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

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)

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. 1. I-V forward characteristics of a thermodiode at different temperatures. The forward voltage drop varies material/geometrical/process variations related to from 0.6 V at room temperature (25 °C) to 0.75 V at –55 the diode manufacturing process. °C and 0.37 V at +150 °C. Bipolar transistors (BJTs) with superior performance are commonly met in analogue, mixed-signal or RF processes. Lateral bipolar When the diode is used as a temperature transistors or the parasitic substrate transistors sensor it is best to operate it at relatively low can also be used as thermotransistors in CMOS forward currents/low power to avoid self- technology.

heating, the influence of the series resistances, The generation of ∆VBE , the base-emitter and to increase the sensitivity to values closer to voltage drop difference can be carried out in –2.2 mV/ °K. To further increase the sensitivity, several ways (Fig. 2) one can use either two or more diodes in series (i) Using one transistor and applying two or, if possible, an on-chip analogue amplifier. different emitter currents (with a known ratio N) The useful temperature range for the diode is at different times through a switch (note that the typically between –100 to 250 oC, however to base current is negligible and the emitter current preserve accurate linearity and be compatible is approximately equal to the collector current). with typical IC junction temperatures, the scale (ii) Using two identical transistors (placed side is generally restricted to –55 to 150 oC for a bulk by side) and operated with two constant currents o silicon process and –55 to 200 C for SOI. Over in their emitters with a known current ratio N. this limited temperature range, thermodiodes (iii) Using two identical constant current offer a low cost way of measuring temperature to sources driving two transistors that sit side by a reasonably good accuracy. side, the two transistors having different areas with a ratio N.

Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. (iv) Using one constant current source driving be directly embedded into the heat source to one transistor and an identical current source detect with high precision the hotspot driving N identical transistors in parallel that sit temperature. Virtually all the CPUs today have side by side. an on-chip thermodiode (or thermotransistor) that Fig. 2 shows schematic circuits that are able can be connected remotely to a temperature

to generate ∆VBE using p-n-p transistors. A sensor which measures accurately the silicon similar implementation is possible using n-p-n temperature, rather than package temperature. transistors and constant current sources attached They also react instantly to temperature changes to the collector terminals. (as the thermal impedance from the heat source to the sensing element is extremely small and 4. IC TEMPERATURE SENSORS certainly negligible when compared to the junction to ambient thermal impedance). The IC Integrated circuit (IC) or smart temperature temperature sensor can send signals to a cooling sensors is a new class of thermal sensors that fan or lower the clock frequency, change the duty has emerged in the last decade and is now one of cycle or stop momentarily the clock when a safe the main driving forces in sensor development. limit of temperature is exceeded. New IC The field was initially driven by automotive temperature sensors are fully programmable, and electronics but has now moved into lower power, contain a smart controller. Not only do they higher volume products such as mobile phones, allow the set up of the safe temperature limits, LCD TVs, computers, and PDAs. the hysteresis cycle but they can also decide on the best combination of fan speed/clock- IE NIE stalling/duty cycle change to respond to I E NIE overheating. The IC temperature sensors today VBE have an overwhelming impact in increasing the

∆VBE system reliability and improving performance without resorting to over-designed, expensive cooling hardware, or running at a reduced performance, based on the worst-case scenario

for the heat dissipation. (i) (ii) Comprehensive articles and application notes describing the benefits of IC temperature sensors IE IE are freely available on the web. A nice example I IE is ‘IC temperature Sensors - Find the hot spot’ NA ∆VBE E A ∆VBE [29]. IC temperature sensors differ from the more traditional resistive or thermocouple sensors in several ways. They are cheaper, easy to integrate

and they generally do not need cold-junction (iii) (iv) compensation or complex, linearization circuits. They can measure both the absolute and Fig. 2. Generation of ∆VBE : (i) using a single transistor, a switch and two different current sources, (ii) using differential temperature, easily. They have a identical transistors and two current sources with different very low thermal mass and they can have a fast currents, (iii) using identical current sources and two transient response, especially when the sensing transistors with different emitter areas, and (iii) using element (thermodiode) is fully buried within the identical current sources and a combination of multiple identical transistors. A combination of these techniques is heat source. In addition, the IC temperature also possible. sensors benefit from on-chip associated electronics, such as signal processing circuits and Such sensors monitor the temperature in CPUs, smart controllers. These advantages make them batteries, power ICs, motherboards, hard disc easily the most attractive class of temperature drives, in essence everything that is exposed or sensors. Nevertheless they have limited range of produces heat. Moreover, unlike any other operation (typically –55 oC to +150 oC) and their thermal sensors, the IC temperature sensors can

Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. accuracy, while in most cases acceptable, is to the extrapolated energy-bandgap voltage in below that of platinum RTDs. silicon at zero temperature, 1.205 V. For this reason the circuit is also referred to as the ‘1.2 V A. Remote Sensing reference’. A reference voltage can have a zero- Remote sensing of the temperature is one of temperature coefficient, by adding up the voltage the most exciting applications of the IC sensors. drop across the base-emitter of a transistor VBE One can mount a remote thermodiode (or to a voltage drop that is proportional with ∆VBE . thermotransistor) inside the same package or, in The former has a negative temperature best case, inside the same piece of semiconductor coefficient which exactly cancels out the positive as that of a system that generates heat. Such a temperature coefficient of the latter. system can be a micro-processor, a power One of the bandgap reference circuits is shown integrated circuit or a battery operating near its in Fig. 4. [30]. maximum junction temperature. The rest of the IC temperature sensor does not need to make C. Vptat And Iptat direct thermal contact to the system but can be Interestingly, the bandgap voltage circuit can mounted away from it. Instead, the IC sensor be easily turned into a VPTAT (Voltage can measure both the temperature of the heat- Proportional To Absolute Temperature) sensor. generating system using the ∆VBE method applied In Fig. 4 one can monitor the voltage drop across to the remote thermodiode (or theromotransistor) the resistor R2, as this voltage is directly and the temperature of its own package. The IC proportional to the absolute temperature and is sensor also contains an A/D converter, logic amplified by R2/R3. A second stage amplifier can blocks that switch the constant current sources be used to further amplify this voltage if and read-out circuitry. The IC temperature required. Summing this voltage to VBE and sensor can send signals to a micro-controller as adjusting the values of R2 and R3 one can obtain a to optimize the electro-thermal performance of stable reference voltage VREF =1.2V. the system for best performance +V DD R2/R 3∆VBE VREF =V BE + +VDD R2/R 3∆VBE Logic R1 R2 Remote block Thermotransistor Remote Q2 AD Q3

Q1 Local

VBE R3 ∆VBE Heat generating Microcontroller system (e.g.

CPU) Fig. 4. Widlar cell provides an output reference voltage that is independent of the temperature. Fig. 3. Remote sensing of temperature using a thermotransistor embedded in a heat generating system An IPTAT (Current Proportional To (such as silicon CPU), an IC temperature sensor and a Absolute Temperature) is a constant current microcontroller. generator that varies linearly with the absolute temperature. without exceeding the maximum junction temperature. D. CMOS Sensors Fig. 3 shows schematically how the remote Advanced CMOS technologies, such as Bi- sensing of the temperature is carried out. CMOS or BCD (Bipolar CMOS DMOS), also contain bipolar structures, but standard CMOS B. Bandgap Reference that is applicable to the largest market of The basic cell of advanced IC temperature integrated circuits does not normally feature such sensors is a bandgap circuit. The bandgap transistors. One idea is to use the variation of circuit provides a reference voltage that is equal the MOSFETs on-state with temperature. This

Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. variation is characterized by two parameters: the 5. CMOS TEMPERATURE SENSORS degradation of the mobility with temperature and OPERATING BEYOND 250 oC the decrease in the threshold voltage with o temperature. The first is more evident at higher SiC diodes can, in principle, go up to 800 C currents while the second is dominant when the as the ni is almost 18 orders of magnitude o gate voltage is very close to the threshold smaller than in silicon (at 300 K) [33] (shown in voltage. Unfortunately both variations are highly Fig. 5). However, their incompatibility with IC non-linear (especially the mobility with technology makes them of little use in practical temperature) and they tend to vary severely from applications. technology to technology and from one device to Interestingly, what limits the application of another. Fortunately in standard CMOS, there silicon thermodiodes to temperatures beyond o are two types of parasitic bipolar transistors. 250 C is not their physical limits, but more often Firstly there are the lateral n-p-n or p-n-p the reliability of the IC process and the normal transistors which are in parallel with the n- operation of the IC. channel MOSFET and p-channel MOSFET, respectively. These transistors are normally inhibited by reverse-biasing (or zero biasing) their base-emitter junctions, but can be used by making the MOS gate inactive and forward- biasing their emitter/base junction. Secondly, there are the substrate parasitic transistors (n-p-n in p-well CMOS technology or p-n-p in n-well CMOS technology). Generally it is accepted that the substrate transistors are more suited for use as thermotransistors because they exhibit a ‘more’ ideal behaviour and have lower sensitivity Fig. 5. Variation of the carrier concentration ni with 1/ T to stress. Wang and Meijer [31], Pertjis et al . for silicon, germanium, GaAs and silicon carbide (4H [32] also showed that in spite of a reduced gain, SiC). The plot is linear with a gradient proportional to the such transistors, display similar performance to bandgap. that of standard bipolar transistors when used as The junction temperature is often limited by temperature sensors. electromigration (EM) of metal contacts, latch-

up in circuits, reliability of the wire bonds, E. Analogue, Digital And Pulse Width reliability of the package, or effects like ionic Modulator (PWM) IC Temperature contamination. At high temperatures, the Sensors performance of CMOS devices deteriorates Analogue temperature sensors available either substantially and parasitic effects such as as VPTAT with gains between 6-20 mV/ oC, or substrate injection or short-channel effects are as IPTAT with gains of 1-10 µA/ o C are very exacerbated. common. Their accuracy varies from 0.5 % to However there are situations where the about 2%. integrated electronics operates below the junction Recently, more sophisticated temperature temperature (e.g. 150 oC), while a part of the sensors with digital output have been developed. smart sensor operates at much higher Such sensors can more easily communicate with temperatures (e.g. 150-600 oC). Temperatures a microcontroller. sensors up to 400 oC have been investigated in Pulse width modulator (PWM) temperature [34-35], but beyond this level, there is very little sensors are based on an output signal that has a reported in the literature. CMOS micro- mark-space ratio dependent on the temperature. hotplates can often go beyond 300 oC (e.g. This can be obtained by using a VPTAT and a microcalorimeters) and for this reason it would type of voltage to frequency converter. be interesting to see what is the actual temperature limit for a thermodiode or a thermotransistor.

Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. Figure 6 (a) shows a micro-hotplate as part of The forward voltage vs temperature ( V-T) plot smart gas sensors [36-37] made using SOI of the thermodiode at three driving currents is membrane technology. The micro-hotplate is shown in Fig. 7. The slope of the thermodiodes is embedded in a thin membrane (~ 5 µm) found to be –1.3 mV/ °C at 65 µA current and containing a silicon island (with a thickness of ~ operate linearly up to 550 °C. The reason for this 0.25 µm) where a thermodiode is placed to linear operation up to 550 °C is due to the use of monitor accurately the temperature of the micro- a thin active silicon layer (i.e. SOI layer) with a hotplate. The top view of the fabricated micro- very low volume of the depletion region. This hotplate along with the diode temperature sensor results in a very low value of saturation current . is shown in Fig. 6(b). The micro-hotplate is The linearity maintained up to high temperatures heated by a resistive heater made of tungsten. At is due to the very low value of the reverse o high temperatures (e.g. 200- 500 C), the gas saturation current, Is. If we further increase reacts with the sensitive layer (placed above the temperature it was found that the slope becomes heater) and changes its resistance. Tungsten is nonlinear. CMOS compatible and can be employed instead Extensive finite element simulations in of Al as a CMOS metal to avoid electro- ‘Sentaurus Device’ and theoretical calculations migration and allow much higher temperature of were carried out to match the experimental operation. Here, it is additionally used as a results and explain in detail the thermodiode heater. Since no electro-migration, no latch-up temperature characteristics. Experimental, and no package issues are present, the micro- numerical and analytical calculations are all in hotplate can run well beyond the junction excellent agreement which is shown Fig. 8. The temperature, being ultimately limited by the full equation of the forward voltage drop in a accuracy of the thermodiode, mechanical stress thermodiode function of the temperature T is and aging. given in equation (8).

65 µµµA 0.8 Silicon nitride Sensing material 1 µµµA Electrode 14nA

0.6

p+ n n+ 0.25 µm 0.4

Micro-heater SiO Thermodiode 2 0.2

Forward voltage drop (V) drop voltage Forward ~ ~ ~ ~ 0.0 0 200 400 600 800 Membrane o Temperature ( C) a) Fig. 7. Voltage vs temperature plot of a Si diode at different driving current.

Thermodiode+ 1.0 micro-heater Experimental Simulated 0.8 Theoretical

0.6

65 µµµA 0.4 1 µµµA 525 100 µm 0.2 14 nA 385 Membrane 285

Forward voltage drop (V) b) 0.0 Fig. 6. (a) Cross sectional view of micro-hotplate 0 200 400 600 800 (drawing not to scale) (b) Fabricated micro-hotplate with o diode temperature sensor. Temperature ( C)

Fig. 8. Experimental, theoretical and simulated V-T plot.

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The contribution of the non-linear terms is 0.5 negligible at low temperature. However at high temperatures (e.g. > 550 °C) the saturation 0.4 current increases because of the rapid growth in the intrinsic carrier concentration. The intrinsic 0.3 carrier concentration increases exponentially 10 -3 with temperature (e.g. at 27 °C ni is 1 ×10 cm 0.2 17 -3 and at 650 °C ni is 1.49 ×10 cm ). At very high Before correction temperatures the saturation current is 0.1 After correction orward voltage(V) drop comparable or even higher than the driving F

current and the consequence of this is that the 0.0 last term dominates over the other terms of 0 20 40 60 80 100 equation (8)). As a result the V-T plot of the Time (hour) thermodiode becomes severely non-linear as Fig. 9. Reliability test of thermodiode at 500 °C. The first curve (solid line) is the measured voltage drop across the shown in Fig. 7. As expected the high diode. The second curve (dotted line) has been corrected temperature nonlinearity occurs earlier for lower by taking into account the slight temperature change (due driving forward currents. This is expected to the change in heater resistance) to more accurately represent the diode characteristic. because in that case the saturation current becomes dominant over the driving forward 20

current at a lower temperature level. Therefore to 15 Change in resistance of RTD extend the temperature range of a thermodiode a Change in voltage of the thermodiode 10 substantial driving forward current is needed. Nevertheless, as already mentioned, a too high 5

V/V V/V (%) 0 ∆ ∆

forward current is undesirable because of the ∆ ∆

self-heating effect, as well as the effect of the -5 parasitic resistors. In addition the sensitivity (the

R/R R/R and -10 ∆ ∆ slope of the V-T curve) is decreased at higher ∆ ∆ current levels as can also be seen in Fig. 7. -15 The long term stability (the drift in the voltage -20 drop when supplied with a constant current) of 0 20 40 60 80 100 the thermodiodes was checked by operating them Time (hour) at 500°C for 100 hours using the tungsten heater Fig. 10. Relative change in tungsten resistance and diode voltage with time. on the membrane. The tungsten heater was operated using a fixed constant current while the diode was driven at current of 65µA. The 7. PROSPECTS maximum deviation in the diode voltage over this The future for IC temperature sensors is very time period was 48mV (~40 °C). Details promising. The market in IC temperature sensors investigation showed that this change was not alone is estimated at over 700 million dollars and due to the deterioration of the diode, but due to is expected to grow to over 2 billion dollars in the change in resistance of the tungsten heater less than 5 years. They account for about 15% of (Fig. 9), which caused a corresponding change in the total sensor market. This is remarkable given, the actual temperature. Accounting for this that their general research profile is quite low change in the temperature, the drift in the diode compared to other categories of smart sensors output voltage is 1-2 mV (~1 °C) (Fig. 9). The such as IC pressure, accelerometer or chemical relative change comparison between ∆R/R (for sensors. The IC temperature sensor field is tungsten) and ∆V/V (for thermodiode) shown in ahead of many other categories of sensors in Fig. 10 confirms that the performance of the terms of revenue growth (with a CAGR diode temperature sensor is more reliable than estimated in excess of 10%) with a continuous that of the tungsten RTD, which might suffer expansion in new fields. The temperature sensors from electro migration or mechanical stress perform real-time thermal management in effects. virtually all IT products, such as PCs,

Authorized licensed use limited to: UNIVERSITAETSBIBL STUTTGART. Downloaded on August 25, 2009 at 03:38 from IEEE Xplore. Restrictions apply. notebooks, power supplies, computer peripherals [8] Z. Wang, K. Tian, Y. Zhou, L. Pan, C. Hu, and industrial equipment. As the price and the L. Liu, “A high-temperature silicon-on- volume of electronics is going down with the insulator stress sensor”, Journal of frequency going up and the packages becoming Micromechanics and microengineering , vol. cheaper and smaller naturally leading to a 18 , pp. 045018-045028, 2008. [9] R. Kersjes, J. Eichholz, J. Langerbein, Y. superlinear increase in the power density, the Manoli, W. Mokwa, “An integrated sensor need for accurate temperature control is no for invasive blood-velocity measurement”, longer desirable but mandatory! Sensors and Actuators A , vol. 37 , pp. 674-

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