# AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

_{2DEG}= 1 × 10

^{13}cm

^{−2}of two-dimensional electron gas in the range of 77–300 K, with mobilities μ = 1.7 × 10

^{3}cm

^{2}/V∙s and μ = 1.0 × 10

^{4}cm

^{2}/V∙s at 300 K and 77 K, respectively. The maximum drain current and the transconductance were demonstrated to be as high as 0.5 A/mm and 150 mS/mm, respectively, for the transistors with gate length L

_{G}= 5 μm. Low-frequency noise measurements demonstrated an effective trap density below 10

^{19}cm

^{−3}eV

^{−1}. RF analysis revealed f

_{T}and f

_{max}values up to 1.3 GHz and 6.7 GHz, respectively, demonstrating figures of merit f

_{T}× L

_{G}up to 6.7 GHz × µm. These data further confirm the high potential of a GaN–SiC hybrid material for the development of thin high electron mobility transistors (HEMTs) and SBDs with improved thermal stability for high-frequency and high-power applications.

## 1. Introduction

## 2. Materials and Methods (Experimental Details)

_{0.25}Ga

_{0.75}N barrier, and a 255-nm GaN channel grown directly on a 62-nm high-quality AlN NL on SiC. The sheet resistance (R

_{Sh}) of the as-grown T-HEMT structure determined from contactless eddy current measurements was 380 ± 10 Ω/□. The band diagram and electron distribution were calculated by a 1D Poisson simulator using the nominal thickness of all layers [20,21]. The results are shown in Figure 1b. The density of the two dimensional electron gas (2DEG) was calculated by integrating an electron distribution in the quantum well. Its value was found to be about 1 × 10

^{13}cm

^{−2}.

_{c}), and the specific resistivity (ρ

_{c}) of ohmic contacts were determined by transmission line method (TLM), demonstrating average values of about 1 Ω × mm and 2 × 10

^{−5}Ω × cm

^{2}, respectively. Schottky contacts were formed from Ni/Au (25/150 nm).

_{SD}= 14 μm, gate length L

_{G}= 5 μm, and gate-source distance L

_{SG}= 5 μm. For comparison, the T-HEMTs with rectangular-type electrodes (see Figure 4), labelled here as DC T-HEMT, were also investigated (see also reference [9]). Similar to RF T-HEMTs, all DC T-HEMTs had the same gate length and gate-source distance of 5 μm, but the channel width was of 200 μm and the drain-source distances were 17.5 μm, 15 μm, and 12.5 μm for three sample transistors labelled DC T-HEMT-1, DC T-HEMT-2, and DC T-HEMT-3, respectively.

_{T}) and the unity maximum unilateral power gain frequency (f

_{max}) were found from de-embedded S-parameter frequency characteristics.

_{L}, were amplified by a low-noise amplifier and analysed using “PHOTON” spectrum analyser (Bruel & Kjaer, Nærum, Denmark). The spectral noise density of drain current fluctuations was calculated in the usual way with ${S}_{I}={S}_{V}{\left(\left({R}_{L}+{R}_{DS}\right)/{R}_{L}{R}_{DS}\right)}^{2}$, where S

_{V}is the drain voltage fluctuations and R

_{DS}is the total drain to source resistance.

## 3. Experimental Results and Discussions

_{2DEG}), mobility (μ

_{2DEG}), and sheet resistance (R

_{Sh}) were determined in the Hall experiments using Van der Pauw (VdP) geometry. The results are summarized in Table 1. Good agreement between the calculated carrier density, an integral of electron distribution in the quantum well (see Figure 1b), measured sheet resistance using contactless eddy current method, and the results of the Hall experiment were found within a deviation interval of 7%.

#### 3.1. Performance of SBDs

_{po}) needed to fully deplete a 2DEG channel was found to be about −3.1 V. The density of 2DEG under Schottky contact was calculated using the integral capacitance technique [28]:

_{P}(V) is the capacitance. The carrier density N dependence on the distance from the surface W was found from C-V data using the following formulas [28]:

_{0}is the vacuum permittivity. The obtained N dependence on the parameter W is shown in Figure 5b. The density of 2DEG was found to be N

_{G-2DEG}= 0.69 × 10

^{13}cm

^{−2}at 300 K. This density is smaller than that found from the Hall measurements due to depletion by the Schottky barrier built-in voltage [29,30].

_{ON}/j

_{OFF}ratio; for example, for SBD with L = 40 µm, the highest achieved value was found to be more than three orders of magnitude, j

_{ON}/j

_{OFF}≥ 3200, taking into account also the reverse-current densities prior to a breakdown which occurred at a voltage of −780 V. Furthermore, a 2.5 times improvement in the maximum current density was obtained in comparison with previously reported SBDs fabricated on standard AlGaN/GaN HEMT structures with a thick GaN:C buffer [8]. Note the dependence of forward current on the distance between ohmic and Schottky contacts indicating good performance of the fabricated ohmic contacts with negligible losses.

#### 3.2. Performance of T-HEMTs

_{D}= 10 V and V

_{G}= +1 V. This translates into an input power value of 2.6 W/mm for T-HEMT with a channel width of 0.4 mm. The drain current in the saturation region fell by 1–2% only. This indicates the advantages of efficient heat removal from the 2DEG channel in AlGaN/GaN with AlN NL that exploits the absence of the buffer layer and high thermal conductivity of the SiC substrate.

_{m}) characteristics at V

_{D}= 5 V for various T-HEMTs are shown in Figure 8b,c. The impact of mesa on the device performance can be identified from the transfer characteristics. Indeed, the circular DC T-HEMT devices demonstrated up to two orders of magnitude larger leakage currents in comparison to those measured for RF T-HEMTs. Both the maximum drain current and the transconductance values were found to be higher for the DC T-HEMTs demonstrating values up to 507 mA/mm and 154 mS/mm, respectively. Meanwhile, RF T-HEMTs demonstrated only 266 mA/mm and 77 mS/mm. The pinch-off region is observed beyond a gate bias of −3 V, which is in good agreement with V

_{po}obtained from C-V measurements.

^{γ}noise with exponent γ = 0.9–1.1. The dependences of the noise S

_{I}/I

^{2}on the gate voltage swing (V

_{G}-V

_{T}) at f = 10 Hz for three representative devices are shown in Figure 9a (here, V

_{T}is the threshold voltage determined from the transfer current voltage characteristics in the linear regime). As seen, noise depends on the gate voltage as (V

_{G}-V

_{T})

^{2}or steeper. It is known that, in many cases, this dependence at high gate voltages may become flat, indicating a contribution of the contact noise. It is seen from Figure 9a that this is not the case for the studied devices and that contacts do not contribute to noise significantly. The effective trap density N

_{T}in the McWhorter model can be estimated from gate voltage noise as follows [9]:

_{Ch}and L

_{G}is the channel area, C is the gate capacitance per unit area, and γ is the attenuation coefficient of the electron wave function under the barrier, taken to be 10

^{8}cm

^{−1}.

^{19}–10

^{20}cm

^{−3}eV

^{−1}. Some of the devices demonstrated N

_{T}< 10

^{19}cm

^{−3}eV

^{−1}. These values are of the same order or even smaller than those reported earlier for AlGaN/GaN HEMTs with a thick buffer layer [9]. Therefore, we conclude that studied T-HEMTs are characterized by the same quality as or even better quality than regular devices with thick buffers.

_{T}) and the unity maximum unilateral power gain frequency (f

_{max}) were found at various voltages down to the threshold voltage. The results are shown in Figure 10. The RF T-HEMTs with a 0.4-mm channel width demonstrated the highest operational frequencies, with values reaching f

_{T}= 1.33 GHz at V

_{GS}= 0 V with V

_{D}= 5 V and f

_{max}= 6.7 GHz at the bias of V

_{G}= −0.8 V and V

_{D}= 7 V. These results revealed a figure of merit (FOM) factor f

_{T}× L

_{G}up to 6.7 GHz × µm, which is comparable with the best value of 9.2 GHz × µm reported for the T-HEMTs in Reference [19]. The performance of RF T-HEMTs can be further improved in our processing via optimization of ohmic contact/access resistance and the reduction of channel length L

_{SD}in tandem with gate length L

_{G}[34,35]. Note that there is up to 3 times difference between the FOM factor of T-HEMTs and that of standard HEMTs, which requires more detailed investigations in the future [36].

## 4. Conclusions

^{13}cm

^{2}with mobility of 1.7 × 10

^{3}cm

^{2}/V∙s and 1.0 × 10

^{4}cm

^{2}/V∙s at 300 K and 77 K, respectively, were found from the Hall measurements. The unterminated and unpassivated SBDs fabricated on these heterostructures exhibited high breakdown voltages up to −780 V, with the critical breakdown field reaching 0.8 MV/cm. Transistors on these heterostructures, so-called T-HEMTs, demonstrated maximum current density and transconductance values up to 0.5 A/mm and 150 mS/mm, respectively, with a negligible reduction in the drain current. This indicates improved thermal management due to a heterostructure design on the SiC substrate without a GaN buffer layer. By systematic low-frequency noise measurements, we estimated the effective trap density, which in T-HEMT structures was below the level of 10

^{19}cm

^{−3}eV

^{−1}. This value is similar to or even smaller than previously reported trap densities in heterostructures with thick GaN:C buffers. This means that avoiding a GaN:C buffer in GaN–AlN-SiC material does not lead to an increase in active (dislocation-related) trap density. The unity current gain cut-off and unity maximum unilateral power gain were measured to be 1.3 GHz and 6.7 GHz, respectively. Using this data, the figure of merit f

_{T}× L

_{G}is estimated at 6.7 GHz × µm. Therefore, we conclude that a buffer-free design did not compromise the quality of the structures or the performance of the devices. Our results confirm the potential of a GaN–SiC hybrid material for the development of HEMTs and SBDs for high-frequency and high-power applications with improved thermal stability.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**(

**a**) Schematic of the thin high electron mobility transistor (T-HEMT) structure cross section with ohmic and Schottky contacts and (

**b**) the calculated band diagram and electron density distribution in the upper layers of the heterostructure.

**Figure 2.**Microscope image of the fabricated Schottky barrier diode (SBD): L is the separation between the Ohmic and Schottky contacts. The scale bar is 50 μm.

**Figure 3.**Microscope image of the radio frequency (RF) T-HEMT (left hand side) and details of the design parameters (right hand side) illustrating the Gate (G), Source (S), and Drain (D) electrodes in a 150-μm pitch implementation.

**Figure 5.**(

**a**) Capacitance-voltage (C-V) characteristics of SBD with L = 20 μm at modulation frequencies of 100 kHz (red line) and 1 MHz (blue line), and (

**b**) carrier distribution N(W) calculated from C-V data using Equations (2) and (3).

**Figure 6.**(

**a**) Reverse current-voltage characteristics of SBDs and (

**b**) current-voltage characteristics of SBDs with L = 5 and 40 μm at low voltages.

**Figure 7.**Breakdown voltage and critical electric field dependences on the distance between ohmic and Schottky contacts: error bars in the critical field data are depicted by the size of the symbols. Inset: images of L = 40 μm SBD before and after breakdown (scale bar is 50 μm).

**Figure 8.**DC characteristics of T-HEMTs under study: (

**a**) DC output characteristics of 0.4 mm wide RF T-HEMT-1 and comparisons of transfer (

**b**) and transconductance (

**c**) characteristics of the RF T-HEMTs and DC T-HEMTs with various values of the channel widths W

_{Ch}. The gate length for all devices is 5 µm.

**Figure 9.**(

**a**) Drain current noise S

_{I}/I

^{2}at frequency f = 10 Hz for T-HEMTs of different channel widths ranging from 0.2 mm to 0.6 mm and (

**b**) the effective trap density N

_{T}as a function of the gate voltage swing (V

_{G}-V

_{T}) for the same transistors.

**Figure 10.**Frequencies f

_{T}and f

_{max}at different biasing conditions extracted from S-parameters measurements of RF T-HEMT-1 with W

_{Ch}= 0.4 mm and L

_{G}= 5 μm.

Hall Measurements | Simulation | Eddy Current Measurements | ||
---|---|---|---|---|

Parameter | 300 K | 77 K | 300 K | 300 K |

N_{2DEG}, ×10^{13} cm^{−2} | 1.00 | 0.96 | 1.0 | - |

μ_{2DEG}, cm^{2}/V∙s | 1.7 × 10^{3} | 1.0 × 10^{4} | - | - |

R_{Sh}, Ω/□ | 375 | 64 | - | 380 ± 10 |

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**MDPI and ACS Style**

Jorudas, J.; Šimukovič, A.; Dub, M.; Sakowicz, M.; Prystawko, P.; Indrišiūnas, S.; Kovalevskij, V.; Rumyantsev, S.; Knap, W.; Kašalynas, I.
AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics. *Micromachines* **2020**, *11*, 1131.
https://doi.org/10.3390/mi11121131

**AMA Style**

Jorudas J, Šimukovič A, Dub M, Sakowicz M, Prystawko P, Indrišiūnas S, Kovalevskij V, Rumyantsev S, Knap W, Kašalynas I.
AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics. *Micromachines*. 2020; 11(12):1131.
https://doi.org/10.3390/mi11121131

**Chicago/Turabian Style**

Jorudas, Justinas, Artūr Šimukovič, Maksym Dub, Maciej Sakowicz, Paweł Prystawko, Simonas Indrišiūnas, Vitalij Kovalevskij, Sergey Rumyantsev, Wojciech Knap, and Irmantas Kašalynas.
2020. "AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics" *Micromachines* 11, no. 12: 1131.
https://doi.org/10.3390/mi11121131