High-Temperature Characterization of AlGaN Channel High Electron Mobility Transistor Based on Silicon Substrate

Abstract

In this paper, it is demonstrated that the AlGaN high electron mobility transistor (HEMT) based on silicon wafer exhibits excellent high-temperature performance. First, the output characteristics show that the ratio of on-resistance (R_ON) only reaches 1.55 when the working temperature increases from 25 °C to 150 °C. This increase in R_ON is caused by a reduction in optical phonon scattering-limited mobility (μ_OP) in the AlGaN material. Moreover, the device also displays great high-performance stability in that the variation of the threshold voltage (ΔV_TH) is only 0.1 V, and the off-state leakage current (I_D,off-state) is simply increased from 2.87 × 10⁻⁵ to 1.85 × 10⁻⁴ mA/mm, under the operating temperature variation from 25 °C to 200 °C. It is found that the two trap states are induced at high temperatures, and the trap state densities (D_T) of 4.09 × 10¹²~5.95 × 10¹² and 7.58 × 10¹²~1.53 × 10¹³ cm⁻² eV⁻¹ are located at E_T in a range of 0.46~0.48 eV and 0.57~0.61 eV, respectively, which lead to the slight performance degeneration of AlGaN HEMT. Therefore, this work provides experimental and theoretical evidence of AlGaN HEMT for high-temperature applications, pushing the development of ultra-wide gap semiconductors greatly.

1. Introduction

Owing to the high electric field strength and electron mobility of the GaN material, GaN HEMTs have been widely researched and applied in power electronics [1,2,3,4,5]. Compared with traditional GaN materials, ultra-wide bandgap (UWBD) materials possess higher bandgaps and critical electric fields; thus, these devices based on UWBD materials have more developmental potential in power electronic applications [6,7,8,9,10,11]. Among these UWBD materials (β-Ga₂O₃, diamond, and AlN), the material growth and device fabrication of AlGaN appear to be much easier due to the relevant GaN-based heterojunction. Moreover, AlGaN HEMTs have been recognized as an excellent candidate for high-temperature applications because of their great thermal stability [12,13]. The outstanding performance of AlGaN HEMTs under extreme temperature conditions makes them an ideal choice for a wide range of applications, including aerospace, electric vehicles, radar and communication, energy conversion, and industrial automation.

For these reasons, many scholars have carried out extensive studies on the device characteristics of AlGaN HEMTs at high temperatures. Numerous valuable achievements have been obtained, including lower drain current attenuation, off-state leakage current, on-resistance degradation, channel mobility attenuation, smaller threshold voltage shift, higher breakdown electric field, saturated drain current, and current gain cut-off frequency of AlGaN channel HEMTs compared with traditional GaN channel HEMTs [14,15,16,17,18,19,20,21,22,23,24,25]. However, most of these reports are based on sapphire and AlN substrates, and there is a lack of research on the performance and related analysis of the AlGaN HEMTs based on silicon (Si-based AlGaN HEMTs) at high temperatures. As is known, the Si-based AlGaN HEMTs have better thermal conductivity and greater commercialization potential due to the use of low-cost substrates. Therefore, it is necessary to investigate the high-temperature characteristics of Si-based AlGaN HEMTs and identify the relevant influence factors for performance degeneration so as to put forward more useful strategies to solve it.

In this work, the technology of pulsed metal–organic chemical vapor deposition (MOCVD) was used to fabricate the 6-inch AlGaN wafer on the Si substrate. Then, the Si-based AlGaN HEMT with 400 nm Al_0.1Ga_0.9N as a channel layer was fabricated and showed a low contact resistance of 0.95 Ω·mm and a high breakdown voltage of 1110 V (with L_GD = 15 μm). In addition, the output and transfer characteristics of this device at high-temperature working conditions were systematically investigated. Using the alternating current conductance method, the trapping effect in the AlGaN channel was characterized by time constants (τ_T) and D_T, which helped to explain the performance degradation. Overall, given the cost advantages of Si-based AlGaN HEMTs, this study provides valuable references for future design optimizations of such devices and also lays the theoretical foundation for their practical applications in harsh environments, such as those involving high temperatures and high powers.

2. Materials and Methods

The pulsed MOCVD system was used to grow AlGaN material on a 6-inch Si substrate. Firstly, an AlN nucleation layer was deposited. Then, a 3 μm graded Al_xGa_1−xN layer, 1 μm C-doped Al_0.1Ga_0.9N layer, 400 nm Al_0.1Ga_0.9N channel layer, 1 nm AlN insertion layer, 24 nm Al_0.4Ga_0.6N barrier layer, and 2 nm GaN cap layer were grown above the AlN nucleation layer as a sequence. The HRXRD rocking curves from (0002) and (10–12) planes of the AlGaN channel layer in this work are 708 and 1147 arcsec, respectively.

The fabrication process of AlGaN HEMTs began with mesa isolation etching. Thereafter, a 30 nm recess was etched accurately using the inductively coupled plasma (ICP) technique, and the Ti/Al alloy was deposited and annealed to form ohmic contact as a source/drain electrode. The Ni/Al alloy was deposited as a gate electrode. Finally, 40 nm Al₂O₃ was grown on the surface of AlGaN HEMTs by plasma-enhanced atomic layer deposition (PEALD) to suppress the effect of surface states. The device tested in this work had a gate width (W_G) of 100 µm, a gate length (L_G) of 4 µm, a gate–source distance (L_GS) of 4 µm, and a gate–drain distance (L_GD) of 15 µm. The cross-sectional structure of AlGaN HEMT is shown in Figure 1. The output and transfer characteristics were tested using the Agilent B1505A high-voltage semiconductor analyzer system (Keysight Technologies, Beijing, China).

3. Results and Discussion

As shown in Figure 2a, the DC output characteristics of Si-based AlGaN HEMTs were measured at 25~200 °C with a growing step of 25 °C under a given V_G of 3 V. At 25 °C, the maximum output current (I_D,max) of the AlGaN HEMTs is 323.14 mA/mm, displaying a gradually decreasing trend as the temperature rises. The I_D,max still maintains values of 212.04 and 189.50 mA/mm at high temperatures of 150 and 200 °C, respectively. So, the ratio R_{ON,150 °C}/R_{ON,25 °C} for the Si-based AlGaN HEMT is only 1.55 in this work, which is significantly lower than the corresponding values of commercial GaN HEMTs on silicon (the values of R_{ON,150 °C}/R_{ON,25 °C} of the GaN systems GS 66516B and GS 66502B are about 2.58). Therefore, the Si-based AlGaN HEMT demonstrates more stable output characteristics compared with GaN HEMTs at high-temperature applications [9,10].

The degeneration of I_D,max at high temperatures may be caused by an increase in ohmic contact resistance (R_C) and 2DEG channel resistance (R_CH). R_ON can be realized as R_ON = R_S + R_D + 2R_C + R_CH. R_S or R_D is the parasitic source/drain resistance. Usually, R_S and R_D are much lower than R_C. As shown in Figure 2b, R_C is calculated using a transmission line model (TLM) at different temperatures. The variation of R_C is very poor with the growing temperature. Compared with the variation of R_ON, the variation of R_C is negligible. However, it is observed that the sheet resistance (R_SH) increases from 947.0 Ω/□ to 1945.7 Ω/□ as the temperature rises from 25 °C to 200 °C, indicating that the rise in R_ON can be attributed to increased channel resistance, which can be interpreted as decreased carrier mobility.

The temperature-dependent C-V characteristics of the Schottky barrier diode with an AlGaN channel are shown in Figure 3a. The electron concentration of the AlGaN channel varied with the temperature-dependent depth profiles and are derived by the following Equations (1) and (2) [26,27], and the relevant results are shown in Figure 3b:

Z = εA/C

(1)

$$n_c=-{\displaystyle \frac{2}{\mathrm{e}\mathsf{\epsilon }{A}^2}}\times {\displaystyle \frac{1}{d\left(1/c^2\right)/dV}}q$$

(2)

$$n_{2DEG}={{\displaystyle \int }}_{-\infty }^{+\infty }{n}_c\left(Z\right)dZ$$

(3)

μ = 1/(R_SH × n_s × q)

(4)

where Z is the depth to the gate, ε is the dielectric constant of the barrier, C is the junction capacitor, and A is the area of the junction capacitor. The dielectric constant used in this work is ${\epsilon }_{r,A{l}_{0.4}G{a}_{0.6}N}=8.66$. The n_2DEG is approximately calculated by Equation (3). As shown in Figure 3c, with a temperature increase, the variation of n_2DEG is very poor.

As shown in Figure 3d, the carrier mobility of the AlGaN channel (μ_AlGaN) is approximately calculated according to Equation (4) [28], which decreases from 691.58 to 339.47 cm²/v·s as the temperature increases from 25 °C to 200 °C, and it is in good agreement with the simulation result obtained by alloy scattering-limited and optical phonon-limited mobility models [13]. As is known, the μ_AlGaN is affected jointly by alloy disorder scattering and optical phonon scattering, where μ_OP is very sensitive to temperature and alloy scattering-limited mobility (μ_alloy) is inverse [13]. When the temperature rises, the influence of μ_OP is gradually reduced while part of μ_alloy still remains stable, leading to the idea that the AlGaN HEMTs have very excellent output characteristics under high-temperature conditions, something which is lacking in the GaN HEMTs because μ_OP is very dominating in terms of carrier mobility.

Furthermore, the transfer characteristics of AlGaN HEMTs were measured at 25~200 °C with a step of 25 °C under a V_D of 10 V and are shown in Figure 4a. V_TH shifts slightly towards a positive direction, from −4.6 V to −4.5 V, as the temperature increases. I_D,off-state also increases marginally from 2.87 × 10⁻⁵ to 1.85 × 10⁻⁴ mA/mm. Additionally, the buffer leakage current (I_buffer) measured with an isolated active-area mesa structure is only 9.5 × 10⁻⁸ mA/mm at 200 °C, which is significantly lower than I_D,off-state. Therefore, an increase in I_D,off-state at high temperatures is attributed to the reverse gate leakage current, as shown in Figure 4b. And it can be suppressed by optimizing the barrier layer or adopting a metal–insulator–semiconductor (MIS) structure.

The AC capacitance technique was used to characterize the trapping effect on AlGaN HEMTs at a room temperature of 25 °C and a high temperature of 200 °C. The frequency ranged from 1 kHz to 300 kHz. As shown in Figure 5a,b, the conductance gradually increases and then decreases with the growing radial frequency. It is observed that only one conductance peak exists for a given V_G at 25 °C in Figure 5a, which can be assumed as only one kind of trap state existing in the AlGaN channel. However, there are two conductance peaks occurring at 200 °C in Figure 5b, indicating that two kinds of trap states exist in the AlGaN channel under high-temperature conditions.

Here, the plots in Figure 5a,b are effectively fitted as these curves by the following equations, respectively:

$${\displaystyle \frac{G_p}{\omega }}={\displaystyle \frac{q\omega {\tau }_T{D}_T}{1+{\left(\omega {\tau }_T\right)}^2}}$$

(5)

$${\displaystyle \frac{G_p}{\omega }}={\displaystyle \frac{q\omega {\tau }_{T,1}{D}_{T,1}}{1+{\left(\omega {\tau }_{T,1}\right)}^2}}+{\displaystyle \frac{q\omega {\tau }_{T,2}{D}_{T,2}}{1+{\left(\omega {\tau }_{T,2}\right)}^2}}$$

(6)

Therefore, the time constant (τ_T) and trap D_T can be extracted from the above equations. The relationship between τ_T and the applied gate voltage is depicted in Figure 5c at 25 °C and 200 °C, respectively, where τ_T is distributed in the range of 1.08~3.13 μs for the device working on 25 °C but two types of values of 0.53~0.77 μs and 6.29~17.2 μs for that on 200 °C, indicating the existence of two kinds of trap states at the same time. So, these states can be calculated by the equation as follows:

$${\tau }_T={\left({\sigma }_T{N}_c{v}_T\right)}^{-1}exp\left({\displaystyle \raisebox{1ex}{$E_T$}\!\left/ \!\raisebox{-1ex}{$kT$}\right.}\right)$$

(7)

Here, the cross-section of the trap states (${\sigma }_T$) is captured as 3.4 × 10⁻¹⁵ cm⁻², and the density of states in the conduction band (N_c) is 2.2 × 10¹⁸ cm⁻³. The average thermal velocity of carriers (v_T) is 2.6 × 10⁷ and 1.69 × 10⁷ cm/s for 25 and 200 °C, respectively. As shown in Figure 5d, D_T from the range of 4.43 × 10¹²~6.67 × 10¹² cm⁻² eV⁻¹ is located at E_T in a range of 0.32~0.35 eV at 25 °C. But when the temperature increases to 200 °C, D_T with a distribution of 4.09 × 10¹²~5.95 × 10¹² and 7.58 × 10¹²~1.53 × 10¹³ cm⁻² eV⁻¹ is obviously located at E_T in the range of 0.46~0.48 eV and 0.57~0.61 eV, respectively. Compared with the reported trapping effects in AlGaN materials on sapphire by Zhao’s work [29], the τ_T and D_T of trap states are much lower in the AlGaN material on silicon, indicating that the AlGaN epilayer material quality has improved significantly.

4. Conclusions

In summary, the high-temperature characteristics of the Si-based AlGaN HEMTs are studied sufficiently in this work. The carrier mobility of the AlGaN channel still remains at 339.47 cm²/v·s when the working temperature reaches 200 °C, and I_D,max still reaches up to 189.50 mA/mm. Moreover, only a positive voltage drift of 0.1 V occurs, and I_D,off-state increases marginally from 2.87 × 10⁻⁵ to 1.85 × 10⁻⁴ mA/mm, strongly proving the outstanding working performance of Si-based AlGaN HEMTs compared with GaN at high temperatures. Finally, the inner trapping effect in Si-based AlGaN materials is investigated using the AC capacitance technique. Two types of trap states are found at a high temperature of 200 °C. The first type of trap state has a D_T ranging from 4.09 × 10¹² to 5.95 × 10¹² cm⁻² eV⁻¹ and is located within an E_T of 0.46 to 0.48 eV. The second type of trap state has a D_T ranging from 7.58 × 10¹² to 1.53 × 10¹³ cm⁻² eV⁻¹ and is located within an E_T of 0.57 to 0.61 eV. Hence, this work is very valuable for Si-based AlGaN materials as the channel for HEMTs, especially for high-temperature applications, and significantly pushes the development of ultra-wide bandgap semiconductors in power electronics.