Condensed Matter Physics Research Center, Department of Physics, Jadavpur University, Kolkata-700032, IndiaCondensed Matter Physics Research Center, Department of Physics, Jadavpur University, Kolkata-700032, India
Abstract: The influence of temperature on the intensity of light emitted by as well as the carrier life time τ of a standard AlGaAs based light emitting diode has been investigated in the temperature range from 345 to 136 K. The open-circuit voltage decay (OCVD) technique has been used for measured the carrier lifetime. Our experimental results reveal a 16% average increase in intensity and a 163.482-19.765 ns variation in carrier lifetime in the above temperature range. Further, theoretical and experimental analysis show that for negligible carrier density the intensity is inversely proportional to carrier lifetime for this sample.
The study of temperature dependence of various parameters in light emitting diode (LED) has importance from both the view points of basic physics as well as different technological applications, particularly in low temperature regions. The performance of an LED is significantly dependent on temperature. LEDs are designed by using semiconductor structures in such a way that the electrical energy is converted to radiant energy and heat energy. LEDs release the radiant energy for the formation of photons due to the recombination of the injected electrons and holes within active regions; otherwise, electron–hole pairs encounter defects in the LED crystal and then recombine non-radiatively which produces an unwanted heating effect and decreases the efficiency of the device[1]. The light intensity varies with the driving of the LED current as well as its environmental temperature and it is known that with the lowering of temperature, intensity increases and carrier lifetime decreases. Characterization of LEDs above Ln2 temperature has been studied where it has been reported that the luminous intensity dropped by about one percent per degree Kelvin rise in temperature[2]. Another important work has related intensity with temperature in the range 243–373 K[3] showing that as the temperature decreased from 293 to 243 K, the relative light intensity of the AlGaInP-based red power LEDs increased by about 10% and the InGaN-based blue LEDs increased by about 9% on an average. Since the data obtained has no unique nature and also the relative light intensity variation is not same for all types of materials, further studies are required on the various types of LEDs for acquiring a better insight. This variation of intensity with temperature is dependent on various parameters such as lifetime, concentration, and distribution state of carriers, but the importance of their inter relation has not yet been studied. As of now, some theoretical works are available where intensity, device efficiency, carrier lifetime, and carrier concentration have been correlated[4]. Till now, no significant experimental study has related intensity and carrier lifetime. In our work, we have tried to relate these two parameters experimentally.
2.
Experimental details
The experimental set-up for the carrier life time measurement by the open circuit voltage decay method (OCVD) is shown in circuit diagram of Fig. 1. In our investigation we used one red diffused LED (RS Components), (L934SRDG 30 mA) made with the AlGaAs materials. The LED was placed inside a bath-type optical cryostat designed in our laboratory. With special care, liquid nitrogen was poured inside the liquid chamber of the cryostat which was pre-evacuated to a pressure 10−4 Torr by using a high vacuum pumping unit (Model No. PU-2 CH-8, manufactured by Vacuum Products & Consultants) to avoid moisture on the sample. The temperature measurement in the range 345 to 136 K was done by using a Chromel-Alumel thermocouple (TC). The TC output was recorded by a Keithley 2000 multimeter with accuracy of the order of ±0.15 K. The minority carriers were injected in the base region of the LED with the help of a voltage pulse applied in the forward direction from a GWINSTEK pulse generator model SFG-1013. A square pulse of amplitude 5 V and of pulse repetition frequency 1.25 kHz was applied through a current-limiting resistance R. The pulse generator connection to the LED was cut off when the pulse amplitude varied from a high level to a low level due to the presence of a p–n diode in the input circuit. The output wave pattern of the OCVD curves was recorded on a 100 MHz Agilent 54622D mixed signal oscilloscope.
Figure
1.
Schematic diagram of the experimental set up used for the OCVD LED measurement.
The I–V measurements were performed by using a Keithley 2400 source measure unit. To measure the intensity of the LED, the experimental set up of Fig. 1 is modified and shown in Fig. 2. In this arrangement the current-limiting resistance and the p–n diode was replaced by a milli ammeter. The LED was driven by a constant current source with a 1 K resistance in series. The output of LED intensity was measured by a BPW 34B photodetector (PIN Photodiode w/Enh Blue Sens, DIL, 60 degree, sensitive area 2.73 × 2.73 mm) which was placed outside the cryostat. The photodetector was reversely biased with a 2.0 V DC source and with a 390 K sensing resistance in series.
Figure
2.
Schematic diagram of the experimental set up used for the LED intensity measurement.
The open-circuit voltage decay (OCVD) technique is one of the most widespread methods for determining carrier lifetime in the bulk of diode structures[5-14]. The open-circuit voltage decay wave shapes for the wide temperature range from 345 to 136 K for the 1.25 kHz frequency supply have been measured. Figure 3 shows the open-circuit voltage decay wave shape at different temperatures.
Figure
3.
Open-circuit voltage decay wave shape at different temperatures in the range 345–136 K.
It is easy to implement, interpretation of experimental data is fairly straightforward and, moreover, very good correlation with real electrical parameters of semiconductor devices is expected. When the diode bias circuit is opened, subsequent excess carrier recombination is detected by monitoring the open circuit voltage. Figure 3 shows the direct pulse generator output superimposed on the OCVD curve. The pulse repetition frequency and the pulse width was always adjusted to allow the OCVD curve to fall to ground level before the appearance of the next forward biasing pulse, as shown in Fig. 3 The decay is characterized by two distinct regions. The first vertical drop is due to the series resistance of the LED. The next division on the horizontal time axis clearly demonstrates an almost linear portion. This is followed by a decay towards the zero voltage. This portion is complicated due to the combined effect of the junction voltage decay and the junction capacitance discharge[6]. With forward bias applied to the diode, there is an excess minority-carrier concentration in the neutral region near the depletion region. Then the excess hole charge Qp in the neutral n-base region, will be[15]
Qp=τpIss(eqpVjKT−1),
(1)
where τp is the hole minority carrier lifetime, Iss is the diode reverse saturation current, qp is the hole charge, Vj is the junction voltage, K is the Boltzmann constant, and T is the temperature.
Also, the excess electronic charge can be expressed as
Qe=τeIss(eqeVjKT−1),
(2)
where τe is the excess electron minority carrier lifetime and qe is the electronic charge. So the total stored charge is given by
Q=Qp+Qe=(τp+τe)Iss(eqVjKT−1)=τIss(eqVjKT−1),
(3)
where τ=τp+τe= effective carrier lifetime and qp=qe=q the electronic charge. To obtain a dynamic relation, the time rate of change of the excess charge must be included. Then the total diode current (Id) will be given by
Id(t)=Qτ+dQdt,
(4)
where Q is given by Eq. (3). This is the fundamental charge control equation for a p–n junction diode which states that the diode current supplies holes to the neutral n region and electrons to the neutral p region at the rate at which the stored charge increases plus the rate at which holes and electrons are being lost by recombination. From the lumped charge approximation, changes of excess carriers in the volume of the diode can be described by
dQdt=Id(t)−Qτ,
(5)
where τ is an effective carrier lifetime. After opening the circuit in time t= 0, the charge stored in the base of the diode decreases, for t > 0, as
Q(t)=Q(0)e−tτ.
(6)
The carrier lifetime can be expressed by the relation[6, 7]
τ=nKTq(dVOCdt)−1,
(7)
The value of dVOCdt was calculated from the linear portion of the OCVD curves at different temperatures. The value of the ideality factor n of the LED was determined by measuring I–V characteristics at different temperatures.
3.2
Ⅰ-Ⅴ characteristics
Temperature dependence I–V characteristics, has an important role in the performance of LEDs[16-20]. The current flows through an LED can be described by the well-known thermionic emission (TE) theory and the I–V relationship of a Schottky diode is given by[16]
I=Isexp(qVnKT)[1−exp(−qVKT)],
(8)
where
Is=AA∗∗T2exp(−qΦBOKT).
(9)
Here, Is is the saturation current, q is the electron charge, V is the applied voltage, T is the absolute temperature, n is the ideality factor, k is Boltzmann's constant, A is the contact area, A∗∗ is the effective Richardson constant, and ΦBO is the Schottky barrier height (SBH). The lnI–V characteristics in the temperature range 345–136 K as shown in Fig. 4.
Figure
4.
Variation of lnI with voltage (a) in the temperature range 345–249 K and (b) in the temperature range 236–136 K
The ideality factor n is a measure of the conformity of the light emitting diode to pure thermionic emission and it is determined from the slope of the linear region of the forward bias lnI–V characteristics through the relation
n=qKTdVd(lnI).
(10)
The values of ideality factor n have been calculated using Eq. (10) and it is found to vary with temperature as shown in Fig. 5.
Figure
5.
Variation of ideality factor n with temperature.
The values of n are presented in Table 1 and the corresponding values of lifetime have also been calculated using Eq. (7) and the change of the carrier lifetime with temperature is shown in Fig. 6.
Table
1.
Variation of carrier lifetime τ, ideality factor n with temperature.
From Figs. 6 and 7 it is clear that both carrier life time and luminescence intensity vary with temperature. There is not much work available to show how the intensity depends on the carrier lifetime. We have experimentally obtained the values of intensity (L) and carrier lifetime (τ), which are plotted in the curve L versus 1τ and shown in Fig. 8. A theoretical relationship between intensity and carrier lifetime has already been obtained which is given by
L=ηncτ(cm−3s−1),
(11)
Figure
8.
Variation of carrier lifetime with intensity.
where η is the efficiency of the device and nc is the carrier density. The relation shows the intensity is dependent on three factors. But the efficiency (η) of this device is nearly constant in the above temperature range[21]. Since density of states as well as carrier density (nc) has temperature dependence[22], then L can vary with nc and carrier lifetime (τ). The experimental curve obtained by us is found to be almost straight line. This indicates that intensity L is inversely proportional to lifetime (τ) and the change of nc is negligible. So the experimental result is in qualitative agreement with the above relation. However further work is needed to verify the relation quantitatively.
4.
Conclusion
The effect of temperature is found to have quite a significant effect on the properties of an LED. As the temperature was decreased from 345–136 K, the relative light intensity of the AlGaAs-based red high power LED increased by about 16% on average. The values of the estimated ideality factor n change from 2.173 at 136 K to 1.878 at 345 K. It appears that there is a general trend by which the ideality factor decreases with the temperature. From the OCVD measurement it is observed that the voltage drop due to series resistance decreases with decreasing temperature. For this reason the slope increases and then the carrier lifetime also increases with temperature. The estimated values are 19.765 ns at 136 K and 163.482 ns at 345 K. This is in good agreement with the theoretical analysis and hence it may be concluded that the intensity L is inversely proportional to the carrier lifetime (τ), where the change of nc is negligible. The data will be especially helpful for application in opto-sensors in below room temperature ambiences like the polar region, middle and upper atmospheres, and other environments. In our future work we shall probe the further implications of our present study for AlGaAs as well as other LED diodes.
Acknowledgments:
The authors acknowledge the Defence Research Development Organization (DRDO), India, for financial assistance, and one of the authors, P. Dalapati is thankful to DRDO for the award of a research fellowship.
References
[1]
Lee Y J, Lee C J, Chen C H. Determination of junction temperature in InGaN and AlGaInP light-emitting diodes. IEEE J Quantum Electron, 2010, 46:1450 doi: 10.1109/JQE.2010.2050866
[2]
Manik N B, Basu A N, Mukherjee S C. Characterisation of the photodetector and light emitting diode at above liquid nitrogen temperature. Cryogenics, 2000, 40:341 doi: 10.1016/S0011-2275(00)00036-9
[3]
Guo Weling, Jia Xuejiao, Yin Fei, et al. Characteristics of high power LEDs at high and low temperature. Journal of Semiconductors, 2011, 32:4007
Bhattacharya D K, Abhai M. Determination of recombination center position from the temperature dependence of minority car-rier lifetime in the base region of p-n junction solar cells. J Appl Phys, 1985, 57:2942 doi: 10.1063/1.335234
[7]
Mathur P C, Sharma R P, Saxena P, et al. Temperature dependence of minority carrier lifetime in single-crystal and polycrystalline Si solar cells. J Appl Phys, 1981, 52:3651 doi: 10.1063/1.329101
[8]
Mahan J E, Ekstedt T W, Frank R I. Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay. IEEE Trans Electron Devices, 1979, 26:733 doi: 10.1109/T-ED.1979.19487
[9]
Vishnoi A, Gopal R, Dwivedi R, et al. Measurement of minority carrier lifetime of solar cells using surface voltage and current transients. Solid-Stare Electron, 1990, 33:41l https://arxiv.org/pdf/1511.07887.pdf
[10]
Milan T, Jaroslav P, Andrej V, et al. Application of open circuit voltage decay to the characterization of epitaxial layer. Electrical Eng, 2004, 55:239
[11]
Sertap K A, Nese K, Sener O. The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell. Solar Energy, 2009, 83:1446 doi: 10.1016/j.solener.2009.03.009
[12]
Vobecky J, Hazdra P, Zahlava V. Open circuit voltage decay lifetime of ion irradiated devices. Microelectron J, 1999, 30:513 doi: 10.1016/S0026-2692(98)00173-6
[13]
Pisarkiewicz T. Photodecay method in investigation of materials and photovoltaic structures. Opto-Electron Rev, 2004, 12:33 https://tw.arxiv.org/pdf/1706.02397
[14]
Salach-Bielecki R, Pisarkiewicz T, Stapinski T, et al. Influence of junction parameters on the open circuit voltage decay in solar cells. Opto-Electron Rev, 2004, 12:79
[15]
Tseng K J, Pan S. Modified charge-control equation for simulation of diode reverse recovery. Electron Lett, 1996, 32:404 doi: 10.1049/el:19960210
[16]
Ravinandan M, Koteswara R P, Rajagopal R V. Temperature dependence of current-voltage (Ⅰ-Ⅴ) characteristics of Pt/Au Schottky contacts on n-type GaN. J Optoelectron Adv Mater, 2008, 10:2787 http://wulixb.iphy.ac.cn/EN/volumn/volumn_522.shtml
[17]
Tunc T, Dokme I, Altindal S, et al. Temperature dependent current-voltage (Ⅰ-Ⅴ) characteristics of Au/n-Si (111) Schottky barrier diodes (SBDs) with polyvinyl alcohol (Co, Ni-doped) interfacial layer. Optoelectron Adv Mater, 2010, 4:947
[18]
Singh R, Arora S K, Renu T, et al. Temperature dependence of current-voltage characteristics of Au/n-GaAs epitaxial Schottky diode. Bull Mater Sci, 2000, 23:471 doi: 10.1007/BF02903886
[19]
Zhu Shiyang, Ru Guoping, Qu Xinping, et al. Double threshold behaviour of Ⅰ-Ⅴ characteristics of CoSi2/Si Schottky contacts. Chin Phys B, 2002, 11:156 doi: 10.1088/1009-1963/11/2/310
[20]
Acharya Y B, Vyavahare P D. Temperature characteristics of the device constant (n) of a light emitting diode. Solid-State Electron, 1999, 43:645 doi: 10.1016/S0038-1101(98)00291-3
[21]
Bergman J P, Hallin C, Janzen E. Temperature dependence of the minority carrier, lifetime in GaAs/AlGaAs double heterostructures. J Appl Phys, 1995, 78:4808 doi: 10.1063/1.360740
[22]
http: //www. ioffe. ru/SVA/NSM/Semicond/AlGaAs/bandstr. html
Fig. 1.
Schematic diagram of the experimental set up used for the OCVD LED measurement.
Table 1.
Variation of carrier lifetime τ, ideality factor n with temperature.
[1]
Lee Y J, Lee C J, Chen C H. Determination of junction temperature in InGaN and AlGaInP light-emitting diodes. IEEE J Quantum Electron, 2010, 46:1450 doi: 10.1109/JQE.2010.2050866
[2]
Manik N B, Basu A N, Mukherjee S C. Characterisation of the photodetector and light emitting diode at above liquid nitrogen temperature. Cryogenics, 2000, 40:341 doi: 10.1016/S0011-2275(00)00036-9
[3]
Guo Weling, Jia Xuejiao, Yin Fei, et al. Characteristics of high power LEDs at high and low temperature. Journal of Semiconductors, 2011, 32:4007
Bhattacharya D K, Abhai M. Determination of recombination center position from the temperature dependence of minority car-rier lifetime in the base region of p-n junction solar cells. J Appl Phys, 1985, 57:2942 doi: 10.1063/1.335234
[7]
Mathur P C, Sharma R P, Saxena P, et al. Temperature dependence of minority carrier lifetime in single-crystal and polycrystalline Si solar cells. J Appl Phys, 1981, 52:3651 doi: 10.1063/1.329101
[8]
Mahan J E, Ekstedt T W, Frank R I. Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay. IEEE Trans Electron Devices, 1979, 26:733 doi: 10.1109/T-ED.1979.19487
[9]
Vishnoi A, Gopal R, Dwivedi R, et al. Measurement of minority carrier lifetime of solar cells using surface voltage and current transients. Solid-Stare Electron, 1990, 33:41l https://arxiv.org/pdf/1511.07887.pdf
[10]
Milan T, Jaroslav P, Andrej V, et al. Application of open circuit voltage decay to the characterization of epitaxial layer. Electrical Eng, 2004, 55:239
[11]
Sertap K A, Nese K, Sener O. The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell. Solar Energy, 2009, 83:1446 doi: 10.1016/j.solener.2009.03.009
[12]
Vobecky J, Hazdra P, Zahlava V. Open circuit voltage decay lifetime of ion irradiated devices. Microelectron J, 1999, 30:513 doi: 10.1016/S0026-2692(98)00173-6
[13]
Pisarkiewicz T. Photodecay method in investigation of materials and photovoltaic structures. Opto-Electron Rev, 2004, 12:33 https://tw.arxiv.org/pdf/1706.02397
[14]
Salach-Bielecki R, Pisarkiewicz T, Stapinski T, et al. Influence of junction parameters on the open circuit voltage decay in solar cells. Opto-Electron Rev, 2004, 12:79
[15]
Tseng K J, Pan S. Modified charge-control equation for simulation of diode reverse recovery. Electron Lett, 1996, 32:404 doi: 10.1049/el:19960210
[16]
Ravinandan M, Koteswara R P, Rajagopal R V. Temperature dependence of current-voltage (Ⅰ-Ⅴ) characteristics of Pt/Au Schottky contacts on n-type GaN. J Optoelectron Adv Mater, 2008, 10:2787 http://wulixb.iphy.ac.cn/EN/volumn/volumn_522.shtml
[17]
Tunc T, Dokme I, Altindal S, et al. Temperature dependent current-voltage (Ⅰ-Ⅴ) characteristics of Au/n-Si (111) Schottky barrier diodes (SBDs) with polyvinyl alcohol (Co, Ni-doped) interfacial layer. Optoelectron Adv Mater, 2010, 4:947
[18]
Singh R, Arora S K, Renu T, et al. Temperature dependence of current-voltage characteristics of Au/n-GaAs epitaxial Schottky diode. Bull Mater Sci, 2000, 23:471 doi: 10.1007/BF02903886
[19]
Zhu Shiyang, Ru Guoping, Qu Xinping, et al. Double threshold behaviour of Ⅰ-Ⅴ characteristics of CoSi2/Si Schottky contacts. Chin Phys B, 2002, 11:156 doi: 10.1088/1009-1963/11/2/310
[20]
Acharya Y B, Vyavahare P D. Temperature characteristics of the device constant (n) of a light emitting diode. Solid-State Electron, 1999, 43:645 doi: 10.1016/S0038-1101(98)00291-3
[21]
Bergman J P, Hallin C, Janzen E. Temperature dependence of the minority carrier, lifetime in GaAs/AlGaAs double heterostructures. J Appl Phys, 1995, 78:4808 doi: 10.1063/1.360740
[22]
http: //www. ioffe. ru/SVA/NSM/Semicond/AlGaAs/bandstr. html
Yi Xiaoyan, Ma Long, Guo Jinxia, Wang Liangchen, Li Jinmin, et al.
Chinese Journal of Semiconductors , 2005, 26(S1): 161-164.
Search
GET CITATION
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
Export: BibTexEndNote
Share:
Article Metrics
Article views: 3901 TimesPDF downloads: 76 TimesCited by: 0 Times
History
Received: 02 February 2013Revised: Online:Published: 01 September 2013
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001 ****P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
Citation:
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001
****
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001 ****P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
Citation:
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001
****
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
The influence of temperature on the intensity of light emitted by as well as the carrier life time τ of a standard AlGaAs based light emitting diode has been investigated in the temperature range from 345 to 136 K. The open-circuit voltage decay (OCVD) technique has been used for measured the carrier lifetime. Our experimental results reveal a 16% average increase in intensity and a 163.482-19.765 ns variation in carrier lifetime in the above temperature range. Further, theoretical and experimental analysis show that for negligible carrier density the intensity is inversely proportional to carrier lifetime for this sample.
The study of temperature dependence of various parameters in light emitting diode (LED) has importance from both the view points of basic physics as well as different technological applications, particularly in low temperature regions. The performance of an LED is significantly dependent on temperature. LEDs are designed by using semiconductor structures in such a way that the electrical energy is converted to radiant energy and heat energy. LEDs release the radiant energy for the formation of photons due to the recombination of the injected electrons and holes within active regions; otherwise, electron–hole pairs encounter defects in the LED crystal and then recombine non-radiatively which produces an unwanted heating effect and decreases the efficiency of the device[1]. The light intensity varies with the driving of the LED current as well as its environmental temperature and it is known that with the lowering of temperature, intensity increases and carrier lifetime decreases. Characterization of LEDs above Ln2 temperature has been studied where it has been reported that the luminous intensity dropped by about one percent per degree Kelvin rise in temperature[2]. Another important work has related intensity with temperature in the range 243–373 K[3] showing that as the temperature decreased from 293 to 243 K, the relative light intensity of the AlGaInP-based red power LEDs increased by about 10% and the InGaN-based blue LEDs increased by about 9% on an average. Since the data obtained has no unique nature and also the relative light intensity variation is not same for all types of materials, further studies are required on the various types of LEDs for acquiring a better insight. This variation of intensity with temperature is dependent on various parameters such as lifetime, concentration, and distribution state of carriers, but the importance of their inter relation has not yet been studied. As of now, some theoretical works are available where intensity, device efficiency, carrier lifetime, and carrier concentration have been correlated[4]. Till now, no significant experimental study has related intensity and carrier lifetime. In our work, we have tried to relate these two parameters experimentally.
2.
Experimental details
The experimental set-up for the carrier life time measurement by the open circuit voltage decay method (OCVD) is shown in circuit diagram of Fig. 1. In our investigation we used one red diffused LED (RS Components), (L934SRDG 30 mA) made with the AlGaAs materials. The LED was placed inside a bath-type optical cryostat designed in our laboratory. With special care, liquid nitrogen was poured inside the liquid chamber of the cryostat which was pre-evacuated to a pressure 10−4 Torr by using a high vacuum pumping unit (Model No. PU-2 CH-8, manufactured by Vacuum Products & Consultants) to avoid moisture on the sample. The temperature measurement in the range 345 to 136 K was done by using a Chromel-Alumel thermocouple (TC). The TC output was recorded by a Keithley 2000 multimeter with accuracy of the order of ±0.15 K. The minority carriers were injected in the base region of the LED with the help of a voltage pulse applied in the forward direction from a GWINSTEK pulse generator model SFG-1013. A square pulse of amplitude 5 V and of pulse repetition frequency 1.25 kHz was applied through a current-limiting resistance R. The pulse generator connection to the LED was cut off when the pulse amplitude varied from a high level to a low level due to the presence of a p–n diode in the input circuit. The output wave pattern of the OCVD curves was recorded on a 100 MHz Agilent 54622D mixed signal oscilloscope.
Figure
1.
Schematic diagram of the experimental set up used for the OCVD LED measurement.
The I–V measurements were performed by using a Keithley 2400 source measure unit. To measure the intensity of the LED, the experimental set up of Fig. 1 is modified and shown in Fig. 2. In this arrangement the current-limiting resistance and the p–n diode was replaced by a milli ammeter. The LED was driven by a constant current source with a 1 K resistance in series. The output of LED intensity was measured by a BPW 34B photodetector (PIN Photodiode w/Enh Blue Sens, DIL, 60 degree, sensitive area 2.73 × 2.73 mm) which was placed outside the cryostat. The photodetector was reversely biased with a 2.0 V DC source and with a 390 K sensing resistance in series.
Figure
2.
Schematic diagram of the experimental set up used for the LED intensity measurement.
The open-circuit voltage decay (OCVD) technique is one of the most widespread methods for determining carrier lifetime in the bulk of diode structures[5-14]. The open-circuit voltage decay wave shapes for the wide temperature range from 345 to 136 K for the 1.25 kHz frequency supply have been measured. Figure 3 shows the open-circuit voltage decay wave shape at different temperatures.
Figure
3.
Open-circuit voltage decay wave shape at different temperatures in the range 345–136 K.
It is easy to implement, interpretation of experimental data is fairly straightforward and, moreover, very good correlation with real electrical parameters of semiconductor devices is expected. When the diode bias circuit is opened, subsequent excess carrier recombination is detected by monitoring the open circuit voltage. Figure 3 shows the direct pulse generator output superimposed on the OCVD curve. The pulse repetition frequency and the pulse width was always adjusted to allow the OCVD curve to fall to ground level before the appearance of the next forward biasing pulse, as shown in Fig. 3 The decay is characterized by two distinct regions. The first vertical drop is due to the series resistance of the LED. The next division on the horizontal time axis clearly demonstrates an almost linear portion. This is followed by a decay towards the zero voltage. This portion is complicated due to the combined effect of the junction voltage decay and the junction capacitance discharge[6]. With forward bias applied to the diode, there is an excess minority-carrier concentration in the neutral region near the depletion region. Then the excess hole charge Qp in the neutral n-base region, will be[15]
Qp=τpIss(eqpVjKT−1),
(1)
where τp is the hole minority carrier lifetime, Iss is the diode reverse saturation current, qp is the hole charge, Vj is the junction voltage, K is the Boltzmann constant, and T is the temperature.
Also, the excess electronic charge can be expressed as
Qe=τeIss(eqeVjKT−1),
(2)
where τe is the excess electron minority carrier lifetime and qe is the electronic charge. So the total stored charge is given by
Q=Qp+Qe=(τp+τe)Iss(eqVjKT−1)=τIss(eqVjKT−1),
(3)
where τ=τp+τe= effective carrier lifetime and qp=qe=q the electronic charge. To obtain a dynamic relation, the time rate of change of the excess charge must be included. Then the total diode current (Id) will be given by
Id(t)=Qτ+dQdt,
(4)
where Q is given by Eq. (3). This is the fundamental charge control equation for a p–n junction diode which states that the diode current supplies holes to the neutral n region and electrons to the neutral p region at the rate at which the stored charge increases plus the rate at which holes and electrons are being lost by recombination. From the lumped charge approximation, changes of excess carriers in the volume of the diode can be described by
dQdt=Id(t)−Qτ,
(5)
where τ is an effective carrier lifetime. After opening the circuit in time t= 0, the charge stored in the base of the diode decreases, for t > 0, as
Q(t)=Q(0)e−tτ.
(6)
The carrier lifetime can be expressed by the relation[6, 7]
τ=nKTq(dVOCdt)−1,
(7)
The value of dVOCdt was calculated from the linear portion of the OCVD curves at different temperatures. The value of the ideality factor n of the LED was determined by measuring I–V characteristics at different temperatures.
3.2
Ⅰ-Ⅴ characteristics
Temperature dependence I–V characteristics, has an important role in the performance of LEDs[16-20]. The current flows through an LED can be described by the well-known thermionic emission (TE) theory and the I–V relationship of a Schottky diode is given by[16]
I=Isexp(qVnKT)[1−exp(−qVKT)],
(8)
where
Is=AA∗∗T2exp(−qΦBOKT).
(9)
Here, Is is the saturation current, q is the electron charge, V is the applied voltage, T is the absolute temperature, n is the ideality factor, k is Boltzmann's constant, A is the contact area, A∗∗ is the effective Richardson constant, and ΦBO is the Schottky barrier height (SBH). The lnI–V characteristics in the temperature range 345–136 K as shown in Fig. 4.
Figure
4.
Variation of lnI with voltage (a) in the temperature range 345–249 K and (b) in the temperature range 236–136 K
The ideality factor n is a measure of the conformity of the light emitting diode to pure thermionic emission and it is determined from the slope of the linear region of the forward bias lnI–V characteristics through the relation
n=qKTdVd(lnI).
(10)
The values of ideality factor n have been calculated using Eq. (10) and it is found to vary with temperature as shown in Fig. 5.
Figure
5.
Variation of ideality factor n with temperature.
The values of n are presented in Table 1 and the corresponding values of lifetime have also been calculated using Eq. (7) and the change of the carrier lifetime with temperature is shown in Fig. 6.
Table
1.
Variation of carrier lifetime τ, ideality factor n with temperature.
From Figs. 6 and 7 it is clear that both carrier life time and luminescence intensity vary with temperature. There is not much work available to show how the intensity depends on the carrier lifetime. We have experimentally obtained the values of intensity (L) and carrier lifetime (τ), which are plotted in the curve L versus 1τ and shown in Fig. 8. A theoretical relationship between intensity and carrier lifetime has already been obtained which is given by
L=ηncτ(cm−3s−1),
(11)
Figure
8.
Variation of carrier lifetime with intensity.
where η is the efficiency of the device and nc is the carrier density. The relation shows the intensity is dependent on three factors. But the efficiency (η) of this device is nearly constant in the above temperature range[21]. Since density of states as well as carrier density (nc) has temperature dependence[22], then L can vary with nc and carrier lifetime (τ). The experimental curve obtained by us is found to be almost straight line. This indicates that intensity L is inversely proportional to lifetime (τ) and the change of nc is negligible. So the experimental result is in qualitative agreement with the above relation. However further work is needed to verify the relation quantitatively.
4.
Conclusion
The effect of temperature is found to have quite a significant effect on the properties of an LED. As the temperature was decreased from 345–136 K, the relative light intensity of the AlGaAs-based red high power LED increased by about 16% on average. The values of the estimated ideality factor n change from 2.173 at 136 K to 1.878 at 345 K. It appears that there is a general trend by which the ideality factor decreases with the temperature. From the OCVD measurement it is observed that the voltage drop due to series resistance decreases with decreasing temperature. For this reason the slope increases and then the carrier lifetime also increases with temperature. The estimated values are 19.765 ns at 136 K and 163.482 ns at 345 K. This is in good agreement with the theoretical analysis and hence it may be concluded that the intensity L is inversely proportional to the carrier lifetime (τ), where the change of nc is negligible. The data will be especially helpful for application in opto-sensors in below room temperature ambiences like the polar region, middle and upper atmospheres, and other environments. In our future work we shall probe the further implications of our present study for AlGaAs as well as other LED diodes.
Acknowledgments:
The authors acknowledge the Defence Research Development Organization (DRDO), India, for financial assistance, and one of the authors, P. Dalapati is thankful to DRDO for the award of a research fellowship.
Lee Y J, Lee C J, Chen C H. Determination of junction temperature in InGaN and AlGaInP light-emitting diodes. IEEE J Quantum Electron, 2010, 46:1450 doi: 10.1109/JQE.2010.2050866
[2]
Manik N B, Basu A N, Mukherjee S C. Characterisation of the photodetector and light emitting diode at above liquid nitrogen temperature. Cryogenics, 2000, 40:341 doi: 10.1016/S0011-2275(00)00036-9
[3]
Guo Weling, Jia Xuejiao, Yin Fei, et al. Characteristics of high power LEDs at high and low temperature. Journal of Semiconductors, 2011, 32:4007
Bhattacharya D K, Abhai M. Determination of recombination center position from the temperature dependence of minority car-rier lifetime in the base region of p-n junction solar cells. J Appl Phys, 1985, 57:2942 doi: 10.1063/1.335234
[7]
Mathur P C, Sharma R P, Saxena P, et al. Temperature dependence of minority carrier lifetime in single-crystal and polycrystalline Si solar cells. J Appl Phys, 1981, 52:3651 doi: 10.1063/1.329101
[8]
Mahan J E, Ekstedt T W, Frank R I. Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay. IEEE Trans Electron Devices, 1979, 26:733 doi: 10.1109/T-ED.1979.19487
[9]
Vishnoi A, Gopal R, Dwivedi R, et al. Measurement of minority carrier lifetime of solar cells using surface voltage and current transients. Solid-Stare Electron, 1990, 33:41l https://arxiv.org/pdf/1511.07887.pdf
[10]
Milan T, Jaroslav P, Andrej V, et al. Application of open circuit voltage decay to the characterization of epitaxial layer. Electrical Eng, 2004, 55:239
[11]
Sertap K A, Nese K, Sener O. The circuit point of view of the temperature dependent open circuit voltage decay of the solar cell. Solar Energy, 2009, 83:1446 doi: 10.1016/j.solener.2009.03.009
[12]
Vobecky J, Hazdra P, Zahlava V. Open circuit voltage decay lifetime of ion irradiated devices. Microelectron J, 1999, 30:513 doi: 10.1016/S0026-2692(98)00173-6
[13]
Pisarkiewicz T. Photodecay method in investigation of materials and photovoltaic structures. Opto-Electron Rev, 2004, 12:33 https://tw.arxiv.org/pdf/1706.02397
[14]
Salach-Bielecki R, Pisarkiewicz T, Stapinski T, et al. Influence of junction parameters on the open circuit voltage decay in solar cells. Opto-Electron Rev, 2004, 12:79
[15]
Tseng K J, Pan S. Modified charge-control equation for simulation of diode reverse recovery. Electron Lett, 1996, 32:404 doi: 10.1049/el:19960210
[16]
Ravinandan M, Koteswara R P, Rajagopal R V. Temperature dependence of current-voltage (Ⅰ-Ⅴ) characteristics of Pt/Au Schottky contacts on n-type GaN. J Optoelectron Adv Mater, 2008, 10:2787 http://wulixb.iphy.ac.cn/EN/volumn/volumn_522.shtml
[17]
Tunc T, Dokme I, Altindal S, et al. Temperature dependent current-voltage (Ⅰ-Ⅴ) characteristics of Au/n-Si (111) Schottky barrier diodes (SBDs) with polyvinyl alcohol (Co, Ni-doped) interfacial layer. Optoelectron Adv Mater, 2010, 4:947
[18]
Singh R, Arora S K, Renu T, et al. Temperature dependence of current-voltage characteristics of Au/n-GaAs epitaxial Schottky diode. Bull Mater Sci, 2000, 23:471 doi: 10.1007/BF02903886
[19]
Zhu Shiyang, Ru Guoping, Qu Xinping, et al. Double threshold behaviour of Ⅰ-Ⅴ characteristics of CoSi2/Si Schottky contacts. Chin Phys B, 2002, 11:156 doi: 10.1088/1009-1963/11/2/310
[20]
Acharya Y B, Vyavahare P D. Temperature characteristics of the device constant (n) of a light emitting diode. Solid-State Electron, 1999, 43:645 doi: 10.1016/S0038-1101(98)00291-3
[21]
Bergman J P, Hallin C, Janzen E. Temperature dependence of the minority carrier, lifetime in GaAs/AlGaAs double heterostructures. J Appl Phys, 1995, 78:4808 doi: 10.1063/1.360740
[22]
http: //www. ioffe. ru/SVA/NSM/Semicond/AlGaAs/bandstr. html
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001 ****P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. Journal of Semiconductors, 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001
****
P. Dalapati, N. B. Manik, A. N. Basu. Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode[J]. J. Semicond., 2013, 34(9): 092001. doi: 10.1088/1674-4926/34/9/092001.