Description:Field of Invention
This invention presents novel device structure for the High Electron MobilityTransistor(HEMT) using partial barrier layer, referred here as PBL-HEMT. The state-of-art design of the transistor achieved improved transconductance/gain linearity at higher current as compared to conventional High Electron Mobility Transistors (HEMTs).The present invention demonstrates optimization of source access resistance using partial barrier layer andGaussian lateral doping in the access regions, which facilitates postponement of carrier velocity saturation by redistribution of electric field.
The objectives of this invention
The objective of the present invention is to demonstrate improved linearity in the novel Aluminium Nitride/Beta-Gallium-Oxide (AlN/β-Ga2O3) PBL-HEMT over conventional Aluminium Gallium Nitride/Gallium Nitride (AlGaN/GaN) HEMTs. Additionally, higher carrier density in the form of Two-Dimensional Electron Gas (2DEG) channel, improved dc and RF characteristics are the important considerations for the analyzed AlN/β-Ga2O3 PBL-HEMT.
Background of the invention
Like other field-effect-transistors (FETs), GaN HEMTs also suffer from poor transconductance (gm), cut-off frequency (fT) and maximum cut-off frequency (fMAX) linearity. This poor linearity affects thelarge signal operations and result in distortion as well as gain compression. Now-a-days, increasing demand of low noise amplifiers (LNA) inmodern communication system like 5G technology is the key motivation to develop LNA with superior linearity. To address the linearity issues in HEMTs, various studies have already done.Palacios et al. [2005],IEEE Trans. Electron Devices, vol. 52, no. 10, pp. 2117–2122, analyzedthe effect of dynamic access resistance on the transconductance (gm) and cut-off frequency (fT) linearity in GaN HEMTs, andshowed n+ doping in source access region for better linearity.However, Fang et al. [2012], IEEE Electron Device Lett. 33, 709, instead found strong interactions between electron and optical phonon for gm and fT nonlinearity in GaN HEMTs. The authors further demonstrated that smaller source resistance (RS) can cause increasednonlinearity in HEMTs.Chen et al. [2016], Solid. State. Electron., vol. 126, pp. 115–124, found self-heating effects responsible for degraded linearity in GaN HEMTs, apart from dynamic access resistances. In another report, improved linearity profiles of fT and fMAX versus drain current (IDS) using graded channel and compositionally graded channel in GaN HEMTs were demonstrated by Bajaj et al. [2017], IEEE Trans. Electron Devices, vol. 64, no. 8, pp. 3114–3119, and Ancona et al. [2019], IEEE Trans. Electron Devices, vol. 66, no. 5, pp. 2151–2157respectively. Recently, Lu et al. 2021demonstrated GaN/InGaN coupling channel to achieve almost constant dynamic source resistance which resulted in improved gm linearity. And, Odabasi et al. [2021], IEEE Trans. Electron Devices, vol. 68, no. 3, pp. 1016–1023, demonstrated laterally gated non-planer GaN HEMTs and demonstrated improved gm linearity using experimental study. Most of them found dynamic access resistance responsible for poor linearity in HEMTs. Some other reports suggested self-aligned structures, graded-channel and multichannel structures of HEMTs, and laterally gated HEMTs with superior linearity, however, at the cost of low breakdown voltage and complex processing steps.
Description of Prior Art
The conventional design of HEMTs offer several best-in-class parameters over other field effect transistors (FETs). These include high critical electric field and so high breakdown voltages, higher sheet charge density in form of 2DEG, high carrier mobility in the channel due to reduced impurity scattering. These properties have made AlGaN/GaN HEMTs most suitable for high power and high frequency switching applications. Due to higher electron mobility, GaN HEMTs have low on-resistance(RON) and associated higher switching speed over silicon MOSFETs.Almost all the previous GaN HEMTs design comprising of a substrate, buffer layer, fist active layer or channel followed by second active layer i.e., barrier layer, and three electrodes for electrical connections as demonstrated in (US5192987A).Fabrication steps of HEMT using aluminum oxide gate and surface passivation using silicon nitride aims to circumvent current collapse phenomenon and higher life span of HEMT has been published (111129140). The improved breakdown voltage and higher output power was achieved using field-plated recess gate technology in AlGaN / GaN HEMT and is disclosed in (113725288). The extension of gate electrode towards drain side helps to redistribute electric field peak values and thus breakdown voltage improves.Due to always ‘ON’ state of depletion-type AlGaN / GaN HEMTs, enhancement-type devices are preferred for safe and failure-proof operations. U.S. Pat. No. 9,379,191 to M. Srivastava et. el. discloses enhancement mode AlGaN / GaN HEMT using AlxTi1-x gate oxide engineering. The device used hybrid gate structure that combines p-GaN technology with the AlxTi1-x gate oxide, and achieved tunable threshold voltage. On the other hand, various heterostructures or heterojunctions using ultrawide bandgap semiconductor have been also disclosed. Among the emerging UWBG semiconductors, the most commonly used is β-Ga2O3and associated FETs. In one the recent disclosure, fabrication method for preparation of gallium oxide / copper gallium oxide heterojunction has been described (WO/2020/124413).Furthermore, a vertical structure of gallium oxide for power applications is provided in the disclosure WO/2021/103953.The breakdown voltage of the device is enhanced using thick intermediate layer of gallium oxide. and highly thermally conductive Ga2O3 substrate provides good thermal performance of the device. It is worth noting that, all these devices have used almost common device structure having thick GaN buffer or channel layerand relatively thin AlGaN barrier layer. Furthermore, all these device structures have poor gain linearity which affects their potential applications in low-noise amplifiers and other emerging areas where gain distortions at higher voltage is avoided.
Summary of the invention
The present invention addresses linearity issue in conventional HEMTs by presenting the novel device structure of AlN / β-Ga2O3 HEMT. In particular, the present invention relates to barrier layer engineering in the conventional HEMT device structure for improved transconductance linearity at higher current values.
In one aspect, the present disclosure provides a heterojunction transistor including asemi-insulating β-Ga2O3 (010) substrate followed by a n-type β-Ga2O3 channel or buffer layer, and a thin AlN barrier layer which is partially etched away over source and drain access regions.
In another aspect, the access region gaps are doped with n-type dopant with Gaussian profile.
In another aspect, the present transistor has a gate; a drain and a source with highly-doped n-type access contact areas.
In another aspect, silicon nitride passivation is included on the upper surface of barrier layer.
Detailed description of the invention
The conventional HEMTsstructure consist of channel or drift layer grown the suitable substrate material. It follows growth of the thin epitaxial layer of gate barrier. The device structures have full-length buffer layer grown on the barrier layer. For instance, full-length AlGaN exists on entire length of GaN buffer in AlGaN/GaN HEMTs. Due to the energy bandgap difference between barrier and buffer layer, there is a band discontinuity at the heterointerface. This band discontinuity is known as band offset, and has a typical value of 0.1 to 0.3 eV in AlGaN/GaN HEMTs. Furthermore, due to strong polarization properties of III-nitrides, charge accumulation in the form of 2DEG occurs at the heterointerface. The resulting sheet charge density (ns) in AlGaN/GaN HEMTs is usually in the order of ~ 1012 cm–2due to the reasons described earlier. It is worth mentioning that large value of band offset or more precisely conduction band offset (CBO) and higher 2DEG density are most preferred parameters to achieve improved dc and RF performance in the HEMTs.
The PBL-HEMT (AlN/β-Ga2O3)offers large value of CBO in the range of 0.5 to 1.5 eV, mainly due to the large energy bandgap difference betweenβ-Ga2O3(Eg = 4.9 eV) and AlN (Eg = 6.0 eV). Additionally, higher 2DEG density  1013 cm–2 is estimated. This is attributed to strong spontaneous polarization charge of the AlN barrier and piezoelectric charge due to strain in thin AlN epitaxial layer. Since β-Ga2O3 does not owns any polarization property, the net polarization charges remain high as compared to AlGaN/GaN HEMTs. It is worth noting that as this 2DEG density is dependent on applied gate voltage and therefore affects electron mobility in access regions due to nonlinear resistance of the access regions. The nonlinear nature of the access resistance stems from the fact that its value increases with rising bias voltages. With increasing gate voltage, 2DEG density also increases which results in reduced electron velocity mainly due to enhanced electron phonon interactions. The resultant effect is bias dependent transconductancecharacteristics in conventional HEMTs, which limits linearity and noise performance.This is also attributed to depletion of charge carriers under the source edge gate and referred to as ‘source choking effect’. On the contrary, PBL-HEMT novel device structure consists of partial barrier layer which alleviates this depletion of charge carriers and offers less pronounced effect on access resistance values. In this way AlN/β-Ga2O3HEMT offers optimized access resistance which results in improved device linearity. This is achieved using manipulation of peak electric field in the source-access region to postpone carrier velocity saturation.
As per the theoretical estimates, for a typical HEMT with 2DEG sheet charge density of ~ 3 × 1012 cm–2, device width of W, and effective electron velocity (veff) of ~ 107 cm/s, the current density is given as:
I_D/W=q.n_s.v_e=5 A/cm (1)
However, there is a significant shortfall in experimental measurement comparing above values of the drain current density. This implies that electron velocity in access regions is less than 107 cm/s at higher gate voltages, as other quantities q and ns are fixed. It happens due to enhanced electron-phonon scattering and early electron velocity saturation in access regions. This is also attributed to pronounced effect of bias dependent access resistance on drain current as described earlier. Since electron velocity got saturation led to mobility degradation at higher electric field, therefore, it is found that partial barrier layer can facilitates decoupling of 2DEG sheet density with gate voltage in access regions and can mitigate above effects. The access region gaps are doped with n-type Gaussian profile with peak concentration along the interface.Additionally, access region length, type of doping profile and doping concentration in access regions can be fine-tuned to further optimize the dynamic source access resistance.
As far as electron transport in β-Ga2O3 is concerned, it shows negative differential conductivity (NDC) beyond electric field values of 200 kV / cm. Ghosh, K. and Singisetti, U., (2017). Journal of Applied Physics, 122(3), p.035702, investigated high-field transport in monoclinic β-Ga2O3 and described velocity-field curves. Therefore, parallel electric-field dependent mobility model accounting for NDC is used, and is given as;
μ_n (E)=(μ_n0+v_sat/E 〖(E/E_C )〗^γ)/(1+〖(E/E_C )〗^γ ) (2)
where low field mobility μn0 = 140 cm2/Vs, criticalfield for electrons EC = 2.25 × 105 V/cm andγ = 2.84 are user-defined values for β-Ga2O3.
Keeping the above observation in considerations, the partial barrier layer in AlN/β-Ga2O3 is introduced. The electric fields in access regions are manipulated using access regions length and doping concentration and doping profile. In this way electron velocity saturation is postponed and dynamic access resistance is optimized. The linearity analysis is performed for two different HEMT samples, one with partial barrier layer and another with full barrier layer length as used in conventional HEMTs.
For fixed drain-source voltage, gate-source voltage is varied and electric field values are extracted for both designs of HEMTs. It is found that, PBL-HEMT offers relatively lower peak values of electric field and also there is significantly less dispersion between different values as compared to conventional HEMT. As a result, there is a ~40 % higher mobility and ~ 50 % lower source access resistance is found in PBL-HEMT. Hence, the novel PBL-HEMT achieved improved linearity by optimizing source access resistance with the help of access region gaps in which electric fields are manipulated to postpone electron velocity saturation.
Brief description of Drawing
The figures accompanied here areincluded to provide further understanding of the present invention. The drawings illustrate exemplary embodiments of the invention.
Figure 1 Cross-sectional view of the PBL-HEMT for improved linearity, in accordance with an exemplary embodiment of the present invention.
Figure 2 Transconductance versus gate voltage for both conventional and PBL-HEMT, illustrating improved linearity in the PBL-HEMT.
Figure 3 Energy-band diagram of AlN/β-Ga2O3HEMT under the gate including the interface of the said buffer and barrier layers.
Figure 4Electric field values versus position in source access region under varying gate voltage and fixed drain voltage, in accordance with an exemplary embodiment of the present invention.

Detailed description of the drawing
As described above the present invention relates to copyright fetching.
Figure 1 shows the cross-sectional view of the AlN/β-Ga2O3HEMT which includes the epitaxial layer arrangement of the device and access area gaps created due to the said partial barrier layer.
Figure 2 illustrates the transconductance versus gate voltage for both conventional and PBL-HEMT, a figure-of-merit called as gate voltage swing (GVS) is used to measure the linearity of both designs.
Figure 3.shows the energy band diagram and 2DEG density for the PBL-HEMT, the data are extracted under the gate of the present disclosure of the simulated structure saved at thermal equilibrium.
Figure 4.illustrates the electric field distribution in source access region at different gate voltages, the figure is in accordance with the disclosure of the present invention.

5 Claims & 4 Figures , Claims:The scope of the invention is defined by the following claims:

Claims:
1. A novel structure of high electron mobility transistor (HEMT), comprising
a) An unintentional doped (UID) β-Ga2O3 buffer layer;
b) A thin partially etched undoped AlN barrier layer on said β-Ga2O3 buffer layer;
c) An access region gaps, which stem from above said partially etched barrier layer,are doped with n-type Gaussian profile;
d) A source, and drain electrode directly contacting said buffer layer; and a gate contact on said barrier layer.
2. As per claim 1, in the said HEMT, there is a charge confinement in form of two-dimensional electron gas (2DEG) at the interface of said buffer and barrier layer.
3. According to claim 2, the charge density in access regions is independent of applied gate voltage.
4. As per claim 1, said buffer layer with lower bandgap material providesgood source and drain ohmic contacts.
5. As per claim 1, electron velocity saturation in access regions is postponed by controlling peak electric field and thus improved linearity is achieved.