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An investigation on the influence of electromagnetic interference induced in conducting wire of universal LEDs
Microelectronics Reliability 47 (2007) 959–966 www.elsevier.com/locate/microrel
An investigation on the influence of electromagnetic interference induced in conducting wire of universal LEDs Han-Chang Tsai
*
Department of Electronic Engineering, Cheng-Shiu University No. 840, Chenrg Ching Rd., Neau Song Township, Kaohsiung County 833, Taiwan, ROC Received 27 April 2006; received in revised form 14 June 2006 Available online 22 August 2006
Abstract Electromagnetic Interference (EMI) has a detrimental effect upon the performance of Optical-Fiber Communication (OFC) systems. The present study considers the case where EMI is induced in a conducting wire (CW), and derives equations to establish the influence of the induced EMI on GaP and GaAsP Light-Emitting Diodes (LEDs). These equations are then verified experimentally. The results indicate that the degree of influence of the EMI upon both LED devices depends upon the interference power, the interference frequency, the induced power, the input resistance of the device, the inverse saturation current, and the ideal factor of the LED. Moreover, it is found that the induced interference current increases with an increasing interference frequency and that the EMI has a greater influence on devices with a lower input impedance. The theoretical results are found to be in good agreement with the experimental data. 2006 Elsevier Ltd. All rights reserved.
1. Introduction LED devices have attracted intense research effort for many years now. Advances in electro-optics technology now enable optical integrated circuits and electronic circuits to be combined within complex systems. It is impossible to shield such systems completely from the effects of Electromagnetic Interference (EMI) [1–10], particularly since the configuration of the wiring within the electronic circuits is such that many of the wires act as straight antenna conducting wires. LEDs are fundamental components within an OFC system [11–17]. Consequently, the present study develops a theoretical approach to investigate the influence of EMI on the reliability of GaP and GaAsP LED devices. Of all the available semiconducting materials, GaAs and GaP, together with their ternary alloy, GaAs1yPy, are undoubtedly the most important III–V compounds. Consequently, in investigating the influence of EMI upon LED *
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0026-2714/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2006.06.005
devices, the current study focuses specifically on the characteristic curves of GaP and GaAsP diodes. Fig. 1 illustrates the basic structures of the GaP and GaAsP LED devices. The interaction processes between a photon and an electron in a solid can be classified as either absorption, spontaneous emission, or stimulated emission. LED devices are p–n junctions which emit spontaneous radiation under an appropriate bias. Of the two compounds considered in the present study, Gallium Arsenide (GaAs) is a direct-bandgap semiconductor (Eg = 1.44 eV), in which p–n junctions with high luminescent efficiencies are readily formed. Conversely, Gallium Phosphide (GaP) is an indirect-bandgap semiconductor (Eg = 2.2 eV), and hence band-to-band transitions do not generally occur. As shown in Fig. 1(a), direct-bandgap LEDs (which emit red light) are generally fabricated on a gallium arsenide substrate, while indirect-bandgap LEDs (which emit orange, yellow or green light) tend to be fabricated on a gallium phosphide substrate, as shown in Fig. 1(b). Group V elements, such as N (nitrogen) and Bi (bismuth), are commonly used as dopants to assist radiative recombination [18–24] by replacing the phosphorous atoms in the
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Fig. 2. Current carrying coil wound on ferromagnetic toroid with air gap.
Fig. 1. Structure of GaP(Red) and GaAsP(Yellow) LEDs.
lattice sites. Although the outer electronic structure of nitrogen is similar to that of phosphorous, the electronic core structures of these two atoms are quite different. This difference generates an electron trap close to the base of the conduction band. Hence, a recombination center, referred to as an ‘‘isoelectronic trap’’, is formed, which greatly increases the probability of radiative transition in indirect-bandgap semiconductors. The ternary GaAs1yPy (Gallium Arsenide Phosphide) alloy is also a commonly applied semiconductor material. This alloy is noteworthy because it changes from a direct-bandgap material (when y < 0.45) to an indirect-bandgap material when the band gap is approximately 2.1 eV. The intensity of the low frequency electromagnetic waves present in a typical indoor environment is greater than that of the high frequency electromagnetic waves. In this study, the intensities of these electromagnetic waves are measured using the CA40 (low frequency) and CA41 (high frequency) gaussmeters manufactured by Chauvin Arnoux (France). The intensity of the low frequency electromagnetic waves is found to be 0.03 lT, while that of the high frequency waves is just 2 V/m (approximately 0.0067 lT). Therefore, the present investigation concentrates primarily on the effects of low frequency electromagnetic wave interference on the current GaP and GaAsP LEDs and only briefly considers the case of high frequency electromagnetic wave interference. The results obtained for low and high frequency EMI are found to be the same.
magnetic toroid containing an air gap. Using the experimental setup shown in Fig. 3, a series of studies are performed to investigate the influence of EMI and Magnetic Field Interference (MFI) on an antenna conducting wire (CW) and to establish the effect of the induced power upon the current–voltage (I–V) characteristics of the current GaP and GaAsP LED devices. The experimental system is shielded in a metal case (not shown) to prevent the influence of external noise. The conducting wire (CW) is positioned in the air gap of the ferromagnetic toroid shown in Fig. 2. The EMI frequency ranges from 0 Hz to 1.8 GHz and the interference amplitude is varied between 0.3 V and 1 V. During the experiments, an AC or DC current is passed through the coil causing a magnetic or electrical energy field to be established within the air gap. The energy field induces a noise voltage within the CW, which is then re-coupled with the electric circuit system. The corresponding response of the circuit system is analyzed using the Superposition Theorem. This particular theorem is specifically chosen here since it enables the CW-induced interfer-
2. Theoretical analysis 2.1. Induced power in straight conducting wire As shown in Fig. 2, the EMI source considered in this study consists of a current-carrying coil wound on a ferro-
Fig. 3. Experimental setup of CW for EMI measurement.
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966
~/ , of the z-directed source can be expressed respectively H by [25,26]:
ΔI
~ Eh ¼ ^hjxl sin hAZ ^ sin hAZ ¼ ~ ~ / ¼ /jb H Eh =g
Veff Vin
RLED
Rr
Antenna Fig. 4. Equivalent circuit model for antenna–receiver combination used to measure field intensity.
ing voltage to be considered in isolation. Fig. 4 presents the equivalent circuit model for the antenna–receiver combination used to measure the field intensity. Note that the ohmic resistance of the CW is incorporated within the RLED term in this circuit model. The time-averaged power received can be written in the form [25–28]: P r ¼ DI 2 ðRr þ RLED Þ ¼ DIV eff ¼ DI ¼
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V eff Rr þ RLED
RLED ¼ Rohmic þ Rin DZ Rohmic ¼ Rs 2pa rffiffiffiffiffiffiffiffi xl0 Rs ¼ 2r
V 2eff Rr þ RLED
ð1aÞ ð1bÞ ð2Þ ð3Þ ð4Þ
where Veff and DI are the induced interference voltage and induced interference current in the LED, respectively; RLED is the resistance of the LED, and comprises the ohmic resistance of the CW (Rohmic) and the dynamic input resistance of the LED (Rin); a and DZ are the radius and length of the wire, respectively; Rs is the surface resistance; x is the angular frequency and is always equivalent to 2pf, where f indicates the frequency in hertz; l0 is the permeability of free space; r is the conductivity of the wire; and Rr is the radiation resistance of a short dipole (DZ < k/4), which from [25,26] is given by 2 DZ Rr ¼ 20p2 for 0 < DZ < k=4 ð5aÞ k 2:4 DZ Rr ¼ 24:7 p for k=4 < DZ < k=2 ð5bÞ k where k is the wavelength of the EMI. It is noted that the value of DZ is very small compared to that of the wavelength, i.e. DZ k. By applying the Maxwell Electromagnetic Field Equations, it is found that the wire antenna is left along the z-axis. Hence, the electric field, E~h , and the magnetic field,
ð6Þ ð7Þ
where AZ is the vector potential along the z-axis, h is the directional angle ofpthe ffiffiffiffiffiffiffi radiation wave with respect to the z-axis, and g ¼ l=e is the intrinsic impedance of the medium. The current experiments assume that the inductive conducting wire acts as a straight dipole antenna, in which the induced currents are equal in magnitude but opposite in direction. Furthermore, the distribution of the current along the length of the wire is assumed to be sinusoidal, and can therefore be expressed as L L IðZÞ ¼ I m sin b jZj ; jZj < ð8Þ 2 2 where Im is the maximum value of the induced current in the dipole antenna, L is the length of the antenna, and b = 2p/k is a phase constant. To obtain the dipole radiation pattern, it is first necessary to define the radiation integral for the z-directed wire antenna along the z-axis, i.e. Z L ejbr 2 AZ ¼ IðZÞe jbZ cos h dz ð9Þ 4pr L2 Substituting Eq. (8) into Eq. (9) gives: cos h cos bL ejbr cos bL 2 2 Im Eh ¼ jg sin h 2pr
ð10Þ
Since the length of an inductive wire antenna in a typical circuit may be as much as L = k/2, the radiated power for the case of bL ¼ p2 can be written as [26]: 2 Z 2p g 2 1 cos t Im dt Pr ¼ 8p t 0 2p k g 2 X t2g g nþ1 ¼ I ð1Þ ¼ 2:44 I 2m 8p m n¼1 8p 2nðn!Þ 0
¼
2 eff
V Rr þ RLED
ð11Þ
and V eff ¼ 0:3117I m
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gðRr þ RLED Þ ¼ c1 I m gðRr þ RLED Þ ð12Þ
wherec1 is a proportionality constant. Substituting Eq. (5b) into Eq. (12) yields the value of the induced interference voltage, Veff. The interference voltage is expressed as a root mean square value and varies as a function of the interference frequency, f. Specifically, the interference voltage (Veff) increases as the interference frequency (f) increases. If the dipole is assumed to be short, expanding the cosine function of Eq. (10) as a Taylor series, and neglecting the higher-order terms, yields the
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H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966
following expressions for the radiated power and the induced interference voltage: " #2 Z 2p Z p 2 1 I 2m 1 bL 2 g sin h r2 sin h dh d/ Pr ¼ 2g 0 ð2prÞ2 2 2 0 ¼
g 2 4 4 V 2eff I mb L ¼ 192p Rr þ RLED
n-region, the steady-state continuity equation reduces to [29]: d2 P n P n P no ¼0 dx2 Dp T p Hence
ð13Þ
and pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V eff ¼ 0:0407I m b2 L2 gðRr þ RLED Þ ¼ c2 I m b2 L2 gðRr þ RLED Þ ð14Þ
where c2 is a proportionality constant. Even though DZ L, Eqs. (1b), (12) and (14) show that the magnitude of the induced interference voltage varies in proportion to the interference frequency. Furthermore, Eq. (1b) shows that the induced current is inversely proportional to the LED resistance, RLED.
qv xxn P n P no ¼ P no ekT 1 e Lp
V
I d ¼ I s eC s V t
Hg ¼
ð15Þ
P rB
ð16Þ
In the interfering magnetic energy system shown in Fig. 3, the voltage is constantly changing, and hence the magnetic field induced in the circuit current is different from that produced in the ferromagnetic toroid. Consequently, an alternation may occur. However, when the alternation has stabilized, a corresponding frequency will be induced. This induced frequency determines the latetime damped resonance produced by the impedance loading of the LED circuit (see f1 and f2 in Table 2) and causes a corresponding interference voltage, Veff. It can be assumed that PrB has the same average power for every unit of time, and hence the coupling efficiency constant between the interference source and the CW can be defined as K¼
Pr P rB
ð17Þ
ð20Þ
In Eqs. (19) and (20), Is is the reverse saturation current, Cs is the ideal factor, Vt = KT/q is the equivalent voltage, q is the charge of an electron, K is the Boltzmann constant, T is the absolute temperature, and V is the forward bias or reverse bias voltage. If the noise voltage of the radio frequency (RF) interference is given by VRF sin xt, then it can be shown that I þRF ¼ I s e
As shown in Fig. 3, an interfering magnetic source is formed by applying a voltage to points A and B on the ferromagnetic toroid. The following expressions can be derived for the magnetic field density (Hg) and the flux density (PrB) in the air gap [25]:
ð19Þ
The total current generated in the diode for eV/Vt 1 is then given by [27]:
2.2. Interfering magnetic energy system
lNI 0 l0 ð2pr0 lg Þ þ llg Z Z 1 1 l0 H 2g dv ¼ H g Bg dv ¼ 2 2
ð18Þ
C s V þV RF sin xt Vt
V
¼ I s eC s V t e
V RF sin xt Vt
V eff
¼ I de V t
ð21Þ
Therefore, the induced current is given by V eff
DI ¼ I þRF I d ¼ I d ðe V t 1Þ
ð22Þ
where the pffiffieffective induced voltage is given by ffi V eff ¼ V RF = 2 and the magnitude of the EMI interference induced via the CW is a function of Veff and Id. Similarly, if the interfering source is MFI, the mean current is expressed as V
V eff
V eff
I þmf ¼ I s eCs V t e V t ¼ I d e V t
ð23Þ
and the induced current is given by V eff
DI ¼ I þmf I d ¼ I d ðe V t 1Þ
ð24Þ
where Veff is the voltage induced by the MFI via the CW. From Eqs. (22) and (24), it is noted that both I+RF and I+mf are proportional to the voltage induced via the CW, i.e. Veff. If two appropriate points, i.e. (V1, I1) and (V2, I2), are taken from the measured I–V curve, then from Eq. (20), it can be shown that: 1
1 1 1 Y
I s ¼ I 11Y I 2
ð25Þ
where Y = V1/V2. Eqs. (20)–(25) enable the characteristic I–V curves of the two LED devices to be simulated with and without EMI conditions, respectively. The simulated results can then be compared to the measured experimental results to verify the accuracy of the developed analytical models.
2.3. Interference of LED device 3. Results and discussion The ideal I–V characteristics of the p–n junction can be derived by assuming that no current is generated within the depletion region, i.e. all of the current originates from the neutral regions. Since there is no electric field in the neutral
During the experimental stage of the present investigation, currents were passed through the coil with frequencies ranging from 600 KHz to 1.2 MHz at a peak voltage of
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966
VP = 1 V, and at DC voltages of 0.3 V–0.7 V, respectively. As discussed in Section 2, the effects of EMI and MFI on the I–V characteristics of the present LEDs were investigated using a wire antenna as the inductive device. Therefore, the induced voltage is in the form of an anti–emf for the electromagnetic wave. Figs. 5 and 7 plot the measured I–V characteristic curves of the GaP and GaAsP LEDs, respectively. The measured values of the non-interfered I–V curves at V = 0.1 V and V = 2.2 V were substituted into Eqs. (20) and (25) to obtain the values of Cs and Is, respectively, for each diode, as shown in Table 1. Using these values of Cs and Is, a simulated I–V characteristic curve for the case of zero interference was generated for the GaP and GaAsP LEDs, as shown by Line 3 in Figs. 6 and 8, respectively. The Veff values for EMI and MFI interference were obtained by Eqs. (21) and (23) based on the measured data plotted in Figs. 5 and 7. The corresponding simulated I–V curves of the LED devices were then constructed using Eqs. (21) and (23) based on the value of Id determined from the simulated non-interference case. The simulated I–V curves for the GaP and GaAsP LED devices under EMI and MFI interference are presented in Figs. 6 and 8, respectively. Except
Fig. 5. Measured I–V characteristic curves of GaP LED with electromagnetic interference and magnetic interference.
Fig. 6. Simulated I–V curves of GaP LED with electromagnetic interference and magnetic interference.
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Fig. 7. Measured I–V characteristic curves of GaAsP LED with electromagnetic interference and magnetic interference.
Table 1 Results of measured curve fitting at V = 1.8 V with Eq. (21) ITEM
GaP-LED
GaAsP-LED
Is Cs Veff1 Veff2 Veff3 Veff4 DI1 DI2 DI3 DI4 K1 K2 K3 K4
1E6 0.110 2.654E3 6.441E4 1.046E3 2.135E3 3E5 7E6 1.1E5 2.2E5 6.729E4 2.075E4 9.536E5 3.892E4
1E6 0.113 3.551E3 2.037E3 8.113E3 1.330E2 1.94E4 1.08E4 3.55E4 5.3E4 5.822E3 1.012E2 2.386E2 5.841E2
1: Magnetic interference DCV = 0.7 V, PrB = 118.32 lw. 2: Magnetic interference DCV = 0.3 V, PrB = 21.73 lw. 3: Electromagnetic interference ACV VP = 1 V and f = 600 Hz, PrB = 120.7 lw. 4: Electromagnetic interference ACV VP = 1 V and f = 1.2 MHz, PrB = 120.7 lw. V: LED input voltage = 1.8 V.
Fig. 8. Simulated I–V curves of GaAsP LED with electromagnetic interference and magnetic interference.
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the range of above the cut-in voltage, Figs. 5–8 confirm that a good agreement exists between the simulated results and the experimental results in all cases. The agreement between the measured results and the simulated results is shown more clearly in Figs. 9 and 10, which plot both sets of results on the same figure for the GaP and GaAsP LEDs, respectively. As discussed above, Figs. 5 and 7 show the influence of the induced EMI and MFI on the measured characteristic curves of the GaP and GaAsP LEDs, respectively. As predicted by Faraday’s Law of Electromagnetic Induction, EMI reduces the diode current, while MFI increases the diode current. The current results demonstrate that the GaAsP LED (Fig. 7) is more susceptible to the effects of EMI than the GaP LED (Fig. 5). Additionally, the measured I–V curves show that the dynamic impedance of the GaP LED is greater than that of the GaAsP LED. This may be explained by considering that if the GaP compound of the III–V materials is required to glow red as in the current study, then the Zn (Zinc) and O (oxygen) elements should be doped. In this situation, the Zn dopant atoms replace the Ga, and the O atoms replace the original P.
Fig. 9. Comparison of measured and simulated I–V curves of GaP LED (shown in Figs. 5 and 6, respectively).
Fig. 10. Comparison of measured and simulated I–V curves of GaAsP LED (shown in Figs. 7 and 8, respectively).
When the two different dopant atoms lie on nearest neighbor sites, an isoelectronic trap is formed, with the result that the dynamic impedance increases. Since the magnitude of the induced voltage is different at each measured value, Table 1 takes only one case as an example to derive the value of the coupling efficiency constant, K. Note that in this table, the parameters of the induced effective voltage and current, and the coupling efficiency constants, are assigned subscripts of ‘‘1’’ to ‘‘4’’. These subscripts refer to the corresponding interference conditions, details of which are presented at the foot of the table. By defining the radiated power as Pr = Veff · DI, the value of the coupling efficiency, K, for each interference condition can be obtained from Eq. (17). The results presented in Table 1 show that the lower the input resistance of the LED device, the greater the value of DI and K. From Eq. (1b), it can be shown that the interference currents induced in the GaAsP LED are higher than those induced in the GaP LED. Moreover, an increasing interference frequency and an increasing interference amplitude both increase the magnitude of the anti–emf. Clearly, the results presented in Table 1 relate specifically to the two LEDs considered in this study. However, the theoretical and experimental method employed to establish these results is equally applicable to similar devices with different wavelengths. When the DCV interfering voltage increases, a variation in the magnetic flux density, Bg, of the ferromagnetic toroid generally results, and this induces a current flow. When Bg becomes constant, it may alternate with the circuit current that implies the induced magnetic field becomes alternate. According to Faraday’s law, when the alternation stabilizes, a corresponding noise frequency will be induced, which may well be in the order of several MHz. Therefore, when simulating the DCV interference, it is necessary to obtain the corresponding frequency value and different Im values associated with the known Veff. In the case of ACV interference, many parameters vary as a function of the frequency, and hence the frequency and Im values are all different even though the VP values are the same. Table 2 presents the simulated I–V characteristics for the current antenna conducting wire. Note that the results in this table are derived from the measured impedance values, the values of Veff, DI and K presented in Table 1, and Eq. (14). The results indicate that the value of parameter A is close to the theoretical value of 0.0407 and imply that the Veff values indicated in Table 1 can be simulated using Eq. (14) such that the same set of simulated I–V curves are obtained. To investigate whether the effects of high frequency EMI on the I–V characteristics of an LED are different from the low-frequency effects discussed above, this study investigated the effects of EMI on a typical LED used for optical fiber communications. During the investigation, a current was passed through the ferromagnetic toroid coil at various frequencies in the range 0 Hz to 1.8 GHz with a peak voltage of VP = 1 V. Fig. 11 shows the obtained results. It can
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966 Table 2 Results of measured curve fitting at V = 1.8 V with Eq. (14) ITEM
GaP-LED
GaAsP-LED
R1 R2 R3 R4 Im1 Im2 Im3 Im4 f1 f2 f3 f4 Veff1 Veff2 Veff3 Veff4 K1 K2 K3 K4 A1 A2 A3 A4
5844.16 6315.79 6741.57 7031.25 1.563E3 3.889E4 1.528E2 1.528E3 1.6e6 1.5e6 600 k 1.2e6 2.654E3 6.441E4 1.046E3 2.135E3 6.729E4 2.075E4 9.536E5 3.892E4 0.0409 0.0423 0.0669 0.0473
1188.9 1260.5 1865.28 2278.48 1.010E2 6E3 4.931E2 3.681E2 1.6e6 1.5e6 600 k 1.2e6 3.551E3 2.037E3 8.113E3 1.330E2 5.822E3 1.012E2 2.386E2 5.841E2 0.0188 0.0194 0.0306 0.0215
1: Magnetic interference DCV = 0.7 V, PrB = 118.32 lw. 2: Magnetic interference DCV = 0.3 V, PrB = 21.73 lw. 3: Electromagnetic interference ACV VP = 1 V, f = 600 Hz, PrB = 120.7 lw. 4: Electromagnetic interference ACV VP = 1 V and f = 1.2 MHz, PrB = 120.7 lw. V: LED input Voltage = 1.8 V, raluminum = 3.54E7, laluminum = lrl0 = l0 = 4pe7, L = 0.01 m, jgj = (2pfl/r)1/2, b = (pflr)1/2, for good conductors of CW. Veff = c2Imb2L2(jgjR)1/2, R = Rr + RLED, c2 = AK, theoretic value A = 0.0407 by Eq. (14). K is the coupling efficiency constant. f1,2 is the noise frequency for DCV Electromagnetic interference.
Fig. 11. Comparison of measured and simulated I–V curves of OFC LED with electromagnetic interference. Note that each pair of lines with an equivalent style relates to the same set of EMI conditions and that the upper line indicates the simulated results while the lower line indicates the experimental results.
be seen that the results are very similar to those of the low frequency EMI case. In other words, a higher interference
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frequency or higher interference amplitude increases the magnitude of the anti–emf voltage. 4. Conclusions This paper has performed experimental and analytical investigations into the influence of EMI and MFI on the I–V characteristic curves of GaP and GaAsP LED devices. Analytical models have been developed to simulate the influence of the interference. A good agreement has been found between the theoretical results and the experimental results. It has been shown that the magnitude of the interference induced via a CW is a function of f, RLED, Rr, Is, Cs, V, Pr, PrB and Veff. Moreover, the results have suggested that EMI has a greater effect on the GaAsP LED than on the GaP LED. It has been suggested that this is a consequence of their different process structures, which causes a difference in the input impedance of the two devices. From the results of the present study, it can be concluded that an increased interference frequency will increase the induced interference current. Furthermore, the results suggest that the smaller the input impedance of the device, the greater the influence of EMI. These conclusions are applicable to both low and high frequency electromagnetic interference radiation. Acknowledgement The current author wishes to acknowledge the invaluable assistance provided by Jing-Yen Lin, Gwo-Jong Horng and Jeng-Shian Yeh during the course of this study. References [1] Busatto G, Fratelli L, Abbate C, Mamzo R, Iannuzzo F. Analysis and optimisation through innovative driving strategy of high power IGBT performances/EMI reduction trade-off for converter systems in railway applications. Microelectron Reliab 2004;44(9–11):1443–8. [2] Carren˜o F, Anto´n MA, Caldero´n Oscar G. Quantum interference effects in resonance fluorescence and absorption spectra of a V-Type three-level atom damped by a broadband squeezed vacuum. Opt Commun 2003;221(4–6):365–85. [3] Hang CY, Mo WW. The effect of attached fragments on dense layer of electroless Ni/p deposition on the electromagnetic interference shielding effectiveness of carbon fiber/acrylonitrile–butadience–styrene composites. Surf Coat Tech 2002;154(1):55–62. [4] Poljak D. Electromagnetic modeling of finite length wires buried in a lossy half-space. Eng Anal Bound Elem 2002;26(1):81–6. [5] Kraz V, Wallash A. The effects of EMI from cell phones on GMR magnetic recording neads and test equipment. J Electrostat 2002; 54(1):39–53. [6] Rudack AC, Pendley M, Levit L. Measurement technique developed to evaluate transient EMI in a photo bay. J Electrostat 2002;54(1): 95–104. [7] Kim EA, Han EG. Effects of catalyst accelerator on electromagnetic shielding in non-electrolytic Cu-plated Fabrics. J App Phys 2000; 87(9):4984–6. [8] Joo J, Lee CY. High frequency electromagnetic interference shielding response of mixtures and multilayer films based on conducting polymers. J App phys 2000;88(1):513–8.
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An investigation on the influence of electromagnetic interference induced in conducting wire of universal LEDs Han-Chang Tsai
*
Department of Electronic Engineering, Cheng-Shiu University No. 840, Chenrg Ching Rd., Neau Song Township, Kaohsiung County 833, Taiwan, ROC Received 27 April 2006; received in revised form 14 June 2006 Available online 22 August 2006
Abstract Electromagnetic Interference (EMI) has a detrimental effect upon the performance of Optical-Fiber Communication (OFC) systems. The present study considers the case where EMI is induced in a conducting wire (CW), and derives equations to establish the influence of the induced EMI on GaP and GaAsP Light-Emitting Diodes (LEDs). These equations are then verified experimentally. The results indicate that the degree of influence of the EMI upon both LED devices depends upon the interference power, the interference frequency, the induced power, the input resistance of the device, the inverse saturation current, and the ideal factor of the LED. Moreover, it is found that the induced interference current increases with an increasing interference frequency and that the EMI has a greater influence on devices with a lower input impedance. The theoretical results are found to be in good agreement with the experimental data. 2006 Elsevier Ltd. All rights reserved.
1. Introduction LED devices have attracted intense research effort for many years now. Advances in electro-optics technology now enable optical integrated circuits and electronic circuits to be combined within complex systems. It is impossible to shield such systems completely from the effects of Electromagnetic Interference (EMI) [1–10], particularly since the configuration of the wiring within the electronic circuits is such that many of the wires act as straight antenna conducting wires. LEDs are fundamental components within an OFC system [11–17]. Consequently, the present study develops a theoretical approach to investigate the influence of EMI on the reliability of GaP and GaAsP LED devices. Of all the available semiconducting materials, GaAs and GaP, together with their ternary alloy, GaAs1yPy, are undoubtedly the most important III–V compounds. Consequently, in investigating the influence of EMI upon LED *
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devices, the current study focuses specifically on the characteristic curves of GaP and GaAsP diodes. Fig. 1 illustrates the basic structures of the GaP and GaAsP LED devices. The interaction processes between a photon and an electron in a solid can be classified as either absorption, spontaneous emission, or stimulated emission. LED devices are p–n junctions which emit spontaneous radiation under an appropriate bias. Of the two compounds considered in the present study, Gallium Arsenide (GaAs) is a direct-bandgap semiconductor (Eg = 1.44 eV), in which p–n junctions with high luminescent efficiencies are readily formed. Conversely, Gallium Phosphide (GaP) is an indirect-bandgap semiconductor (Eg = 2.2 eV), and hence band-to-band transitions do not generally occur. As shown in Fig. 1(a), direct-bandgap LEDs (which emit red light) are generally fabricated on a gallium arsenide substrate, while indirect-bandgap LEDs (which emit orange, yellow or green light) tend to be fabricated on a gallium phosphide substrate, as shown in Fig. 1(b). Group V elements, such as N (nitrogen) and Bi (bismuth), are commonly used as dopants to assist radiative recombination [18–24] by replacing the phosphorous atoms in the
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Fig. 2. Current carrying coil wound on ferromagnetic toroid with air gap.
Fig. 1. Structure of GaP(Red) and GaAsP(Yellow) LEDs.
lattice sites. Although the outer electronic structure of nitrogen is similar to that of phosphorous, the electronic core structures of these two atoms are quite different. This difference generates an electron trap close to the base of the conduction band. Hence, a recombination center, referred to as an ‘‘isoelectronic trap’’, is formed, which greatly increases the probability of radiative transition in indirect-bandgap semiconductors. The ternary GaAs1yPy (Gallium Arsenide Phosphide) alloy is also a commonly applied semiconductor material. This alloy is noteworthy because it changes from a direct-bandgap material (when y < 0.45) to an indirect-bandgap material when the band gap is approximately 2.1 eV. The intensity of the low frequency electromagnetic waves present in a typical indoor environment is greater than that of the high frequency electromagnetic waves. In this study, the intensities of these electromagnetic waves are measured using the CA40 (low frequency) and CA41 (high frequency) gaussmeters manufactured by Chauvin Arnoux (France). The intensity of the low frequency electromagnetic waves is found to be 0.03 lT, while that of the high frequency waves is just 2 V/m (approximately 0.0067 lT). Therefore, the present investigation concentrates primarily on the effects of low frequency electromagnetic wave interference on the current GaP and GaAsP LEDs and only briefly considers the case of high frequency electromagnetic wave interference. The results obtained for low and high frequency EMI are found to be the same.
magnetic toroid containing an air gap. Using the experimental setup shown in Fig. 3, a series of studies are performed to investigate the influence of EMI and Magnetic Field Interference (MFI) on an antenna conducting wire (CW) and to establish the effect of the induced power upon the current–voltage (I–V) characteristics of the current GaP and GaAsP LED devices. The experimental system is shielded in a metal case (not shown) to prevent the influence of external noise. The conducting wire (CW) is positioned in the air gap of the ferromagnetic toroid shown in Fig. 2. The EMI frequency ranges from 0 Hz to 1.8 GHz and the interference amplitude is varied between 0.3 V and 1 V. During the experiments, an AC or DC current is passed through the coil causing a magnetic or electrical energy field to be established within the air gap. The energy field induces a noise voltage within the CW, which is then re-coupled with the electric circuit system. The corresponding response of the circuit system is analyzed using the Superposition Theorem. This particular theorem is specifically chosen here since it enables the CW-induced interfer-
2. Theoretical analysis 2.1. Induced power in straight conducting wire As shown in Fig. 2, the EMI source considered in this study consists of a current-carrying coil wound on a ferro-
Fig. 3. Experimental setup of CW for EMI measurement.
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966
~/ , of the z-directed source can be expressed respectively H by [25,26]:
ΔI
~ Eh ¼ ^hjxl sin hAZ ^ sin hAZ ¼ ~ ~ / ¼ /jb H Eh =g
Veff Vin
RLED
Rr
Antenna Fig. 4. Equivalent circuit model for antenna–receiver combination used to measure field intensity.
ing voltage to be considered in isolation. Fig. 4 presents the equivalent circuit model for the antenna–receiver combination used to measure the field intensity. Note that the ohmic resistance of the CW is incorporated within the RLED term in this circuit model. The time-averaged power received can be written in the form [25–28]: P r ¼ DI 2 ðRr þ RLED Þ ¼ DIV eff ¼ DI ¼
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V eff Rr þ RLED
RLED ¼ Rohmic þ Rin DZ Rohmic ¼ Rs 2pa rffiffiffiffiffiffiffiffi xl0 Rs ¼ 2r
V 2eff Rr þ RLED
ð1aÞ ð1bÞ ð2Þ ð3Þ ð4Þ
where Veff and DI are the induced interference voltage and induced interference current in the LED, respectively; RLED is the resistance of the LED, and comprises the ohmic resistance of the CW (Rohmic) and the dynamic input resistance of the LED (Rin); a and DZ are the radius and length of the wire, respectively; Rs is the surface resistance; x is the angular frequency and is always equivalent to 2pf, where f indicates the frequency in hertz; l0 is the permeability of free space; r is the conductivity of the wire; and Rr is the radiation resistance of a short dipole (DZ < k/4), which from [25,26] is given by 2 DZ Rr ¼ 20p2 for 0 < DZ < k=4 ð5aÞ k 2:4 DZ Rr ¼ 24:7 p for k=4 < DZ < k=2 ð5bÞ k where k is the wavelength of the EMI. It is noted that the value of DZ is very small compared to that of the wavelength, i.e. DZ k. By applying the Maxwell Electromagnetic Field Equations, it is found that the wire antenna is left along the z-axis. Hence, the electric field, E~h , and the magnetic field,
ð6Þ ð7Þ
where AZ is the vector potential along the z-axis, h is the directional angle ofpthe ffiffiffiffiffiffiffi radiation wave with respect to the z-axis, and g ¼ l=e is the intrinsic impedance of the medium. The current experiments assume that the inductive conducting wire acts as a straight dipole antenna, in which the induced currents are equal in magnitude but opposite in direction. Furthermore, the distribution of the current along the length of the wire is assumed to be sinusoidal, and can therefore be expressed as L L IðZÞ ¼ I m sin b jZj ; jZj < ð8Þ 2 2 where Im is the maximum value of the induced current in the dipole antenna, L is the length of the antenna, and b = 2p/k is a phase constant. To obtain the dipole radiation pattern, it is first necessary to define the radiation integral for the z-directed wire antenna along the z-axis, i.e. Z L ejbr 2 AZ ¼ IðZÞe jbZ cos h dz ð9Þ 4pr L2 Substituting Eq. (8) into Eq. (9) gives: cos h cos bL ejbr cos bL 2 2 Im Eh ¼ jg sin h 2pr
ð10Þ
Since the length of an inductive wire antenna in a typical circuit may be as much as L = k/2, the radiated power for the case of bL ¼ p2 can be written as [26]: 2 Z 2p g 2 1 cos t Im dt Pr ¼ 8p t 0 2p k g 2 X t2g g nþ1 ¼ I ð1Þ ¼ 2:44 I 2m 8p m n¼1 8p 2nðn!Þ 0
¼
2 eff
V Rr þ RLED
ð11Þ
and V eff ¼ 0:3117I m
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi gðRr þ RLED Þ ¼ c1 I m gðRr þ RLED Þ ð12Þ
wherec1 is a proportionality constant. Substituting Eq. (5b) into Eq. (12) yields the value of the induced interference voltage, Veff. The interference voltage is expressed as a root mean square value and varies as a function of the interference frequency, f. Specifically, the interference voltage (Veff) increases as the interference frequency (f) increases. If the dipole is assumed to be short, expanding the cosine function of Eq. (10) as a Taylor series, and neglecting the higher-order terms, yields the
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following expressions for the radiated power and the induced interference voltage: " #2 Z 2p Z p 2 1 I 2m 1 bL 2 g sin h r2 sin h dh d/ Pr ¼ 2g 0 ð2prÞ2 2 2 0 ¼
g 2 4 4 V 2eff I mb L ¼ 192p Rr þ RLED
n-region, the steady-state continuity equation reduces to [29]: d2 P n P n P no ¼0 dx2 Dp T p Hence
ð13Þ
and pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V eff ¼ 0:0407I m b2 L2 gðRr þ RLED Þ ¼ c2 I m b2 L2 gðRr þ RLED Þ ð14Þ
where c2 is a proportionality constant. Even though DZ L, Eqs. (1b), (12) and (14) show that the magnitude of the induced interference voltage varies in proportion to the interference frequency. Furthermore, Eq. (1b) shows that the induced current is inversely proportional to the LED resistance, RLED.
qv xxn P n P no ¼ P no ekT 1 e Lp
V
I d ¼ I s eC s V t
Hg ¼
ð15Þ
P rB
ð16Þ
In the interfering magnetic energy system shown in Fig. 3, the voltage is constantly changing, and hence the magnetic field induced in the circuit current is different from that produced in the ferromagnetic toroid. Consequently, an alternation may occur. However, when the alternation has stabilized, a corresponding frequency will be induced. This induced frequency determines the latetime damped resonance produced by the impedance loading of the LED circuit (see f1 and f2 in Table 2) and causes a corresponding interference voltage, Veff. It can be assumed that PrB has the same average power for every unit of time, and hence the coupling efficiency constant between the interference source and the CW can be defined as K¼
Pr P rB
ð17Þ
ð20Þ
In Eqs. (19) and (20), Is is the reverse saturation current, Cs is the ideal factor, Vt = KT/q is the equivalent voltage, q is the charge of an electron, K is the Boltzmann constant, T is the absolute temperature, and V is the forward bias or reverse bias voltage. If the noise voltage of the radio frequency (RF) interference is given by VRF sin xt, then it can be shown that I þRF ¼ I s e
As shown in Fig. 3, an interfering magnetic source is formed by applying a voltage to points A and B on the ferromagnetic toroid. The following expressions can be derived for the magnetic field density (Hg) and the flux density (PrB) in the air gap [25]:
ð19Þ
The total current generated in the diode for eV/Vt 1 is then given by [27]:
2.2. Interfering magnetic energy system
lNI 0 l0 ð2pr0 lg Þ þ llg Z Z 1 1 l0 H 2g dv ¼ H g Bg dv ¼ 2 2
ð18Þ
C s V þV RF sin xt Vt
V
¼ I s eC s V t e
V RF sin xt Vt
V eff
¼ I de V t
ð21Þ
Therefore, the induced current is given by V eff
DI ¼ I þRF I d ¼ I d ðe V t 1Þ
ð22Þ
where the pffiffieffective induced voltage is given by ffi V eff ¼ V RF = 2 and the magnitude of the EMI interference induced via the CW is a function of Veff and Id. Similarly, if the interfering source is MFI, the mean current is expressed as V
V eff
V eff
I þmf ¼ I s eCs V t e V t ¼ I d e V t
ð23Þ
and the induced current is given by V eff
DI ¼ I þmf I d ¼ I d ðe V t 1Þ
ð24Þ
where Veff is the voltage induced by the MFI via the CW. From Eqs. (22) and (24), it is noted that both I+RF and I+mf are proportional to the voltage induced via the CW, i.e. Veff. If two appropriate points, i.e. (V1, I1) and (V2, I2), are taken from the measured I–V curve, then from Eq. (20), it can be shown that: 1
1 1 1 Y
I s ¼ I 11Y I 2
ð25Þ
where Y = V1/V2. Eqs. (20)–(25) enable the characteristic I–V curves of the two LED devices to be simulated with and without EMI conditions, respectively. The simulated results can then be compared to the measured experimental results to verify the accuracy of the developed analytical models.
2.3. Interference of LED device 3. Results and discussion The ideal I–V characteristics of the p–n junction can be derived by assuming that no current is generated within the depletion region, i.e. all of the current originates from the neutral regions. Since there is no electric field in the neutral
During the experimental stage of the present investigation, currents were passed through the coil with frequencies ranging from 600 KHz to 1.2 MHz at a peak voltage of
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966
VP = 1 V, and at DC voltages of 0.3 V–0.7 V, respectively. As discussed in Section 2, the effects of EMI and MFI on the I–V characteristics of the present LEDs were investigated using a wire antenna as the inductive device. Therefore, the induced voltage is in the form of an anti–emf for the electromagnetic wave. Figs. 5 and 7 plot the measured I–V characteristic curves of the GaP and GaAsP LEDs, respectively. The measured values of the non-interfered I–V curves at V = 0.1 V and V = 2.2 V were substituted into Eqs. (20) and (25) to obtain the values of Cs and Is, respectively, for each diode, as shown in Table 1. Using these values of Cs and Is, a simulated I–V characteristic curve for the case of zero interference was generated for the GaP and GaAsP LEDs, as shown by Line 3 in Figs. 6 and 8, respectively. The Veff values for EMI and MFI interference were obtained by Eqs. (21) and (23) based on the measured data plotted in Figs. 5 and 7. The corresponding simulated I–V curves of the LED devices were then constructed using Eqs. (21) and (23) based on the value of Id determined from the simulated non-interference case. The simulated I–V curves for the GaP and GaAsP LED devices under EMI and MFI interference are presented in Figs. 6 and 8, respectively. Except
Fig. 5. Measured I–V characteristic curves of GaP LED with electromagnetic interference and magnetic interference.
Fig. 6. Simulated I–V curves of GaP LED with electromagnetic interference and magnetic interference.
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Fig. 7. Measured I–V characteristic curves of GaAsP LED with electromagnetic interference and magnetic interference.
Table 1 Results of measured curve fitting at V = 1.8 V with Eq. (21) ITEM
GaP-LED
GaAsP-LED
Is Cs Veff1 Veff2 Veff3 Veff4 DI1 DI2 DI3 DI4 K1 K2 K3 K4
1E6 0.110 2.654E3 6.441E4 1.046E3 2.135E3 3E5 7E6 1.1E5 2.2E5 6.729E4 2.075E4 9.536E5 3.892E4
1E6 0.113 3.551E3 2.037E3 8.113E3 1.330E2 1.94E4 1.08E4 3.55E4 5.3E4 5.822E3 1.012E2 2.386E2 5.841E2
1: Magnetic interference DCV = 0.7 V, PrB = 118.32 lw. 2: Magnetic interference DCV = 0.3 V, PrB = 21.73 lw. 3: Electromagnetic interference ACV VP = 1 V and f = 600 Hz, PrB = 120.7 lw. 4: Electromagnetic interference ACV VP = 1 V and f = 1.2 MHz, PrB = 120.7 lw. V: LED input voltage = 1.8 V.
Fig. 8. Simulated I–V curves of GaAsP LED with electromagnetic interference and magnetic interference.
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the range of above the cut-in voltage, Figs. 5–8 confirm that a good agreement exists between the simulated results and the experimental results in all cases. The agreement between the measured results and the simulated results is shown more clearly in Figs. 9 and 10, which plot both sets of results on the same figure for the GaP and GaAsP LEDs, respectively. As discussed above, Figs. 5 and 7 show the influence of the induced EMI and MFI on the measured characteristic curves of the GaP and GaAsP LEDs, respectively. As predicted by Faraday’s Law of Electromagnetic Induction, EMI reduces the diode current, while MFI increases the diode current. The current results demonstrate that the GaAsP LED (Fig. 7) is more susceptible to the effects of EMI than the GaP LED (Fig. 5). Additionally, the measured I–V curves show that the dynamic impedance of the GaP LED is greater than that of the GaAsP LED. This may be explained by considering that if the GaP compound of the III–V materials is required to glow red as in the current study, then the Zn (Zinc) and O (oxygen) elements should be doped. In this situation, the Zn dopant atoms replace the Ga, and the O atoms replace the original P.
Fig. 9. Comparison of measured and simulated I–V curves of GaP LED (shown in Figs. 5 and 6, respectively).
Fig. 10. Comparison of measured and simulated I–V curves of GaAsP LED (shown in Figs. 7 and 8, respectively).
When the two different dopant atoms lie on nearest neighbor sites, an isoelectronic trap is formed, with the result that the dynamic impedance increases. Since the magnitude of the induced voltage is different at each measured value, Table 1 takes only one case as an example to derive the value of the coupling efficiency constant, K. Note that in this table, the parameters of the induced effective voltage and current, and the coupling efficiency constants, are assigned subscripts of ‘‘1’’ to ‘‘4’’. These subscripts refer to the corresponding interference conditions, details of which are presented at the foot of the table. By defining the radiated power as Pr = Veff · DI, the value of the coupling efficiency, K, for each interference condition can be obtained from Eq. (17). The results presented in Table 1 show that the lower the input resistance of the LED device, the greater the value of DI and K. From Eq. (1b), it can be shown that the interference currents induced in the GaAsP LED are higher than those induced in the GaP LED. Moreover, an increasing interference frequency and an increasing interference amplitude both increase the magnitude of the anti–emf. Clearly, the results presented in Table 1 relate specifically to the two LEDs considered in this study. However, the theoretical and experimental method employed to establish these results is equally applicable to similar devices with different wavelengths. When the DCV interfering voltage increases, a variation in the magnetic flux density, Bg, of the ferromagnetic toroid generally results, and this induces a current flow. When Bg becomes constant, it may alternate with the circuit current that implies the induced magnetic field becomes alternate. According to Faraday’s law, when the alternation stabilizes, a corresponding noise frequency will be induced, which may well be in the order of several MHz. Therefore, when simulating the DCV interference, it is necessary to obtain the corresponding frequency value and different Im values associated with the known Veff. In the case of ACV interference, many parameters vary as a function of the frequency, and hence the frequency and Im values are all different even though the VP values are the same. Table 2 presents the simulated I–V characteristics for the current antenna conducting wire. Note that the results in this table are derived from the measured impedance values, the values of Veff, DI and K presented in Table 1, and Eq. (14). The results indicate that the value of parameter A is close to the theoretical value of 0.0407 and imply that the Veff values indicated in Table 1 can be simulated using Eq. (14) such that the same set of simulated I–V curves are obtained. To investigate whether the effects of high frequency EMI on the I–V characteristics of an LED are different from the low-frequency effects discussed above, this study investigated the effects of EMI on a typical LED used for optical fiber communications. During the investigation, a current was passed through the ferromagnetic toroid coil at various frequencies in the range 0 Hz to 1.8 GHz with a peak voltage of VP = 1 V. Fig. 11 shows the obtained results. It can
H.-C. Tsai / Microelectronics Reliability 47 (2007) 959–966 Table 2 Results of measured curve fitting at V = 1.8 V with Eq. (14) ITEM
GaP-LED
GaAsP-LED
R1 R2 R3 R4 Im1 Im2 Im3 Im4 f1 f2 f3 f4 Veff1 Veff2 Veff3 Veff4 K1 K2 K3 K4 A1 A2 A3 A4
5844.16 6315.79 6741.57 7031.25 1.563E3 3.889E4 1.528E2 1.528E3 1.6e6 1.5e6 600 k 1.2e6 2.654E3 6.441E4 1.046E3 2.135E3 6.729E4 2.075E4 9.536E5 3.892E4 0.0409 0.0423 0.0669 0.0473
1188.9 1260.5 1865.28 2278.48 1.010E2 6E3 4.931E2 3.681E2 1.6e6 1.5e6 600 k 1.2e6 3.551E3 2.037E3 8.113E3 1.330E2 5.822E3 1.012E2 2.386E2 5.841E2 0.0188 0.0194 0.0306 0.0215
1: Magnetic interference DCV = 0.7 V, PrB = 118.32 lw. 2: Magnetic interference DCV = 0.3 V, PrB = 21.73 lw. 3: Electromagnetic interference ACV VP = 1 V, f = 600 Hz, PrB = 120.7 lw. 4: Electromagnetic interference ACV VP = 1 V and f = 1.2 MHz, PrB = 120.7 lw. V: LED input Voltage = 1.8 V, raluminum = 3.54E7, laluminum = lrl0 = l0 = 4pe7, L = 0.01 m, jgj = (2pfl/r)1/2, b = (pflr)1/2, for good conductors of CW. Veff = c2Imb2L2(jgjR)1/2, R = Rr + RLED, c2 = AK, theoretic value A = 0.0407 by Eq. (14). K is the coupling efficiency constant. f1,2 is the noise frequency for DCV Electromagnetic interference.
Fig. 11. Comparison of measured and simulated I–V curves of OFC LED with electromagnetic interference. Note that each pair of lines with an equivalent style relates to the same set of EMI conditions and that the upper line indicates the simulated results while the lower line indicates the experimental results.
be seen that the results are very similar to those of the low frequency EMI case. In other words, a higher interference
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frequency or higher interference amplitude increases the magnitude of the anti–emf voltage. 4. Conclusions This paper has performed experimental and analytical investigations into the influence of EMI and MFI on the I–V characteristic curves of GaP and GaAsP LED devices. Analytical models have been developed to simulate the influence of the interference. A good agreement has been found between the theoretical results and the experimental results. It has been shown that the magnitude of the interference induced via a CW is a function of f, RLED, Rr, Is, Cs, V, Pr, PrB and Veff. Moreover, the results have suggested that EMI has a greater effect on the GaAsP LED than on the GaP LED. It has been suggested that this is a consequence of their different process structures, which causes a difference in the input impedance of the two devices. From the results of the present study, it can be concluded that an increased interference frequency will increase the induced interference current. Furthermore, the results suggest that the smaller the input impedance of the device, the greater the influence of EMI. These conclusions are applicable to both low and high frequency electromagnetic interference radiation. Acknowledgement The current author wishes to acknowledge the invaluable assistance provided by Jing-Yen Lin, Gwo-Jong Horng and Jeng-Shian Yeh during the course of this study. References [1] Busatto G, Fratelli L, Abbate C, Mamzo R, Iannuzzo F. Analysis and optimisation through innovative driving strategy of high power IGBT performances/EMI reduction trade-off for converter systems in railway applications. Microelectron Reliab 2004;44(9–11):1443–8. [2] Carren˜o F, Anto´n MA, Caldero´n Oscar G. Quantum interference effects in resonance fluorescence and absorption spectra of a V-Type three-level atom damped by a broadband squeezed vacuum. Opt Commun 2003;221(4–6):365–85. [3] Hang CY, Mo WW. The effect of attached fragments on dense layer of electroless Ni/p deposition on the electromagnetic interference shielding effectiveness of carbon fiber/acrylonitrile–butadience–styrene composites. Surf Coat Tech 2002;154(1):55–62. [4] Poljak D. Electromagnetic modeling of finite length wires buried in a lossy half-space. Eng Anal Bound Elem 2002;26(1):81–6. [5] Kraz V, Wallash A. The effects of EMI from cell phones on GMR magnetic recording neads and test equipment. J Electrostat 2002; 54(1):39–53. [6] Rudack AC, Pendley M, Levit L. Measurement technique developed to evaluate transient EMI in a photo bay. J Electrostat 2002;54(1): 95–104. [7] Kim EA, Han EG. Effects of catalyst accelerator on electromagnetic shielding in non-electrolytic Cu-plated Fabrics. J App Phys 2000; 87(9):4984–6. [8] Joo J, Lee CY. High frequency electromagnetic interference shielding response of mixtures and multilayer films based on conducting polymers. J App phys 2000;88(1):513–8.
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