ON CHIP MEASUREMENT OF IC(VBE) CHARACTERISTICS FOR HIGH ACCURACY BANDGAP APPLICATIONS

The EG and Xrl coefficients are sufficient to completely characterise the temperature dependence of lc(VBE) relationship of bipolar transistors (BJT). They are usually obtained from measured VSE(T) values, using least square algorithm at a constant collector current. This method involves an accurate measurement of VBE and of the operating temperature. We propose in this paper, a configurable test structure dedicated to the extraction of the temperature dependence of Ic(VBE) characteristic for the BJT designed with bipolar or BiCMOS processes. This allows a direct measurement of the die temperature and consequently an accurate measurement of VB~(T). First, the classical extraction method is explained. Then, the implementation technique of the new method is discussed and finally, an improvement of a bandgap design is presented. I. INTRODUCTION UE to the considerable expansion of portable D applications like cellular telephone, PALM, electronic organiser and others, great interest is given to design methodologies for low consumption. As a consequence, we observe a continuous decrease of the values of the bias current and the supply voltage used in mixed integrated applications. Due to their very low consumption and their wide temperature range of operation, bandgap references are widely used in various types of analog circuits for signal processing such as A/D and D/A converters, voltage regulators and a number of measurement devices. Designing high stability circuits such as the bandgap references demands to obtain a very good correlation between the physical parameters and their representation model used in the electrical simulators. Characterising these parameters for low voltage and current operating points over a wide range of temperature is out of the current foundry measurement protocols. At low operating bias conditions parasitic effects are exhausted, this greatly modifies the stability conditions at high temperature of critical designs such as bandgap references. This stability may be deduced from the BJT's saturation current equation commonly used in electrical simulators (I) where q is the electron charge, k is the Boltzman constant and EG is the forbidden energy band value at 0 K of silicon including the band gap narrowing. XTI is a constant parameter, related to the temperature dependence of the mobility of minority carriers in the base region and To is the reference temperature. EG and XT, are among the most difficult parameter