Abstract:
A method for providing accurate temperature sensing of a substrate utilizing the PN junction of a transistor formed on the substrate is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/796,600, filed Apr. 27, 2007, which is a divisional of U.S. application Ser. No. 11/096,701, filed Mar. 31, 2005, now U.S. Pat. No. 7,237,951. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention pertains to a temperature sensing apparatus, in general, and to an easily calibrated temperature sensing apparatus, in particular. 
         [0004]    2. Description of the Related Art 
         [0005]    A typical approach to measuring temperatures is to utilize a PN diode junction as a temperature sensor. In integrated circuit applications, the PN junction is typically provided by using a bipolar transistor integrated into the substrate. 
         [0006]    In investigating the properties of PN junction temperature sensors, I have determined that certain inaccuracies result from the standard methodology utilized to sense temperatures of substrates of microprocessors. 
       SUMMARY OF THE INVENTION 
       [0007]    In accordance with the principles of the invention, an improved method of determining the temperature of substrates is provided. 
         [0008]    In accordance with the principles of the invention two methods of providing improved and more accurate temperature sensing are provided. 
         [0009]    In a first methodology in accordance with the principles of the invention, non constant β characteristics of a sensing transistor are compensated in the current provided to the transistor emitter. 
         [0010]    In a second methodology in accordance with the principles of the invention, the transistor base current is utilized to determine the temperature of the PN junction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will be better understood from a reading of the following detailed description of illustrative embodiments of the invention in which like reference indicator are utilized to identify like elements, and in which: 
           [0012]      FIG. 1  illustrates a temperature sensing configuration to which the invention may be advantageously applied; 
           [0013]      FIG. 2  illustrates a temperature sensing transistor; 
           [0014]      FIG. 3  illustrates the characteristic curve of a PNP transistor&#39;s β characteristic; 
           [0015]      FIG. 4  illustrates a sensing transistor operated in accordance with one principle of the invention; and 
           [0016]      FIG. 5  illustrates a sensing transistor operated in accordance with another principle of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates a typical temperature sensing configuration utilized as part of a temperature sensing and controlling arrangement for use with highly integrated devices such as microprocessors. The configuration includes a PN junction  101  that is subjected to two current levels I 1  and I 2 , by a temperature sensing and control circuit  103 . 
         [0018]    As shown in  FIG. 2 , PN junction  101  typically comprises a bipolar transistor. The bipolar transistor is known to give a transfer equation of 
         [0000]        V   be   =ηKT/q  ln  I   c   /I   o    
         [0019]    Using this knowledge it is possible to determine the temperature of a transistor by driving it with two different currents whose ratio is M. In so doing, the difference in V be  between current I c1  and I c2  is: 
         [0000]      Δ V   be   =ηKT/q  ln  M,    
         [0000]    where η (emission coefficient), K (Boltzmann&#39;s Constant), q (electron charge), and ln M are all constants. Thus T (in Kelvin) is directly proportional to ΔV be . 
         [0020]    In many circuits, however, the standard bipolar transistor available is a substrate PNP (P source/drain, N well, P substrate), thus we are unable to drive or control the collector current as the collector is tied via the silicon substrate to circuit ground. 
         [0021]    Since only the emitter and base terminals are available, the current industry standard practice is to drive the emitter with currents I E1  and I E2  whose ratio is M. 
         [0022]    If β 1 |I E1 =β 2 |I E2  then the collector ratio is also M and temperature is easily determined. 
         [0023]    In high performance CMOS processes it is unlikely that β 1 =β 2 , further in these processes β is typically very low (0.5-2.0). This is shown in the graph of  FIG. 3  by curve  301 . In this case I E1 /I E2 ≠I C1 /I C2    
         [0024]    The problem is indicated if the equation for ΔV be  is modified to read: 
         [0000]      Δ V   be   =ηKT/q  ln [ I   E2 (β 1 +1)β 2   ]/[I   E1 (β 2 +1)β 1 ] 
         [0025]    If for example β 1 =0.7 and β 2 =0.8 we may record an error of 10° C. when using the industry approach of driving the emitter. This error is not tolerable when system requirements are errors of 1° C. or less. 
         [0026]    In accordance with a first method to correct for this error, beta correction is utilized. 
         [0027]    We wish to control I C2 /I C1 =M thus, M=(I E2 −I B2 )/(I E1 −I B1 ) and we can show 
         [0000]        I   E2   =MI   E1 +( I   B2   −MI   B1 ) 
         [0028]    If β 1 =β 2  the second term goes to zero and we drive I E2 /I E1 =M. However, for instances in which β 1 ≠β 2  we modify the current drive to satisfy the above equation. 
         [0029]    In accordance with the principles of the invention, a method and circuit implementation to achieve the above equation is as follows: 
         [0030]    1. Drive I E1  and record I B1 , V be1  and create MI B1    
         [0031]    2. Drive MI E1  and record I B2    
         [0032]    3. Add I B2 −MI B1  current to MI E1    
         [0033]    4. Record V be2    
         [0034]    5. ΔVbe=V be2 −V be1    
         [0035]    6. Compute temperature 
         [0036]    This arrangement of transistor  101  is shown in  FIG. 4 . 
         [0037]    In accordance with the principles of the invention, the base drive may be utilized. It is a little known and/or used fact that base current also follow an exponential equation 
         [0000]        V   be   =ηKT/q  ln  I   B   /I   o  and thus, Δ V   be   =ηKT/q  ln  I   B2   /I   B1    
         [0038]    In accordance with this equation, accurate temperatures may be recorded by referencing the emitter of transistor  101  to a common voltage V and current driving the base with current I B1  and I B2  such that I B2 /I B1 =M as shown in  FIG. 5 . 
         [0039]    The invention has been described in terms of illustrative embodiments of the invention. It will be apparent to those skilled in the art that various changes may be made without departing from the spirit of scope of the invention. It is not intended that the invention be limited by the embodiments disclosed and described.