An investigation into EMI-induced noise in nanometer multi-quantum well InGaN LEDs

Optics Communications 273 (2007) 311–319 www.elsevier.com/locate/optcom

An investigation into EMI-induced noise in nanometer multi-quantum well InGaN LEDs Han-Chang Tsai

*

Department of Electronic Engineering, Cheng-Shiu University, No. 840 Cherng-Ching Road, Niao-Sung, Kaohsiung County 83305, Taiwan Received 15 March 2006; received in revised form 9 January 2007; accepted 17 January 2007

Abstract This study investigates the low-frequency noise induced by electromagnetic radiation interference (EMI) in a nanometer multi-quantum well InGaN LED (NMQLED). Theoretical models of the noise spectra and the EMI are constructed. In general, a good agreement is identified between the experimental and theoretical results. Both sets of results reveal that the magnitude of the EMI-induced noise is related to the pulse height, the output load, the parasitic capacitance, the interference frequency and the interference amplitude. It is shown that the harmonic noise increases with an increasing interference amplitude and frequency. The techniques presented in this study provide a systematic approach for obtaining the interference noise and signal-to-noise ratio (SNR) in LEDs and similar wavelengthbased semiconductor devices.  2007 Elsevier B.V. All rights reserved. Keywords: Noise; Harmonic wave; Nanometer; Multi-quantum well; InGaN LED; Radiated power; Conducting wire; EMI; SNR

1. Introduction Nanometer technology [1–7] has attracted intensive research efforts in recent years [8–12] and will undoubtedly continue to do so in the coming decades. The nanotechnology field encompasses the fundamental principles of various scientific domains, including those of physics, chemistry, biology and materials science. The dimensions of industrial and commercial products are reducing on a seemingly continuous basis. Nanotech products offer many advantages, including: (1) reduced material and space requirements; (2) reduced energy consumption during production and operation; (3) lower energy losses and heating effects; and (4) smaller structures with enhanced functionality. The trend toward device miniaturization is particularly evident in the information industry, in which the internal structures of semiconductor elements are typically less than *

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0030-4018/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.01.038

65 nm. The characteristics of these semiconductor elements are significantly influenced by electrical, magnetic and thermal effects. These effects become particularly pronounced as the density of the electronic components and integrated circuits within these semiconductor elements increases. Therefore, it is essential to develop a thorough understanding of the physical properties and principles which govern their operation at the nanometer scale. With the proliferation of electronic appliances nowadays, and the increasing use of wireless communications for a variety of purposes, the environment is becoming saturated with electromagnetic signals. Irrespective of whether these electromagnetic signals are generated by electronic products or by wireless communication devices, the resulting electronic noise influences the performance of nearby electronic apparatus and results in a phenomenon known as electromagnetic interference (EMI). The effect of EMI becomes increasingly severe as the characteristic size of the semiconductor components used in these apparatus reduces. Accordingly, a branch of electrical science has emerged whose goals include studying the generation of

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H.-C. Tsai / Optics Communications 273 (2007) 311–319

EMI and exploring potential methods for reducing the electromagnetic susceptibility (EMS) of these products. To safeguard public health and to ensure the normal operation of electronic products, many countries around the world have introduced norms and testing standards for EMI. These standards include, but are not restricted to, professional norms (e.g. STD, FCC, SAE, IEC, ISO and ESD), European and American norms (e.g. CISPR, VDE and EN), and Taiwanese norms (i.e. CNS). CISPR standards 13, 14, 22, 21 and 24 and EN standards 55013, 55014, 55022, 55021 and 55024 provide EMC norms for product families in the IA (Information Appliance) and ISM (Industrial Scientific and Medical) fields, and cover such diverse issues as conducted emissions, disturbance power, harmonic current emissions, voltage fluctuation and flicker, immunity, test sites, modus operandi, test equipment, and so on. Light emitting diodes (LEDs) have been commercialized since the 1960s. LEDs are characterized by a high shock toleration, long life, low power consumption and low heat release, and are applied in a wide variety of applications, including printers, scanners, sensors, optical communication devices, pilot lamps, traffic lights, large-scale illuminated panels, backlight modules in LCD computer monitors, and so forth. In many of these applications, it is desirable to maximize the light output of the photoelectric elements in the LED. In general, the light output varies as a function of the input current. Therefore, stabilizing the input current is an essential concern in LED circuit design. In practice, it is virtually impossible to fully shield semiconductor devices such as LEDs from the effects of EMI. Therefore, the objective of the current study is to investigate the effect of EMI on the input current of LED devices. In a normal indoor environment, the intensity of the low frequency electromagnetic waves is greater than that of the high frequency electromagnetic waves, i.e. 0.03 and 0.0067 lT, respectively. (Note that the intensity data reported here were measured by the current authors in an indoor environment using CA40/43 instrumentation (Chauvin Arnoux, France)). Accordingly, this paper focuses primarily on the effects of low frequency electromagnetic wave interference on LED devices [13–18]. The theoretical and experimental investigations conducted in this study consider the particular case of the effect of EMI on the noise induced in a nanometer quantum well InGaN LED (NMQLED). However, the current approach is equally suitable for determining the characteristic influence of EMI on the noise spectrum of any wavelengthbased semiconductor device. The methods and results presented in this study are intended primarily as a reference for electronic engineers involved in the design of LED circuits for the backlit modules of LCD display units. 2. Experiments Fig. 1 shows the experimental set-up used to measure the induced interference noise spectrum. This study

A

Shielding B

CW Printer

Low Noise Amplifier R1 VDC

C

DUT LED

C R

Oscilloscope or Signal Analyzer

Personal Computer

Fig. 1. Experimental set-up for noise measurement.

investigates the noise characteristics of a NQWLED when interfered with by a conducting wire (CW) positioned in the air gap of a ferromagnetic toroid. In the current experiments, an EMI source was simulated by applying a voltage across points A and B of the coil wound on the ferromagnetic toroid. The resulting magnetic field in the air gap induced a current in the conducting wire, which was amplified by a 66 dB low-noise pre-amplifier such that it could be detected by an oscilloscope and then connected in series with the LED circuit. In other words, the EMI interference was detected initially as radiated noise and was then coupled in series with the LED circuit as conducted noise. The EMI signals were transmitted to the oscilloscope via the circuit coupling and were then passed to a spectrum analyzer to generate the time domain and spectrum plots of the interference signals. During the experiments, the EMI frequency was varied from 200 to 900 Hz and the interference amplitude was varied between 0.2 and 1.6 V. The measurement system was shielded in a metal case to suppress the effects of external noise. Both the low noise amplifier and the VDC source were battery-powered. The signal analyzer (model HP E4440A) was controlled by a PC via an IEEE-488 bus. 3. Theoretical analysis 3.1. EMI-induced power spectrum Noise is generally investigated in terms of 1/f noise, 1/f1.5 noise or traps [19–27], or by the surface thermal noise model [28]. This study uses the above noise theorem to investigate the power spectrum induced by the electromagnetic radiation interference in the measurement system shown in Fig. 1. When the CW induces a pulse voltage in the LED circuit, it generates a noise spectrum of the form shown in Fig. 2 with a periodic pulse of period T. This response pulse can be analyzed by taking the discrete Fourier transformation of function A (the pulse height), i.e.

H.-C. Tsai / Optics Communications 273 (2007) 311–319 0.1

R1

0.08

C

C

MEASUREMENT SIMULATION

0.06 0.04

Voltage

313

Veff

Cd

rd

R Signal Analyzer

0.02 0 -0.02 -0.04

Fig. 4. Equivalent circuit of experimental system.

-0.06 -0.08 -0.1

-T0

0

T0

Seconds Fig. 2. Measurement and simulation results for typical periodic pulse function generated by periodic EMI signal with period T.

Z 0  Z T0 1 AeaðT 0 þtÞ ejxnt dt þ Aeat ejxnt dt T T 0 0 A a  jxn  aT 0 ¼ ð1 þ e Þðcos xnT 0  1Þ T a2 þ x 2 n2  ð1Þ þið1  eaT 0 Þ sin xnT 0 2   A 2 ð1 þ e2aT 0  2eaT 0 Þð1  cos xnT 0 Þ S 2n ¼ 2 2 ð2Þ T ða þ x2 n2 Þ Sn ¼

where n is an integer, x ¼ 2pf ¼ 2p=T , a is the attenuation factor of the exponential function, and A is the amplitude of the noise pulse generated by the CW-induced EMI. The amplitude spectrum of the EMI current can be obtained by plotting Sn against discrete frequencies, xn. The square of Sn has dimensions A2 and corresponds to the current power spectrum S ik ðfn Þ over 2T [28].

Let C d ¼ C in the Fig: 5

ð3Þ

where DVc is the variation in the voltage across the capacitor, and RL is the output load. Taking the Fourier series expansions of DVc and ik gives: 1 ik 1 X ¼ an expðjxn tÞ ð4Þ C C n¼1 dðDV cn Þ DV cn 1 þ ¼ an expðjxn tÞ dt RL C C

ð5Þ

DV cn ¼ bn expðjxn tÞ

Active region (MQW):5pairs

3 4 5 6

dDV c DV c ik þ ¼ dt RL C C

From which it can be shown that:

Blue

1 2

The current experiments were performed using an InGaN LED with a wavelength of 555 lm. As shown in Fig. 3, this LED has a five-layer quantum well, with a thickness of between 2 and 3 nm. Fig. 4 shows the equivalent circuit of the LED device under the imposed EMI effect. Applying Norton’s theorem, the circuit can be simplified to the form shown in Fig. 5, in which the current induced by the EMI is represented by ik. From Kirchhoff’s current law, the circuit equation is given by

Name p layer

Material

Thickness

Mg doped Gan

0.2 µm

MQW layer (5pair)

GaN barrier

InGaN well Si doped GaN

n layer Buffer layer Nucleation layer substrate

where:

0.1-0.15 µm

bn ¼

2 µm

Undoped GaN

1 µm

Low temp. GaN

30 µm

sapphire

430 µm

ð6Þ

a n RL 1 þ jxn RL C

ð7Þ

Thus, the noise power spectrum, S DV c ðfn Þ, of the diode voltage induced by the EMI is given by:

Device test structure ~0.2 µm

Mg doped GaN

P layer

InGaN well (20~30Å) GaN barrier (70~180Å) Si doped GaN

i =Veff/R1

iL

iC

N layer

Cd 1~2 µm ~1µm ~50µm ~320µm

RL

Undoped GaN buffer layer Low temp. GaN nucleation layer Sapphire substrate

Fig. 3. Structure of InGaN LED.

Fig. 5. Simplified diagram obtained by applying Norton’s theorem to Fig. 4.

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H.-C. Tsai / Optics Communications 273 (2007) 311–319

S DV c ðfn Þ ¼ 2T bn bn S DV c ðfn Þ ¼ S ik ðfn Þ

ð8Þ R2L

ð9Þ

1 þ xnRL C

where: S ik ðfn Þ ¼ 2T an an ¼ 2TS 2n

ð10Þ

Fig. 6 presents the equivalent circuit model for the antenna–receiver combination used to measure the field intensity in the current study. Note that the ohmic resistance of the CW is incorporated within the RLED term of the model. The time-averaged power received can be written in the form [17,29,30]:

From Eqs. (4) and (9), it can be shown that: 2

S DV c ðfn Þ ¼ 2T

A

P r ¼ DI 2 ðRr þ RLED Þ ¼ DIV eff ¼

R2L 2

1 þ ðx1 nRL CÞ  1 2 2aT 0 aT 0  2 2 ð1 þ e  2e Þð1  cos xnT Þ 0 T a þ x 2 n2 

V eff Rr þ RLED RLED ¼ Rohmic þ Rin DZ Rohmic ¼ Rs 2pr1 rffiffiffiffiffiffiffiffi xl0 Rs ¼ 2r DI ¼

ð11Þ

The total noise power can then be obtained by summing S DV c ðfn Þ over all possible integers, n. In order to identify the relative magnitudes of the various harmonic components, this study initially determines the value of A (the pulse height) from the measured power spectral intensity of the fundamental harmonic and then evaluates the power spectral intensities of the higher-order harmonics. 3.2. Electromagnetic interference As shown in Fig. 1, the interference source in this study takes the form of a current-carrying coil wound on a ferromagnetic toroid with an air gap. A conducting wire (CW) was positioned in the air gap to induce an EMI voltage of magnitude Veff. The magnetic field density and flux density in the air gap are given, respectively, by [29]: lNI 0 l0 ð2pr0  lg Þ þ llg Z Z 1 1 2 ¼ l0 H g dv ¼ H g Bg dv 2 2

Hg ¼

ð12Þ

P rB

ð13Þ

In Eqs. (12) and (13), l0 is the permeability of free space, l is the permeability of the ferromagnetic material, I0 is the current flowing within the coil, r0 is the mean radius of the toroid, lg is the width of the narrow air gap, and PrB is the magnetic energy in the air gap. The value of PrB represents the average energy per second of the interference on the CW, and can either be calculated from Eq. (13) or measured directly.

RLED

Rr

Antenna

Receiver

Fig. 6. Equivalent circuit model for antenna–receiver combination used to measure field intensity.

ð15Þ ð16Þ ð17Þ ð18Þ

where k is the wavelength of the EMI. The value of DZ is very small compared to that of the wavelength, i.e. DZ  k. By applying Maxwell’s electromagnetic field equations, it can be shown that the wire antenna is left along the Z-axis. The radiated power and the CW-induced interference voltage, Veff, can be written, respectively, as [17,30]: "   #2 Z 2p Z p 2 1 I 2m 1 bL 2 g sin h r2 sin h dh Pr ¼ 2g 0 ð2prÞ2 2 2 0

V eff Veff

ð14Þ

In Eqs. (14)–(18), Veff and DI are the interference voltage and interference current induced in the device, respectively, and can be obtained directly by measurement as shown in Fig. 2; RLED is the input resistance of the device, comprising the ohmic resistance of the coil (Rohmic) and the dynamic input resistance of the device (Rin); r1 is the radius of the coil; DZ is its length; Rs is the surface resistance; x is the angular frequency 2pf, where f is the frequency in Hertz; l0 is the permeability of free space; r is the conductivity of the conducting wire; and Rr is the radiation resistance of a short dipole [17,29,30]. It is noted that:  2 2 DZ for 0 < DZ < k=4 ð19Þ Rr ¼ 20p k  2:4 DZ Rr ¼ 24:7 p for k=4 < DZ < k=2 ð20Þ k

g 2 4 4 V 2eff I mb L ¼ 192p Rr þ RLED pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ¼ 0:0407I m b2 L2 gðRr þ RLED Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ c2 I m b2 L2 gðRr þ RLED Þ

d/ ¼

Iin

Vin

V 2eff Rr þ RLED

ð21Þ

ð22Þ

where c2 is a proportionality constant, b is the phase constant, L is the length of the dipole, g is the intrinsic impedance of the medium, Im is the maximum induced current within the dipole, and RLED is the input resistance. From Eqs. (19) and (22), it can be shown that the magnitude of the induced interference is proportional to the interference frequency, even when DZ  L. Eq. (15) shows that the

H.-C. Tsai / Optics Communications 273 (2007) 311–319 C

i

Y¼

1 ðRx þ jnxLÞ jnxC x

magnitude of DI is inversely proportional to RLED, i.e. the lower the value of RLED, the higher the magnitude of DI. Eq. (22) is derived by considering the correlation between the electromagnetic field density of the ferromagnetic toroid and the conducting wire antenna. Moreover, a function of the inductance and the capacitance of the ferromagnetic toroid can be used to derive the theoretic equation corresponding to Fig. 7, as described in the following. Any periodic signal can be represented by the following complex Fourier series: 1 1 X X V ðtÞ ¼ C n ejnxt ¼ C 0 þ 2jC n j cosðnxt þ \C n Þ ð23Þ n¼1

where: Cn ¼

1 T

Z

T 2

1 R2x þ n2 x2 L2

V ðtÞejnxt dt

ð24Þ

T2

The experimental input signal of the ferromagnetic toroid has the form of a periodic square wave, as shown in Fig. 8. In this case, the following Fourier transform applies: Z 1 t0 V 0 t0 sin 12 nxt0 j1nxt0 Cn ¼ V 0 ðtÞejnxt dt ¼ e 2 ð25Þ 1 T 0 T nxt0 2 and V 0 t0 sin 12 nxt0 1 T nxt0 2 1 \C n ¼ nxt0 2 Substituting Eqs. (26) and (27) into Eq. (23) gives: 1 V0 X np

T þ K n cos nxt0 þ for t0 ¼ V ðtÞ ¼ 2 2 2 n¼1 Kn ¼

ð29Þ

ð30Þ

n¼1

Fig. 7. Equivalent circuit of ferromagnetic toroid.

n¼1

¼

 ½Rx þ jðnxR2x C x  nxL þ n3 x3 L2 C x Þ ¼ jY j\h1 IðjnxÞ ¼ YV ðtÞ ¼ C 0 Y 0 1 X þ 2K n jY jcosðnxt þ \C n þ h1 Þ

R

L

1 ðRx þ jnxLÞ þ jnxC x

315

ð26Þ ð27Þ

ð28Þ

From Eq. (12), it can be shown that:   Z 1 1 l0 lNIðjxLÞ ~ B  d~ s¼ V eff ¼   Dzd ð31Þ dt s dt l0 ð2pr0  lg Þ þ llg 1 X l0 lN ðDzdÞ 2K n jY jnxsinðnxt þ \C n þ h1 Þ ð32Þ ¼ l0 ð2pr0  lg Þ þ llg n¼1 1 X ¼ 2KK n jY jnxsinðnxt þ \C n þ h1 Þ n¼1

K¼

l0 lN Dzd l0 ð2pr0  lg Þ þ llg

ð33Þ

Fig. 9 shows the equivalent circuit obtained by applying Thevenin’s theorem to the circuit shown in Fig. 4. The Vout signal input into the oscilloscope or signal analyzer is then given by V out ¼ V th

Ri K 1 K 3 \h3 Ri ¼ V eff   Rth þ Ri R1 K 2 \h2 K 4 \h4

ð34Þ

where K 1 ¼ R1 ==rd for a small value of Cd. 1 ¼ K 2 \h2 Q2 ¼ K 1  j xC "  2 #12 1 2 K2 ¼ K1 þ xC

ð35Þ

1  xC K1 Q3 ¼ K 2 \h2 ==R ¼ K 3 \h3 ¼ a þ bj ( 2 1 R K RðK þ RÞ þ K3 ¼  1 2 1 1 x2 C 2 ðK 1 þ RÞ2 þ xC  2 ) K 1R R  ðK 1 þ RÞ þ xC xC K1R R  ðK þ RÞ xC 1 h3 ¼ tan1 xC K 1 RðK 1 þ RÞ þ x2RC2

h2 ¼ tan1

Fig. 7 shows the equivalent circuit of the ferromagnetic toroid. The following admittance can be obtained:

ð36Þ ð37Þ

ð38Þ

Rth

Vi(t) Vth

V0

Ri

Vout

t t0

T

Fig. 8. Square wave electromagnetic interference on ferromagnetic toroid.

Fig. 9. Simplified diagram obtained by applying Thevenin’s theorem to Fig. 4.

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H.-C. Tsai / Optics Communications 273 (2007) 311–319

Q4 ¼ ðR1 ==rd þ X C Þ==R þ X C þ Ri ¼ Q3 þ X C þ Ri ¼ Rth þ Ri   1 ¼ a þ Ri þ j b  ¼ K 4 \h4 xC "  2 #12 1 2 K 4 ¼ ða þ Ri Þ þ b  xC h4 ¼ tan1

4. Results and discussion The induced voltage is amplified by a factor of 2000 such that the signal can be detected by an oscilloscope. Fig. 2 shows the typical periodic pulse function generated by a periodic EMI signal with a period T. The following parameters are measured in the simulations conducted using Eq. (41): V0 = 1 V, l0 = 4pE7 H/m, lr = 4000, N = 500, F = 400 Hz, Rx = 0.52 X, r0 = 0.09 m, lg = 0.005 m, R1 = 156.5 X, Cx = 6.558E10 F, C = 100E6 F, Dz = 0.01 m, d = 0.001 m, rd = 1050 X, C3 = 0.02, XC = 1/(2pfC), R = 10,000 X, L = 1E4 H, x = 2pf and Ri = 50 X. Fig. 2 shows a high degree of agreement between the measured curve and the simulated curve. Fig. 10 presents typical measured and simulated noise power spectral intensities of the DUT for EMI in the form of a square wave with VP = 1.6 V and a frequency of f = 400 Hz. Again, a good agreement is observed between the two sets of results. To further investigate the effect of EMI on the noise spectrum, the amplitude of the AC interference signal with an offset voltage of VP was varied between 0.2 and 1.6 V. Some of the measured values were Cd = 5.24 · 1011 F, RL = 43 X (LED on), and f = 400 Hz. An appropriate value of a = 6438 was chosen to simulate the noise spectrum shown in Fig. 10. Initially, a measured data point was selected for fitting purposes to obtain the induced current, A, of the EMI induced by an AC interference amplitude of VP = 1.6 V and frequency f = 400 Hz, as shown in Fig. 10. Fig. 10 compares the experimental results obtained for the harmonic wave of the noise power spectral intensity with the simulation results obtained using Eq. (11) for a simulated current of fixed value A = 774 lA. Fig. 11 compares the experimental and simulation results obtained for the other harmonic components in the noise spectrum for VP = 0.2–1.6 V and simulated values of A ranging from 88 to 774 lA. In all cases, the simulated results are in good agreement with the experimental data. Table 1 compares the measured and simulated peak values of the harmonic

ð39Þ

1 b  xC a þ Ri

ð40Þ

From the above, it can be shown that: 1 X 2KK n jY jnxK 1 K 3 Ri sinðnxt þ\C n þh1 þh3 h2 h4 Þ V out ¼ R1 K 2 K 4 n¼1 1 X C 3 KK n jY jnxK 1 K 3 Ri V out ¼ sinðnxt þ\C n þh1 þh3 h2 h4 Þ R1 K 2 K 4 n¼1 ð41Þ

where the constant C3 is the effective inducted coefficient. 0

Measurement Simulation

S Vc (fn)(dBmA)

-20

-40

-60

-80

-100

2000

4000

6000

8000

10000

Frequency (Hz) Fig. 10. Measured and simulated noise power spectral intensities of DUT for EMI signal of VP = 1.6 V at f = 400 Hz.

SVc (fn)(dBmA)

-10 -15

All f=400Hz 1.Vp=1.6V

-20

2.Vp=1.4V 3.Vp=1.2V

-25

4.Vp=1.0V

-30 5.Vp=0.8V

-35

6.Vp=0.6V

-40

7.Vp=0.4V

-45

8.Vp=0.2V

Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation

-50 0

2000

4000

6000

Frequency (Hz) Fig. 11. Measured and simulated results at 400 Hz with VP = 1 V for odd-order harmonics. Simulation results computed using Eq. (11) shown as filled symbols.

H.-C. Tsai / Optics Communications 273 (2007) 311–319

493 lA, i.e. the noise power spectral intensity varies only as a function of the AC interference frequency and the other parameters remain constant. Fig. 12 indicates the measured values using solid lines, with labels 1–6 indicating the fundamental amplitude and odd-order harmonics, respectively. Using Eq. (11), the measured value of Sn(f) can be used to obtain the interference current A. The simulation results are shown in Fig. 12 using filled-symbols, with the fundamental amplitude indicated by labels 1–6. Although a –3 dB difference is observed between the measurement results and the simulation results, the dominant influence of the frequency is clear. By tuning the amplitude parameter, the same accuracy as that shown in Fig. 11 can be obtained. Furthermore, by tuning the amplitude parameter in Eq. (11), the same accuracy as that shown in Fig. 12 can be obtained in Fig. 13 and Table 3. Figs. 11 and 12 reveal that the difference between the experimental results and the simulation results for the variable amplitude case is less than that in the variable frequency case, i.e. the theoretical results for the variable amplitude case are more accurate than those for the variable frequency case. The reason for this may be that the parasitic capacitance and dynamic input resistance are not constants for the variable frequency case. In general, the simulation and experimental results indicate that the magnitude of the EMI induced by the CW is a function of the amplitude, frequency and period of the interference, the parasitic capacitance of the device, the input load, the output load, and the parameter, a. Table 3 shows that the attenuation factor varies directly with the interference frequency. The quantified value of the EMI with the noise spectrum can be obtained from Fig. 2 or 10. Specifying the reference signal Si as 1 lA, the signal-to-noise ratio (SNR) of the circuit can be computed, as shown in Table 4. The approach outlined above is suitable for determining the characteristic influence of EMI on the noise spectrum of any wavelength device.

Table 1 Comparison of measurement and simulation results for maximum noise power spectral intensity for variable VP VP (V)

Frequency (Hz)

Measurement (dB mA)

Simulation (dB mA)

Frequency fixed, amplitude variable (first harmonic wave) 0.2 400 30.3 30.18286 0.4 400 24 24.01545 0.6 400 20.4 20.41313 0.8 400 17.8 17.80289 1.0 400 15.8 15.76089 1.2 400 14.1 14.06235 1.4 400 12.6 12.61608 1.6 400 11.3 11.2977

components of the EMI at different amplitudes and a constant frequency of f = 400 Hz, i.e. the harmonic value varies only as a function of the interference amplitude and the other parameters remain constant. Table 2 compares the experimental and simulation results obtained for the noise power spectral intensity when the AC interference frequency is varied from 200 to 900 Hz and the AC interference amplitude is fixed at a value of

Table 2 Comparison of measurement and simulation results for maximum noise power spectral intensity for variable frequency Frequency (Hz)

Measurement (dB mA)

317

Simulation (dB mA)

Amplitude fixed, frequency variable (first harmonic wave) 200 19.5 17.76983 300 17.6 16.20373 400 16.3 15.21558 500 15.3 14.56911 600 14.6 14.15694 700 14 13.91784 800 13.5 13.81078 900 13 13.80555

SVc (fn)(dBmA)

-10

1 2 3 4 5 6

-20

All Vp=1V 1.F=800Hz 2.F=700Hz 3.F=600Hz 4.F=500Hz

-30

5.F=400Hz 6.F=300Hz

-40

0

1000

2000

3000

4000

5000

6000

7000

Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation

8000

Frequency (Hz) Fig. 12. Measured and simulated results for frequencies in range 300–800 Hz with VP = 1 V for odd-order harmonics. Simulation results computed using Eq. (11) shown as filled symbols.

318

H.-C. Tsai / Optics Communications 273 (2007) 311–319 1 2 3 4 5 6

-10 -15

SVc (fn)(dBmA)

-20

All Vp=1V 1.F=800Hz 2.F=700Hz

-25

3.F=600Hz -30

4.F=500Hz

-35

5.F=400Hz

-40

6.F=300Hz

-45

Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation Measurement Simulation

-50 0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Frequency (Hz) Fig. 13. Experimental and simulated results for odd-order harmonics at 300–800 Hz with Table 3 (simulation results computed using Eq. (11) shown as solid symbols).

Table 3 Comparison of experimental and theoretical results for maximum noise power spectral intensity for various values of frequency and amplitude Frequency (Hz)

Attenuation factor (a)

Induced current A (lA)

Various values of amplitude and frequency (first harmonic wave) 300 7011 458 400 8311 547 500 8611 579 600 8801 599 700 9148 630 800 9438 660

The EMI experiments performed in this study use a conduction wire with a length of 1 cm. If the conduction wire in a practical electrical circuit has a length of more than 1 m, the EMI effect will potentially be magnified by a factor of 100 times. This will have a significant effect on the circuit; particularly if the EMI effect is induced at the receiving terminal of a communication system. Hence, the effects of EMI on wiring circuits merit further attention here. For example, assuming an induced current value of 0.2315 lA in Table 4, a conducting wire of length 1 cm, and a load resistance of 1.2065 kX, the induced voltage is 279.3 lV, which, as stated above, may then be amplified 100 times in a practical system. According to EN55011, 55013, 55022 (European Standards) and CISPR 11, 13, 22 (InterTable 4 Relative SNR of induced current by EMI VP (V)

Induced current A (lA)

Induced current Ni = A/2000 (lA)

Relative SNR (dB lA)

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2

774 665 563 463 366 271 179 88

0.387 0.3325 0.2815 0.2315 0.183 0.1355 0.0895 0.044

8.24 9.56 11.01 12.71 14.75 17.36 20.96 27.13

Where SNR = 20 log(Si/Ni), Si = 1 lA, Ni = A/2000 and f = 400 Hz.

Measurement (dB mA/Hz1/2)

Simulation (dB mA/Hz1/2)

17.6 16.3 15.3 14.6 14 13.5

17.664 16.292 15.3 14.597 14.003 13.483

national Special Committee on Radio Interference), the intensity of EMI should be limited to less than 110 dB lV over the interference frequency range of 0.009–0.050 MHz, equivalent to a voltage of 316227 lV. Under these conditions, the length of the conducting wire in the system should not exceed 11.32 cm. 5. Conclusions This study has employed a combined experimental and theoretical approach to characterize the noise spectrum of a multi-quantum well InGaN LED subject to interference by a periodic EMI via a CW. The results have shown that the EMI has a significant effect on the noise spectrum of the NMQLED. An analytic formula has been developed to describe the measured noise spectrum. The experimental and theoretical results are in good agreement in both the time domain and the frequency domain. A periodic interference signal acting on the device induces a burst magnitude of induced voltage which generates an observable noise spectrum in the LED. The simulation and experimental results have shown that the magnitude of the EMI induced by the CW is a function of f, V0, l0, lr, N, Rx, r0, lg, R1, Cx, Cd, Ri, Dz, d, rd, R, L, A, Rr, g, RLED, r1, T0, RL and C. The EMI experiments performed in this study use a conduction wire with a length of 1 cm. However, in practical applications, the EMI may be enhanced

H.-C. Tsai / Optics Communications 273 (2007) 311–319

by a factor of 100–1000 times. Hence, in accordance with CISPR and EN norms, the results presented in this study imply that the conducting wires used in practical circuits should not exceed a length of 11.32–1.132 cm. This result represents an important guideline for circuit designers. The magnitude of the induced interference current increases at higher values of the interference frequency and interference amplitude. The EMI amplitude has a more dominant effect than the EMI frequency. However, a variable EMI frequency influences both the attenuation factor and the induced wave type. The methods outlined in the current study to evaluate the quantified values of the EMI and SNR provide a convenient and systematic approach for carrying out the EMI analysis of general wavelength-based semiconducting devices. Acknowledgement The current author wishes to acknowledge the invaluable assistance provided by Kuo-Chang Wang, Ying-Zhi Chen and Zhi-Rong Zhang throughout the course of this study. References [1] A.G. Michette, S.J. Pfauntsch, A. Erko, A. Firsov, A. Svintsov, Optics Communications 245 (2005) 249. [2] V. Parinda, B.P. Singh, P. Taneja, P. Ayyub, Optics Communications 233 (2004) 297. ˜A ˜ hme, B. Rauschenbach, Microelectronic Engi[3] K. Zimmer, R. BA neering 78–79 (2005) 324. [4] A. Khalil, M. Miroslav, N. Shohei, A. Arous, M. Declan, Microelectronic Engineering 78–79 (2005) 39. [5] Z. Yong, X.H. Mao, J.A. Chen, W. Ding, X.Y. Gao, Z.M. Zhou, Journal of Magnetism and Magnetic Materials 292 (2005) 255. [6] Y.Z. Xue, Y.J. Wu, F. Zhu, Y.F. Zhang, Physica E 25 (2005) 395. [7] W.B. Young, Microelectronic Engineering 77 (2005) 405.

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