Tunneling current in Si-doped n type-GaAs heterostructures infrared emitter

Abstract

In the present work, we measured the forward bias current-voltage (I-V) characteristics of Si-doped n type gallium arsenide (GaAs) heterostructures infrared emitter over a wide temperature range from 350 to 77 K. Results showed that the slopes of the exponential curve changed slowly with temperature. The analysis of the various tunneling mechanisms indicated that the tunneling current varied approximately as a function of ~ exp(- aE_g + beV) where the parameters a and b varied indistinctively with temperature and voltage. The dependence of forward tunneling current on the temperature and bias can be explained by thermally induced band gap shrinkage and bias induced route change respectively. These results will be helpful for application of the optoelectronics device in both high and low temperature ambiences.

Key words： infrared emitter; tunneling; characteristic energy; band gap; thermal stress

Introduction

Gallium arsenide (GaAs) based infrared emitters are reliable optoelectronic devices, which has important applications in defense research, space optical communication, and the pumping sources of solid-state lasers, etc [ 1]. Such devices are not only suitable for low temperature applications, but also important for controlling parameters like angular position, speed and acceleration of motorized gimbals presented in Control Moment Gyroscope (CMGs) which helps to optimize spacecrafts orientation in space application [ 2– 4]. So, it is important to study the physical processes responsible for controlling the different parameters of the device.

GaAs is a direct band gap semiconductor with the band gap E_g = 1.443 eV and wavelength l_g = 860 nm at room temperature where p-n junctions are readily formed and have high luminance efficiency. Recently the diodes have made by doping with silicon (Si) for more luminance efficiency. Metalorganic vapor phase epitaxy (MOVPE) is one of the premier techniques for the fabrication of GaAs-based structures for optoelectronic device applications [ 5]. It is more important that in solution-grown GaAs, Si can act as either a donor (replacing Ga), or an acceptor (replacing As), depending on the growth conditions. In Si-doped diodes, the main luminescent transition occurs between the conduction band and acceptor level. The peak wavelength of the transition can lie between 910 and 1020 nm, and these Si-doped diodes have now become “standard” near-infrared emitting devices [ 6]. The effect of heavy doping (>10¹⁹ cm^-3) in GaAs is an important issue deciding its optical and electrical properties not only in view of a fundamental understanding but also for manipulation of the device applications. The high doping affects the band gap shrinkage and extends into the gap by a band tail [ 5, 7]. The electrical characterization of these devices can provide important information about the current transport through the narro-band gap p–n heterojunction and layer materials.

Several publications on the dominance of tunneling current in GaN based blue light emitting diodes (LEDs) are available [ 8– 10]. These literatures show that the variation of ideality factor with temperature for all type of LEDs is not same and also their characteristic energies are different. Many other workers also successfully apply this current transport model to explain the current transport process in different heterojunctions devices [ 11, 12]. There is not enough study on the carrier transport mechanism of the commercially available infrared emitters from high to low temperature ambiences. In this paper, by studying the temperature dependent characteristics of forward current in GaAs based infrared emitter, we try to find out the various mechanisms responsible for the variation of the same as a function of temperature and bias voltage. Also, we intend to identify the probable carriers, which are dominant in the tunneling current in such devices. Since these devices have wide applications, this study will be extremely useful to select the proper optoelectronics devices for designing the efficient instruments in the field of space science and defense research.

Experiment

In our investigation, we used one GaAs based infrared emitter (LD 261, 30 deg.) procured from RS Components, having a peak wavelength of 950 nm. The infrared emitter was placed inside a bath type optical cryostat designed by our laboratory [ 3]. 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 and Consultants) to avoid moisture on the sample. The temperature measurement in the range of 350–77 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 ±0.14 K. Current-voltage (I-V) measurements were performed by Keithley 2400 source measure unit. The details of the experimental set up are also available in our previous work [ 13].

Results and discussion

Forward I-V characteristics

Typical forward I-V curves are shown in Fig. 1. The most striking feature of forward I-V characteristics is the very weak dependence on temperature, which is indicated by the slope of I-V curves on a semilogarithmic plot, in contrast to the standard Shockley model of a p-n diode.

Fig.1 I-V characteristics of GaAs based infrared emitter in the temperature range of 350–77 K

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In fact, these curves are composed of two successive linear dependent segments with different slopes which are determined by the bias levels. The first segment is defined as low-bias (I) and the second segment as medium-bias (II) respectively. Both components can be well approximated by the exponential functions [10,14].

(1a) I_1=I_{\mathrm{S1}}\left[\exp \left(e{V}_{\mathrm{j}}/E_1\right)-1\right],V_{\mathrm{j}}\ge 0,

(1b) I_2=I_{\mathrm{S2}}\left[\exp \left(e{V}_{\mathrm{j}}/E_2\right)-1\right],V_{\mathrm{j}}\ge 0,

where e is electron charge, I_S1, I_S2 are pre-exponential factors, E₁, E₂ are the characteristic energies for low and medium bias levels respectively. In general, for a p-n junction diode, the I-V characteristics can be expressed by I = I_Sexp(eV/nKT–1), where n is ideality factor, K is Boltzmann constant and T is temperature. When the value of n lies between 1 and 2, the current flow is mainly diffusion recombination current; while if n is larger than 2, tunneling mechanism becomes dominant. If we assume the characteristic energies E₁ = n₁KT for low bias and E₂ = n₂KT for medium bias regions, then these energies represent the tunneling transparency of the energy barrier at the junction interface [ 8]. So, it is clear that the characteristic energies are directly proportional to both ideality factor (n) and temperature (T). Since n is increasing with lowering of temperature it indicates that the effect of tunneling is becoming more important. But the characteristic energy being a function of both n and T, is only weakly dependent on temperature, as n decreases with increase of T and vice versa. This weak dependences of E₁ and E₂ on temperature indicate that the dominant transport mechanism is associated with tunneling of carriers rather than thermal diffusion. Similar conclusion for different devices has been arrived at by other workers [ 10] also. The values of the characteristic energies E₁, E₂ and ideality factors n₁, n₂ as obtained from our data are listed in Table 1.

Tab.1 Experimental values of n₁, n₂, E₁, E₂ (for both low-bias and medium-bias, refer Eq. (1)), E₁/E₂, peak wavelength l_max and the band gap energy E_g at different temperatures

temperature/K	n1	n2	E1/meV	E2/meV	E1/E2	lmax/nm	Eg/eV
77	3.27	2.422	21.716	16.07	1.35	844	1.469
110	2.88	2.207	27.32	20.94	1.30	851	1.457
148	2.58	2.023	32.93	25.787	1.28	857	1.447
181	2.35	1.834	36.68	28.63	1.28	869	1.427
205	2.19	1.787	38.72	31.59	1.22	881	1.407
228	2.07	1.728	40.71	33.38	1.22	894	1.387
250	1.949	1.673	42.02	36.07	1.17	903	1.373
270	1.832	1.651	42.66	38.45	1.11	927	1.337
291	1.755	1.627	44.05	40.83	1.10	948	1.308
311	1.69	1.6	45.33	42.92	1.05	974	1.273
330	1.608	1.564	45.77	44.51	1.03	986	1.258
350	1.562	1.528	47.15	46.13	1.02	994	1.247

Figure 2 shows a plot of the characteristic energies E₁, E₂ and ideality factors n₁, n₂ versus temperature from 350 to 77 K.

Fig.2 Variations of characteristic energies (E) and ideality factors (n) as function of temperature

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Since the variation of the characteristic energy over the temperature range is of the order of a few meV, it indicates that the dominant transport mechanism is associated with tunneling of carrier rather than thermal diffusion.

However, the pre-exponential factors I_S1 and I_S2 themselves are temperature dependent which are illustrated in Fig. 3 for the low and medium voltage current components.

Fig.3 Temperature dependencies of pre-exponential factors I_S1 of low-voltage current component (close square) and I_S2 of medium-voltage current component (close triangle) with temperature

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The pre-exponential factors I_S1 varies from about 10^-7 A at 350 K to 10^-12 A at 77 K, whereas the variation obtained for I_S2, is from 10^-13 to 10^-26 A over the same temperature range. The effect of temperature on forward tunneling current, based on Eq. (1) could be included in the expression of I_S, which equals to the tunneling current at forward low-bias [ 8]. In case of medium-bias level, our experimental data show that the trend of change of I_S is quite similar to that of low-bias region particularly below 150 K. Hence we presume that the changes of I_S in both the cases are correlated with tunneling mechanism below 150 K. Similar variation of I_S at higher temperature cannot be correlated with tunneling mechanism, as its effect gradually decreases above 150 K. This is also evident from the lowering of the value of ideality factor with rise in temperature given in Table 1. A similar work was also done by Eliseev et al. [ 10] and they reported that the dependence of I_S below 100 K for Nichia NLPB-500 double-heterostructure blue LEDs is very similar for both bias polarities, providing yet another evidence for the tunneling nature of the current [ 10]. While Eliseev et al. have investigated the tunneling nature of the current for two different polarities, whereas we have studied it in a single polarity, but for two bias regions.

Tunneling current through p-n junction

The weak temperature dependent slope of I-V curves in the semilogarithmic plot suggests the involvement of tunneling transport across the junction, and associated impurity-level radiation emission is an evidence of tunneling to those levels. This mechanism of ‘‘excess current’’ in heavily doped junctions was considered by Morgan [ 15], where the bias-dependent shift of the emission peak was usually observed. Such types of process may occur in the space-charge region of the junction through one usually called diagonal tunneling or several intermediate (deep-center) states. Following the tunneling probability calculations, the forward-bias excess current involving deep level is obtained in the following form [ 15]:

(2) I\sim \exp \left\{{\displaystyle \frac{-\mathrm{4}\mathit{\gamma }}{\mathrm{3}}}{\left[{\displaystyle \frac{e\left(V_{\mathrm{b}}-V\right)}{E_{\mu }}}\right]}^{\mathrm{3}/\mathrm{2}}\right\},

where

(3) \gamma ={\left[{\displaystyle \frac{\mathrm{1}+r}{{\left(\mathrm{1}+r^{\mathrm{1}/\mathrm{3}}\right)}^{\mathrm{3}}}}\right]}^{\mathrm{1}/\mathrm{2}}.

It equals very nearly one half for reasonable values of the mass ratio r = m_l/m_n (here m_l is light hole mass and m_n is electron effective mass), V_b is the built-in potential barrier height and E_{\mu } is a bias-dependent characteristic tunneling energy, associated with the electric field in the junction. For an asymmetric step junction, Eq. (2) may be rearranged in the same manner of Eq. (1), namely,

(4) I=I_{\mathrm{S}}\left[\exp \left(e{V}_{\mathrm{j}}/E_{\mathrm{T}}\right)-\mathrm{1}\right],V_{\mathrm{j}}\ge \mathrm{0},

where E_T is a characteristic energy constant, given by

(5) E_{\mathrm{T}}\approx {\displaystyle \frac{\mathrm{4}eh}{\mathrm{\pi }}}\left(\sqrt{{\displaystyle \frac{N_{\mathrm{I}}}{m_{\mathrm{T}}^{*}{ϵ}_{\mathrm{r}}{ϵ}_{\mathrm{0}}}}}\right),

with N_I, the ionized impurity concentration, {ϵ}_{\mathrm{r}} , the static dielectric constant, {{ϵ}_{}}_0 , the vacuum permittivity and h is Plank’s constant. The tunneling effective mass m_{\mathrm{T}}^{*} is the reduced effective mass of light holes and electrons for an interband (diagonal) tunneling or effective mass of the carriers of one type for band-deep level tunneling. Assuming {ϵ}_{\mathrm{r}} ≈ 12.9, N_I = 10¹⁷ cm^-3 [ 16] and m_{\mathrm{T}}^{*} = 0.067m₀, we obtain E_T = 91.38 meV which is quite different from the experimental value (47.15 meV at 350 K to 21.716 meV at 77 K) due to the uncertainties associated with determination of the series resistance. These values have been used to make only a rough order of magnitude estimate of characteristic energy (E_T). Therefore, the low-voltage component may be explained as electron tunneling into n-GaAs, followed by the radiative transition via Si-related levels. In order to tentatively interpret the medium-voltage current component (with parameter E₂ between 46.13 at 350 K to 16.07 at 77 K) we take m_{\mathrm{T}}^{*} = 0.082m₀. This gives an estimate of E_T = 36.66 meV which is close to the experimental value of characteristic energy. Hence, the medium-voltage current component may be explained by tunneling of hole to deep levels on the p side of the junction.

Staircase model for doped infrared emitter

Beside the diagonal tunneling mechanism in which both electrons and holes tunnel to the junction region and successively recombine radioactively, another possible mechanism of carrier transport through the p-n junction is due to multistep tunneling. A multistep tunneling-recombination model has been proposed by Riben and Feucht to describe tunneling currents in forward bias in Ge-GaAs heterojunctions [ 12]. The model, instead of a single-step transition, consists in loss of electron energy through a number of steps before reaching finally to the valence band. We suggest that this mechanism may also determine the forward current in the Si doped GaAs based infrared emitter investigated in this paper. For the multistep tunneling-recombination mechanism (assuming the traps are uniformly distributed in energy and space), the forward current is given by

(6) I=A{N}_{\mathrm{t}}{\mathrm{e}}^{\left[-C{Q}^{\mathrm{1}/\mathrm{2}}\left(V_{\mathrm{b}}-V\right)\right]},

where

(7) C={\displaystyle \frac{\mathrm{8}}{\mathrm{3}h}}\sqrt{{\displaystyle \frac{m_{\mathrm{n}}{ϵ}_{\mathrm{S}}}{N_{\mathrm{d}}}}}.

Here A is a constant, N_t is the density of trapping levels, V_b is the built-in voltage of the diode, {ϵ}_{\mathrm{S}} is the dielectric constant of GaAs and l/Q is the number of the tunneling recombination steps required to traverse the diode depletion region. The value of V_b is estimated from capacitance-voltage (C-V) measurement. Figure 4 shows a plot of 1/C² versus V.

Fig.4 1/C² as function of reverse bias voltage (V) for GaAs infrared emitter at room temperature

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The extrapolated intercept of the l/C² versus V curve on the voltage axis gives the value of V_b. We have found the value of V_b is equal to 1.3 V at room temperature. Due to doping the GaAs infrared emitter has also its band gap energy altered from that of pure GaAs and is found to be 1.308 eV (see Table 1) at room temperature. This compares favorably with the measured value of V_b. Using typical values of the relevant material parameters and the fit of Eq. (6), we give the value of Q of the order of 0.195, which means that ~ 5 tunneling steps are required to traverse the depletion region. However, the assignment of a definite number should only be taken as a tentative one. The values of Q ranging from 0.001 to 0.3 have been reported by other workers [ 11]. Such a staircase path which consists of a series of tunneling transitions between trapping levels (in the diode space charge region) has a series of vertical steps. The carrier loses energy by transferring from one level to another as illustrated in Fig. 5.

Fig.5 Proposed model for tunneling recombination in Si-doped n type-GaAs infrared emitter. The quantities E_C, E_V and E_F denote the conduction band, the valence band and Fermi energy, respectively

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Band gap shrinkage effect due to doping and temperature

The interesting fact is that the tunneling recombination in GaAs infrared emitter is step dependent. For such mechanism, the tunneling probability P_t for each kind of carriers of I_S can be described by a horizontal tunneling model as follows [ 8]

(8) I_{\mathrm{S}}\propto P_{\mathrm{t}}=\exp \left(-{\displaystyle \frac{\omega {\phi }^{3/2}}{E}}\right).

With \omega ={\displaystyle \frac{\mathrm{8}\mathrm{\pi }{\left(\mathrm{2}{m}_{\mathrm{T}}^{*}\right)}^{\mathrm{1}/\mathrm{2}}}{\mathrm{3}eh}} , \phi is the tunneling barrier and E is the effective electrical field. At low forward bias, the electrical field E across a step p-n junction is given by E ≈ 2E_geW, where W is the width of space-charge region (SCR) (taking W ≈ (2ϵ_rϵ₀E_g/e²N_I)^1/2). If we assume \phi ≈ E_g, the carrier tunneling probability at very low forward bias is given by I_S ∝P_S∝ exp (–aE_g), with a ≈ (8p/3h)( m_{\mathrm{T}}^{*} ϵ_rϵ₀/N_I)^1/2. This expression suggests that the effect of temperature on forward tunneling current can be traced down to the band gap shrinkage effect or band gap narrowing (BGN). For the BGN effect, both width and height of the tunneling barrier would be reduced.

When GaAs is highly doped by Si, the BGN effect will be dominant for such type of devices. In general, the BGN is proportional to the electron concentration of the form n^1/3, and can be represented by [ 5, 7]

(9) \mathrm{\Delta }{E}_{\mathrm{g}}=E_{\mathrm{g}}\left(\mathrm{d}\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{d}\right)-E_{\mathrm{g}}\left(\mathrm{p}\mathrm{u}\mathrm{r}\mathrm{e}\right)=-B{n}^{\mathrm{1}/\mathrm{3}}.

The value of B has been adjusted to give the measured value of E_g at higher electron concentrations and the minus sign signifies the band gap shrinkage at higher concentrations. The empirical relation for BGN for GaAs can be written as

(10) \mathrm{\Delta }{E}_{\mathrm{g}}=-\mathrm{2.75}\times {\mathrm{10}}^{-\mathrm{8}}{n}^{\mathrm{1}/\mathrm{3}},

where ΔE_g is in eV and n is in cm^-3.

Assuming the general expression of temperature dependent threshold voltage V_th = hc/l_max, where c and l_max are the speeds of light in free space and peak wavelength, we can estimate the value of temperature dependent band gap E_g using the relation E_g = 1.24/l_max. The estimated values of l_max and E_g are listed in Table 1 and the variations of both with temperature are shown in Fig. 6.

Fig.6 Variation of peak wavelength (l_max) and band gap (E_g) in the temperature range from 350 to 77 K. The continuous line represents the theory, while dots the experimental points

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Many workers have measured the variation of peak wavelength with temperature and obtained an approximate linear correlation between the peak wavelength and the temperatures [ 17]. But for GaAs based infrared emitter, the peak wavelength shifts non-linearly with temperature and changes by about 15% towards the lower end of the infrared spectrum when the temperature is lowered from 350 to 77 K, this implies that the band gap of the device changes, and it is found to vary from 1.247 to 1.469 eV for the same temperature range.

In order to test whether the Varshni equation with modified values of its parameters can explain the measured temperature dependence of band gap energy of the doped GaAs, we write down the corresponding equation as follows

(11) E_{\mathrm{g}}^{\mathrm{d}}\left(T\right)=E_{\mathrm{g}}^{\mathrm{d}}\left(\mathrm{0}\right)-{\displaystyle \frac{{\alpha }^{\prime }{T}^{\mathrm{2}}}{{\beta }^{\prime }+T}},

where E_{\mathrm{g}}^{\mathrm{d}}\left(T\right) is the energy gap at temperature T, E_{\mathrm{g}}^{\mathrm{d}}\left(\mathrm{0}\right) is the energy gap at 0 K, {\alpha }^{\prime } is the empirical constant and {\beta }^{\prime } is approximately the 0 K Debye temperature. Treating E_{\mathrm{g}}^{\mathrm{d}}\left(\mathrm{0}\right) , {\alpha }^{\prime } , and {\beta }^{\prime } in Eq. (11) as free parameters, we tried to reproduce the experimental data in Fig. 6. It is found that Eq. (11) with E_{\mathrm{g}}^{\mathrm{d}}\left(\mathrm{0}\right) , {\alpha }^{\prime } , and {\beta }^{\prime } having values as 1.475 eV, 8.5 ´ 10^-4 eV/K and 195 K fairly reproduces the general trend of observation and hence may be used for the purpose of extrapolation to obtain a rough estimate of the band gap energy of the device at any arbitrary temperature.

A comparison of the values of parameters given above with those of pure GaAs, which are given as 1.519 eV, 5.41 ´ 10^-4 eV/K and 204 K [ 18], directly indicates that these values are different for different doping concentration.

Further, if we evaluate the temperature dependence of the band gap energy, a more quantitative comparison will reveal the difference between the two systems. Differentiating E_{\mathrm{g}}^{\mathrm{d}}\left(T\right) with respect to temperature, we get as follows

(12) {\displaystyle \frac{\mathrm{d}{E}_{\mathrm{g}}^{\mathrm{d}}\left(T\right)}{\mathrm{d}T}}=-{\displaystyle \frac{{\alpha }^{\prime }T\left(T+\mathrm{2}{\beta }^{\prime }\right)}{{\left(T+{\beta }^{\prime }\right)}^{\mathrm{2}}}}.

Validity of Eq. (12) implies that at any given temperature in the above range, one can obtain the value of the change of band gap energy with respect to temperature. However, the values of {\alpha }^{\prime } and {\beta }^{\prime } are not same for pure and doped GaAs, and hence the temperature dependence of the band gap energy should not be same. The trend of the change of temperature dependence band gap energy is illustrated in Fig. 7 for pure and doped GaAs.

Fig.7 Variation of temperature dependence band gap energy with temperature

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It can be observed from Fig. 7 that the magnitude of the temperature coefficient of band gap of doped GaAs increases from about 61% to 58%, when the temperature increases from 77 to 350 K.

BNG effect on tunneling current and nature of the carriers

For forward tunneling current using relation described as above, section temperature and bias coefficients have been obtained, and substituting them into Eq. (1) yields

(13) I=\exp \left(-\alpha E_{\mathrm{g}}+\beta eV-\mathrm{1}\right),

with b = 1/E_T ≈ (p/4h)( m_{\mathrm{T}}^{*} ϵ_rϵ₀/N_I)^1/2. Equation (13) indicates that the forward tunneling current, which is dependent on bias as well as temperature, is regulated by two independent parameters a and b. The theoretical value of the ratio a/b is a constant independent of both temperature and bias voltage, and it is given by the number ~ 10.667. Moreover, the exact values of m_{\mathrm{T}}^{*} and N_I are not so important to calculate the value of a/b.

Fig.8 Band gap dependence of forward tunneling current at different voltages of medium-bias region

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The semilog plots of forward current versus band gap at different voltages of medium-bias region is shown in Fig. 8, which clearly shows that the data points can be linearly fitted and the fitted lines are parallel to each other. The slopes of the fitted lines in Fig. 8 gives the value of a = 327.34 eV^-1 (here a = ln(I₁/I₂)/|ΔE_gT|, where |ΔE_gT| = [E_g1(T₁) – E_g2(T₂)] ≈ 0.029 eV and the subscripts 1 and 2 refer to data points at two different temperatures at a fixed bias). Using the reciprocal value of E_T, we can estimate the approximate value of b ≈ 31.59 eV^-1 (at 205 K) at medium bias region. The experimental value a/b equals to 10.362 at medium-bias region, which quite agrees with the theoretically predicted value of 10.667. Then the experimental value a/b demonstrates the validity of Eq. (13) for in and around this temperature.

If we consider the ratio of characteristics energies E₁/E₂ for both regions, then using Eq. (5) we get E₁/E₂ = {\left(m_{\mathrm{T}\mathrm{1}}^{*}/m_{\mathrm{T}\mathrm{2}}^{*}\right)}^{\mathrm{1}/\mathrm{2}} , i.e., the ratio of characteristic energies depends on the square root of the effective mass of the tunneling entities. In GaAs, the effective mass m_{\mathrm{T}}^{*} for light holes, heavy holes and electrons, are 0.082m₀, 0.51m₀ and 0.067m₀, respectively [ 19]. The average experimental value of E₁/E₂ yields ~ 1.18 over the entire temperature range, which is seen to be approximately consistent with the ratio of the square root of the effective masses for light holes and electrons of pure GaAs (~ 1.11). This seems to indicate that the tunneling current in low-bias region might be dominated by electron tunneling via intermediate states while light hole tunneling prevails in medium-bias region. For the low temperature end, the ratio is 1.35 at 77 K which is larger than 1.18. If we replace the light hole mass by the heavy hole mass, the ratio becomes 2.75 even much larger than the low temperature value, 1.35. One reason for this may be ascribed to the approximate relation that the effective electron mass is proportional to the band gap energy [ 20]. Estimating the constant of proportionality from the case of pure GaAs, the corrected value for doped GaAs shows that the change in the ratio will be 2.70, which indicates a change in the right direction but of insufficient magnitude. Again for GaN based blue light-emitting diodes, Yan et al. [ 8] reported that the tunneling current in low-bias region might be dominated by electron tunneling via intermediate states while heavy hole tunneling prevails in medium-bias region. So, the carriers involved in transport phenomena are different for different materials. They may also depend on the temperature as well as the voltage bias of the device.

Conclusions

In conclusion, the measurement of the forward I-V characteristics shows that the slope of the exponential curve changes relatively slowly as a function of temperature, which seems to suggest the involvement of various mechanisms including tunneling responsible for transport. It is shown that the results can be interpreted by assuming a multistep tunneling recombination mechanism. Also in forward I-V characteristics, we can identify low-bias and medium-bias exponential components. The low-bias component may be explained as electron tunneling into n-GaAs, followed by the radiative transition via Si-related levels. And the medium-bias component may be explained by tunneling of light hole to deep levels on the p side of the junction. The general trend of light intensity emitted by many LED is to increase with lowering of temperature. In this case, tunneling of electron is much dominant to enhance the process of radiative transition. This finding, together with the important striking feature of I-V characteristics, i.e., the observed tunneling current found in this investigation, should be taken into consideration in respect to design, fabrication and interpretation of the performance of GaAs heterojunctions infrared emitter devices at different temperatures. This work is expected to stimulate further theoretical and experimental study to achieve better understanding of the tunneling mechanism and the exact nature of carriers in various types of heterojunctions devices.

Acknowledgements

The authors acknowledge the Defence Research Development Organization (DRDO), India, for financial assistance, and one of the authors, Pradip Dalapati is thankful to DRDO for the award of a research fellowship.

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