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Gate Leakage Mechanisms in AlGaN/GaN and AlInN/GaN HEMTs: Comparison and Modeling Sreenidhi Turuvekere, Naveen Karumuri, A. Azizur Rahman, Arnab Bhattacharya, Senior Member, IEEE, Amitava DasGupta, Member, IEEE, and Nandita DasGupta, Member, IEEE
Abstract— The gate leakage mechanisms in AlInN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are compared using temperature-dependent gate current–voltage (IG –VG ) characteristics. The reverse bias gate current of AlInN/GaN HEMTs is decomposed into three distinct components, which are thermionic emission (TE), Poole–Frenkel (PF) emission, and Fowler–Nordheim (FN) tunneling. The electric field across the barrier in AlGaN/GaN HEMTs is not sufficient to support FN tunneling. Hence, only TE and PF emission is observed in AlGaN/GaN HEMTs. In both sets of devices, however, an additional trap-assisted tunneling component of current is observed at low reverse bias. A model to describe the experimental IG –VG characteristics is proposed and the procedure to extract the associated parameters is described. The model follows the experimental gate leakage current closely over a wide range of bias and temperature for both AlGaN/GaN and AlInN/GaN HEMTs. Index Terms— AlGaN/GaN high electron mobility transistor (HEMT), AlInN/GaN HEMT, gate leakage current, leakage current modeling, parameter extraction.
I. I NTRODUCTION
A
lGaN/GaN high electron mobility transistors (HEMTs) have been widely studied over the last two decades and several breakthrough performances have been demonstrated. Current gain cut-off frequency (fT ) as high as >100 GHz and output power density close to 10 W/mm have already been demonstrated with the basic device structure [1]–[2]. Power density >30 W/mm and fT >200 GHz have also been achieved with special device structures [3]–[4]. Strong spontaneous and piezoelectric polarization in this system has given an additional degree of freedom while designing these devices. Two-dimensional electron gas (2DEG) concentration >1013 cm−2 has been achieved with the proper choice of Al mole fraction and thickness of the barrier layer. Despite the excellent performance of these devices, the built-in strain in the barrier layer because of lattice mismatch Manuscript received March 15, 2013; accepted July 2, 2013. This work was supported by the Department of Science and Technology, Government of India. The review of this paper was arranged by Editor T. Palacios. S. Turuvekere, N. Karumuri, A. DasGupta, and N. DasGupta are with the Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai 600036, India. (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). A. A. Rahman and A. Bhattacharya are with the Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2013.2272700
and the additional strain during the operation of the device due to the inverse piezoelectric nature of AlGaN is a matter of concern while assessing the reliability of the device. It has been shown that high reverse gate voltage results in permanent structural breakdown [5]. Use of lattice matched barrier layer improves the reliability of the device as it is free from initial stress. Lattice matched Al0.83 In0.17 N (cited as AlInN hereafter) has emerged as a strain-free barrier layer on GaN and has offered several other advantages over AlGaN barrier layer [6]. Even in the absence of piezoelectric polarization, its higher spontaneous polarization coefficient gives rise to higher 2DEG concentration than that for AlGaN barriers with normally used Al mole fractions. Also because of the lower free surface potential of AlInN, the barrier layer can be made thinner without compromising 2DEG concentration. This has enabled sub-100 nm gate lengths without invoking shortchannel effects. Its chemical and thermal stability is wellestablished [7]. HEMT devices with gate lengths of 30 nm have exhibited fT as high as 250–300 GHz and microwave power >10 W/mm in X-band has been achieved with 0.25 μm gate length device [8]–[10]. While III-nitride system enjoys several attractive physical properties, gate leakage is one of the major problems plaguing these devices [11]–[16]. The 2DEG in the channel of an HEMT device is normally controlled by the gate Schottky diode. Because HEMT devices on these materials are normally on devices with high 2DEG concentration, large negative bias is necessary to turn off the device. Thus the gate leakage becomes significant in deciding the standby power dissipation and the reliability of the device. The observed reverse leakage current in AlGaN/GaN and AlInN/GaN HEMTs are much larger than that predicted by thermionic emission (TE). A wide range of bias and temperature dependence of gate leakage has been studied and reported for AlGaN/GaN HEMTs to explain the gate leakage mechanism [11]–[13]. Poole–Frenkel (PF) emission has been said to be the dominant leakage mechanism for the gate current conduction at higher temperature [11], [12], while Fowler–Nordheim (FN) tunneling is also observed at very low temperatures [11]. At large reverse bias, impact ionization at lower temperature, hopping conduction along the surface at moderate temperature and temperature-assisted tunneling at higher temperatures have been reported [13]. In case of AlInN/GaN HEMT, thinner barrier and higher electric field across the barrier results in carrier tunneling through the barrier, leading to larger gate leakage current. The very few reports available on the gate
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leakage mechanisms for AlInN/GaN HEMTs are restricted either to a small bias and/or temperature range [14]–[16]. In these studies, PF emission in the reverse bias [14], [15] and tunneling current via dislocations in the forward bias [16] have been reported. However, the gate encounters a large reverse bias during the normal operation of the device and understanding the leakage current mechanism over a wide range of gate bias is necessary. To the best of our knowledge, there are no reports on the study of gate leakage mechanisms in AlInN/GaN HEMT for a wide range of bias and temperature. In this paper, for the first time, we study and compare the gate leakage mechanisms in AlGaN/GaN and AlInN/GaN HEMTs over a wide range of bias and temperature, propose a model to describe the gate current–voltage (IG –VG ) characteristics, and explain the procedure to extract the associated parameters. In Section II, the process steps for fabrication of AlGaN/GaN and AlInN/GaN HEMT and the device characterization techniques are described. Temperature dependence of experimental IG −VG characteristics of AlGaN/GaN and AlInN/GaN are presented and compared in Section III. In Section IV, the model to describe the experimental IG −VG characteristics is proposed and the procedure to extract the associated model parameters is explained. Finally, the conclusions are drawn in Section V.
Fig. 1. Gate current density–voltage characteristics of AlGaN/GaN HEMT measured at different temperatures. Inset: the reverse current on linear scale to illustrate current saturation below threshold voltage.
diodes fabricated on the same die. The measurements were carried out at different temperatures from 223 to 473 K using Cascade Microtech Summit 12000 AP thermal chuck probe station. The gate current was measured at the gate terminal while setting VDS = 0.
II. E XPERIMENT The HEMT structures used in this paper are grown on c-plane sapphire substrates. The layer structure for AlGaN/GaN HEMT includes GaN buffer layer of 1-μm thickness, AlN spacer layer of 1-nm, and AlGaN barrier layer of 24-nm thickness with Al mole fraction of 26%. The layer structure for AlInN/GaN HEMT includes GaN buffer layer of 2-μm thickness, AlN spacer layer of 1-nm, AlInN barrier layer of 10-nm thickness with Al mole fraction of 83% and Si-doped GaN capping layer of 2-nm thickness. The samples were first degreased in organic solvents. Following this, mesa etching was carried out in BCl3 /Cl2 inductively coupled plasma with photoresist as mask. The mesa etch depth was 300 nm on both the samples. After mesa etching, the resist mask was stripped and the samples were immersed in ammonium sulfide solution with excess sulfur [(NH4 )2 SX , 40% sulfur]. This not only removes the native oxide but also passivates the surface and avoids the formation of oxide. After this step, photoresist was coated and windows were opened for ohmic contacts. Before loading the samples into the metallization chamber, the samples were immersed in HCl: H2 O (1:1) to remove any native oxide. Ti/Al/Au (30 nm/150 nm/50 nm) was then evaporated and lifted-off. The contact activation annealing was carried out in a conventional furnace at 550 °C for 5 min in nitrogen ambient. Finally, Schottky contacts were patterned, Ni/Au (20/75 nm) was evaporated and lifted-off. The devices were not passivated. The gate length, source-to-gate spacing, and source-to-drain spacing are 4, 10 and 30 μm, respectively. I–V characterization was carried out on dark using Agilent’s B1500A semiconductor device analyzer. Capacitance–Voltage (C–V ) characterization was carried out on large area Schottky
III. R ESULTS AND D ISCUSSION In this section, the models for different components of current are presented. The models are validated using the extracted values of individual current components. The procedure to obtain each of the current components from the raw data is described in detail in Section IV. A. Analysis of Gate I–V Characteristics of AlGaN/GaN and AlInN/GaN HEMTs 1) AlGaN/GaN HEMT: The gate current density (J)–voltage (V ) characteristics of AlGaN/GaN HEMT at different temperatures are shown in Fig. 1. A significant increase in reverse current with increase in temperature is observed. Also, the reverse current increases with the increase in the reverse bias (in region I) and saturates beyond threshold voltage (VG < −4 V, in region II), which is seen clearly on the linear scale shown in the inset of Fig. 1. In the lower forward bias region, a small increase in the gate current is observed with increase in temperature and the curves for different temperature merge at higher forward bias. a) Thermionic emission current: To analyze the gate current mechanism, forward bias is considered first. Considering TE as dominant phenomenon in this bias range, the J –V characteristics of a Schottky contact is described by the expression [17] qV −1 (1) JTE = J0 exp ηkT where
qφb J0 = A T exp − kT ∗
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Fig. 2. Variation of barrier height ( and ) and ideality factor (• and ◦) with temperature obtained from the forward IG –VG characteristics of AlGaN/GaN (solid symbols) and AlInN/GaN (open symbols) HEMT. Inset: typical plots of ln(J ) versus V for AlGaN/GaN and AlInN/GaN HEMTs.
J0 being the reverse saturation current density, A∗ the effective Richardson’s constant, T the absolute temperature, q the electron charge, φb the Schottky barrier height, η the ideality factor, and k the Boltzmann’s constant. A typical plot of ln(J ) versus V is shown in the inset of Fig. 2. It can be seen from the figure that ln(J ) versus V is a straight line as predicted by (1) signifying J ≈ JTE in this region of operation. The parameters, J0 (and hence φb ), and η can be extracted from the intercept and slope of the ln(JTE ) versus V for V > 3 kT, respectively. The extraction procedure is described in detail in Section IV. The extracted values of φb and η for different temperatures are plotted in Fig. 2. It is seen from the figure that φb increases monotonically with increase in temperature, while η does not show a monotonic behavior with temperature. b) Poole–Frenkel emission current: The reverse leakage current of these devices are significantly higher than that predicted by TE. To understand the reverse leakage mechanism, PF emission is considered as has been reported earlier [11], [12]. The current density (JPF )–electric field (E) dependence of PF emission is given by [18] √ q φt − (q E/πεi ) (2) JPF = C Eexp − kT where C is a constant, φt is the barrier height for the electron emission from the trap state and εi is the permittivity of the semiconductor at high frequency. Equation (2) can be rearranged as √ ln (JPF /E) = m(T ) E + c(T ) where
q q m(T ) = kT πεi qφt + ln(C). c(T ) = − kT
(3)
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Fig. 3. Plot of electric field as a function of gate voltage for AlGaN/GaN and AlInN/GaN HEMT. Inset: the typical C–V and nS versus VG characteristics for AlGaN/GaN HEMT.
The electric field across the barrier is calculated using the expression [12] E=
q(σb − n s ) ε
(4)
where σb is the bound charge at the hetero-interface, which is the sum of the piezoelectric polarization charge in the barrier and the difference between spontaneous polarization charge in the barrier and the buffer, n s is the 2DEG concentration at the hetero-interface and ε is the permittivity of the barrier. To calculate the electric field using (4), it is essential to know the value of σb and 2DEG concentration as a function of gate voltage. The value of σb used for calculating the electric field is 1.44 × 1013 cm−2 [19]. C–V profiling is one of the techniques used to extract n s as a function of gate voltage [12]. In this paper, the 2DEG concentration is obtained by integrating the C–V characteristics of a large area Schottky diode fabricated on the same die. A plot of electric field as a function of gate voltage is shown in Fig. 3. Inset of Fig. 3 shows the measured C–V characteristics and n S versus VG characteristics for AlGaN/GaN HEMT. The electric field across the barrier saturates below threshold voltage when n s becomes negligible with respect to σb . At zero gate bias, the value of n s is smaller than that of σb and hence, the electric field across the barrier is nonzero. The importance of this is discussed later in this section. √ Equation (3) suggests that the plot of ln(JPF /E) versus E should yield a straight line if the leakage current is because of PF emission. Further, the plot of the y-intercepts (c(T )) as a function of q/kT should yield another straight line whose slope gives the value of φt . The plot of ln( JPF /E) versus √ E for different temperatures is shown in Fig. 4. The plot of c(T ) versus q/kT is shown as inset. The data points fit well with straight lines for both the plots indicating PF emission. The value of φt extracted from the slope of the plot c(T ) versus q/kT is 0.17 eV, which is slightly lower than the values reported in the literature [11].
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Fig. 4. PF emission plot for the Reg I of the IG –VG characteristics of AlGaN/GaN HEMT. Inset: the plot of c(T ) versus q/kT .
c) Trap-Assisted tunneling current: The electric field across the barrier does not go to zero at zero gate bias, as mentioned earlier. This predicts a nonzero current at zero gate bias because of electric field dependence of PF emission current, which is not possible. Yan et al. [12] have proposed that forward defect-assisted tunneling current flows from gate to the channel to compensate for the PF emission current flowing from the channel to the gate near zero bias. Further, it is shown that this defect-assisted tunneling current has the same temperature dependence as that of PF emission current. In this paper, this additional current component, represented as JTAT , is modeled with an exponential function which takes the expression similar to that of TE current. The expression for JTAT is given by q(V − V0 ) JTAT = J02 exp −1 (5) η2 kT where J02 , V0 , and η2 are the parameters used to fit the experimental characteristics near zero bias. A typical plot of ln( JTAT ) versus (V −V0 ) is shown in Fig. 5. It is seen from the figure that the data points fit well on a straight line validating the use of (5). d) AlInN/GaN HEMT: The gate J –V characteristics of AlInN/GaN HEMT at different temperatures are shown in Fig. 6. The forward characteristics resemble that of AlGaN/GaN HEMT. However, a significant difference is observed in the reverse bias region. Apart from the strong temperature-dependent lower reverse bias region (Reg I) and saturation region at higher reverse bias beyond threshold voltage (Reg III), a weak temperature-dependent and strong bias-dependent region in the moderate reverse bias (Reg II) is observed. The behavior in the forward bias region and the Reg I of the reverse bias is similar to that of AlGaN/GaN HEMT. It is found that the experimental data fit well with the characteristics predicted by TE in the forward bias and the PF emission in the Reg I of the reverse bias region. Fig. 2 shows the extracted values of φb and η for AlInN/GaN HEMTs at different temperatures. The behavior of both φb and η are
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Fig. 5. Typical ln(JTAT ) versus (VG –V0 ) plots for AlGaN/GaN and AlInN/GaN HEMTs.
Fig. 6. Gate current density–voltage characteristics of AlInN/GaN HEMT measured at different temperatures.
similar to that of AlGaN/GaN case. Fig. 7 shows the PF emission curves for different temperatures. The electric field is calculated using (4) with σb = 2.8 × 1013 cm−2 [19]. The plot of the y- intercept as a function of temperature is shown as inset in Fig. 7. It can be seen clearly that all the curves fit well on a straight line indicating PF emission in Reg I. The value of φt extracted from the slope of the plot c(T ) versus q/kT is 0.54 eV, which is higher than the value reported in the literature [14]. To analyze the gate current in Reg II of the reverse bias, FN tunneling is assumed because of strong electric field dependence and weak temperature dependence in this region. The JFN –E dependence of FN tunneling is given by [18] B 2 (6) JFN = AE exp − E with
8π 2m ∗n (qφeff )3 B= 3qh
where A is a constant, m ∗n is the conduction-band effective mass in semiconductor, h is the Planck’s constant, and φeff is
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Fig. 7. PF emission plot for Reg I of the IG –VG characteristics of AlInN/GaN HEMT. Inset: the plot of c(T ) versus q/kT .
Fig. 8. FN tunneling plots for Reg II of the IG –VG characteristics of AlInN/GaN HEMT.
the effective barrier height. Rearranging (6), we get ln JFN /E 2 = ln(A) − B/E
(7)
which indicates that the plot of ln( JFN /E 2 ) versus 1/E yields a straight line. Fig. 8 shows that it is indeed a straight line and the value of the effective barrier height extracted from the slope is 2.3 eV using the effective mass of electron in AlInN as 0.4m e , where m e is the free electron mass. The extracted values of barrier height varied slightly (12%) with temperature. Apart from these three current components (TE, PF, and FN) for AlInN/GaN devices, at low reverse bias, a defect-assisted tunneling current has also been considered as discussed in the case of AlGaN/GaN devices. B. Comparison of Gate J–V Characteristics of AlGaN/GaN and AlInN/GaN HEMTs To compare the J –V characteristics, the reverse bias region is considered first. As discussed in the previous
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section, the current in the lower bias region for AlGaN/GaN HEMTs increases with increase in field because of PF emission, and in the higher bias region, the current saturates because of saturation of electric field beyond the threshold voltage. However, for AlInN/GaN HEMTs, it is seen that the reverse bias characteristics has an intermediate region where the current increases with increase in bias because of FN tunneling. Conduction band (CB) edge diagrams, obtained from Sentaurus device simulator [20] as shown in Fig. 9, are used to explain this behavior in AlGaN/GaN and AlInN/GaN HEMTs. Fig. 9(a) shows CB edges of AlGaN/GaN and AlInN/GaN HEMTs at thermal equilibrium. Fig. 9(b) and (c) represents the cases at low reverse bias and at a voltage close to the threshold voltage, respectively. As seen Figs. 3 and 9(a) the electric field across the AlInN barrier is higher than that across the AlGaN barrier. A continuum of states is believed to be present in these materials, which originates from the conductive dislocations in the barrier layer. This is shown as a band of states marked in gray and labeled as E dis in Fig. 9(a) and (b). Assuming the trap states are very close to the metal Fermi level, the continuum of states is at a height equal to φt from the metal Fermi level [11], [15]. In the lower reverse bias region, the current conduction in both AlGaN/GaN and AlInN/GaN is dictated by PF emission. The PF emission mechanism corresponds to the activation of carriers from a trap state to the continuum of states because of thermal energy. The barrier for the emission of electron from trap state to continuum of states decreases with increase in electric field similar to that of Schottky barrier lowering. The extracted values of zero field barrier height for electron emission, φt , for AlGaN/GaN and AlInN/GaN are 0.17 eV and 0.54 eV, respectively. As the gate voltage is increased further, the electric field across the barrier increases. At a critical electric field, when the barrier width at the metal Fermi level becomes < 5 nm, the electrons at the metal Fermi level tunnel through the AlInN barrier as shown in Fig. 9(c), and the FN tunneling current starts to dominate. The value of φeff extracted from the slope of FN plot in Fig. 8 is 2.3 eV. The critical electric field required for FN tunneling to dominate can be estimated as E critical = φeff /5 nm. The value of E critical for AlInN/GaN device is 4.6 MV/cm which is reached at a gate voltage −4.5 V as seen in Fig. 3. This is also validated from Fig. 6, where we observe that FN component indeed starts to dominate around the same gate voltage. This region of operation is found only in case of AlInN/GaN HEMT and not in AlGaN/GaN HEMT because the electric field across the barrier in AlGaN is not sufficiently large. Assuming φeff = 1.4 eV for AlGaN/GaN HEMT, E critical = 2.8 MV/cm. As seen in Fig. 3, in case of AlGaN/GaN HEMT, the electric field saturates before it reaches the critical field, primarily because of the lower value of σb . Thus FN tunneling current is insignificant. At a given temperature, the AlInN/GaN HEMT has higher gate leakage current compared to the AlGaN/GaN device. This is because the higher spontaneous polarization charge in the AlInN/GaN system results in greater electric field, which in turn results in the increased PF emission component and
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leakage current mechanism in AlGaN/GaN HEMTs. The PF emission component has strong temperature dependence and only at very low temperatures the FN tunneling component may be observed because of the reduced PF emission component [11]. On the other hand, FN tunneling will be the dominant component of current in AlInN/GaN HEMTs at large reverse bias voltages near the threshold voltage. This is true irrespective of the thickness of the barrier layer. It has been reported that for a higher barrier thickness of AlInN (20 nm) than in our experiment, only PF emission component is significant even at larger reverse bias of −5 V [14]. This is because a thicker AlInN layer results in a higher threshold voltage and n s is significantly large even at VG = −5 V, resulting in lower electric field. Thus, FN component will be dominant only at larger reverse bias when the thickness of the barrier layer is increased. In the forward bias region, both AlGaN/GaN and AlInN/GaN HEMTs show similar behavior. As shown in Fig. 2, the extracted barrier heights increase with increase in temperature. However, the values of φb are much smaller than that predicted by TE model. This reduction in φb may be attributed to the presence of a tunneling current via dislocations [16]. As the temperature is increased, the TE component increases resulting in increase in the extracted φb . However, even at 473 K, the extracted value of φb is much less than expected, indicating that tunneling current is still dominant at this temperature. The ideality factor when tunneling is the dominant phenomenon is given by [16] E 00 E 00 (8) coth η= kT kT where E 00 is the characteristic tunneling energy, which should be a temperature-independent parameter. Thus η should decrease monotonically with increase in temperature and then saturate at unity. However, in our case η first decreases with increase in temperature as expected but again starts to increase for T >373 K as shown in Fig. 2. It is suggested that increase in temperature >373 K activates more traps/defects which modifies the value of E 00 . This is possibly the reason for the increase in η in both AlGaN/GaN and AlInN/GaN devices for T > 373 K. Also, this increased activation of traps at higher temperature can modify the value of φeff resulting in a mild temperature dependence of JFN seen in the Reg II of J –V characteristics of AlInN/GaN devices in Fig. 6. While the extracted ideality factor shown in Fig. 2 has increased at 423 K and 473 K, the current in the FN plots at these temperatures are slightly higher than the rest as seen in Fig. 8. IV. M ODEL F ORMULATION AND PARAMETER E XTRACTION Fig. 9. Simulated conduction band edge diagram of AlGaN/GaN and AlInN/GaN HEMT at (a) equilibrium, (b) small gate voltage (Reg I), and (c) gate voltage near threshold.
also the dominance of the FN tunneling component at higher reverse bias. It can also be concluded that because of the lower electric field, PF emission component is expected to be the dominant
A. Model Formulation The gate leakage current in AlGaN/GaN and AlInN/GaN HEMTs can be modeled as a voltage and temperaturedependent current source I in series with a resistance R, where I = Area ∗ (JTE + JPF + JFN + JTAT )
(9)
where, JTE is the TE current density, JPF is the PF emission current density, JFN is the FN tunneling current density,
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and JTAT is the trap-assisted tunneling current density. The expressions for JTE , JPF , JTAT , and JFN are given by (1), (2), (5), and (6), respectively. Fig. 10(a) and (b) compares the model with experimental data at 473 K for AlGaN/GaN and AlInN/GaN HEMT, respectively. Various components of the diode currents are also shown for reference. It can be seen that the model follows the experimental data points closely over the entire region of operation for AlGaN/GaN HEMT. For AlInN/GaN HEMT, there is a slight deviation in the region close to and below threshold voltage (−6 V). This discrepancy is observed for all temperatures and the predicted current is always lower than the experimental current. The possible reason for underestimation of current could be as follows. The electrons tunneling from metal to GaN have large kinetic energy. These electrons, while losing kinetic energy, could generate electron–hole pairs, which results in higher current than that predicted by FN tunneling. This effect is accounted for in device simulators such as Sentautus with the help of a multiplication factor greater than unity [20]. The I –V characteristics at 473 K is shown in Fig. 10 for the sake of clarity. Excellent agreement between the model and experimental points has also been obtained at all other temperatures. B. Parameter Extraction The parameters associated with the proposed model include those of the TE ( J0 and η), PF emission (c and m), FN tunneling ( A and B) and trap-assisted tunneling (V0 , J02 , and η2 ) components of current as well as the value of resistor R. The procedure to extract these parameters is described below. The parameters for a particular current component are extracted in the region where it is dominant. 1) TE Current Parameters: As shown in Fig. 10, the TE current is the dominant component in the lower forward bias region. In this region, when the applied bias is higher than 3 kT, (1) can be rearranged as qV . (10) ln(J ) = ln(J0 ) + nkT A plot of ln( J ) versus V yields a straight line. The intercept of the straight line gives the value of J0 , and η can be extracted from the slope of the straight line. 2) PF Emission Current Parameters: It is seen from Fig. 10 that the PF emission current is dominant at lower reverse bias region. In this region of operation, JPF can be approximated as the difference between the measured current density ( J ) and the TE component (JTE ), which is calculated by substituting the extracted parameters in (1). JPF is now used to obtain the PF plot for the lower reverse bias region (shown in Figs. 4 and 7). As evident from (3), the parameters c and m are the intercepts and slopes of these plots, respectively. 3) FN Tunneling Current Parameters: The FN tunneling current component may be dominant in the higher revese bias region close to the threshold voltage. In this region, JFN can be approximated as JFN = J − JTE − JPF . The extracted TE and PF emission parameters are substituted in (1) and (3) to obtain JTE and JPF , respectively. JFN is now used to obtain
Fig. 10. I –V characteristics of (a) AlGaN/GaN and (b) AlInN/GaN HEMT comparing the model with the experiment. Various current components in the model are shown for reference.
the FN plot (shown in Fig. 8). The parameters A and B are obtained from the intercepts and slopes of the straight line fits respectively as seen from (7). In AlGaN/GaN HEMT, JFN is very small signifying negligible FN tunneling. 4) Trap-Assisted Tunneling Current Component Parameters: In the reverse bias region close to the origin, the current component JTAT becomes significant. This current component can be expressed as JTAT = J − JTE − JPF − JFN . This current is represented by (5). The value of V0 is the voltage at which the sum of JTE , JPF , and JFN starts to deviate from the experimental curve. The value of η2 is extracted from the slope of ln(J TAT ) versus (V − V0 ) (shown in Fig. 5) using a similar procedure described previously for the TE current. The value of J02 is calculated by equating JTAT to the sum of JPF and JFN at zero gate bias. 5) Series Resistance: In the forward bias region, the current has to increase exponentially with applied voltage. However, the current on the semilogarithmic I –V characteristic plot deviates from the straight line at larger forward bias because of the additional voltage drop across R. To extract its value, the current in the lower forward bias region is extrapolated
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towards higher current. The voltage difference between the experimental curve and the extrapolated curve for a given current is the voltage drop across R. The ratio of this voltage to the current at which the voltage difference is measured gives the value of R. V. C ONCLUSION The IG –VG characteristics of AlGaN/GaN and AlInN/GaN HEMTs are analyzed and possible mechanisms for gate leakage current are proposed. The gate leakage current mechanisms in AlGaN/GaN and AlInN/GaN HEMTs are compared using conduction band edge diagrams. While TE and PF emission are observed in AlGaN/GaN HEMT, an additional FN tunneling component exists in AlInN/GaN HEMT. Trapassisted tunneling current component is also observed in both the set of devices. While higher value of σb in AlInN/GaN system makes it an attractive alternative to AlGaN/GaN, the same is also responsible for higher gate leakage current. A model to describe the gate I –V behavior of AlGaN/GaN and AlInN/GaN HEMT is proposed and validated. Procedure to extract the associated model parameters is described. The model follows the experimental characteristics closely for a wide range of bias and temperature for AlGaN/GaN and AlInN/GaN HEMTs. Although the results presented in this paper are for devices with 4 μm gate length, similar results have been obtained for devices with gate lengths varying from 2.5 to 40 μm, as gate current density is used for analysis. The proposed model can be, in general, used for any III-nitridebased HEMT. R EFERENCES [1] V. Kumar, W. Lu, R. Schwindt, A. Kuliev, G. Simin, J. Yang, M. A. Khan, and I. Adesida, “AlGaN/GaN HEMTs on SiC with fT of over 120 GHz,” IEEE Electron Device Lett., vol. 23, no. 8, pp. 455–457, Aug. 2002. [2] Y.-F. Wu, D. Kapolnek, J. P. Ibbetson, P. Parikh, B. P. Keller, and U. K. Mishra, “Very-high power density AlGaN/GaN HEMTs,” IEEE Trans. Electron Devices, vol. 48, no. 3, pp. 586–590, Mar. 2001. [3] Y.-F. Wu, A. Saxler, M. Moore, R. P. Smith, S. T. Sheppard, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh, “30-W/mm GaN HEMTs by field plate optimization,” IEEE Electron Device Lett., vol. 25, no. 3, pp. 117–119, Mar. 2004. [4] K. Shinohara, A. Corrion, D. Regan, I. Milosavljevic, D. Brown, S. Burnham, P. J. Willadsen, C. Butler, A. Schmitz, D. Wheeler, A. Fung, and M. Micovic, “220 GHz fT and 400 GHz fmax in 40-nm GaN DH-HEMTs with re-grown ohmic,” in Proc. IEEE IEDM, Dec. 2010, pp. 30.1.1–30.1.4. [5] J. A. del Alamo and J. Joe, “GaN HEMT reliability,” Microelectron. Rel., vol. 49, nos. 9–11, pp. 1200–1206, Sep./Nov. 2009. [6] F. Medjdoub, J. F. Carlin, C. Gaquiere, N. Grandjean, and E. Kohn, “Status of the emerging InAlN/GaN power HEMT technology,” Open Electr. Electron. Eng. J., vol. 2, pp. 1–7, Jan. 2008. [7] F. Medjdoub, M. Alomari, J.-F. Carlin, M. Gonschorek, E. Feltin, M. A. Py, N. Grandjean, and E. Kohn, “Thermal stability of 5 nm barrier in InAlN/GaN HEMTs,” in Proc. ISDRS, Dec. 2007, pp. 1–2. [8] D. S. Lee, J. W. Chung, H. Wang, G. Xiang, G. Shiping, P. Fay, and T. Palacios, “245-GHz InAlN/GaN HEMTs with oxygen plasma treatment,” IEEE Electron Device Lett., vol. 32, no. 6, pp. 755–757, Jun. 2011. [9] D. S. Lee, G. Xiang, G. Shiping, D. Kopp, P. Fay, and T. Palacios, “300-GHz InAlN/GaN HEMTs with InGaN back barrier,” IEEE Electron Device Lett., vol. 32, no. 11, pp. 1525–1527, Nov. 2011. [10] N. Sarazin, E. Morvan, M. A. di Forte Poisson, M. Oualli, C. Gaquiere, O. Jardel, O. Drisse, M. Tordjman, M. Magis, and S. L. Delage, “AlInN/AlN/GaN HEMT technology on SiC with 10-W/mm and 50% PAE at 10 GHz,” IEEE Electron Device Lett., vol. 31, no. 1, pp. 11–13, Jan. 2010.
[11] H. Zhang, E. J. Miller, and E. T. Yu, “Analysis of leakage current mechanisms in Schottky contacts to GaN and Al0.25 Ga0.75 N/GaN grown by molecular-beam epitaxy,” J. Appl. Phys., vol. 99, pp. 023703-1–023703-6, Jan. 2006. [12] D. Yan, H. Lu, D. Cao, D. Chen, R. Zhang, and Y. Zheng, “On the reverse gate leakage current of AlGaN/GaN high electron mobility transistors,” Appl. Phys. Lett., vol. 97, no. 15, pp. 153503-1–153503-3, Oct. 2010. [13] S. Arulkumaran, T. Egawa, H. Ishikawa, and T. Jimbo, “Temperature dependence of gate–leakage current in AlGaN/GaN high-electronmobility transistors,” Appl. Phys. Lett., vol. 82, no. 18, pp. 3110–3112, Mar. 2003. [14] E. Arslan, S. Butun, and E. Ozbay, “Leakage current by Frenkel–Poole emission in Ni/Au Schottky contacts on Al0.83 In0.17 N/AlN/GaN heterostructures,” Appl. Phys. Lett., vol. 94, no. 14, pp. 142106-1–142106-3, Apr. 2009. [15] W. Chikhaoui, J.-M. Bluet, M.-A. Poisson, N. Sarazin, C. Dua, and C. Bru-Chevallier, “Current deep level transient spectroscopy analysis of AlInN/GaN high electron mobility transistors: Mechanism of gate leakage,” Appl. Phys. Lett., vol. 96, no. 7, pp. 072107-1–072107-3, Feb. 2010. [16] E. Arslan, S. Altindal, S. Ozcelik, and E. Ozbay, “Tunneling current via dislocations in Schottky diodes on AlInN/AlN/GaN heterostructures,” Semicond. Sci. Technol., vol. 24, no. 7, pp. 075003-1–075003-6, Jul. 2009. [17] E. H. Rhoderick and R. H. Williams, “Current-transport mechanisms,” in Metal—Semiconductor Contacts, 2nd ed. Oxford, U.K.: Clarendon, 1988, pp. 98–100. [18] S. M. Sze, “MIS diode and charge-coupled devices,” in Physics of Semiconductor Devices, 2nd ed. New York, NY, USA: Wiley, 2001, pp. 402–407. [19] Electronic Archive. (2013, Jul.). New Semiconductor Materials, Characteristics and Properties, Washington, DC, USA [Online]. Available: http://www.ioffe.ru/SVA/NSM/Semicond/ [20] Sentaurus Device User Guide, Version F-2011.09, Synopsys, Mountain View, CA, USA, Sep. 2011, pp. 587–588.
Sreenidhi Turuvekere is currently pursuing the Ph.D. degree in GaN-based HEMT devices with the Indian Institute of Technology Madras, Chennai, India. His current research interests include fabrication and characterization of wide band-gap semiconductor devices for RF and power electronics applications.
Naveen Karumuri has been pursuing the Ph.D. degree with the Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai, India, since 2010. His current research interests include modeling and characterization of GaN-based electronic devices.
A. Azizur Rahman received the M.Sc. degree in energy science from Madurai Kamaraj University, Madurai, India. He is a Scientific Staff Member with the Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India.
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. TURUVEKERE et al.: GATE LEAKAGE MECHANISMS IN AlGaN/GaN AND AlInN/GaN HEMTs
Arnab Bhattacharya (M’94–SM’06) received the Ph.D. degree from the University of WisconsinMadison, Madison, WI, USA. He is an Associate Professor in the Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India.
Amitava DasGupta (S’85–M’88) received the Ph.D. degree from the Indian Institute of Technology (IIT) Kharagpur, India, in 1988. He has been a Faculty Member in the Department of Electrical Engineering, IIT Madras, since 1993 and is currently a Professor.
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Nandita DasGupta (M’08) received the Ph.D. degree from the Indian Institute of Technology (IIT) Madras, Chennai, India, in 1988. She has been a Faculty Member with the Department of Electrical Engineering, IIT Madras, since 1993 and is currently a Professor.
