APPLIED PHYSICS LETTERS 104, 073504 (2014)
Temperature sensor based on 4H-silicon carbide pn diode operational from 20 C to 600 C Nuo Zhang,1,2,a) Chih-Ming Lin,2,3 Debbie G. Senesky,4 and Albert P. Pisano1,2,3 1
Department of Electrical Engineering & Computer Sciences, University of California, Berkeley, California 94720, USA 2 Berkeley Sensor and Actuator Center, University of California, Berkeley, California 94720, USA 3 Department of Mechanical Engineering, University of California, Berkeley, California 94720, USA 4 Department of Aeronautics and Astronautics, Stanford University, California 94305, USA
(Received 22 August 2013; accepted 30 January 2014; published online 18 February 2014) A high-performance temperature sensor based on 4H-SiC pn diode is demonstrated. The sensor is capable of stable operation in a temperature range from 20 C to 600 C. The forward voltage of the pn diode has a linear dependence on temperature variation at a constant current, and recombination current dominates the current flowing in the 4H-SiC pn diode in the measured current range. At a forward current density of 0.44 mA/cm2, the device achieves a sensitivity of 3.5 mV/ C. This type of temperature sensor can be integrated with SiC power management and control circuitry to create a sensing module that is capable of working at extremely high C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4865372] temperatures. V An integrated sensing module capable of operating at high temperatures up to 600 C would be beneficial to a number of industrial applications, such as geothermal power plants, industrial gas turbines, and aerospace systems.1,2 The ability to place a sensing unit at the crucial hot spots enables real-time monitoring of these systems. As a result, the maintenance costs of the systems can be reduced. In addition, realtime monitoring can help to detect and predict the failures of critical components in a timely fashion. One key design consideration of such sensing modules is the ease of integrating the sensor with the supporting circuitries. Integrated circuit (IC) compatible temperature sensors based on semiconductor diodes,3 bipolar transistors,4 and junction field-effect transistors (JFETs)5 have been reported. Among these devices, the simplest temperature sensor that can be integrated with a circuit is based on semiconductor diodes. In comparison with Si, silicon carbide (SiC) is a promising semiconductor for harsh environment sensing applications due to its excellent electrical and physical properties.1,6–8 The wide bandgap energy (3.2 eV for 4H-SiC) and low intrinsic carrier concentration allow SiC semiconductor device to be functional at much higher temperatures. Moreover, high breakdown field (3–5 MV/cm), high-saturated electron velocity (2 107 cm/s) and high thermal conductivity (3–5 W/cm C) enable SiC devices to work under extreme conditions.9,10 SiC Schottky diodes have been previously demonstrated as viable temperature sensors that can work up to 400 C.11–13 However, SiC Schottky diode suffers from reliability issues of the Schottky contact as well as high leakage current at elevated temperatures. Fortunately, SiC pn junction is very stable and theoretically permits device operation at junction temperatures exceeding 800 C.14 Hence, SiC pn diode can be a suitable alternative for operation at elevated temperatures.
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In this Letter, a high-performance temperature sensor based on 4H-SiC pn diode is demonstrated and experimentally characterized. This compact temperature sensor has the advantage of requiring no reference temperature point which is necessary for thermocouples. Also, the electronics can ultimately be placed near the temperature sensing element on the same chip. The sensor demonstrated stable operation in a temperature range from 20 C up to 600 C. In fact, the highest temperature is not limited by the device itself but by the measurement equipment. The device with an active area of 2.25 104 cm2 achieves a sensitivity of 3.5 mV/ C, which is higher than the reported sensitivities of SiC Schottky diode12 and SiC JFET5 temperature sensors. The temperature sensor based on 4H-SiC pn diode structure described in this work is illustrated in Figure 1(a). Both pn diode terminals are accessible at the top for easy circuit integration. A 1-lm-thick N þ SiC epitaxial layer doped at 1019 cm3 combined with a 0.3-lm-thick P SiC layer doped at 1.8 1018 cm3 forms the pn junction. The device is electrically isolated by a lightly doped N-region epitaxially grown on 4H-SiC substrate. The device was fabricated on a 4 off-axis Si-face n-type 4H-SiC wafer. The temperature sensor was fabricated using surface micro-machining techniques. First, transformer coupled plasma (TCP) etching steps were used to define the N-type SiC mesa and to isolate the device. Next, plasma enhanced chemical vapor deposition (PECVD) of silicon dioxide (SiO2) was performed for surface passivation. Then, the passivation oxide was patterned using reaction ion etch (RIE). After that, E-beam evaporation was used to deposit Ni for N-type SiC contacts, and Ni/Ti/Al metal stack for P-type SiC contacts. After each metal deposition, a lift-off process was used to pattern the contacts, and a rapid thermal annealing (RTA) step at high temperature was performed to obtain low resistive ohmic contacts. The specific contact resistances for N þ SiC and P SiC are 1.38 104 X cm2 and 2.18 103 X cm2, respectively. Figure 1(b) shows the scanning electron microscope (SEM) image of the fabricated temperature sensor. The
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Appl. Phys. Lett. 104, 073504 (2014)
FIG. 2. Measured I-V curves of the 4H-SiC pn diode temperature sensor with an active area of 2.25 104 cm2 at different temperatures (20–600 C). FIG. 1. (a) Cross-sectional schematic and (b) SEM image of the temperature sensor based on 4H-SiC pn diode.
dimension of the metal contact pads is 130 lm 130 lm, and the dimension of the N þ SiC mesa is 150 lm 150 lm. The active area of the device is 2.25 104 cm2, and the total area of the device is 5.44 104 cm2. The current I of the pn diode at a given applied biasvoltage V can be expressed using the following equation:15,16 qV I ¼ I0 enkT 1 ; (1) where q is the electric charge, k is Boltzmann constant, and n is the ideality factor. When diffusion current dominates, n ¼ 1 and Eg I0 ¼ qANC NV ðLDN NN A þ LDP NPD Þe kT . When recombination current dominates, n ¼ 2 and I0 ¼ qAnsei W . A is the cross-sectional area of the device, NC and NV are the effective density of conduction and valence band states, DN and DP are the diffusion coefficients of electron and hole, LN and LP are the diffusion lengths of electron and hole, ND and NA are the doping concentrations and P-SiC, Eg is the bandgap energy, Eg pffiffiffiffiffiffiffiffiffiffiffiin ffi N-SiC ni ¼ NC NV e2kT is the intrinsic carrier concentration, W is the depletion width, and se is the effective carrier lifetime. Typically, recombination current dominates at low current levels in 4H-SiC pn diode, resulting in n ¼ 2.16 At forward bias (qVF kT), the forward voltage of the pn diode can be calculated by Eg 2kT I 2kT Ise kT ln ln ¼ VF ¼ lnðNC NV Þ þ : qWA q q I0 q q (2)
of the device from room temperature up to 600 C. The peak temperature was not limited by the device, but by the hightemperature probe station which can only heat up to 600 C. The figure shows that, by taking advantages of SiC material properties, stable device performance can be achieved at extremely high temperatures. From Figure 2, the ideality factor n of the fabricated device at low current levels was extracted to be 2.08 under room temperature and has a low variation of around 15% over the temperature range. The extracted ideality factors and leakage currents at 20 C, 50 C, and 100 C are indicated on the graph. For a given forward current level, the voltage decreases with increasing temperature. Figure 3 illustrates the forward voltage versus temperature of the fabricated device at different forward current densities, respectively. The graph shows that the forward
If the temperature dependence of se, W, NC, and NV is ignored, the theoretical sensitivity of the temperature sensor based on 4H-SiC pn diode can be expressed as16 dVF 2k Ise ln 7:67mV=K: (3) dT qWA q The fabricated device was characterized at different temperatures. Figure 2 presents the I-V measurement results
FIG. 3. Measured forward voltage versus temperature at different forward current densities.
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FIG. 4. Measured and modeled sensitivity versus forward current density of the 4H-SiC pn diode temperature sensor. The extracted ideality factor and carrier lifetime are indicated on the graph.
Appl. Phys. Lett. 104, 073504 (2014)
reported devices based on SiC Schottky diodes13 and SiC JFET5 structures. The sensitivity of the proposed device is also higher than the reported 4H-SiC pn diode betavoltaic cell16 mainly due to the smaller depletion width and less impact of shunt resistance. The forward bias sensing mode can be used in the entire range from 20 C to 600 C and the forward voltage can be converted into a measurable variable by using a sensing circuit.11 In summary, a high performance temperature sensor based on 4H-SiC pn diode has been designed, fabricated, and characterized. It shows stable and reliable operation from room temperature up to 600 C. Under forward bias condition, the sensitivity of the sensor changes from 2.2 mV/ C at a forward current density of 0.89 A/cm2, to 3.5 mV/ C at a forward current density of 0.44 mA/cm2. Hence, a higher sensitivity can be achieved with a lower forward current level. The results reported in this letter show that the IC compatible temperature sensor based on the 4H-SiC pn diode is a promising technology for harsh environment sensing applications. 1
voltage of the device has a linear temperature dependence at all forward current levels, and it decreases with increasing temperature. By calculating the slopes of these linear relationships, temperature sensitivities can be obtained. At a forward current density of 0.89 A/cm2, a sensitivity of 2.2 mV/ C is achieved. At a lower current density of 0.44 mA/cm2, the sensitivity increases to 3.5 mV/ C. From Eq. (3), it can be observed that the absolute value of dVF/dT, which is the sensitivity, is higher at lower current level. Figure 4 shows the sensitivity versus forward current density relationship. A linear relationship is observed, and n ¼ 2.0 can be extracted from the slope of the fitted curve showing that recombination current dominates the current flowing in the 4H-SiC pn diode in the measured current range. The results show good agreement with the model described in Eq. (3). Given the doping concentrations of the Nþ and P regions, the corresponding depletion width is 0.047 lm. The carrier lifetime se ¼ 0.334 ns can also be extracted from the sensitivity versus forward current density relationship. The low carrier lifetime is mainly caused by the high density of interface traps between the passivation oxide and SiC. In addition, the experimental results indicate that the proposed 4H-SiC pn diode achieves better sensitivities and higher operation temperatures in comparison with previously
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