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Theoretical analysis of the radiationeffect on the transient behavior ofoptoelectronic integrated devicesM. Ashry a , Mohamed B. El-Mashade b , Sh. M. Eladl a & M. S.Rageh aa Radiation Engineering Department , National Center forRadiation Research and Technology, Atomic Energy Authority ,Egyptb Electrical Engineering Department, Faculty of Engineering ,Azhar University , EgyptPublished online: 01 Feb 2007.

To cite this article: M. Ashry , Mohamed B. El-Mashade , Sh. M. Eladl & M. S. Rageh (2004)Theoretical analysis of the radiation effect on the transient behavior of optoelectronic integrateddevices, Radiation Effects and Defects in Solids: Incorporating Plasma Science and PlasmaTechnology, 159:8-9, 453-460

To link to this article: http://dx.doi.org/10.1080/10420150410001670297

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Radiation Effects & Defects in SolidsAugust–September 2004, Vol. 159, pp. 453–460

THEORETICAL ANALYSIS OF THE RADIATIONEFFECT ON THE TRANSIENT BEHAVIOR OFOPTOELECTRONIC INTEGRATED DEVICES

M. ASHRYa,∗, MOHAMED B. EL-MASHADEb, SH. M. ELADLa and M. S. RAGEHa

aRadiation Engineering Department, National Center for Radiation Research and Technology, AtomicEnergy Authority, Egypt; bElectrical Engineering Department, Faculty of Engineering,

Azhar University, Egypt

(Received 21 January 2004; In final form 5 February 2004)

Theoretical analysis of the radiation effect on transient behavior of an optoelectronic integrated device composed of aheterojunction phototransistor and a light emitting diode is studied theoretically. First, the transient behavior and therise time of this device before radiation are investigated based on the frequency response of the constituent devicesand the optical feedback inside the device. Second, the effect of neutron irradiation flux on the transient behavior ofthis device is theoretically studied. The results show that, by increasing the optical feedback inside the device, therise time in the amplification mode is increased along with an increasing output, while that in the switching modecan be reduced effectively, and the neutron irradiation reduces the transient response and the rise time in both theamplification and switching modes. This type of model can be exploited as optical amplifier, optical switching device,and other applications.

Keywords: Optoelectronic integrated device; Optical functional device; Optical amplification mode; Opticalswitching mode

1 INTRODUCTION

There has been much interest in optoelectronic integrated devices (OEIDs) [1, 2], which areimportant for optical information processing and optical computing. An important exampleof such a device is shown in Figure 1, where a heterojunction phototransistor (HPT) and alight-emitting diode (LED) or a laser diode (LD) are directly integrated. The HPT converts aninput light to the amplified current and the LED or LD driven by current emits an intensifiedoutput light [3].

In addition to the input light, the HPT also responds to the light emitted from the LEDor LD, this is called an optical feedback inside the device. This optical feedback plays animportant role in realizing various optical functions such as light amplification and opticalswitching [4, 5]. In the amplification mode, the output light changes linearly with input light;while in the switching mode, the output light jumps abruptly from the low-current state to thehigh-current state.

∗ Corresponding author.

ISSN 1042-0150 print; ISSN 1029-4953 online © 2004 Taylor & Francis LtdDOI: 10.1080/10420150410001670297

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454 M. ASHRY et al.

FIGURE 1 Block diagram of OEID with optical feedback.

In this article, a theoretical analysis of the transient behavior upon pre-irradiation of theOEIDs consisting of HPT and LED is carried out using the same method as used in Ref. [3].The effect of radiation on the transient behavior of these devices is studied by replacingthe pre-irradiation sensitive parameters, which do not include the radiation factors, in thetransient behavior characteristic equations by the post-irradiation parameters, which includethe radiation factors, which in turn affect the device modeling.

2 THEORETICAL ANALYSIS

2.1 Pre-irradiation Transient Behavior

The characteristics of an optoelectronic device depend on both the frequency (wavelength) andthe modulation signal frequency of an input light. The former is called the spectral response,and the latter is the frequency response. The block of the OEID with optical feedback is shownin Figure 1, and the frequency response of the optical gain G(ω) of the OEID can be expressedas [3]

G(ω) = g(ω)η(ω)

1 − k(ω)g(ω)ηf(ω), (1)

where g(ω) denotes the conversion gain of each HPT, η(ω) is the external quantum efficiencyof the LED, ηf(ω) is the internal quantum efficiency of the LED for the feedback light, andk(ω) is the ratio of the photons which reach the HPT to those emitted by the LED.

The frequency response of the conversion gain of the HPT is expressed as [3]

g(ω) = g0

1 + jω/ωβ

, (2)

where g0 = β0ηh0 denotes the conversion gain of the HPT at low frequency regime, and β0 andηh0 are the current gain and the quantum efficiency of the HPT in the low frequency regime,where β0[cosh(wb/l) − 1]−1, l is the pre-irradiation diffusion length in the base region, wb isthe base width, and ωβ is the beta cutoff frequency. The frequency response of an LED can beexpressed as [3].

ηsp(ω) = ηsp0

1 + jω/ω1, (3)

where ηsp0 denotes the quantum efficiency for the spontaneous emission in the low frequencyregime, ω1 is the cutoff frequency of the LED where ω−1

1 = τ0 is the minority carrier lifetime.The frequency response of the OEID can thus be expressed as:

G(ω) = g0η0/((1 + jω/ωβ)(1 + jω/ω1))

k0g0ηf0/((1 + jω/ωβ)(1 + jω/ω1)), (4)

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RADIATION EFFECT ON OEID 455

where η0 denotes the quantum efficiency for the output, and ηf0 for the feedback light at lowfrequency regime and fb = k0g0ηf0.

The Laplace transform of the optical gain can be obtained as

P0(s) = γg0η0ωβω1Pi

s(s − λ1)(s − λ2), (5)

where γ = ν0/νi and ν0 and νi are the frequencies of the output and input lights, respectively,and

λ1 = −(ωβ + ω1) + √(ωβ + ω1)2 − 4(1 − fb)ωβω1

2, (6)

λ2 = −(ωβ + ω1) − √(ωβ + ω1)2 − 4(1 − fb)ωβω1

2. (7)

The output response of the OEIDs for fb �= 1 can be obtained from the inverse Laplace ofEq. (5) as

P0(t) = γg0η0ωβω1Pi

λ1λ2

[1 + λ2

λ1 − λ2exp(λ1t)

], (8)

P0(t) = γg0η0Pi

1 − f[1 − exp(−(1 − f )(ωβt))], (9)

P0(t) = γg0η0Pi

fb − 1[exp((fb − 1)(ωβt)) − 1]. (10)

2.2 Rise Time Characteristics

The rise time of the OEIDs can be given for fb �= 1 as

T = 1

λ1ln

[g0η0 − 0.9(1 − fb)G

g0η0

]. (11)

The rise time at fb = 1 is

Ts = 0.9Gs

g0η0ωβ

. (12)

2.3 Radiation Effect Characteristics

In this section, the radiation effect on the OEIDs is studied by replacing all the above parame-ters, which are considered to be pre-irradiation parameters, by the post-irradiation parametersthat include the radiation factors. The minority carrier lifetime parameter τ0 is sensitive to theneutron irradiation flux, and hence all factors including this parameter, such as (g0, η0, ω1), aresensitive to the neutron irradiation flux. Nonradiative recombination centers are introduced bythe neutron irradiation, which compete with radiative centers for excess carriers resulting in areduction of the minority carrier lifetime. The pre-irradiation value τ0 of the total lifetime canbe written as [6].

1

τ0= 1

τ0R+ 1

τ0NR, (13)

where τ0R and τ0NR are the pre-irradiation values of lifetime associated with radiative andnonradiative recombination processes, respectively. It is the reduction in τ0NR which is respon-sible for the reduction in total lifetime. A variety of recombination centers can act as sites

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456 M. ASHRY et al.

for nonradiative recombination. If these centers are introduced during exposure to a radiationfluence, f (n cm−2), at a rate determined by the constant k (n cm−2 s−1), then one can expressthe reduction in minority carrier lifetime to a post-irradiation value of τ in the followingmanner [7, 8]:

τ0

τ= 1 + τ0kf,

l2

l′2 = 1 + l2KLf, (14)

where l′ is the post-irradiation diffusion length and l is the pre-irradiation diffusion length, KLis the minority carrier diffusion length damage constant (depend on material, type of radiation,injection level, and temperature), and f is the particle fluence.

As regards the HPT, it is often convenient to express the transistor damage as a gain damagefactor, Kb. Thus,

1

β− 1

β0= �(1/β) = Kbf. (15)

Gain after a given irradiation can be calculated:

β = β0

1 + β0Kbf. (16)

A simple relation between Kb and k, is derived as:

Kb = k

ωT, (17)

where ωT is the gain bandwidth product of HPT, and ωT = β0ωβ .

3 RESULTS AND DISCUSSIONS

In this article, the device parameters used in the following calculations are the same as thoseones used by Zhu et al. [3], and the input light is assumed as a step function in time andωβ = 108 Hz, ω1 = 1010 Hz = 1/τ0, and g0 = β0ηh0 = 100.

The transient behavior of the OEID in the amplification mode at fb = 0 and different valuesof kf is shown in Figure 2. It can be seen that the output light of the OEID approaches a definitevalue proportional to the input light. Thus, the OEID is stable in the amplification mode. Fromthe same figure, it is clear that the output light of the OEID will be degraded when exposed to

FIGURE 2 Transient behavior of OEID in the amplification mode at fb = 0 and different values of kf.

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RADIATION EFFECT ON OEID 457

FIGURE 3 Transient behavior of OEID in the amplification mode at fb = 0.5 at different values of kf.

irradiation flux corresponding to kf = 108, which implies that the radiation particles reducethe minority carrier lifetime and hence the LED efficiency η0 and the optical gain of the HPTwhich cause the light output of the device to decrease. By increasing the quantity kf from 108

to 1010 (which implies an increasing of the irradiation flux because k is constant), the lightoutput of the device will exhibit further decrease.

The same behavior is shown in Figures 3 and 4 at fb = 0.5 and fb = 0.8, respectively. Itis clear that by increasing the optical feedback inside the device in the amplification mode,the output light will increase, but the device requires more time to arrive at saturation than thetime required at lower optical feedback.

The output response of the OEID in the switching mode at fb = 5 and different values ofkf is shown in Figure 5. It is clear that the output light increases exponentially with time, whichcorresponds to the jump of the device response from the lower value state to the upper valuestate in the switching mode. From the same figure, it is clear that the output light of the OEIDwill be degraded when exposed to an irradiation flux corresponding to kf = 108, and thedevice requires more time to arrive at the switching state than the time required if the deviceis not exposed to irradiation flux. Further exposure of the device to radiation will reduce theefficiency of the LED and the optical gain of the HPT, so that the device needs optical feedbackto compensate for this degradation. So, it takes a time to raise the output light to jump abruptlyfrom the low-current state to the high-current state, which retards the switching state.

The same behavior is shown in Figures 6 and 7 at fb = 10 and fb = 15, respectively. Itis clear that an increase of the optical feedback inside the device in the switching mode will

FIGURE 4 Transient behavior of OEID in the amplification mode at fb = 0.8 at different values of kf.

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458 M. ASHRY et al.

FIGURE 5 Transient behavior of OEID in the switching mode at fb = 5 at different values of kf.

FIGURE 6 Transient behavior of OEID in the switching mode at fb = 10 at different values of kf.

increase the output light, but the device arrives at the switching state at a shorter time. Exposureof the device to irradiation will retard the switching state.

Figure 8 shows the dependence of the rise time on the optical feedback. It is clear that byincreasing the optical feedback, the increase in the rise time in the amplification mode is due to

FIGURE 7 Transient behavior of OEID in the switching mode at fb = 15 at different values of kf.

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RADIATION EFFECT ON OEID 459

FIGURE 8 Dependence of rise time on optical feedback at different values of kf.

FIGURE 9 Dependence of optical gain on the optical feedback for switching mode at different values of kf.

the increase in the difference between the output light powers at the initial and the final states.In addition, the exposure of the device to irradiation flux will decrease the rise time value. Incontrast to the amplification mode, the rise time in the switching mode decreases withincreasing the optical feedback as shown in Figure 9. By increasing the optical feedback,the decrease in the rise time in the switching mode is due to the increase of the derivative ofthe output.

FIGURE 10 Derivative of output of OEID at different values of fb.

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460 M. ASHRY et al.

The time dependence of the derivative of the output is shown in Figure 10. In the amplifi-cation mode with fb < 1, the output of the OEID changes with a decreasing derivative. In theswitching mode with fb ≥ 1, the output of the OEID changes with a constant or increasingderivative. It can also be seen that at a given time, the derivative increases with increasingoptical feedback.

4 CONCLUSIONS

An OEID composed of a HPD and a LED is studied theoretically based on the frequencyresponse of a phototransistor and a LED. The results show that, by increasing the optical feedback inside the device, the rise time in the amplification mode (fb < 1) is increased along withan increasing output, while that in the switching mode (fb ≥ 1) can be reduced effectively, andthe neutron irradiation reduces the output response and the rise time in both the amplificationand switching modes. This type of model can be exploited for optoelectronic amplifiers andoptical switching applications.

References

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[2] Noda, S. and Sasaki, A. (1993). Vertical and direct integration of heterojunction phototransistors and laser diodes.Fiber Integrated Opt., 12, 319–345.

[3] Yu Zhu, Susumu Noda, and Akia Sasaki. (1995). Theoretical analysis of transient behavior of optoelectronicintegrated devices. IEEE Transactions on Electron Devices, 42(4).

[4] Jit, S. and Paul, B. B. (2001). A new optoelectronic integrated device for light amplifying optical switch (LAOS).IEEE Trans. Electron Devices, 48(12), 2732–2739.

[5] Noda, S., Shibata, K.,Ahamdy,V. and Sasaki. (1993). Functional switching device by vertical and direct integrationof six heterojunction phototransistors and two laser diodes. OSA Proc. Switching, 16, 47–50.

[6] Hajghassem, H. S., Brown, W. D. and Williams, J. G. (1992). Modeling the effect of neutron irradiation of LEDs.Solid-State Electronics, 35(1), 51–55.

[7] Barnes, C. E. and Wiczer, J. (1984). Report No. SAND-84-0771, Sandia National Laboratories, Albuquerque,NM.

[8] Aukerman, L., Millea, M. F. and McColl, M. (1966). IEEE Trans. Nucl. Sci., Ns-13, 174.

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