An experimental study on the performance of two temperature sensors based on 4h sic diodes

An Experimental Study on the Performance of Two Temperature Sensors Based on 4H SiC Diodes Procedia Engineering 168 ( 2016 ) 729 – 732 Available online at www sciencedirect com 1877 7058 © 2016 The Au[.]

Procedia Engineering 168 ( 2016 ) 729 – 732

1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi: 10.1016/j.proeng.2016.11.262

ScienceDirect

30th Eurosensors Conference, EUROSENSORS 2016

An experimental study on the performance of two temperature

sensors based on 4H-SiC diodes

S Raoa,*, G Pangalloa, F.G Della Cortea

a Università degli Studi “Mediterranea”, Dipartimento di Ingegneria dell’Informazione, delle Infrastrutture e dell’Energia Sostenibile (DIIES),

Via Graziella Feo di Vito, 89122 Reggio Calabria, Italy

Abstract

The performance of two temperature sensors based on 4H-SiC diodes are investigated Both devices show a good linear dependence

on temperature of the difference between the forward bias voltages measured on two identical diodes (Schottky or p-i-n) yet biased

at different constant currents The Schottky diodes-based sensor shows a high sensitivity (S=5.11mV/K) whereas the p-i-n structure has a highly linear output proportional to the absolute temperature

© 2016 The Authors Published by Elsevier Ltd

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

Keywords: Terms—Schottky diodes, p-i-n diodes, power semiconductor devices, sensors silicon carbide, temperature sensors, wide band gap

semiconductors;

1 Introduction

In the last decade, Schottky, p-i-n and p-n diodes have been explored for the realization of temperature sensors [1-4] The favorable chemical and physical properties, e.g high thermal conductivity (3-5 W/cm°C) and high critical

electric field for breakdown setup (E c = 2-5 MV/cm), make 4H-silicon carbide (SiC) a suitable material for high power and high temperature applications [5,6] Recently, diode-temperature sensors based on 4H-SiC were designed and fabricated to work in hostile and harsh environments [7-11] However, the accuracy of such single-diode sensors is

affected by the non-linear behavior with temperature of the saturation current, I S, in particular when the bias current

intensity, I D , is comparable with I S[1,12]

* Corresponding author Tel.: +39 09651693274;

E-mail address: sandro.rao@unirc.it

© 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

In this work, we present two proportional-to-absolute-temperature (PTAT) sensors, overcoming therefore the

non-linear effects of I S, based on 4H-SiC Schottky and p-i-n diodes, respectively The sensors have been realized using two

identical diodes integrated on the same chip and biased with different currents kept constant over the considered

temperature range

2 Sensors structure

In our setup, two p-i-n diodes, D 1 and D 2, with almost identical I D-VDcharacteristics, were driven by two external

and independent current sources, providing constant I D1 and I D2 currents [Fig 1(a)] over the whole temperature range

The two currents are described by the analytical expression:

¸

·

¨

§

−

−

2

,

1

2 , 1 2 , 1 2 , 1 2 , 1

kT I R qV S D

D S D e I

where I S1,2 , R S1,2 , V D1,2 and Ș 1,2are the saturation current, the series resistance, the diode voltage drop and the ideality

factor for D 1 and D 2, respectively

If the two diodes show the same ideality factor (Ș 1=Ș2=Ș), the difference between the voltage drops across the two

diodes (V D2 - VD1) can be written from (1) as:

1

D

q

kT V V

where r=I D2/ID1 is the bias current ratio

Eq (2) indicates that, for a fixed I D2/ID1 ratio, the sensor output, ǻV D, is linearly proportional to T if Ș is highly

stable with temperature and the contribution of R S Â(I D2-ID1) can be considered negligible

Fig 1 Electrical circuit of the PTAT sensor (a) Schematic cross section of the 4H-SiC integrated Schottky (b) and p-i-n diode (c)

The 4H-SiC diodes were fabricated by CNR-Institute for Microelectronics and Microsystems, unit of Bologna (I)

Standard photolithography and wet chemical etching were used to pattern, 150×150 µm2 Ti/Al Schottky contacts,

spaced each other by about 155 µm [Fig 1(b)] The circular p+-type anode regions were obtained simultaneously by

ion implantation of Al through a SiO2 mask designed to pattern the vertical p-i-n diodes with p+-type area of 7.54×10−4

cm2 The distance between the two p-i-n diodes is ~190 µm [Fig 1(c)]

The microchip contains several diodes with the common cathode consisting of a commercial n+-4H-SiC substrate

[13] The chip was packaged and the Ti/Al metal contacts were bonded using thin Al wires, 50 µm in diameter, to a

custom printed circuit board (PCB) to allow an electrical connection to the measurement set-up

In Fig 1 (b) and (c), the schematic cross sections of the PTAT sensors, each consisting of two geometrically similar

integrated diodes biased by different currents kept constant over the considered temperature range, T=293-573 K, are

shown

3 Experimental results and discussion

The devices have been tested in a thermostatic oven (Galli G210F030P) setting the reference temperature through its internal PID digital microcontroller Two calibrated and certified resistance temperature detectors (RTDs) based

on platinum wire (PT100), with an accuracy of ±0.3 K, were placed in contact with the PCB, through a thermal conductive paint, very close to the device under test in order to monitor, during measurements, the temperature set points

A comparison of the measured ǻV D , in a range from (down to) T=293 K up to (from) 573 K for several values of

ID1 and current ratio, r, is reported in Fig 2 together with the best-linear fitting of the experimental data

The plots show both the high linear dependence of ǻV D on T and the corresponding sensitivities calculated from the slope of the ǻV D-T characteristics To evaluate numerically how well the linear model truly fits the experimental

data, the coefficient of determination (R 2) [14] was calculated For Schottky diodes-based PTAT sensors all of the

characteristics show a good degree of linearity (R 2 > 0.998) for I D1 ranging from 0.5 mA up to 1.3 mA and for different

current ratios, r As reported in Fig 2(a), for I D1 =1.3 mA and r=4.9, the calculated sensitivity is S=2.85 mV/K, increasing for higher r and lower bias currents I D1 For r=18.4 and I D1 =0.5 mA we get a high value of S=5.11 mV/K Here the sensor shows its maximum linearity, R 2 =0.9995, corresponding to a rmse, with respect to the best-linear

model, of ±0.6 K

Whereas, the p-i-n diodes-based PTAT sensor, Fig.2 (b), shows a good degree of linearity (R 2 > 0.999) for I D1

ranging from 180 ȝA up to 3.3 mA, with 1.1<r<42.5 The sensitivity is very low for I D1 =3.3 mA and r=1.1, (S=33 µV/K) and increases for higher r For I D1 =180 µA and r=42.5 we get the highest sensitivity of S=610 µV/K

Fig 2 Measured (symbols) and modelled (lines) voltage differences vs temperature at different bias currents, I D1 , and current ratios, r, for

Schottky PTAT sensor (a) and p-i-n PTAT sensor (b).

An extended analysis of sensors performance are reported in Fig 3, showing both R 2 and S for different values of the current ratio (r) The PTAT sensor based on 4H-SiC Schottky diodes has a sensitivity of about one order of

magnitude higher compared to the p-i-n structure since both Schottky diodes have been biased in the linear region of

the I-V characteristics where the contribution of the series resistance (R s) is dominant In this case, the sensitivity,

namely the temperature derivative of Eq 2, is increased of (dR s/dT)Â(ID2-ID1) It is worth noting that Rs reduces the

linearity of the ǻV D-T characteristics (Fig 3 (b)), however specific bias currents allow to maximize the sensor linearity

as obtained for I D1 =500 µA and r~18

On the other hand, the p-i-n diodes-based PTAT sensor was biased in the exponential region where the contribution

of R s can be considered negligible leading to a highly linear sensor with a sensitivity in the order of typical PTAT devices

Fig 3 Comparison between sensitivity (a) and coefficient of determination (b) for the considered temperature range of 293-573 K vs current

ratio, r I D1 =500 µ A for Schottky diodes, and I D1 =420 µ A for p-i-n diodes

4 Conclusions

Two high temperature sensors based on two integrated 4H-SiC Schottky and p-i-n diodes were fabricated and characterized Measurements, performed in a temperature range from 293 up to 573 K, showed that the PTAT sensor based on Schottky diodes has a high sensitivity (S = 5.11 mV/eC) thanks to the additive of the diode series resistance, whereas the p-i-n diodes-based PTAT device has a higher linearity in a wider range of biasing currents

The proposed sensors are independent from the saturation current and show a good repeatability maintaining a stable output over different cycles of measurements

Acknowledgements

Dr Roberta Nipoti from CNR-IMM-UOS Unit of Bologna (Italy) is gratefully acknowledged for providing the Schottky and p-i-n diodes and for helpful discussions

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