference level other than ground, or connected separately to two different reference levels including ground. 
         [0036]    In certain embodiments, the temperature diode  106  is covered by an opaque layer  324  to prevent light from reaching temperature diode  106 , since a photocurrent will be created by light. This photocurrent will add to the diode forward current when the diode is turned on by a negative voltage through the bond pad  110 , and therefore distort the I-V curves shown in  FIGS. 6 and 8  if the light level varies. The opaque layer  324  can be a metal layer such as tungsten or aluminum, or color filter material. There may be openings (not shown) in the opaque layer  324  to facilitate circuit connections and avoid shorts. 
         [0037]    In some implementations, not just diode  106  but also the region surrounding the diode  106  is covered by opaque layer  324 . For example, since the drain region  314  of the transistor  102  also forms a p-n junction diode connecting to the temperature diode  106 , its current also contributes to the total current of the temperature diode. Therefore, the opaque layer  324  may cover the drain region  314  as depicted in  FIG. 3 . Those skilled in art will appreciate that the opaque layer  324  can cover a larger region than that shown in  FIG. 3 . For example, the opaque layer  324  can cover the temperature diode  106 , the region of transistor  102  in addition, or even the entire circuit  100 . 
         [0038]      FIG. 4  is a schematic diagram depicting an exemplary external circuit connected to bond pad  110  (see  FIG. 1 ). Power supply  400  supplies a positive voltage V dc  to bond pad  110  through a known resistor  402  (having a resistance R 1 ). The positive voltage at the anode of temperature-sensor diode  106  turns off the diode and turns on diode-connected transistor  102 . The actual turn-on voltage of diode-connected transistor  102  depends upon the characteristics of transistor  102 , including the threshold voltage V t . Diode-connected transistor  102  turns on because gate  316  and drain  314  are tied together. Therefore, a current I 0  flows only through diode-connected transistor  102  and resistor  104 . The current I 0  is equal to V 1 /R 1 , where V 1  is the voltage across resistor  402 . Since the impedance R 2  of diode-connected transistor  102  is significantly smaller than the resistance value of resistor  104 , the voltage drop V 2  across diode-connected transistor  102  is negligible compared to the voltage drop V 0  across device-identification resistor  104 . Therefore, the resistance value of device-identification resistor  104  can be calculated as (V dc −V 1 )/I 0 , or R 1 (V dc −V 1 )/V 1 . On the other hand, if the impedance R 2  of the transistor  102  is comparable to the resistance value of resistor  104 , the resistance value of device-identification resistor  104  can be calculated as 
         [0000]      ( V   dc   −V   1   −V   2 )/ I   0 , or  R   1 ( V   dc   −V   1 )/ V   1   −R   2 . 
         [0039]    An integrated circuit that uses a device-identification and temperature-sensor circuit can provided with a resistor  104  having a device-specific resistance value, so that different integrated circuits each have a unique value for resistor  104 . If a calculated resistance value for resistor  104  matches or substantially matches (e.g., within a statistical margin of error) the known resistance value associated with a particular integrated circuit, the identity or type of the integrated circuit can be determined based on the match. The list of known resistance values can cover variety of integrated circuits. By way of example only, the list can be implemented as a look-up table stored in a memory. An apparatus that includes the identified integrated circuit can then automatically select the correct hardware or software configuration for the identified integrated circuit, or perform operations specific for that integrated circuit. 
         [0040]    Use of the temperature sensor to measure temperature is illustrated in  FIG. 5 . A negative voltage (−V dc ) applied as shown by a power supply  400  turns off diode-connected transistor  102  and prevents any current flow along the path of the device-identification circuit, which includes resistor  104 . The negative voltage at the cathode of temperature-sensor diode  106  places the diode in a forward-biased state, and a forward current I 1  will flow through diode  106  and through resistor  402 . The current I 1  through resistor  402  is given by V 1 /R 1 , where V 1  is the voltage across resistor  402  and R 1  is the resistance value of resistor  402 . Since the drain region of the transistor  102  is n-type and is inside a p-type well, a forward-biased current will flow into the bond pad  110  as well when the negative voltage is applied from the power supply  400 . Because the size of the temperature diode is much larger than the transistor  102 , the current flowing from the drain region of the transistor  102  is much smaller than the current flowing from the temperature diode. Therefore, the current I 1  is close to the current flowing through temperature-sensor diode  106 . The voltage V 3  across temperature-sensor diode  106  can then be calculated by subtracting V 1  from (−V dc ). Since the relationship between I 1  and V 3  across temperature-sensor diode  106  is temperature-dependent, the temperature can be determined by comparing the data set (I 1 , V 3 ) with voltage/current data sets determined at different temperatures. The data sets are obtained and may be stored in a memory (not shown). By way of example only, the pre-determined data sets can be stored as a look-up table, which is described in more detail in conjunction with  FIGS. 6-9 . 
         [0041]    In various embodiments, the circuitry implementing device identification and temperature sensing is designed so that only one of the two elements in circuit  100  is turned on and operating at any given time. The threshold voltage of the diode-connected transistor  102  may be non-zero and positive. For example, the threshold voltage can be one volt above zero, so that the impact of the leakage current of diode-connected transistor  102  on temperature measurement is small and insignificant when the voltage becomes negative. One way to increase the threshold voltage of diode-connected transistor  102  is to implant a different type of dopant into the channel. For a NMOS transistor, the dopant can be boron, for example. 
         [0042]    At any time, either (but not both of) the device-identification circuit  101  or the temperature sensor  106  may be operating in an ON state. The device-identification circuit  101  may have a first polarity and the temperature sensor may have a second polarity different from the first polarity, where polarity is defined as a positive voltage change or a negative voltage change relative to a reference voltage. 
         [0043]    Once a device is identified using the device-identification circuit  101 , temperature-sensor diode  106  can periodically or continuously monitor the temperature of the integrated circuit while the integrated circuit is operating. For example, when the integrated circuit is an image sensor that is included in a security camera, the security camera can monitor the temperature of the image sensor while capturing images or video. If the temperature of the image sensor rises above a device-specific threshold indicating the temperature is too high, the camera can shut down automatically for a period of time to prevent damage to the image sensor due to high temperature. 
         [0044]      FIG. 6  illustrates simulated I-V curves based on the circuit shown in  FIGS. 4 and 5 . One method that can be used to determine the temperature of an integrated circuit using temperature-sensor diode  106  is to compare different current values obtained at a constant voltage. The current values at different temperatures are obtained along the vertical line A-A when the voltage is constant at −0.7V.  FIG. 7  shows the relationship between the diode current and the temperature when the voltage is at −0.7V in an embodiment of the invention. The temperature of the device can be obtained by measuring the device current and locating the corresponding temperature in curve  700 . 
         [0045]    The current values can be included in a look-up table saved in a memory. For example, if the integrated circuit is an image sensor, the look-up table can be saved in a memory in an image-capture device. When the temperature is to be measured, the diode current can be calculated using the approach described earlier. Then the temperature of the image sensor can be obtained by matching the diode current with one of the diode currents stored in the look-up table. If the measured current falls in between two current values in the look-up table, a linear (or nonlinear) interpolation may be performed. 
         [0046]    Another approach to determining temperature is comparing different voltages at a constant current.  FIG. 8  depicts a relationship between diode current and temperature for different voltages at different temperatures obtained along line B-B in an embodiment of the invention. The voltage values at different temperatures are obtained along the vertical line B-B when the current of the power supply is constant at −0.002 A.  FIG. 9  shows the relationship between the diode voltage and the diode temperature when the current is −0.002 A. Therefore, when the temperature is to be measured, the diode voltage can be calculated using the approach described earlier, and the temperature of the image sensor can be obtained by matching the calculated diode voltage against look-up table values and interpolating if necessary. 
         [0047]    Temperature-sensor diode  106  and diode-connected transistor  102  may be designed so that when diode-connected transistor  102  is in an ON state, the leakage current from temperature-sensor diode  106  is small compared to the current I 0  flowing through the diode-connected transistor  102  and resistor  104 . In addition, transistor  102  may have a lower impedance than the resistance value of resistor  104 , allowing the calculation of the resistance of resistor  104  to be accurate. 
         [0048]      FIG. 10  is a cross-sectional view of a portion of a second integrated circuit that includes device identification and temperature sensor circuit  100  in an embodiment of the invention. The integrated circuit shown in  FIG. 10  is identical to the integrated circuit of  FIG. 3  except the p-type well  302  in  FIG. 3  is replaced with two separate, individual p-type wells  1000  and  1002 . The p-type well  1000  is used to form the temperature-sensor diode  106  and the p-type well  1002  is used to form the diode-connected transistor  102 . The separate wells  1000 ,  1002  can reduce the crosstalk between the temperature-sensor diode  106  and diode-connected transistor  102 . The temperature-sensor diode  1004  is formed by the p-type well  1000  and n-select region  1006 . Both p-type wells  1000 ,  1002  are tied to ground through p-select region  308 . 
         [0049]    In some embodiments, the temperature diode  1006  is covered by an opaque layer  324  to prevent light from impinging on the region of temperature diode  1006 , thereby preventing generation of a photocurrent through the diode. This photocurrent would increase the diode forward current when the diode is turned on by a negative voltage applied to the bond pad  110 , so that the I-V curves shown in  FIGS. 6 and 8  would change as the light level varies. The opaque layer  324  can be a metal layer such as tungsten or aluminum, or a color filter material. Typically there will be openings (not shown) in the opaque layer  324  to allow connections within the circuit  100  to be made without causing any shorts. 
         [0050]    Since the drain region  314  of the transistor  102  also forms a p-n junction diode connecting to the temperature diode  1006 , its current also contributes to the total current of the temperature diode. Therefore, opaque layer  324  may cover the drain region  314  as well, as shown in  FIG. 10 . More generally, opaque layer  324  can cover a larger region than that shown in  FIG. 10 . For example, opaque layer  324  may cover the temperature diode  1006  as well as transistor  102 , and the region surrounding these components, or, indeed, the entire circuit  100 . 
         [0051]    Refer now to  FIG. 11 , which depicts a simplified block diagram of an image-capture device in an embodiment in accordance with the invention. Image-capture device  1100  is implemented as a digital camera in  FIG. 11 . Those skilled in the art will recognize that a digital camera is only one example of an image-capture device that can utilize an image sensor incorporating the present invention. Other image-capture devices, such as, for example, cellphone cameras, digital video camcorders, and other hand-held devices can be used with the present invention. 
         [0052]    In digital camera  1100 , light  1102  from a subject scene is received at an imaging stage  1104 . Imaging stage  1104  can include conventional elements such as a lens, a neutral density filter, an iris and a shutter. Light  1102  is focused by imaging stage  1104  to form an image on image sensor  1106 . Image sensor  1106  captures one or more images by converting the incident light into electrical signals. By way of examples only, image sensor  1106  can be implemented as a CCD image sensor or a CMOS image sensor. Image sensor  1106  includes device identification and temperature sensor circuit  100  shown in  FIG. 1 . 
         [0053]    Digital camera  1100  further includes processor  1108 , memory  1110 , display  1112 , and one or more additional input/output (I/O) elements  1114 . Although shown as separate elements in the embodiment of  FIG. 11 , imaging stage  1104  may be integrated with image sensor  1106 , and possibly one or more additional elements of digital camera  1100 , to form a compact camera module. 
         [0054]    Processor  1108  may be implemented, for example, as a microprocessor, a central processing unit (CPU), an application-specific integrated circuit (ASIC), a digital signal processor (DSP), or other processing device, or combinations of multiple such devices. Various elements of imaging stage  1104  and image sensor  1106  can be controlled by timing signals or other signals supplied from processor  1108 . 
         [0055]    Memory  1110  can be configured as any type of memory, such as, for example, random access memory (RAM), read-only memory (ROM), Flash memory, disk-based memory, removable memory, or other types of storage elements, in any combination. Memory  1110  can store the list of known resistance values and integrated circuits that correspond to the resistance values that can be used when identifying an integrated circuit. 
         [0056]    A given image captured by image sensor  1106  may be stored by processor  1108  in memory  1110  and presented on display  1112 . Display  1112  is typically an active-matrix color liquid-crystal display (LCD), although other types of displays may be used. The additional I/O elements  1114  may include, for example, various on-screen controls, buttons or other user interfaces, network interfaces, or memory card interfaces, or even voice command controls. 
         [0057]    Driver circuit  1116  may include a power supply and a resistor (not shown). The power supply and resistor can be implemented as V AC  and resistor  402  shown in  FIGS. 4 and 5 . Thus, the power supply may be used to apply voltages to common node  108  ( FIG. 1 ). 
         [0058]    Processor  1108  controls driver circuit  1116  either to calculate a resistance value of the resistor (i.e., resistor  104  in  FIG. 1 ) in the device-identification circuit by providing a positive voltage, or to measure the temperature of image sensor  1106  by providing a negative voltage. Once the resistance value of the resistor (i.e., resistor  104 ) in the device-identification circuit is determined, processor  1108  can recognize image sensor  1106  and set up the correct camera file and timing for image sensor  1106  automatically. By controlling driver circuit  1116 , processor  1108  can continuously or periodically monitor the temperature of image sensor  1106  using device identification and temperature sensor circuit  100  and pre-determined comparison data sets stored in memory  1110 . Based on the temperature measured, processor  1108  can control driver circuit  1116  to operate the image sensor  1106  in a safe or optimal manner. For example, if the temperature of image sensor  1106  is too high, processor  1108  can control driver circuit  1116  to turn off the power supply to the image sensor  1106  to prevent damage thereto, or to start a cooling process if there is a cooler attached to image sensor  1106 . In addition, processor  1108  can perform one or more algorithms to improve the image quality by compensating for the measured temperature. For example, the dark current may be estimated based on the temperature measurement (since the dark current in the image sensor  1106  increases with temperature) and subtracted from image signals. 
         [0059]    It is to be appreciated that the digital camera shown in  FIG. 11  may comprise additional or alternative elements of a type known to those skilled in the art. For example, a thermoelectric cooling unit can be attached on the back of the image sensor  1106  inside the digital camera. The cooling unit can be used to cool the image sensor whenever it is needed based on the temperature reading. Elements not specifically shown or described herein may be selected from those known in the art. As noted previously, the present invention may be implemented in a wide variety of image-capture devices. Also, certain aspects of the embodiments described herein may be implemented at least in part in the form of software executed by one or more processing elements of an image-capture device. Such software can be implemented in a straightforward manner given the teachings provided herein, as will be appreciated by those skilled in the art. 
         [0060]    The invention has been described in detail with particular reference to certain embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, the structure of the device-identification and temperature-sensor circuit has been described as having certain conductivity types. In particular, an NMOS transistor  102  built in a p-type well. However, other embodiments in accordance with the invention are not limited to this construction. The conductivity types can be reversed in other embodiments. The identification resistor is described as made by polysilicon material, but other materials can also be used to make the resistor. In addition, both the device-identification resistor and the temperature sensor are shown as connected to ground, but they can be tied to suitable reference voltage. 
         [0061]    And even though specific embodiments of the invention have been described herein, it should be noted that the application is not limited to these embodiments. In particular, any features described with respect to one embodiment may also be used in other embodiments, where compatible. And the features of the different embodiments may be exchanged, where compatible.