A 45 µW Bias Power, 34 dB Gain ReflectionAmplifier Exploiting the Tunneling Effect for RFID

Applications

Francesco Amato, Christopher W. Peterson, Brian P. Degnan and Gregory D. DurginSchool of Electrical and Computer Engineering

Georgia Institute of Technology, Atlanta, Georgia 30332–0250Email: f.amato@gatech.edu, kpeterson@gatech.edu, degs@ece.gatech.edu, durgin@gatech.edu

Abstract—RFID applications have power constraints that limitRF tags to short range communications. This article presents thedesign procedures, validated by experimental results, to makea low-powered reflection amplifier that exploits the quantummechanical tunneling effect to dramatically enhance the rangeof passive or semi-passive tags. A return gain of 34.4 dB withbias power of 45 µW at 5.45 GHz and a return gain of 22.1 dBwith a bias power of 47 µW at 5.55 GHz have been observed forimpinging RF power levels as low as -70 dBm. These results allow,for certain devices, a factor of 7 range improvement to the RFIDlink while keeping the bias power 10 times lower than any otheravailable reflection amplifier. This prototype could play a key rolein enhancing RFID communication ranges without significantlyaffecting the low power budget typical for RFID technology; thedesign can be implemented in currently available semi-passivetags and opens the door for a new generation of long-range,passive/semi-passive transponders.

Keywords—RFID; reflection amplifier; quantum tunneling; tun-nel diode; low powered RFID; backscatter radio; radio propagation;modulation factor.

I. INTRODUCTION

Radio Frequency Identification (RFID) is the most promi-nent application of backscatter modulation: an RFID readertransmits a radio frequency (RF) signal to a tag consistingof an antenna and an integrated circuit (IC). The circuitreceives power from the reader and responds by modulatingits load impedance ZL; by varying the load ZL, the reflectioncoefficient changes. Modulation is possible by changing theamplitude or phase of the reflection coefficient at the interfacebetween the tag antenna and the input load. The IC drawspower either from the RF energy of the reader signal (passivetag) or from a battery (semi-passive tag); both passive andsemi-passive tags are not equipped with amplifiers.

Backscatter radio links are challenging because the tagsinvolved in the communication with a reader have little or nopower storage available; therefore, their use is restricted toshort-range applications that collect power from the transmit-ter. The forward link is limited by the minimum signal strengthrequired for the tag to power up. The return link is limited bythe reader sensitivity to the backscattered power. Semi-passivetags are already return link limited and current results haveshown ranges increased up to 21.25 m at UHF [1]. PassiveRFID systems are currently forward link limited: the maxi-mum distance is determined by the tag power requirements.Nevertheless, with the improving sensitivity of tag ICs [2],

Fig. 1: State-of-the-art reflection amplifiers compared with theresults obtained in this work. Comparisons have been made interms of return gains and required biasing DC power.

[3], the return link might become the weakest link in passiveRFID communications as well.

To increase the link ranges, improvements are needed eitheron the reader sensitivity or on the tag power requirementsor both. On the tag side, [4] and [5] have suggested theuse of reflection amplifiers with active loads to improvethe communication range of RFIDs. Reflection amplifiers arecharacterized by a negative load impedance ZL that, at thecost of a certain amount of biasing power, can amplify thebackscattered RF signal. Several successful prototypes basedon field-effect transistors (FET) are available in the literature[6]–[13] and early results obtained through the tunneling effectwere presented in [4]. In Tab. I, the performances of FETreflection amplifiers are summarized and a comparison withthe proposed prototype in terms of bias requirements and gainis plotted in Fig. 1.

We present the design and testing of a reflection amplifierthat improves both the forward and the return link of anRFID tag by exploiting the tunneling effect of tunnel diodes.Experimental results have shown that an impinging RFsignal of -70 dBm is reflected with a return gain as high

TABLE I: Overview of reflection amplifiers available in literature.

Reference DC Bias Power [mW] Gain [dB] Bias voltage [V] Bias current [mA] RF Input power [dBm] Technology Freq. [GHz]

This Work 0.045 34.37 0.08 0.566 -70 Tunnel diode [14] 5.45

This Work 0.047 22.07 0.09 0.525 -60 Tunnel diode [14] 5.55

[6] 0.325 10.2 0.755 0.431 -50 BFT25A 0.920

[7] 2 13 2.5 0.8 -55 BJT BPF405 5.25

[8] 6.3 11.48 0.7 9 - FET NE32584c 5.8

[9] 12.75 31.5 85 0.150 -15 Silicon Avalance 7.4

[10] 18 10.2 3 6 - FET CFY30 4.5

[11] 209.3 14 2.3 91 -75 pHMET 21.2

[12] 330 14 2.2 - -45 OMMIC D01PH 21.2

[13] - 8.1 - - - FET NE32584c 6.26

Reflection

Amplifier

DC block

RF choke

tunnel

diode

Charge Pump

or Battery

Demodulator

Microcontroller

modulated

DC bias

RFin

RFout

Fig. 2: Semi-passive or passive RFID tag block diagram withreflection amplifier used as single modulating load.

as 34 dB at the cost of a total biasing power of 45 µW.To the best of our knowledge, these are the highest gainand the lowest bias power requirements reported in theresearch literature encouraging further investigation to achievelink improvements in both current semi-passive and futurepassive RFID applications. Measurements have shown that abackscatter signal can be modulated by biasing on and offthe reflection amplifier and that both amplitude shift keying(ASK) and phase shift keying (PSK) communications arepossible. The use of the reflection amplifier as modulatingload ZL for passive or semi-passive RFIDs is depicted inFig. 2: a micro-controller, powered-up by a local coin battery,biases the tunnel diode on and off in order to modulate andamplify the RF signal impinging on the tag antenna. In passivetags, a charge pump can substitute the coin battery as thepower requirements to bias the tunnel diode are extremely low.

In this work, general considerations on the RFID link bud-get are followed by a survey of the state-of-the-art in reflectionamplifiers (Section II). After introducing the fundamentals ofa tunnel diode, the procedures to design a tunnel diode-basedreflection amplifier are outlined (Section III) and experimentalresults at microwave frequencies are reported (section IV).

II. RATIONALE

The communication between a reader and a tag can bemodeled as a monostatic or bistatic radar and can therefore becharacterized by the radar range equation [15]. A link budget

analysis for RFID systems is detailed in [16]. Assuming perfectmatching, no fade margin, and line of sight between the readerand the tag, the forward link is limited by the tag sensitivity

Pt = PTGTRGt

(λ

4πrmax

)2

, (1)

where Pt is the tag sensitivity, PT is the power transmittedby the reader; GTR is the load-matched, free-space gain ofthe transmitter/receiver antenna; Gt is the load-matched, free-space gain of the tag antenna; λ is the carrier-frequencywavelength; and rmax is the maximum achievable reader-to-tag distance.

After powering-up, a sufficient amount of power must bescattered back from the tag to the reader to transfer informa-tion. In a monostatic backscatter link, where a single antennais used at the reader for both transmission and reception, thereturn link is limited by the reader sensitivity PRmin .

PRmin= PTG

2TRG

2t

(λ

4πrmax

)4

M, (2)

where M is the modulation factor.

For an RFID tag switching its load ZL among two states(A and B), the modulation factor M in (2) is a function of thecorrespondent reflection coefficients ΓA and ΓB [17]

M =1

4|ΓA − ΓB |2 , (3)

with reflection coefficients defined as

ΓA,B =ZLA,B

− Z∗ant

ZLA,B+ Z∗

ant

, (4)

with ZLA,Bbeing the load impedance presented at the RF

port of the tag in state A or B and Zant the complex antennaimpedance. A passive RFID tag can switch its loads betweena short impedance (ZLA

= 0, ΓA = −1) and an impedancematched to the antenna (ZLB

= Z∗ant, ΓB = 0); resulting

in a modulation factor M = 0.25. Semi-passive tags, as theyhave less power constraints to power-up the IC, can switchtheir load between an open (ZLA

= ∞, ΓA = 1) and a short(ZLB

= 0, ΓB = −1); resulting in a maximum achievablemodulation factor M = 1.

Fig. 3: Reader requirements to guarantee a backscatter com-munication among passive or semi-passive tags of differentsensitivities Pt for maximum transmitted power PT = 1 W .The horizontal line represents the minimum reader sensitivitycurrently available [18].

By combining (1) and (2), it is possible to highlight whatreader sensitivity is required to read a tag at the maximumpossible distance

PRmin=MP 2

t

PT. (5)

By fixing the maximum transmitted power PT to 1 W, theminimum reader sensitivity PRmin

versus modulation factorM can be plotted (Fig. 3) for different values of tag sensitivityPt. The increasing sensitivity of passive RFID tag ICs willeventually make the reader the weakest node of an RFIDsystem for both passive and semi-passive tags: passive tags(M = 0.25) with sensitivity of -24 dBm would have a longercommunication range, but will require improved reader sen-sitivities or modulation factors higher than 0.25. Data fromsemi-passive tags (M = 1) with sensitivity of -27 dBm willnot be received by currently available readers. One way toimprove the return link consists in modifying the tag circuitryto increase the modulation factor. Values of M grater than oneare possible by replacing the passive load ZL with an activeone and a reflection amplifier can be used for this purpose.

A. Related works

The benefits of a reflection amplifier in RFID commu-nications have been presented in [13]. The authors craftedtwo RFID transponders: a two-element array connected totwo microstrip reflection amplifiers mounting pseudomorphicheterojunction FET (NE32584c) and a passive four-elementarray. When biased with the appropriate amount of voltage,the former topology showed a backscattering field level 4.5dB higher than the latter at a center frequency of 6.04 GHz.Advantages and disadvantages of using reflection amplifierswith RFID transponders have been highlighted in [12]. At Kband (18 - 27 GHz), tuning at the wanted frequency can bedifficult as parasitic components can produce a 2-3 GHz shift

from the center frequency and an additional frequency shift(750 MHz) is observed when varying the biasing voltage level.The authors measured a maximum gain of 14 dB at 21.2 GHzusing a total biasing power of 209.3 mW. Moreover, the returngain is completely absent outside the main band resulting inno amplification of unwanted signals. Amplitude modulation aswell as phase modulation can be performed with transpondersequipped with a reflection amplifier load: the amplitude of thereturn link signal can be modulated by simply turning on andoff the biasing voltage of the load and a change in phase canbe triggered by varying the bias level.

Several researchers have built upon these early resultssuggesting different types of FET-based reflection amplifiers[6]–[13]. Tab. I and Fig. 1 give an overview of the resultsreported in literature and focus the attention on both theachieved gains and the power requirements: gains as high as31.5 dB have been reported for biasing power requirementsfalling in the range between 0.33 mW and 1 W.

III. DESIGN OF REFLECTION AMPLIFIERS WITH TUNNELDIODES

Tunnel diodes have been chosen to investigate the benefitsof quantum tunneling in RFID applications. After a shortintroduction about their behaviour, this section illustrates thedesign procedures used to manufacture a tunnel diode-basedreflection amplifier.

A. Tunnel Diodes

A tunnel diode is the first manufactured semiconductorwhere quantum tunneling was observed [19], [20]. As withother semiconductors, a tunnel diode simply consists of a p-njunction. However, the junction voltage barrier is extremelythin and is formed between two very heavily-doped regions(high concentration of donors and acceptors). Heavy dopingresults in the tunnel diode characteristic curve shown in Fig. 4.Normally, the forbidden energy gap of a p-n junction isolatesthe electrons on the two sides of the junction because theirenergy is not sufficient to surmount the potential barrier. If thejunction region is thin enough, there is a finite probability thatelectrons of the n-type region tunnel to the empty states of thevalence band in the p-type region when an external biasingvoltage is applied [21]. It is from the quantum mechanicaltunneling effect that the device takes its name. By applyinglow forward voltage, the tunneling effect occurs (region Iof Fig. 4); tunneling charges produce a sharp increase ofcurrent with a slight variation of voltage up to a maximumcurrent i1. By increasing the applied bias, fewer and fewerelectrons are able to tunnel through the barrier (region IIof Fig. 4) and the amount of current decreases to i2. Withlarger values of forward bias, the current remains small untilthe minority carrier injection takes place, giving rise to thenormal exponential forward characteristic of a p-n junctiondiode (region III of Fig. 4). In Region II, the decreasing currentas effect of the increasing bias gives to the tunnel diode anatural negative resistance −R that can be used to design areflection amplifier.

A transmission line of impedance Z0 terminating on areflection amplifier with negative resistance −R displays areflection coefficient |Γ| greater than unity:

0 0.1 0.2 0.3 0.4 0.5−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

Forward5bias5[V]

Cur

rent

5[mA

]

I III

i

i

1

2

II

5R5=55slope

1

Fig. 4: Ideal characteristic curve of a tunnel diode.

TABLE II: Coefficients aia1 a2 a3 a4 a5 a6

0.0126 0.0068 -2.6201 24.3383 -84.2936 101.8188

|Γ| =∣∣∣∣ (−R)− Z0

−R+ Z0

∣∣∣∣ =

∣∣∣∣R+ Z0

R− Z0

∣∣∣∣ , (6)

and a return power gain |s11|, in dB, of:

|s11| = 10 log |Γ|2 = 20 logR+ Z0

R− Z0(7)

Therefore, when connected to a matched antenna, the re-flection amplifier gives a modulation factor M with magnitudegreater than 1 and an amplified reflected signal.

B. Simulation

To assist with the design of the reflection amplifier, atesting platform for extracting SPICE models of tunnel diodeshas been developed [22] by using MATLAB, a precisioncurrent source, and a picoammeter. In this work, tunnel diodemodel MBD5057-E28 [14] has been used and its small signalequivalent circuit has been defined (Fig. 5a) with Lp = 0.4nH and Cp = 0.08 pF being the package parasitics reported in[23], C = 0.3 pF being the internal junction capacitance, andG a voltage controlled source defined by 8:

i(v) = a1v + a2v2 + a3v

3 + a4v4 + a6v

5 + a6v6 (8)

with the coefficients ai extrapolated through [22] and listed inTab. II.

The MBD5057-E28 measured characteristic curve is plot-ted in Fig. 5b: the voltage range spanning from 0.07 V to0.29 V has a non-uniform slope resulting in two separatedintervals with negative resistances: from 0.07 V to 0.16 V(R ' −465Ω) and from 0.21 V and 0.29 (R ' −426Ω).In the region between 0.16 V and 0.21 V, no significant RFamplification of an impinging signal is expected. For thisparticular device model, the reduced span of uniform negative

(a)

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

Vbias

[V]

Cu

rre

nt

[mA

]

(b)

Fig. 5: a) Tunnel diode equivalent circuit; b) MBD5057-E28measured characteristic curve (at 300 K) used during thedesign process.

l1 l2 l3 l4

C1 C2 TDL

L rL

w

VbiasRFin/out

Ɵ

Fig. 6: Schematic of the microstrip reflection amplifier. Thefinal geometrical and electrical parameter are listed in Tab. III

resistance region limits amplification to only low power RFsignals. Although low power levels are typically encounteredin RFID communications, a more uniform characteristic curvecan extend the results obtained in this work to higher inputpower levels.

In order to use a tunnel diode as reflection amplifier, thefollowing criteria need to be met [24]: bias the tunnel diodein the negative resistance region of its characteristic curve;present the transmission line with a suitable negative resis-tance at the center frequency f0; and reduce Γ to acceptablelimits at frequencies away from f0; maintain stability. AgilentAdvanced Design System (ADS) has been used to design thereflection amplifier. The microstrip structure of the fabricatedreflection amplifier is shown in Fig. 6. An external powersupply, Vbias, biases the tunnel diode at the desired voltageto amplify and reflect back the RF signal input. The radialstub isolates the biasing network from the input RF signal andthe capacitor C2 is used as DC block. Transmission lines ofdifferent lengths li with fixed width w and a capacitor C1 areused for network matching and for tuning the circuit at thecentral frequency f0.

Setting f0 = 5.8 GHz as the desired central frequencyof operation, the parameters in Fig. 6 have been determinedthrough simulations to obtain an amplified reflected RF signal.The final circuit parameters are listed in Tab. III.

C. Fabrication

The reflection amplifier has been realized on a 4 layer board[25]. The first inner layer (0.7 mil of copper) has been usedas the microstrip line ground plane. The top layer with 1.4mil of copper is separated from the ground plane by a 6.7 mil

TABLE III: Board dimensions and component values

Parameter Sizew 13.5 mill1 50 mill2 300 mill3 0 mill4 30 milθL 37.5

rL 177 mil

Component Value [pF]C1 0.3C2 6.8

C1

C2

Tunnel DiodeRFin

VBias

Bias Tee

a)

b)

Fig. 7: Fabricated reflection amplifiers with a) tunnel diode #1and b) tunnel diode #2.

FR4 substrate. The substrate (FR408 [26]) has a permittivityεr = 3.66 and loss tangent tan δ = 0.0127 at 5 GHz. Via holesof 13 mil diameter and square pads of 15 mil side have beenused. Two tunnel diodes (#1 and #2) have been mounted ontwo identical boards (Fig. 7). The tunnel diodes (MBD5057-E28 [14]) operate up to 18 GHz and are fabricated usingrapid thermal diffusion on Germanium substrate; JohansonTechnology S603DS 0603 capacitors have been used.

IV. EXPERIMENTAL RESULTS

The measurement setups shown in Fig. 8 have been usedto collect experimental data. An ammeter measured the currentdrawn by the amplifiers while a Vector Network Analyzer(VNA) displayed the resonant frequencies; a signal analyzermeasured the RF level of a signal generator output amplified bythe reflection amplifier. Attenuators A1, A2 and A3 have beenused to attenuate the RF signal impinging on the reflectionamplifier and prevent the creation of harmonics. The electricalparameters of each additional component (cables, circulatorand attenuators) used are listed in Tab. IV.

Vector

Analyzer

AVbias

Reflection

Amplifier

AVbias

Reflection

Amplifier

1 2

3

Signal

Generator

Spectrum

Analyzer

A1 A2 1 2

a)1 b)

A3+

Fig. 8: Setups used to collect experimental data. a) Measure-ments on VNA E5071B with avg. 16 and b) measurementswith circulator, signal generator E8247C, and signal analyzerCXA-N9000A. Signal analyzer setup: resolution BW 3 kHz,video BW 100 kHz, avg. 10, span 1 MHz.

TABLE IV: Electrical properties of cables, attenuators andcirculator at 5.8 GHz used for data collection

Part Att. [dB]Cable 1 RG142B/U 1.3Cable 2 RG142B/U 1.3

Attenuator A1 VAT-10+ 10Attenuator A2 VAT-30W2+ 10Attenuator A3 VAT-30W2+ 29.3

Circulator CS-6.000Insertion Loss [dB] 0.4

Isolation [dB] 23

Experimental results clearly show that the designed reflec-tion amplifiers can be used in backscatter communications forboth phase and amplitude modulations by varying the biasingvoltage over time. The one port S-parameter data |s11|, ob-tained for different biasing voltages, are shown in Fig. 9a and9b for tunnel diode #1 and #2, respectively. When the biasingvoltage is off, no amplification, nor resonance of the reflectionamplifiers occur in the frequency band spanning from 3 GHz to7 GHz. When the biasing voltage forces the tunnel diode intothe negative resistance region, gains are observed. The twoboards amplify an RF input signal at two slightly differentfrequencies (5.45 GHz and 5.55 GHz) as consequence ofthe different length of the device leads; moreover, parasiticcomponents, such as bonding wire inductance, contribute tothe detuning from the expected central frequency of 5.8 GHz.In Fig. 9a, it is highlighted how a reflection gain exists onlyaround the center frequency while it is absent outside the mainband that could otherwise give unwanted signal amplification.As expected by the characteristic curve in Fig. 5b, both thereflection amplifiers show an high gain for two biasing voltages(around 0.09 V and 0.22 V) and the amplification is small orabsent for biasing voltages around 0.16 V.

The effect of biasing on the s11 phase is shown in Fig.10. A drastic change is evident when the biasing voltage isswitched from 0 to 0.08 V for tunnel diode #1 and from 0 to0.09 V for tunnel diode #2. The reflection amplifier mountingtunnel diode #1, at the peak gain frequency of 5.45 GHz, hasa phase of −140 for Vbias = 0.08 V and a phase of 91 withno bias (∆φ1 = 49). The reflection amplifier mounting tunneldiode #2 at the peak frequency of 5.55 GHz, has a phase of−133 for Vbias = 0.09 V and a phase of 80 with no bias(∆φ2 = 53).

A useful way to test the reflection amplifiers consists inconnecting the devices to a circulator to effectively separate

(a)

(b)

Fig. 9: S-parameter sweeps for reflection amplifiers mountinga) tunnel diode #1 and b) tunnel diode #2 at different biasinglevels. Power input Pin = −50 dBm.

the input power from the output power; the measurement setupshown in Fig. 8b has been used to record the data reported inFigg. 11, 12 and 13.The sensitivity of the manufactured reflection amplifiers todifferent levels of input power is shown in Fig. 11. Thehighest return gains (34.4 dB and 22.1 dB) are obtained forinput powers of -70.0 dBm and -60.0 dBm respectively. Thetunnel diodes have been biased at 0.08 V and 0.09 V andcurrents of 566 µA and 525 µA have been measured for atotal biasing power of 45 µW and 47 µW respectively. Theseresults represent, at the best of our knowledge, the highestgains and the lowest power requirements for any reflectionamplifier ever reported in literature (Fig. 1 and Tab. I).In Fig. 12, the bandwidths of the reflection amplifiers areshown. At frequencies away from the center, no amplificationsare observed, preventing the device from amplifying unwantedsignals.Finally in Fig. 13, the effects of the biasing voltage on the

Fig. 10: S-parameter phase sweeps for the two reflectionamplifiers when no bias or optimal bias are applied. Powerinput Pin = −50dBm

−70 −65 −60 −55 −50 −45 −40 −35

1

4

7

10

13

16

19

22

25

28

31

34

Pin

[dBm]

Re

turn

Ga

in [

dB

]

Tunnel Diode #1

Tunnel Diode #2

Fig. 11: Return gains as function of the RF power, Pin, at thereflection amplifier input. Tunnel Diode #1: input frequency5.45 GHz, Vbias = 0.08 V, Ibias = 566 µA. Tunnel Diode #2:input frequency 5.55 GHz, Vbias = 0.09 V, Ibias = 525 µA.

output gains have been highlighted for both the amplifiers atthe respective central frequencies (5.45 GHz, 5.55 GHz) andoptimum input powers (-70 dBm, -60 dBm). As expected bythe tunnel diode measured characteristic curve (Fig. 5b), twopeak gains are observed at two different voltage intervals.

V. CONCLUSIONS

The tunneling effect exploited in the presented prototypegave high return gains (34.4 dB and 22.1 dB) with low biaspower requirements (45 µW and 47 µW) at 5.45 GHz and5.55 GHz respectively for RF input power levels as low as-70.0 dBm and up to -40.0 dBm. Differences in the resultsare attributed to fabrication errors and to different lengths ofthe tunnel diode leades.The same results can be obtained with

5.22 5.27 5.32 5.37 5.42 5.47 5.52 5.57 5.62 5.67 5.72 5.77

−5

−2

1

4

7

10

13

16

19

22

25

28

31

34

Freq [GHz]

Re

turn

Ga

in [

dB

]

Tunnel Diode #1

Tunnel Diode #2

Fig. 12: Reflection amplifier gains as function of the inputfrequency. Tunnel Diode #1: input power -70 dBm, Vbias =0.08 V. Tunnel Diode #2: input power -60 dBm, Vbias = 0.09V.

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28

−6

−2

2

6

10

14

18

22

26

30

34

Vbias

[V]

Re

turn

Ga

in [

dB

]

Tunnel Diode #1

Tunnel Diode #2

Fig. 13: Reflection amplifier gains as function of the Vbias.Tunnel Diode #1: input frequency: 5.45 GHz, input power: -70 dBm. Tunnel Diode #2: input frequency: 5.55 GHz, inputpower: -60 dBm

higher RF input powers when employing tunnel diodes witha more uniform characteristic curve and encourage furtherinvestigation to achieve link improvements in both currentsemi-passive and future passive RFID applications.The high gains and low bias powers achieved in this work aredue to the heavy doping of the p-type and n-type semicon-ductors forming the tunnel diode. The thin junction resultingfrom it gives a finite probability for electrons to tunnel throughthe potential barrier when an extremely low bias voltageis applied. At slightly higher voltages, the tunneling effectfades generating a natural negative resistance exploited inthis work. The prototype can play a key role in enhancingRFID communication ranges without significantly affecting

the low power budget typical for this technology. It can beimplemented not only in currently available semi-passive RFIDtag models, but it opens the door for a new generation of longrange passive tags; moreover, retrodirective designs will benefitfrom the integration of tunnelig effect-based technologies anda new class of less power-hungry amplifiers will compete withcurrently available CMOS devices.

ACKNOWLEDGEMENT

This work was funded in part by National Science Foun-dation Grant #1408464.

REFERENCES

[1] S. Thomas and M. Reynolds, “A 96 Mbit/sec, 15.5 pJ/bit 16-QAMmodulator for UHF backscatter communication,” in RFID (RFID), 2012IEEE International Conference on, Apr. 2012, pp. 185–190.

[2] S. Hemour and K. Wu, “Radio-frequency rectifier for electromagneticenergy harvesting: Development path and future outlook,” Proceedingsof the IEEE, vol. 102, no. 11, pp. 1667–1691, Nov 2014.

[3] Impinj. (2014, Nov.). [Online]. Available: http://www.impinj.com/products/tag-chips/monza-r6/

[4] F. Amato and G. D. Durgin, “A tunnel diode reflection amplifier forRFID antennas,” Apr 2013, poster presentation, Orlando FL.

[5] B. Smida and S. Islam, “Full-duplex wireless communication basedon backscatter amplifier,” in Communications Workshops (ICC), 2014IEEE International Conference on, June 2014, pp. 91–95.

[6] J. Kimionis, A. Georgiadis, A. Collado, and M. Tentzeris, “Enhance-ment of rf tag backscatter efficiency with low-power reflection am-plifiers,” Microwave Theory and Techniques, IEEE Transactions on,vol. 62, no. 12, pp. 3562–3571, Dec 2014.

[7] P. Chan and V. Fusco, “Full duplex reflection amplifier tag,” Mi-crowaves, Antennas Propagation, IET, vol. 7, no. 6, pp. 415–420, Apr.2013.

[8] ——, “Bi-static 5.8 GHz RFID range enhancement using retrodirectivetechniques,” in Microwave Conference (EuMC), 2011, pp. 976–979.

[9] G. Dalman, F. Zappert, and C. Lee, “Relaxing-avalanche-mode reflec-tion amplifier,” Electronics Letters, vol. 8, no. 9, pp. 243–244, May1972.

[10] A. Lazaro, A. Ramos, R. Villarino, and D. Girbau, “Time-domainUWB RFID tag based on reflection amplifier,” Antennas and WirelessPropagation Letters, IEEE, vol. 12, pp. 520–523, 2013.

[11] H. Cantu, V. Fusco, and S. Simms, “Microwave reflection amplifier fordetection and tagging applications,” Microwaves, Antennas Propaga-tion, IET, vol. 2, no. 2, pp. 115–119, Mar. 2008.

[12] H. Cantu and V. Fusco, “A 21 GHz reflection amplifier MMIC forretro-directive antenna and RFID applications,” in MM-Wave Productsand Technologies, 2006. The Institution of Engineering and TechnologySeminar on, Nov. 2006, pp. 66–70.

[13] S. Chung, S. Chen, and Y. Lee, “A novel bi-directional amplifierwith applications in active van atta retrodirective arrays,” IEEE Trans.Microwave Theory and Techniques, vol. 51, no. 2, Feb. 2003.

[14] Aeroflex Metelics. (2005, Dec.). [Online]. Avail-able: http://ams.aeroflex.com/Metelics/pdfiles/MBD Series PlanarBack Tunnel Diodes.pdf

[15] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. NewYork: Wiley, 1997.

[16] J. Griffin and G. Durgin, “Complete link budgets for backscatter-radio and RFID systems,” Antennas and Propagation Magazine, IEEE,vol. 51, no. 2, pp. 11–25, Apr. 2009.

[17] J. T. Prothro and G. D. Durgin, “Improved performance of a radiofrequency identification tag antenna on a metal ground plane,” Master’sthesis, Georgia Institute of Technology, Atlanta, 2007.

[18] Motorola. (2012). [Online]. Available: http://www.motorolasolutions.com/web/Business/Products/RFID/RFID\%20Readers/FX9500/Documents/FX9500 Specifications.pdf

[19] L. Esaki, “New phenomenon in narrow germanium p-n junctions,”Phys. Rev., vol. 109, pp. 603–604, Jan. 1958. [Online]. Available:http://link.aps.org/doi/10.1103/PhysRev.109.603

[20] ——, “Long journey into tunneling,” Proceedings of the IEEE, vol. 62,no. 6, pp. 825–831, June 1974.

[21] G. N. Roberts, “Tunnel diodes operation and application,” ElectronicTechnology, vol. 37, no. 6, pp. 217–222, June 1960.

[22] B. Degnan, “Esaki diode test data,” http://users.ece.gatech.edu/∼degs/content/rfid1.tgz, accessed: 18 DEC 2014.

[23] Aeroflex Metelics. (2003, Oct.). [Online]. Available: http://ams.aeroflex.com/metelics/pdfiles/E28.pdf

[24] M. McPhun, “U.h.f. tunnel-diode amplifier,” Electrical Engineers, Pro-ceedings of the Institution of, vol. 114, no. 4, pp. 428–434, Apr. 1967.

[25] OshPark. (2014, Nov.). [Online]. Available: http://support.oshpark.com/support/solutions/articles/122263-4-layer-board-stackup-and-specifications

[26] Isola Group. (2014, Apr.). [Online]. Avail-able: http://www.isola-group.com/wp-content/uploads/2014/04/FR408-High-Performance-Laminate-and-Prepreg-Data-Sheet.pdf