101
Device and Circuit Simulation, Measurement and Modeling
4.3.10 Unipolar Heterostructure Diode for Small Signal
Rrectification and DC Power Supply in RFID
Transponders
Scientist
B. Münstermann, A. Poloczek, W. Prost, T. Feldengut
(Fraunhofer-Institute (IMS) Duisburg
R. Geitmann, A. Eckhardt
Technical Assistance:
Introduction
The current density J0 across an electronic barrier is given by thermionic emission and may vary by
several orders of magnitude depending on its barrier height q   B . In a number of applications such
as small signal rectification, a high current density at small amplitudes is required. Allyn et al [1]
demonstrated that the band gap discontinuity at a heterojunction may be used an electronic barrier
with as a tunable barrier height and hence tunable I-V charateristic. In this contribution, we will
investige the performance of nn-heterostrucuture diodes with a low barrier height for small-signal
RF detection. Especially, the possible application for RF-to-DC power conversion in ultra high
frequency radio frequency identification (RFID) transponders [2-3] will be discussed.
InP-based unipolar nn-diode
N-n-heterostructure diodes consist of a saw tooth type of conduction band barrier sandwiched
between two highly n-doped contact layers. On InP-substrate, we used quaternary
In0.52(GayAl1-y)0.48As layer, which is for all Al-compositions y lattice matched to InP. The barrier
height can be adjusted by the maximum Al-content ymax in the barrier. In this work ymax = 0.7 is
investigated. According to Anderson’s model a barrier height of ΔWL = 0.35 eV is provided.
Energy E
(z)
WF
depth z
Fig. 1
W0
(b)
InGaAs
1E19
250 nm
n-contact
spacer
InGaAs
InGaAs
1E18
-
100 nm
5 nm
saw
tooth
barrier
InGaAlAs
-
dB
1E18
1E19
5 nm
100 nm
20 nm
spacer
InGaAs
n-contact
InGaAs
n+-contact InGaAs
WL
(a)
top contact
substrate
InP:Fe (100)
350 µm
(c)
Nn-diode: (a) symbol and polarity, (b) conduction band evolution, (c) layer stack on
InP-substrate. The saw tooth barrier thickness dB was 200 nm and the maximum Alcontent was ymax = 0.7 (DU 863)
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Bi-Annual Report 2008/2009 - Solid-State Electronics Department
The layer stack was grown by molecular beam epitaxy in a Varian GenII apparatus on s. i. InP:Fe
substrate (cf. fig. 1c). During growth of the In0.52(GayAl1-y)0.48As barrier (0 < y ≤ 0.7, the Indium
cell BEP was kept constant at 3.10-7 Torr. The sum of the Al- and Ga-cell BEP was also kept
constant while both, the Ga- and the Al- cell temperature were inversely ramped in order to reach
an outmost linear slope of the Al-/Ga-composition within the whole saw tooth barrier. The device
processing was done with optical contact lithography, wet chemical etching, and lift-off. A coplanar
contact pattern enabling on-wafer RF measurements is used.
DC/RF characteristics and Device model
The nn-diode I-V characteristic is dominated by the barrier height due to the conduction band
discontinuity ΔWL at the In0.52(Al0.7Ga0.3)0.48As/In0.53Ga0.47As heterojunction. The best fit of
simulated data to experimental reverse and forward I-V characteristics is obtained for a barrier
height of 0.25 eV which is about 0.1 eV less than the nominal data.
To analyze the device capacitance a small signal equivalent circuit as shown in fig. 2 was used
including the parasitic capacitance Cpad = 20 fF and the inductance Ls = 45 pH. The intrinsic diode
is described by the series resistor R and the capacitance C(VD), and the parallel resistance R(VD).
The device capacitance C(VD) has been determined by fitting the parameters to the measured rfdata. The extracted values increases from 0.8 fF/µm2 to 1.1 fF/µm2. For a first guess of the
switching speed of the device, the speed-index s might be used:
(2)
s
t
I

V C
 ps / V  .
The nn-diode with ymax = 0.7 reached a speed index of s = 50 ps/V at a bias as low as VD = 0.15 V
which shows its potential for low-signal microwave applications. Based on the DC and small-signal
data a large-signal device model has been developed. The device model is based on a standard pnjunction model included in the “Advanced Design System” software and shows good agreement
between measured and modeled DC and RF characteristics (800 MHz to 6 GHz). The scalability of
the derived model has been tested on emitter sizes between 16 µm2 and 120 µm2 and can be used to
predict the behavior of the needed low capacitance devices for application in RFID-rectifiers.
pad
VD
Fig. 2
RS intrinsic device
L S = 45 pH
Cpad
20 fF
VD,int
C(V D)
R(V D)
Small Signal equivalent model of an InP-based unipolar n-n-heterostructure diode
Radio frequency identification transponder
In Radio Frequency Identification (RFID) systems the required power for driving the transponder
circuits has to be generated by the received RF-signal power. Therefore voltage rectifier circuits are
needed to convert the high frequency signal into a stable DC supply voltage [4, 5]. Several stages
Device and Circuit Simulation, Measurement and Modeling
103
are implemented in a cascaded charge-pump structure in order to generate sufficient DC voltage for
the operation of integrated circuits (cf. fig. 3). The power efficiency and the minimum input voltage
requirements of the rectifier determine the maximum distance between the remotely powered
device and the base station. For efficient rectification diodes with relatively high switching speed at
a low forward bias are required. The investigated nn-heterostrucuture diode exhibits at VD = 0.15 V
an ideality factor of n = 1.2, a current density of 20 µA/µm², and a rectification factor of GR = 30.
Fig. 3
UHF voltage multiplier/rectifier
In fig. 4 a transient simulation of the ac current id(Vd(t)) within the initial 10 µs is compared to the
device DC I-V characteristics with 5 µm² emitter area. The observed voltage drop in forward
direction is reduced to 150 mV, while in the negative regime the blocking effect is illustrated. The
maximum current of 100µA (20µA/µm2) is used to load the following capacitor in the charge-pump
architecture. Optimization has shown that a drastic reduction of the emitter size to 5µm2 of the
diodes increases Vin and provides a voltage supply of 1.5 V for a minimum received rf-power of 13,6 dBm.
Fig. 4
I-V-characteristics and transient diode current of the device in the rectifier circuit. The
observed forward voltage drop is 150 mV for a 5 µm2 emitter area device
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Bi-Annual Report 2008/2009 - Solid-State Electronics Department
In tab. 1 the performance of the multistage rectifier using nn-diodes is compared with previous
publications based on Si-Schottky diodes. The maximum operating range of the FRID for a basestation which transmits 3.28 W RF power at a carrier frequency of 864 MHz has been calculated.
Using the nn-diodes the operating range of the system can be improved by 30%. The conversion
efficiency is for the given load conditions is 17%, which is an excellent value compared to the
conventional rectifiers.
Tab. 1
this work
min RF-Power
for 1.5 V, 5µA
-13,6 dBm
max. distance
to base station
7,5 m
RF to DC
efficiency
17 %
[10]
-10,5 dBm
5,4 m
12 %
[11]
-11,3 dBm
5,74 m
10 %
Performance data of the investigated cascaded charge-pump circuit for remote RFID
DC power supply based on nn-diodes in comparison to works using Si-Schottky diodes
[10, 11].
Conclusion
A unipolar nn-heterostructure diode on InP-substrate is developed with an In0.52(GayAl1-y)0.48As saw
tooth barrier layer with 0 < y < 0.7. The electronic transport across the barrier can be described by
thermionic emission theory. In this study the device with a low barrier height is investigated for
AC-to-DC conversion in remote powered RFID requiring high current densities and high
rectification factors at low bias. In comparison to a standard Si-Schottky diode 3 dBm less RFpower is needed while the efficiency increases from 12 % to 17 %.
References
[1]
[2]
[3]
[4]
[5]
C. L. Allyn, A. C. Gossard and W. Wiegmann; „New rectifying semiconductor structure by
molecular beam epitaxy“, Appl. Phys. Lett. 36(5) 373-375, 1990.
U. Karthaus, M. Fischer; “Fully Integrated Passive UHF RFID Transponder IC With 16.7µW Minimum RF Input Power”; IEEE J Sol.-State Circ.. vol. 38, no. 10, 2003.
R. E. Barnett, J. Liu, S. Lazar; A RF to DC Voltage Conversion Model for Multi-Stage
Rectifiers in UHF RFID Transponders, IEEE J. Sol.-State Circ.. vol. 44, no. 2, 2009.
J.-P. Curty, N. Joehl, C. Dehollain, and M. J. Declercq; “Remotely Powered Addressable
UHF RFID Integrated System”, IEEE Sol.-State Circ. Vol. 40, no. 11, p. 2193, 2005.
T. Feldengut, R. Kokozinski, S. Kolnsberg; “A UHF Voltage Multiplier Circuit Using a
Threshold-Voltage Cancellation Technique”, PRIME Conf. Proc., p.288-291 July 2009.