Simulation Study of the Impact of Traps in GaN Substrates on the Electrical Characteristics of AlGaN/GaN HEMTs with Thin Channel Layers

Our paper describes a novel way to design gallium nitride (GaN) high electron mobility transistors (HEMTs). Because GaN HEMTs are valuable in everyday radio-frequency (RF) systems, such as radar and satellite communications, and enhanced performance is needed for advanced systems, improving their design is a great advantage. Native GaN substrates have potential for HEMTs because they have a homo-epitaxial growth layer with a low defect density. We have studied the effects of trap concentration in the GaN substrates and how a trade-off relationship develops. We have investigated how traps in a GaN substrate affect GaN HEMTs by a device simulation. We focus on the trade-off relationship that exists between leaks, transient response and low-frequency small-signal-gain that hinders RF performance. Our results confirm that an AlGaN back-barrier structure is highly effective for minimizing leakage and improving trade-off performance in GaN substrates


Introduction
GaN high electron mobility transistors (HEMTs) are promising candidates for high-power and high-frequency radio-frequency (RF) systems such as radar, satellite communications, and base stations because GaN semiconductors feature highly critical electric fields and a high saturation velocity [1][2][3].Although RF amplifiers that use GaN HEMTs have already been commercialized, advanced systems require increased performance.It is possible to improve the RF performances of GaN HEMTs, which can be achieved by exploiting the III-Nitride semiconductors' intrinsically excellent physical properties.
GaN HEMTs have traditionally used AlGaN/GaN epitaxial layers that are grown on SiC or Si substrates.The lattice-mismatched substrates cause crystal defects to be introduced because of hetero-epitaxial growth.Defects in the epitaxial layers are known to strongly affect the electrical characteristics as electron traps [4][5][6][7][8][9].The AlGaN/GaN heterostructures with their many threading dislocation densities have a large gate-leakage current [4].The traps in the buffer layer and at the semiconductor's surface degrade transient-response characteristics such as a current collapse, drain-lag and gate-lag [5][6][7][8][9].For GaN HEMTs on SiC substrates, the suppressed trap effect requires a thick GaN buffer layer with a thickness of around 1 μm because of the decreased number of defects that have been introduced from the difference of the lattice constants between the GaN and SiC substrates [7].However, adopting a thick GaN epitaxial layer has the disadvantage of increased drain-leakage currents, which are a limiting factor for the high-frequency operation of GaN HEMTs with a short gate length.Intentionally introducing the traps into the GaN buffer layer is a way to decrease the drain-leakage current [7].Therefore, the traps play a role in reducing the drain-leakage current and degradation of the transient responses.This trade-off restricts the performances of the GaN HEMTs that have been fabricated on the lattice-mismatched substrates.
Semi-insulating GaN substrates are promising native candidates in GaN HEMTs for RF applications [3].The homoepitaxial method of growing GaN layers on GaN substrates is expected to achieve not only low defect density but also the thin channel layer that is desirable for a high-frequency operation with a short gate length.The AlGaN/GaN epitaxial layers that were grown on GaN substrates have either low reverse leakage currents or a high electron mobility compared to the epitaxial layers that were grown on the lattice-mismatched substrates [10], [11].Many papers reported on GaN HEMTs fabricated on GaN substrates [12][13][14][15][16][17][18][19][20][21][22][23][24][25].A high power density of 9.4 W/mm at 10 GHz was demonstrated by operating with a high drain bias of 50 V [13].A remarkable current-collapse reduction was obtained using GaN HEMTs with an extremely low dislocation density on GaN substrates [16][17][18][19]22].Additionally, a degradation of the direct current (DC) and dynamic performances after 3-MeV proton irradiation became lower using metal-insulator-semiconductor HEMTs on the GaN substrates [20].An excellent high power-added-efficiency of 82.8% at 2−3 GHz was demonstrated recently using GaN substrates with a reduced Si contamination at the interface for the epitaxial layers [25].The reported results indicate that GaN substrate usage improves DC and RF performances.
GaN substrates for a high-frequency RF amplifier are required to have high resistance to suppress parasitic conduction loss at RF operations.GaN semiconductors can increase the resistance by introducing iron (Fe) or carbon (C) atoms, which create the deep level in the GaN bandgap [26][27][28][29].The traps capture the conducting electrons and release them after a long characteristics time, so with the deep levels, the traps increase the resistivity but degrade the transient response that affects the RF performances.When the gate length for the high-frequency operation shortens, that reduced thickness of the epitaxial layers effectively suppresses the drain-leakage current [30].The transient response from the off-to-on state bias worsens as the thin channel layers decrease [31].These simulation results show that precise control of the traps is necessary for obtaining the GaN HEMTs' maximum performance on a GaN substrate.However, the traps' impacts on a GaN substrate for GaN HEMTs with thin channel layers are not clear.
In this paper, a technology computer-aided design (TCAD) simulation investigates the traps' effects for GaN HEMTs on a GaN substrate.We focused on the trade-off relationship between the drain-leakage current and the transient response from off-to-on state bias (corresponding to current collapse), when the trap concentration in the GaN substrate and channel-layer thickness change.Moreover, the power gain (S21) in the small signal response is calculated in a low-frequency range.The low-frequency dispersions in the gain destroy the linearization of GaN HEMTs and cause long-term memory effects on RF systems such as a Doherty power amplifier in cellular base stations [32][33][34].

Device structures of GaN HEMTs on GaN substrates
The simulation was performed by using the Atlas of Silvaco device simulators in TCAD [35]. Figure 1 shows a schematic-cross sectional GaN HEMT structure on GaN substrates for the simulation.The epitaxial layers include the buffer, channel, and barrier and are located on the GaN substrates.The buffer, channel, and GaN substrate layers contain common acceptor traps.For the trap parameters, the energy level below the conduction band and the capture cross section were fixed at 0.6 eV and 5.0 × 10 −15 cm 2 for all GaN layers and the substrate, respectively [36].Approximately 0.6 eV for the trap's energy level is a common value for the experimental results and is observed for the GaN grown on not only SiC but also GaN substrates [8,9,27,[36][37][38][39][40].The trap concentration of the channel layer was fixed to be at as low as 1.0 × 10 15 cm −3 [37].The Al content and barrier layers' thickness were 0.2 and 10 nm, respectively.The channel-layer thickness was changed from 0.02 to 1.0 μm because it is an important parameter for the short-channel effect.This study used two kinds of buffer layers: one is the GaN buffer layer, which is usually used for traditional GaN HEMTs, and the other is an AlGaN back-barrier layer, which aims to confirm two-dimensional electron gas (2DEG) to the channel layer and improve the short-channel effects [41][42][43].The Al content of the back-barrier layers were 0.08.Both buffer layers had the same trap concentration of 2.0 × 10 15 cm −3 and thickness of 0.1 μm.The GaN substrates were fixed to be as thick as 100 μm, which is similar to a realistic value.However, the trap concentration of GaN substrates was changed from 10 15 to 10 16 cm −3 .Heavily doped thin layers were located under the source and drain electrodes to ensure the ohmic characteristics (not shown in Fig. 1).The gate had a T-shaped structure, and its length and work function were 0.6 μm and 5.0 eV, respectively.The distances from the source to the gate electrodes and from the gate to the drain electrodes were 0.6 and 2.0 μm, respectively.Additionally, the gate width was 1 mm.

Simulation conditions
The DC characteristics, transient response, and small signal S parameters were calculated without a self-heating effect, and some figures of merit were selected to study trade-off relationships.The typical simulation results are shown in Fig. 2. Figure 2(a) shows the drain current (IDS) that depends on the drain voltage (VDS).The drain current at VDS = 40 V and VGS = 2 V had a maximum value and was adopted as the figure of merit (IDSMAX) in IDS. Figure 2(b) shows IDS calculated as a function of gate voltage (VGS).Because the pinch-off voltage is approximately −1 V, GaN HEMTs are in the off-state condition at VGS = −5 V.The drain-leakage currents increase as VDS increases from 10 to 40 V.We adopted IDS at the off-state condition (VDS = 40 V and VGS = −5 V) as a figure of merit for the drain-leakage current.This is called IDSOFF, and IDSOFF is 3.0 × 10 −3 A/mm in Fig. 2(b).Depending on the time, in the transient responses, IDS was calculated when VDS and VGS were changed from the off-state bias (VGSQ = −5 V) to the on-state bias (VDSP = 5 V and VGSP = 0 V). Figure 1(c) shows normalized IDS as a function of time.The start VDS in the off-state bias (VDSQ) was 20 or 40 V. IDS was normalized by DC value at the on-state bias.The normalized IDS was below the DC value until approximately 1 ms.The IDS from capturing the electron in the deep levels was small.The normalized IDS increased to the DC value after 1 ms, and reached the DC values at 0.1 s.The characteristic time of the trap was evaluated using the Schockley-Read-Hall defect model [44].Because the characteristic time for the trap energy level was 41 ms, the changes in IDS in Fig. 2(c) result from the traps.The normalized IDS for VDSQ = 40 V was lower than that for VDSQ = 20 V. The normalized IDS at 1 ms for VDSQ = 40 V was adopted as a figure of merit of the transient response.The power gain (S21) in the S parameters was calculated as an on-state bias (VGS = 0 V).Figures 2(d) and (e) show the real and imaginary parts of S21 for a low-frequency range.The real parts of S21 increase monotonically from 100 to 1000 Hz.However, the imaginary part of S21 has a zenith that is caused by the traps, and the peak frequency corresponds to the region of increasing IDS in the real parts of S21.As a figure of merit of S21, the peak magnitude in the imaginary part at VDS = 40 V was adopted.

Dependence on channel thickness
The thin channel layer is useful for high-frequency operations.This section discusses the effects of the traps in GaN substrates by calculating channel-thickness dependence.Figure 3 shows the IDS-VGS curves and transient-response characteristics as a function of channel thickness for relatively high trap concentration of 2.0 × 10 16 cm −3 in the GaN substrates.The channel thickness was changed from 0.02 to 1.0 μm. Figure 3(a) shows how IDS depends on VGS at high VDS of 40 V.Under the on-state condition, IDS has characteristics that are similar for all channel thicknesses, because 2DEG is located on the channel side of the hetero interface, between the barrier and channel layer, and the electron concentration in 2DEG is almost independent of the channel thickness.In subthreshold regions, IDS decreases drastically when the channel-layer thickness decreases from 1.0 to 0.1 μm.Under the pinch-off voltage for a channel-layer thickness less than 0.1 μm, IDS is almost the same as the very low value of 10 −10 A/mm.Therefore, GaN HEMTs with a thin channel layer effectively suppress the drain-leakage current.Figure 3(b) shows the transient responses from the off-state bias (VDS = 40 V, VGS = −10 V) to the on-state bias (VDS = 5 V, VGS = 0 V).Normalized IDS increases from 1 ms and reaches the DC value for all channel-layer thicknesses.When the channel-layer thickness is thinner, the normalized IDS below 1 ms decreases.Therefore, traps in the GaN substrate have a negative effect on the transient response for the thin channel layer, although the trap concentration is constant.As shown in Figs.3(a) and (b), GaN HEMTs with a thin channel layer are found to effectively suppress the drain-leakage current but are worse for the transient response.
Figure 4 shows two-dimensional (2D) plots of the current density at the off-state bias (VDS = 40 V, VGS = −5 V) for 0.02-, 0.7-and 1.0-μm channel thicknesses.In the case of a thin 0.02-μm channel layer, the current density is extremely low for all regions between the source and drain.The drain-leakage currents flow from the drain to the source through the region and (c), the depletion layer, which extended from the gate electrode, is considered to reduce the electrons within the 0.2-μm thickness.Therefore, the 0.02-μm channel-layer thickness is sufficient thin to suppress of the drain-leakage current that flowed under the gate.A larger drain-leakage current flows for a thicker channel layer because of a wider space for the current flow under the gate.Figure 5 shows 2D plots of the ionized trap density at 1ms in the transient response for 0.02-, 0.7-and 1.0-μm channel thicknesses.In the case of the thinner channel layer, the ionized traps with a high density in the GaN substrates were extended to a wider area between the source and drain.The ionized traps with negative charges increase the conduction band energy and decrease the drain current as shown in Fig. 3(b).
Figures 3(a) and 4 show that the drain-leakage current can be suppressed for a channel-layer thickness less than 0.1 μm because the thin channel prevents current flow in the region under the gate electrode.However, the thin channel layer degrades the transient response dramatically as shown in Fig. 3 (b), because the GaN substrate with a high trap concentration gets close to 2DEG in the channel layer.Therefore, it is necessary to investigate a GaN substrate with a lower trap concentration for the GaN HEMTs with the thin channel layers to satisfy both the drain-leakage current suppression and successful transient response.

Effects of the trap concentration in GaN substrates for GaN HEMTs with thin channel layers
This section discusses the characteristics of GaN HEMTs with a thin 0.02-μm GaN channel layer that was simulated by changing the trap concentration in the GaN substrates.Additionally, we investigate for GaN HEMTs both with and without an AlGaN back-barrier layer.Figure 6 shows the IDS-VGS curves, transient response, and imaginary parts of S21 for a normal GaN HEMT structure without a back-barrier layer.The trap concentration in the GaN substrates was changed from 10 15 to 10 16 cm −3 .As shown in Figs.6(a) and (b), both the drain-leakage current and IDS at 1 ms in the transient response decrease when the trap concentration increases.For a high trap concentration of 10 16 cm −3 , the drain-leakage current can be suppressed, although the transient response degrades; that is, the normalized IDS is as small as 0.78 times the DC value.However, in the case of a lower trap density, the transient response has better characteristics, although the drain-leakage current has larger values.Both the moderate suppression of the drain-leakage current and positive transient response are satisfied for the 5.0 × 10 15 cm −3 trap concentration, although it seems that there is room for an improved transient response.substrates becomes thinner as the trap concentration in the GaN substrates increases.Figure 8(c) illustrates how the ionized trap density in the GaN substrates increases according to the trap concentration.As shown in Fig. 8(a) and (c), when the trap concentration in the GaN substrates increases, growing the ionized traps with a negative charge contributes to raising the conduction-band energy in the buffer and substrates.This conduction energy decreases the electron concentration in the buffer and substrate, and the drain-leakage current decreases.This result indicates that the normalized IDS at 1 ms decreases because the ionized traps with higher concentrations increase with the higher trap concentrations in the GaN substrates.This mechanism, in which the ionized traps in the GaN substrate lift the conduction-band energy, is similar to the suppressed drain-leakage current.Therefore, there is a trade-off relationship between the drain-leakage current and transient response.Additionally, the trade-off relationship appears between the drain-leakage current and low-frequency S21, which degrades as the trap concentration in the GaN substrates increases.
The back-barrier is known to escalate the conduction band by inserting it between the channel layer and substrate in GaN HEMTs [41]- [43].The back-barrier layer can be considered for use rather than the traps in the GaN substrates.Therefore, the buffer layer was replaced with the back-barrier layer.The Al content of the back-barrier is 0.08, and its 0.1-μm thickness for the back-barrier layer is the same as that for the GaN buffer layer.Note that the back-barrier layer contained traps with the same 2.0 × 10 15 cm −3 concentration as that for the GaN buffer layer.Figure 9  Figure 10 shows the 2D plots of the current density at the off-state bias (VDS = 40 V and VGS = −5 V) for GaN HEMTs with the back-barrier layer.The scale is the same as that in Fig. 7 for GaN HEMTs without back-barrier layers.The leakage current does not flow for all of the trap-concentration cases.The back-barrier layers are extremely effective in suppressing the drain-leakage current even for the low 10 15 -cm −3 trap concentration in the GaN substrates.Figure 11 shows the conduction-band energy, electron concentration, and ionized trap density under the gate edge of the drain side at 1 ms in the transient responses from the off-state to on-state biases.The origin of the position axis is the surface of the barrier layer (the same as Fig. 8).The conduction-band energy in the GaN substrate increases when the trap concentration in the GaN substrates increases.The growth of the conduction-band energy causes the conduction-band energy to rise at the surface of the channel layer.This growth indicates that the normalized IDS at 1 ms in the transient responses decreases as the trap concentration in the GaN substrates increases, as shown in Fig. 9(b).trap concentration in GaN substrates increases.Figure 12(c) shows the IDSOFF [IDS at the off-state bias (VDS = 40 V and VGS = −5 V)] both with and without the back-barrier layers.IDSOFF is decreased by applying the back-barrier layers.The drain-leakage current improves dramatically at the 10 15 -cm −3 low-substrate trap concentration.Figure 12(d) shows the peak magnitudes in the imaginary parts of S21 as a function of the trap concentration in GaN substrates.The back-barrier layers make the peak magnitude small.As shown in Fig. 12(b) and (d), the transient response and low-frequency S21 do not change much with or without the back-barrier layers.Therefore, the back-barrier layers contribute toward the suppression of the drain-leakage current rather than the improvement of the transient response and low-frequency S21.
Figure 13 illustrates the trade-off relationships between the figures of merit.Figure 13(a) shows IDSOFF versus normalized IDS at 1 ms in the transient responses.IDSOFF increases when the normalized IDS at 1 ms increases.This correlation indicates that there is a trade-off relationship between IDSOFF and normalized IDS at 1 ms and that the back-barrier layer can ease this trade-off relationship.As shown in Fig. 13(b), the peak magnitudes of S21 correlate with the transient responses because these effects result from the traps in GaN substrates.GaN HEMTs with a back-barrier layer have good S21 values for the same transient response.As shown in Fig. 13(c), IDSOFF improves (decreases) as the S21 values degrade, i.e., there is a trade-off relationship.Both the low drain-leakage current and small magnitude of imaginary S21 parts are obtained using the back-barrier layers.

Characteristics of GaN HEMTs with a back-barrier layer and a barrier layer with high Al content
In the previous section, the back-barrier structures can reduce the negative effects of small IDS in the transient responses and the large S21 signal at a low frequency, and the low drain-leakage current continues.However, as is evident from Fig. 12(a), the maximum drain current decreased by adopting the back-barrier layer.In this section, GaN HEMTs that have a high Al content in the barrier layer have been studied to increase the maximum drain current.
Figure 14 shows the characteristics of the GaN HEMTs with 0.22 Al content in the barrier layer.The structure is the same as that calculated in Section B, except for the barrier's Al content.The trap concentration in the GaN substrates is as low as 10 15 cm −3 to obtain a satisfactory transient response and low-frequency S21.The back-barrier layer is adopted to reduce the drain-leakage current.From 14(a) of the IDS-VDS curves, the maximum 1.2 A/mm drain current is obtained at VDS = 40 V and VGS = 2 V.The drain-leakage current is suppressed to be lower than 10 −10 A/mm for IDS in the off-state condition, as shown in the IDS-VGS curves of Fig. 14(b).As these curves in a subthreshold region do not depend on VDS, the back-barrier layer is useful in suppressing the drain-leakage current.The trade-off relationships between the figures of merit maintain good positions in Fig. 13, in addition to increasing the maximum drain current.
Figure 15 shows the characteristics when the gate length was changed from 0.5 to 0.05 μm for the GaN HEMTs with the back-barrier and high-Al-content barrier layers.Figure 15(a) shows how IDS depends on VDS at VGS = 2 V.The maximum IDS increases from 1.3 to 1.6 A/mm as the gate length decreases.Figure 15(b) shows IDS versus VGS of the high VDS of 40 V.When the gate length is shortened from 0.5 to 0.05 μm, the slopes in the subthreshold region increase, and the drain-leakage current occurs in the low VGS because of the short-channel effects.However, the drain leakage current below VGS = −3 V is suppressed to be lower than 5 × 10 −6 A/mm, even for 0.05 μm, which is the shortest gate length.The transient responses at high VDSQ of 40 V have good characteristics, with around 0.98 at 1 ms for all gate lengths, as shown in Fig. 15(c).Figure 15(d) shows the imaginary parts of S21 in the low-frequency region at the on-state bias (VDS = 40 V In this work, the GaN HEMT structures on GaN substrates with thin channel layers were investigated in terms of the trade-off relationship between the drain-leakage current, transient response from the off-state bias to the on-state bias, and low frequency S21 using TCAD simulations.The dependence on the channel-layer thickness was calculated for a relatively high trap concentration of 2 × 10 16 cm −3 in the GaN substrates.The drain-leakage current was suppressed at less than the channel thickness of 0.1 μm, and the transient response are degraded.This finding indicates that the thin channel layer is not preferable for RF applications in the case of GaN substrates with a high trap concentration.Because the transient responses degraded owing to the presence of ionized traps in the GaN substrates, dependencies on the trap concentration were calculated for the GaN HEMTs with a 0.02-μm thin channel layer.The low trap concentration of 10 15 cm −3 in the GaN substrates improves both the transient response and low-frequency S21 characteristics.The drain-leakage current of 10 −3 A/mm is extremely large at high VDS of 40 V because of the leaks in the GaN substrates, which indicates that a satisfactory trade-off relationship cannot be obtained just by adopting the low trap concentration in GaN substrates for GaN HEMTs with a thin channel layer.To suppress the drain-leakage current without increasing the trap concentration, the AlGaN back-barrier layer is adopted.A significantly decreased drain-leakage current by seven orders is obtained by the back-barrier layer rather than the buffer layers, especially at a low trap concentration of 10 15 cm −3 in GaN substrates.For the back-barrier layer, the weak point is the deceased maximum drain current.We investigated how the GaN HEMTs increase the Al content from 0.20 to 0.22 in the barrier layers.The GaN HEMTs have high IDS and good trade-off relationships between the drain-leakage current, transient response, and low-frequency S21.Finally, we confirmed that this structure can operate for a short gate length of 0.05 μm despite the threshold-voltage decrease.

Fig. 1
Fig.1 Schematic cross-sectional structure of GaN HEMTs on GaN substrates for simulation.

Fig. 2 Fig. 3 ( 12 V DS = 40 V
Fig.2 Typical characteristics calculated.(a) I DS −V DS curves, (b) I DS −V GS curves, (c) transient response from off to on-state bias, and (d) real and (e) imaginary part S 21 in low frequency region.Channel thickness and trap concentration of GaN substrates are 1.0 μm and 2 × 10 16 cm −3, respectively.

Figure 6 (
c) shows the imaginary parts of S21 at the on-state bias (VDS = 40 V and VGS = 0 V).The peak frequencies for the different trap concentrations in the GaN substrates are almost

Figure 7
shows the 2D plots of the current density at the off-state bias (VDS = 40 V and VGS = −5 V) for various trap concentrations in the GaN substrates.The depletion region spreads into the GaN substrate through the channel and buffer layers because of the thin 0.02-μm channel layer.In the channel region, the current flows from the drain to the source suppress in all cases of trap concentrations.The current flows in the GaN substrate, as shown in Fig.7(a), because traps as low as 10 15 cm −3 are not sufficient to suppress the leakage current.When the trap concentrations in the GaN substrates increase, the leakage current decreases.Figure8shows the conduction band energy, electron concentration, and ionized trap density under the gate edge of the drain side at 1 ms in the transient responses from the off-state bias (VDS = 40 V and VGS = −10 V) to the on-state bias (VDS = 5 V and VGS = 0 V).The origin of the position axis is the barrier layer's surface.The conduction-band energy in the substrate increases when the trap concentration in the GaN substrates increases.As shown in Fig.8(b), the region with the highest electron concentration in the buffer and

Fig. 6 (Fig. 7 3 .Fig. 8
Fig.6 (a) I DS −V GS curves, (b) transient response from off-state to on-state bias, (c) imaginary parts of S 21 in low frequency region as parameters of trap concentrations in GaN substrates (N TSUB ).Channel thickness is as thin as 0.02 μm.
shows the IDS-VGS curves, transient responses, and imaginary parts of the low-frequency S21 for the GaN HEMTs with the back-barrier layer.The trap concentration in the GaN substrates changed from 10 15 to 10 16 cm −3 .As shown in the IDS-VGS characteristics of Fig. 9(a), the drain-leakage current suppresses sufficiently even for the low 10 15 -cm −3 trap concentration.When the trap concentration in the GaN substrates increases, the transient responses and low-frequency S21 characteristics degrade, which are results that are similar to those of the structures without the back-barrier, as shown in Fig. 6(b) and (c).Therefore, back-barrier layers are considered not to have a large impact on the transient response and low-frequency S21.
Figure 11(b) shows the electron-concentration profiles for various trap

Fig. 9 (Fig. 10 3 for
Fig.9 (a) I DS −V GS curves, (b) transient response from off to on-state bias, (c) imaginary S 21 in low frequency as parameters of trap concentration in GaN substrates (N TSUB ) for GaN HEMTs with AlGaN back-barrier layers.Channel thickness is 0.02 μm.

Fig. 11
Fig.11 Depth profiles of (a) conduction band energy (EC) , (b) electron concentration (nE) and (c) trap ionized density (nTI)under gate edge of drain side in transient response for AlGaN back-barrier for various trap concentration in GaN substrates (N TSUB ).Time in transient response is at 1 ms.Position is defined distance from hetero interface.Bias is changed from off-state (V DS = 40 V and V GS = −10 V) to on-state bias (V DS = 5 V and V GS = 0 V).Channel thickness is 0.02 μm.

Figure 14 (
c) shows the transient responses for VDSQ of 20 and 40 V.The normalized IDS at 1 ms is as high as 0.98 even if there is high VDSQ of 40 V. Figure14(d)shows the imaginary parts of S21 in the low-frequency region.The peak magnitudes decrease as VDS increases from 10 to 40 V. Figures12 and 13include the data for GaN HEMTs with the 0.22 high Al content and back-barrier layer (called back-barrier 2).The maximum IDS becomes as large as GaN HEMTs without the back-barrier layer, as shown in Fig.12(a).However, the drain-leakage current and transient response maintain almost the same values as the GaN HEMTs with the back-barrier layers.The peak magnitude in the imaginary parts of S21 becomes slightly high, as shown in Fig.12(d).

Fig. 13
Fig.13 Trade-off relationships between (a) drain-leakage current and normalized I DS at 1 ms, (b) peak magnitude in imaginary S 21 and normalized I DS at 1 ms, (c) drain leakage current and peak magnitude in imaginary S 21 .Back barrier 2 has higher 0.22-Al content than that of back-barrier 1. Normalized I DS at 1 ms is defined in transient response from off to on-state bias.Channel thickness is 0.02 μm.

Figures Figure 1
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Figure 11 Depth
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Figure 14 Characteristics
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