Cmos Temperature Sensors - Concepts, State-Of-The-Art and Prospects

CMOS TEMPERATURE SENSORS - CONCEPTS, STATE-OF-THE-ART AND PROSPECTS Florin Udrea, Sumita Santra, Julian W. Gardner* Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK Tel: +44 (0)1223 748319; Fax: +44 (0)1223 748348, E-mail: [email protected] *School of Engineering, University of Warwick, Coventry CV4 7AL, UK Abstract–The paper reviews the state-of-the-art in management in the power hungry circuits. Again, IC temperature sensors. It starts by revisiting the steady improvement in the semiconductor semiconductor theory of thermodiodes and process technology is driving the upper thermotransistors, continues with the introduction of temperature limit of circuit operation from IC temperature sensors, the concepts of VPTAT – 125 °C to 200 °C or even 250 °C in the case of ICs Voltage Proportional To Absolute Temperature and IPTAT (Current Proportional To Absolute fabricated in Silicon On Insulator (SOI) process Temperature) and discusses the possibility of use of technology. Moreover, the new generation of parasitic bipolar transistors as temperature sensors CMOS compatible micro-hotplates often require in pure CMOS technology. The next section much higher temperatures (between 200 and demonstrates the very high operating temperature of 600 °C) of operation, where the heating portion is a ‘special’ thermodiode, well beyond the typical IC kept isolated from on chip circuit area by silicon junction temperature. This is achieved with a surrounding dielectric. This kind of structure is diode embedded in an SOI CMOS micro-hotplate. A useful in high temperature gas sensors [1-6] or discussion on the temperature limits of integrated shear stress sensors [7-8] where accurate on chip temperature sensors is also given. The final section outlines the prospects of IC temperature sensors. temperature measurement and control is extremely important. Silicon based RTDs, diodes and transistors are currently used as the best 1. INTRODUCTION temperature sensors for IC compatibility. Temperature sensors are one of the fastest Temperature sensors are also necessary in other growing fields in the sensors market because of sensors, such as flow sensors [9], pressure the abundance of applications where temperature sensors [10-11], IR detectors [12-14], humidity must be monitored and controlled, including sensors [15-16] etc. Cryogenic operation of ICs personal computers, mobile phones, gaming [17-19] is also another important use for CMOS consoles, automobiles, medical equipments, temperature sensors. In this application not only process industries, nuclear plants, within that the sensor has to endure very low different sensors and many others. A large temperatures, but also there are situations when variety of temperature sensors have been it is immersed in a strong magnetic field and/or a developed to match these widely varying radio frequency field. technical and economic requirements of these The basic and most accurate CMOS applications. Discrete temperature sensors in the temperature sensor is the silicon p-n junction form of resistive temperature detectors (RTDs) diode (i.e. thermodiode). The silicon bipolar (e.g. platinum thermometer), thermocouples (e.g. junction transistor (BJT) can also be used as a bimetallic layer) and thermistors (e.g. metal temperature sensor when operated in a diode oxides) have been extensively used in the configuration (base and collector shorted). industries and laboratories over the last many Extensive work on BJT temperature sensors has decades. These temperature sensors are fairly been carried out by a group of Delft University accurate and can be used in wide range of [20-22]. Often temperature sensors are integrated temperatures (~ up to 2000 °K). However, in the with smart processing circuits to form IC last couple of years CMOS temperature sensors temperature sensors [23-26], because have become increasingly popular because of temperature measurement alone is not sufficient. rapid steady growth of Integrated Circuit (IC) The temperature reading must be interpreted industry and the necessity of effective thermal properly and processed so that appropriate 978 -1-4244 -2004 -9/08/$25.00 © 2008 IEEE 31 Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. actions can be taken to counteract any unwanted In the case of forward conduction at low temperature fluctuations. temperatures (below 300 °C), equation (1) This paper will review the concepts of becomes different CMOS temperature sensors I = I s exp (qV kT ) (4) (thermodiode, BJT, MOS transistor as BJT etc). The paper also describes the state-of-the-art From equations (1) and (2) we can see that 2 smart IC temperature sensor. The detail analysis the diode saturation current is proportional to ni , of a thermodiode (with low reverse saturation and the saturation current can be expressed as current), which has the potential to use for high −qV g I CT η e kT temperature measurement (>300 °C), will also be s = (5) discussed. The paper concludes with a discussion where C is a constant that includes the density of of the future prospects of CMOS temperature states, effective masses of electrons and holes, sensors. carrier mobility, doping density, recombination life time, junction area etc. In the above 2. THERMODIODE equation, η (Si ~ 3.5) is a process dependent The application of diodes and transistors as parameter. modulating thermal sensors was first proposed in The voltage across the diode is (from equation (1)) 1962 by McNamara [27]. A clear description of the operation of diodes and transistors as thermal kT I V = ln +1 sensors has been given in Meijer [20]. Potential q I s (T ) advantages of thermodiodes over other types of kT I kT I s (T ) thermal sensors include their compatibility with = ln + ln + 1 (6) q I (T ) q I IC technology and low manufacturing cost. It is s well known that the diode forward voltage Using equation (5) and (6) the voltage can be decreases linearly with temperature when driven expressed as by a constant forward current. This property is kT kT I kT I s (T ) exploited to use it as a temperature sensor. V = Vg − η ln T + ln + ln +1 (7) q q C q I The ideal current I voltage V equation of a p-n junction diode is given by To develop the equation for V, equation (7) is qV qV written at two temperatures, an arbitrary D p Dn kT kT I = qA ( pn − np )e −1 = Is (T )e −1 (1) temperature T and a specified reference Lp Ln temperature Tr (keeping the current constant). ηkT where Dn and Dp are the diffusivity of the V + r −V (T ) ηkT g r electrons and holes respectively, Ln and Lp are r q V = Vg + − T diffusion length for the electrons and holes q Tr respectively. The saturation current I , constant s ηk T kT I + I (T ) at a particular temperature T, is related to the + T −T −T ln + ln s (8) q r T q I I T junction area A and different junction r + s ( r ) parameters. np and pn are the minority carrier in Therefore, the voltage across the diode is the sum p-type and n-type respectively at equilibrium, of a constant term (first term), a term they can be expressed as: proportional to absolute temperature (second 2 2 term) and two nonlinear terms. Neglecting the ni ni n p = , pn = (2) two nonlinear terms the temperature gradient can N a N d be expressed as where n is intrinsic carrier concentration in the i dV ηkT r 1 semiconducting material. = −Vg + −V (Tr ) (9) dT q Tr ni has a very strong dependence on temperature [28] For silicon, taking V(Tr) ≅ 0.6V at 300 °K the −qV g bandgap, Vg = 1.14 V, the temperature gradient 2 3 kT of the diode can be calculated as –2.1 mV/ °K. In ni ∝ T e (3) 32 Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. practice this varies from –1.2 to –2.2 mV/ °K 3. THERMOTRANSISTORS depending on the forward driving current (as The voltage between the base and emitter of V(Tr) is not constant and varies with the forward the transistor, VBE , is temperature-dependent with current as given by equation (1), technology, the a relationship similar to the diode expression. In physical parameters of the junction and the fact a thermodiode can be simply obtained by geometry). The lower absolute value of this short-circuiting the base to the collector of the coefficient (i.e. –1.2 mV/ °K) is generally transistor and the only active junction is that expected when the forward current is higher formed between the base and emitter diffusion leading to higher forward voltage V(Tr) and the regions. Often in an IC, the diodes are made of junction exhibits parasitic series resistances. bipolar transistors with the base and collector Fig. 1 shows the diode I-V forward connected together. A more sophisticated and characteristics of a silicon thermodiode at considerably more accurate method to measure different temperatures. The voltage drop temperature with one diode is to apply first a decreases with increase in temperature for the high forward bias current IC1 to its terminals and same current level. then, secondly, a low forward bias current IC2 . The difference in the voltage drop ∆VBE now only depends upon the ratio of these two currents rather than the geometrical or material factors. Writing the well-known equation from (4) for two values of the current IC1 and IC2 and, knowing that the saturation current is the same, one can find a relationship between the voltage drop difference and temperature ( IC1 >> IC2 and N = IC1 /IC2 ): I kT C1 kT ∆VBE = (VBE1 −VBE2 ) = ln = ln( N) (10 ) q I C2 q In this way one removes all the Fig.

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