High linearity AlGaN/GaN HEMTs with Au-free Ti/Al/Ni/Ti ohmic contacts for Ka-band applications

In this study, AlGaN/GaN HEMTs with Au-free Ti/Al/Ni/Ti ohmic contacts were fabricated. The device presents a contact resistance (R c) of 0.64 Ω·mm and high linearity characteristics. The two-tone measurement at 28 GHz shows that the 2 × 50 μm device exhibits an excellent third-order intercept point (OIP3) value of 41.64 dBm at V DS = 28 V, and an OIP3/P DC of 24.2. An OIP3 of 46.59 dBm was achieved when the device’s gate width was increased to 8 × 50 μm at V DS = 48 V. These results demonstrate that AlGaN/GaN HEMTs with Ti/Al/Ni/Ti ohmic contacts have potential for Ka-band applications.


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][3][4] However, these higher frequency signals tend to attenuate more rapidly over distance, necessitating devices capable of high-power output for high-frequency operations.][7] It is considered a highly promising candidate for 5G communication applications, particularly in the Kaband.In order to prevent signal interference from adjacent channel in the crowded 5G bands, power amplifiers (PAs) used in base stations require high linearity, which is an important indicator for GaN HEMTs.In wireless communication, when two or more signals of different frequencies mutually amplitude modulate each other, they produce intermodulation distortion (IMD), which can interfere with adjacent channels, thereby increasing spectrum occupancy.Among these intermodulation distortions, the third-order intermodulation distortion (IM3) has the greatest impact because it is closest to the main signal and is challenging to filter out.Therefore, a critical issue is how to reduce the IM3 of the device and improve the third-order intercept point (OIP3) to enhance linearity.
[10] Additionally, other methods have been reported to enhance the linearity of the device, including gate dielectric, 11) graded-channel, 12) N-Polar GaN MISHEMTs, 13) selective-area charge implantation, 14) and SiN x passivation. 15)To make AlGaN/GaN HEMTs more competitive in commercialization, reducing production costs and improving device performance is necessary.However, conventional ohmic contact metals using a Ti/Al/Ni/Au structure often exhibit rough surface morphology and sharp edge acuity after high-temperature annealing (above 800 °C), which is due to the formation of the viscous AlAu 4 phase. 16)Compared to Ti/Al alloy, Ti/Al/Ni/Ti has the advantage of better surface roughness. 17,18)Therefore, we chose Ti/Al/Ni/Ti as the metal for ohmic contacts.Moreover, literature reports the use of rapid thermal annealing (RTA) in N 2 atmosphere to form TiN from the final layer of Ti and N 2 , resulting in devices with lower contact resistance and improved microstructure.The Au-free ohmic contacts of AlGaN/GaN HEMTs can be used as CMOScompatible GaN-based power devices. 19)[22][23][24] However, reports on improving device linearity through Au-free ohmic contacts are limited.
In this study, we fabricated AlGaN/GaN HEMTs with Aufree Ti/Al/Ni/Ti ohmic contacts to reduce the cost of device fabrication and improve the device linearity.][27] In order to investigate the effect of different bias voltages and gate widths (W g ) on device linearity, we conducted measurements and analysis in the Ka-band.
Figure 1 shows the schematic diagram of an AlGaN/GaN HEMT with Au-free Ti/Al/Ni/Ti ohmic contacts.The AlGaN/GaN heterostructure used in this paper was grown on a SiC substrate using metal-organic chemical vapor deposition.It includes an unintentionally doped GaN layer of 1 μm, an AlN interlayer of 1 nm, and an AlGaN barrier layer of 25 nm.Additionally, the epitaxial structure's carrier mobility is 1500 cm 2 V −1 s −1 , with a sheet carrier density of 1 × 10 13 cm −2 .Device fabrication began with patterned ohmic recesses.Using stepper photolithography to define patterned recess, the recess areas are lines of 1 μm within the device's active region.After the recess pattern was completed, the AlGaN layer was etched by inductively coupled plasma (ICP) with Cl 2 gas.This method helps in thinning the barrier, which can reduce the contact resistance (R c ).Before depositing the Au-free ohmic metal, diluted hydrofluoric acid (HF) was used to remove natural oxides.Then, a Ti/Al/Ni/Ti (20/120/20/50 nm) ohmic metal was deposited using an Egun evaporator and annealed at 710 °C for 60 s in N 2 ambient.Next, device isolation was achieved using boron ion (B 11+ ) implantation.The R c of the device with Ti/Al/Ni/Ti ohmic contacts was measured to be 0.64 Ω•mm by using transmission line measurements (TLM).Afterward, a 250 nm SiN x film was deposited as passivation layer by plasmaenhanced chemical vapor deposition (PECVD).The next step was to define the gate foot position using stepper photolithography and to etch the SiN x in the gate region by ICP.Subsequently, the gate length was defined by a second stepper photolithography step, which was shifted by 200 nm.Finally, the Γ-shaped gate was formed by E-gun evaporation of Ni/Au (50/500 nm).After the gate fabrication was completed, the first SiN x layer was removed using ICP, and then a second SiN x layer was deposited using PECVD to protect the gate.In the end, via holes were opened using ICP for air-bridge interconnection.In this experiment, all devices have a gate length (L g ) of 300 nm, a gate stem height of 250 nm, and a source-drain spacing (L SD ) of 2.5 μm.
The I DS -V DS characteristics of AlGaN/GaN HEMTs with Ti/Al/Ni/Ti ohmic contacts for a gate width (W g ) of 2 × 50 μm are shown in Fig. 2(a).In this study, saturation drain current (I DSS ) is defined as I DS at V GS = 0 V, which can also be observed in Fig. 2(a).The characteristics of the G m dependence on the gate bias for 2 × 50 μm devices with Ti/Al/Ni/Ti ohmic contacts are shown in Fig. 2(b).When the gate-source voltage (V GS ) swept from −4 to 1 V, the maximum transconductance (G m,max ) of the 2 × 50 μm device at V DS = 20 V, 28 V, and 48 V was 213.6 mS mm −1 , 208.5 mS mm −1 , and 196.5 mS mm −1 , respectively.We adopt the gate voltage swing (GVS) as an indicator of the flatness of the G m curve because a G m curve with a wider gate voltage range will result in a device with high linearity characteristics.Therefore, a larger GVS indicates a wider gate voltage range and a flatter G m curve.A flatter G m curve results in a smaller G m ″. 28,29) In this study, we defined as the voltage range when G m,max decreases by 10%.The corresponding GVS values for the device with a gate width of 2 × 50 μm were 1.40 V, 1.45 V, and 1.50 V at V DS = 20 V, 28 V, and 48 V, respectively.Furthermore, as the gate width of the device increases to 8 × 50 μm, the corresponding GVS value increases from 1.50 to 1.70 V at V DS = 48 V.The relationship between OIP3, G m , and G m ″ is shown in Eq. (1). 30)Moreover, the relationship between IM3 and G m ″ is expressed in Eq. (2) 30) µ  where G m and G m ″ are the transconductance and the second derivative of G m , respectively.G ds is the drain-source conductance, R L is the load impedance, and A is the amplitude of the input signal.According to Eqs. ( 1) and ( 2), the OIP3 value is inversely proportional to G m ″ and directly proportional to the third power of G m .Moreover, it is inversely proportional to the square of G ds .Simultaneously, the level of IM3 is directly proportional to G m ″.Therefore, we expect that a device with a flatter G m profile will improve OIP3 and lower IM3. Figure 2(c) shows the I DS -V GS curves of the device with a gate width of 2 × 50 μm at different V DS bias.It can be observed that there is no significant difference in the I DSS values of the device at V DS = 20 V and V DS = 28 V, which were 431.6 mA mm −1 and 436.8 mA mm −1 , respectively.When V DS is increased to 48 V, the I DSS of the device decreases to 418.8 mA mm −1 .Compared to the I DS -V GS curves at V DS = 20 V and V DS = 28 V, the I DS -V GS curve at V DS = 48 V is flatter.The off-state drain current characteristics of the device with a 2 × 50 μm gate width are shown in Fig. 2(d).The device with Ti/Al/Ni/Ti ohmic contacts exhibits a breakdown voltage of 78 V with a drain current of 0.1 mA mm −1 at V GS = −5 V. Compared to the device with Ti/Al/Ni/Au ohmic contacts, the breakdown voltage improved from 45 to 78 V.According to a previous study, 18) the device with Ti/Al/Ni/Ti ohmic contacts exhibits smoother edge sharpness and surface morphology compared to the device with Ti/Al/Ni/Au ohmic contacts.Figure 2(e) shows optical micrograph images of the device's surface with Ti/Al/ Ni/Ti (20/120/20/50 nm) ohmic contacts and Ti/Al/Ni/Au (20/120/25/100 nm) ohmic contacts.It is also observed that the device with Ti/Al/Ni/Ti ohmic contacts has a smoother surface morphology, while the device with Ti/Al/Ni/Au ohmic contacts has a rougher surface.
The S-parameters of the 2 × 50 μm device at V DS = 20 V were measured using a Keysight E8361A PNA Network Analyzer in the frequency range from 100 MHz to 67 GHz.After de-embedding, the cut-off frequency ( f T ) and maximum oscillation frequency ( f max ) of the device were estimated to be 28 GHz and 167 GHz, respectively, as shown in Fig. 3.
Before the two-tone measurement, the source and load terminations of the device were both tuned for maximum output power.For the device with a gate width of 2 × 50 μm, the load-pull measurement results show the maximum output power density (P out,max ) of 1.6 W mm −1 , the maximum power-added efficiency (PAE max ) of 17.5%, and the transducer gain of 8.7 dB at V DS = 20 V.Then, a two-tone signal was introduced with a frequency of 28 GHz and a tone spacing of 5 MHz.Finally, the OIP3 values of the 2 × 50 μm device under different bias voltages and the 8 × 50 μm device at V DS = 48 V were measured using a scalar loadpull system.Figures 4(a)-4(c) show the OIP3 values for a 2 × 50 μm device under different V DS .These devices were measured at V DS = 20 V, 28 V, and 48 V, respectively, with the drain-source current (I DS ) biased at class A operation.It can be observed that OIP3 values of 38.95 dBm, 41.64 dBm, and 42.69 dBm were achieved for the 2 × 50 μm devices at V DS = 20 V, 28 V, and 48 V, respectively.Furthermore, the 2 × 50 μm devices exhibit output power at the 1 dB gain compression point (OP 1dB ) of 17.01 dBm, 18.19 dBm, and 19.01 dBm at V DS = 20 V, 28 V, and 48 V, respectively.The results show that the OIP3 of the device depends on the 071001-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd increase in V DS .In the ideal case, the drain-source resistance (R ds ) increases with the drain voltage. 31)The R ds is defined as the inverse of drain to source conductance.Therefore, G ds will decrease with increasing drain voltage.Moreover, the square of G ds is inversely proportional to OIP3, as shown in Eq. ( 1). Figure 4(d) shows the IM3 levels of 2 × 50 μm devices at V DS = 20 V, 28 V, and 48 V, power-backed off from the 1 dB compression point (P 1dB ) of each device.The IM3 levels of the devices at 10 dB back-off from the P 1dB were −31.33 dBm, −34.57dBm, and −35.69 dBm, respectively.Compared to V DS = 20 V and 28 V, the device exhibits lower IM3 levels at V DS = 48 V.The IM3 of the device decreased as V DS increased, which can be attributed to high bias voltage operation allowing for a larger load line resistance, resulting in a smaller V GS swing.A smaller V GS swing leads to fewer changes in the nonlinear parameters. 32)n addition, the linearity figure of merit (FOM) OIP3/P DC is also a useful quality factor that can be used to compare the linearity and bias conditions of different devices.The linear  where P DC is DC power consumption. 33)The device with a gate width of 2 × 50 μm has OIP3/P DC values of 18.1, 24.2, and 18.5 at V DS = 20 V, 28 V, and 48 V, respectively, as shown in Table I.The results indicate that the device achieves the highest OIP3/P DC ratio at V DS = 28 V.
][34][35][36] are summarized in Table I.It can be observed that the device's OIP3 improves with the increase in bias voltage and gate width.When the bias voltage is increased to 48 V, the OIP3/P DC may decrease due to excessive DC power consumption.
It was found that Au-free ohmic contact devices have a flat G m profile as compared to Au-based devices, which improves the device's linearity.Additionally, the Ti/Al/Ni/Ti ohmic contact metal surface is smoother, which increases the device's breakdown voltage and allows operation at higher biases.As a result, with the increase in drain voltage, the G ds decreases, leading to an improvement in OIP3 at 28 GHz.
In conclusion, we successfully fabricated AlGaN/GaN HEMTs with Ti/Al/Ni/Ti ohmic contacts.Using Au-free    071001-5 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd

Fig. 2 .
Fig. 2. (a) I DS -V DS , (b) G m -V GS , and (c) I DS -V GS characteristics for 2 × 50 μm AlGaN/GaN HEMTs with Ti/Al/Ni/Ti ohmic contacts.(d) Breakdown voltage measurement results and (e) optical micrograph images of the surface for the 2 × 50 μm device with Ti/Al/Ni/Ti and Ti/Al/Ni/Au ohmic contacts.

Figure 5 (
a) shows the two-tone measurement results for the device with a gate width of 8 × 50 μm at 28 GHz.The linearity characteristics of the 8 × 50 μm device were measured at V DS = 48 V, and the I DS was biased for class A operation.The results show that the OIP3 of device reached 46.59 dBm and the OIP3/P DC was 13.7.Furthermore, the 8 × 50 μm device exhibits an OP 1dB of 23.56 dBm.

Figure 5 (
b) shows the IM3 levels of the 8 × 50 μm device according to the power back-off from the P 1dB .The IM3 level of the device at 10 dB back-off from the P 1dB was −40.91 dBm at V DS = 48 V. Compared to the 2 × 50 μm device, the 8 × 50 μm device exhibits lower IM3 values.The linearity metrics of this study and Au-based AlGaN/GaN HEMTs reported in literature

Fig. 4 .
Fig. 4. Comparison of OIP3 values for the 2 × 50 μm device with Ti/Al/Ni/Ti ohmic contacts at (a) V DS = 20 V, (b) V DS = 28 V, and (c) V DS = 48 V at 28 GHz, and (d) the level of IM3 versus power back-off from P 1dB curves.
Ti/Al/Ni/Ti ohmic contacts not only reduces costs but also improves the device's linearity, leading to an improvement in OIP3 when the device bias is increased.The devices show excellent OIP3 values and high OIP3/P DC linearity metrics at Ka-band.The two-tone measurement results at 28 GHz show that the 2 × 50 μm device exhibits an excellent OIP3 value of 41.64 dBm and an OIP3/P DC of 24.2 at V DS = 28 V.When the gate width of the device is increased to 8 × 50 μm, the OIP3 reaches 46.59 dBm and the OIP3/P DC is 13.7 at V DS = 48 V.These results demonstrate that high-linearity AlGaN/GaN HEMTs with Ti/Al/Ni/Ti ohmic contacts have potential for Ka-band applications.

Table I .
Comparison of linearity metrics with other Au-Based AlGaN/GaN HEMTs.
©2024The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd