Autonomous Temperature Sensor Based on a

Photovoltaic Energy Harvesting System

M. Ferri∗, D. Pinna∗, E. Dallago∗, P. Malcovati∗, G. Ricotti†∗Department of Electrical Engineering, University of Pavia, Pavia, Italy

†STMicroelectronics, Cornaredo (Milano), Italy

E-Mail: {massimo.ferri, daria.pinna, enrico.dallago, piero.malcovati}@unipv.it, giulio.ricotti@st.com

Abstract—In this paper we present an autonomous tempera-ture sensor supplied by a on chip photovoltaic energy harvester.Then system is realized in a BCD SOI technology. The energyharvesting elements consist of a 34 trench-insulated p-n junctions,while the sensing system consists of a bandgap reference circuit,including an integrated high precision temperature sensor, anda high voltage low drop-out voltage regulator (LDO). The entiresystem operates also at low illumination levels and tolerates awide variation of the voltage produced by the micro-photovoltaiccell chain.

I. Introduction

Energy harvesting technologies and systems are emerging

as the new challenge in the research and industrial field,

growing at rapid pace. A wide range of applications can

involve energy harvesting technologies, including distributed

wireless sensor nodes [1]–[3] for structural health monitoring,

embedded and implanted sensor nodes for medical applica-

tions, battery recharging in large systems, monitoring environ-

mental parameters, monitoring tire pressure in cars, powering

unmanned vehicles, and running security systems in household

conditions. Modern ultra-low-power integrated circuits [4]–

[7] have reached such a level of integration and processing

efficiency that the power consumption of the electronics in

many applications is compatible with the amount of wasted

energy [8]–[11] available in the environment.

Breaking down the barriers of traditional sensors, wireless

devices based on energy harvesting eliminate long cable runs

as well as battery maintenance. Combining processors with

sensors, the wireless nodes can record and transmit data, use

energy in an intelligent manner, and automatically change their

operating mode as the application may demand. Harvesting

energy from the environment in the form of vibrations, strain,

or light, these devices use background recharging of a battery

or a super-capacitor to maintain an energy reserve. Recent

applications include piezoelectric powered damage tracking

nodes for helicopters as well as solar powered strain and

seismic sensor networks for bridges.

Photovoltaic phenomena [12], [13] allows us to retrieve the

highest amount of energy with respect to any other type of

harvesters, but when the source is the sun, power collecting

becomes an intrinsically discontinuous process, forcing the

adoption of storage elements in order to supply the system

during the dark period. Moreover the light energy source

usually features several noise components, such as the 50-

500 mV

1 V

17.5V

Bandgap LDO

TemperatureSensor

3.5 V

3.3 V

Vsensor

Fig. 1. Block diagram of the proposed system

Hz modulation of a light bulb, or, simply, the refraction and

absorption by air molecules.

In this paper, we present a photovoltaic energy harvesting

power source, realized in a 0.35-μm BDC SOI technology

[14], which supplies an autonomous temperature sensor. The

system, whose block diagram shown in Fig. 1, consists of a

series of 34 trench insulated p-n junctions, a bandgap reference

circuit, including an integrated high precision temperature sen-

sor, and a high voltage low drop-out voltage regulator (LDO).

The regulator allows us to deliver a constant 3.3-V power

supply voltage to a load also in low environment illumination

conditions, as the large number of micro-photovoltaic cells in

series ensures that, also considering a degradation of almost

80% of the voltage produced by each cell, the generated

voltage is enough to properly operate the system.

II. IntegratedMicro Solar Cells

In order to convert the incident light power into electrical

power, we designed a chain of 34 micro-photovoltaic cells.

Fig. 2 shows the structure of each photovoltaic cell, imple-

mented in a p-well insulated from the common p-substrate

by an oxide trench. The geometry of the n-diffusion realized

in the p-well consists of a series of short-circuited rows, thus

maximizing both the area and the perimeter of the diodes and

creating several photovoltaic structures connected in parallel.

The BCD SOI technology and the configuration realized allow

us to create series structures and provide a voltage higher

than 3.3 V, eliminating the parasitic diode between each

single cell and the chip substrate. In particular, since the open

circuit voltage Voc obtained for each illuminated cell is almost

530 mV, the entire chain can provide up to 17.5 V. This

voltage value ensures that the system can operate, ideally,

with an open circuit voltage as low as 20% of the nominal

value. This voltage reduction can be caused by a condition of

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275

N-Diffusion

P-Substrate

Trench

Single Solar Cell PhotovoltaicString

500 mV

1 V

17.5 V

Fig. 2. Block diagram of the integrated photovoltaic energy harvesting system

low illumination or by a large power request from the load.

Moreover, the series connection of photovoltaic cells allows

us to obtain directly all the reference voltages required for the

entire system. The geometrical dimensions of each cell are

385 μm × 245 μm. The width of the depletion region in the

p-n junction is given by

xdr =

√3εSiΦbi

q

(1

Na+

1

Nd

), (1)

where Na is the p-well doping concentration, Nd the n-

diffusion doping concentration, εSi is the dielectric permittivity

of silicon, q the charge of the electron and Φbi is the built-in

potential, given by

Φbi = VT ln

⎛⎜⎜⎜⎜⎝NaNd

n2i

⎞⎟⎟⎟⎟⎠ , (2)

where VT = kT/q is the thermal voltage. Substituting the

values of each variable in (1), xdr results equal to 3.2 μm.

In order to avoid overlapping between the depletion regions,

the width of the n-diffusion strips and the space among the

strips have been set to 5 μm.

In order to estimate the photogenerated current available

for the design of the system, we realized several micro-

photovoltaic cells in a 0.35-μm standard CMOS technology

on a test chip. The structure that we tested features as an

area of 0.5 mm × 0.5 mm. The power curve that we obtained

with 300 W/m2 of incident light power at 30 ◦C is shown

in Fig. 3 with a short-circuit current of 8.5 μA. Since the

doping concentrations can be assumed as the same in both

technologies, while the area of the BCD SOI photovoltaic

cell is almost 2.8 times smaller than the measured standard

CMOS photovoltaic cell, for the BCD SOI photovoltaic cell

we can estimate a short-circuit current (Isc) of about 2.5 μA.

Measurements on the realized BCD SOI photovoltaic cell are

in good agreement with this estimation. Indeed, Fig. 3 shows

also the power curve of the BCD SOI cell with 300 W/m2 of

incident light power and constant temperature of 30 ◦C.

0 100 200 300 400 500 6000

1

2

3

4

5

6

7

8

9

Photogenerated Voltage [mV]

Pho

toge

nera

ted

Cur

rent

[μA

]

Standard CMOSBCD SOI

Fig. 3. Power curve of photovoltaic cells realized in standard CMOS(0.25 mm2 of area) and BCD SOI (0.09 mm2 of area) technologies with300 W/m2 of incident light power

Vdd

VB,1

VB,2

M6

M4 M5

M7

M3M8

M1 M2

R1

R2

M9

Q1 (x16) Q2 (x2)

Vbandgap

Ban

dgap

Ref

eren

ce C

ircui

t

Ms

Tem

pera

ture

Sen

sor

Vsensor

Fig. 4. Schematic of the bandgap reference circuit

III. Bandgap Reference Circuit

The bandgap circuit is necessary to generate a temperature

independent voltage reference. The circuit operates on the

principle of compensating the negative temperature coefficient

of Vbe with the positive temperature coefficient of the ther-

mal voltage VT . The temperature coefficient of Vbe, at room

temperature, is −2.2 mV/◦C, while the positive coefficient

of the thermal voltage is 0.086 mV/◦C. Therefore a full

compensation, at room temperature, is obtained by combining

the two terms to achieve Vbandgap = Vbe +mVT , where m must

be equal to 25.6. If this condition is satisfied, the resulting

output voltage, approximately equal to 1.2 V, is at first order

temperature independent. Fig. 4 shows the schematic of the

bandgap reference circuit used in the proposed system. The

circuit is particularly critical, since it has to manage the vari-

ations of the supply voltage, due to illumination reduction or

output power changes, providing a constant reference voltage,

equal to 1.2026 V, to the LDO. The current that flows in

transistors M4 and M5 is mirrored in transistor M3, thus biasing

the two external branches with Id,M3= Id,M8

+ Ib,Q1+ Ib,Q2

.

Since Id,M4= Id,M5

, it results that Vgs,M6= Vgs,M7

. The bipolar

transistors, with emitter area ratio equal to 8, drain the same

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276

25 30 35 40

Temperature [°C]

45 50 55 601.2

1.22

1.24

1.26

1.28

1.3

1.32

1.34

Sen

sor

Out

put V

olta

ge [V

]

IdealExperimental

Fig. 5. Temperature sensor output voltage

current, leading to a ΔVbe = VT ln (8). The resulting current

flowing trough the bipolar transistors is

IR1=

VT ln (8)

R1

. (3)

At 27 ◦C IR1is equal to 1 μA. Since the same current is

mirrored in M3 and M8, the total power consumption of the

circuit is

Ptot =(Id,M3

+ Id,M4+ Id,M5

+ Id,M8+ Ic,Q1

+ Ic,Q2

)Vdd. (4)

The power supply voltage of the bandgap reference circuit

(Vdd) corresponds to the photogenerated voltage of the 7th

micro photovoltaic cell of the series chain (about 3.5 V). The

used bandgap reference circuit does not require any opera-

tional amplifier. The output voltage is fixed by the feedback

loop including transistor M8, which compensates any eventual

variation of Vbe,Q1,Q2. The cascade transistors M6 and M7 are

used to increase the gain of the loop. The voltage drop across

resistor Rs (Vsensor) is proportional to the absolute temperature

and it is used as temperature sensor in the proposed system.

Fig. 5 shows the value of Vsensor as a function of temperature.

The variation of Vsensor is closely related to the temperature

dependent current IR1, which is mirrored in the Rs branch. The

sensitivity achieved by the temperature sensor is 3.8 mV/◦C.

IV. LDO Circuit

The proposed LDO circuit is shown in Fig. 6. The circuit

consists of an error amplifier (M6, M7, M4, M5, M10), an

output stage (M1, M2, M8, M9, Ma, Mb), a pass transistor

(Mc) and a resistive divider (R1, R2). The circuit is basically

an operational amplifier with resistive feedback. The output

voltage (VLDO) is an amplified version of the bandgap voltage

(Vbandgap). Transistor M1 mirrors the current of the bandgap

circuit, biasing the output stage composed of transistors M8

and M9.The power supply voltage of the error amplifier is

the photogenerated voltage of 7th photovoltaic cell series

chain (Vdd), while the output stage and the pass transistor

are supplied by the maximum voltage generated by the pho-

tovoltaic cell chain, corresponding to the 34th cell (Vdd,high).

The measured value of Vdd,high is 17.44 V. Transistors M1 and

M8 are standard transistors with a breakdown voltage between

drain and source Vds,break equal to 3.3 V. Therefore, in order

to protect them from the high voltage, we used a cascode

topology realized with the high-voltage DMOS transistors Ma

and Mb. These DMOS transistors, indeed, can withstand up

Vbandgap

Vdd

Vdd

VB,2

VB,1

Vdd

Vdd,high

M2 M9

M10

Ma Mb

Mc

M4 M5

M1 M6M7 M8

R1

R2

VLDO

IL

Fig. 6. Schematic of the LDO circuit

Fig. 7. Layout of the system, corresponding to the microphotograph reportedin Fig. 1

to 80 V of Vds. Transistor Mc is a DMOS too, in order to

avoid breakdown in high illumination conditions (or low load

power request). The transfer function of the LDO circuit can

be written asVLDO

Vbandgap=

A1 − Aβ

, (5)

where

A = gm,M5

(rds,M5

||rds,M7

)gm,M8

rds,M9, (6)

and

β =R2

R1 + R2

. (7)

Assuming Aβ � 1, we obtain

VLDO =

(1 +

R2

R1

)Vbandgap, (8)

and, hence, starting from a bandgap reference voltage of 1.2 V,

we obtain an output voltage equal to 3.3 V. The total current

consumption of the proposed LDO is 3.3 μA, while the total

power consumption depends on the incident light.

V. Measurement Results

The proposed system has been implemented in a 0.35-

μm BCD SOI technology. The chip area is 4 mm2. Fig. 7

shows the layout of the realized chip, corresponding to the

microphotograph reported in Fig. 1. Fig. 8 shows the short-

circuit current (Isc) as a function of the incident light power.

The 2010 IEEJ International Workshop on AVLSI, Pavia, Italy

277

0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

Incident Light Power [W/m2]

Pho

toge

nera

ted

Cur

rent

[μA

]

Fig. 8. Short-circuit current (Isc) as a function of the incident light power

0 0.5 1 1.5 2 2.5 3 3.52

2.5

3

3.5

4

Load Current (IL) [μA]

Reg

ulat

ed V

olta

ge (

VLD

O)

[V]

Fig. 9. Regulated voltage (VLDO) as a function of the load current (IL) with600 W/m2 of the incident light power

The regulated voltage (VLDO) as a function of the load current

(IL) is shown in Fig. 9. The system delivers a regulated

output voltage of 3.3 V for load currents up to 500 nA with

600 W/m2 of the incident light power (3.3 μA are consumed

by the LDO circuit). The value of IL can be increased simply

by using micro-photovoltaic cells with larger area or by

associating to the system an accumulation device (battery or

supercapacitor). Tab. I summarizes the transistors dimensions,

while Tab. II reports the achieved performance in simulation

and measurements.

TABLE ITransistor Dimensions

Transistor W [μm] L [μm] Circuit Block

M2, M9 6 4 LDOMb, Ma 6 2 LDOMc 24 2 LDOM1, M3, M10 10 4 LDOM4, M5 10 2 LDOM6, M7 4 4 LDOM8 0.5 0.35 LDOM3, M4, M5, M8 10 4 BandgapM6, M7, M9 5 2 BandgapM1, M2 5 4 BandgapMs 20 4 Sensor

TABLE IIPerformance Summary

Parameter Simulation Measurement

Bandgap voltage (Vbandgap) 1.199 V 1.202 VOutput voltage (VLDO) 3.357 V 3.369 VVdd,high 17 V 17.4 VVdd 3.5V 3.8 VTemperature sensor sensitivity 4.2 mV/◦C 3.8 mV/◦CMaximum load current (IL) — 500 nA

VI. Conclusions

In this paper we presented an autonomous temperature

sensor with a photovoltaic energy harvester and a voltage

regulator. The voltage regulator consists of a bandgap ref-

erence circuit and a high voltage LDO circuit. The realized

chip has been extensively simulated and measured, showing a

good agreement between simulated and experimental results.

Presently, we are designing an integrated solution in order to

obtain a completely autonomous wireless sensor node.

Acknowledgments

This work was supported by the Italian Ministry of Uni-

versity under FIRB project RBAP065425 “Analog and Mixed-

Mode Microelettronics for Advanced Systems”. The BCD SOI

technology has been provided by the R&D Department of

STMicroelectronics, Cornaredo (Milano), Italy..

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