MULTI-FINGER HIGH-ELECTRON MOBILITY TRANSISTOR

Abstract

A multi-finger high-electron mobility transistor and a method of manufacturing such a transistor, and an electronic device including such a transistor is provided. According to an aspect of the present disclosure, an etching step for reducing donor layer thickness and/or performing an ion implantation is used for locally reducing the 2DEG concentration.

Claims

1. A multi-finger high-electron mobility transistor (HEMT), comprising: a semiconductor body comprising a channel layer configured to hold a two-dimensional electron gas (2DEG), and a donor layer configured to supply electrons to form the 2DEG; a plurality of gate fingers; a plurality of drain fingers; wherein each gate finger is associated with a respective first region of the channel layer that is arranged underneath that gate finger, and a respective second region of the channel layer that is arranged in between that respective first region and a drain finger among the plurality of drain fingers that is associated with the gate finger; and wherein the 2DEG in absence of any voltage applied to the HEMT in a part of the second region associated with a gate finger among the plurality of gate fingers has a concentration that is lowered, by ion-implanting and/or etching the donor layer, relative to a different part of the second region associated with the gate finger, respectively.

2. The HEMT according to claim 1, wherein the concentration of the 2DEG is lowered by ion implantation; wherein the part of the donor layer corresponding to the part of second region associated with the gate finger among the plurality of gate fingers of which the 2DEG is lowered relative to the different part of the second region associated with the gate finger, respectively, comprises a third concentration of implanted ions, and wherein the part of the donor layer corresponding to the different part of the second region comprises a fourth concentration of implanted ions, wherein the third concentration is greater than the fourth concentration.

3. The HEMT according to claim 1, wherein the concentration of the 2DEG is lowered by etching; wherein the part of the donor layer corresponding to the part of the second region associated with the gate finger among the plurality of gate fingers of which the 2DEG is lowered relative to the different part of the second region associated with the gate finger, respectively, has a third thickness, and wherein the part of the donor layer associated with the different part of the second region has a fourth thickness, and wherein the third thickness is smaller than the fourth thickness.

4. The HEMT according to claim 1, wherein the concentration of the 2DEG in absence of any voltage applied to the HEMT in a part of the second region associated with a gate finger among the plurality of gate fingers is different for more than two other parts of the second region associated with that gate finger.

5. The HEMT according to claim 1, further comprising one or more fieldplates associated with a gate finger among the plurality of gate fingers, wherein the one or more fieldplates are connected to a source or gate of the HEMT, and wherein the one or more fieldplates extend at least above respective segments of the second region associated with the gate finger.

6. The HEMT according to claim 1, wherein the 2DEG concentration in absence of any voltage applied to the HEMT in the second regions associated with gate fingers that are arranged in a central portion of the HEMT is lower than that in the second regions associated with gate fingers that are arranged in an outer portion of the HEMT.

7. The HEMT according to claim 1, wherein the HEMT is a GaN-based HEMT, and wherein the channel layer comprises an Al.sub.xGa.sub.1-xN layer and the donor layer an Al.sub.yGa.sub.1-yN layer, wherein x < y, wherein 0 <= x <= 0.10 and 0.10 <= y <= 0.50.

8. The HEMT according to claim 2, wherein the third concentration lies in a range between 4e12 and 12e12 #/cm.sup.2, and wherein the fourth concentration lies in a range between 0 and 4e12 #/cm.sup.2.

9. The HEMT according to claim 2, wherein ions used for the ion-implanting the donor layer are ions out of the group consisting of Argon ions, Nitrogen ions, Boron ions, Silicon ions, and Phosphorus ions.

10. The HEMT according to claim 2, further comprising one or more fieldplates associated with a gate finger among the plurality of gate fingers, wherein the one or more fieldplates are connected to a source or gate of the HEMT, and wherein the one or more fieldplates extend at least above respective segments of the second region associated with the gate finger.

11. The HEMT according to claim 2, wherein the 2DEG concentration in absence of any voltage applied to the HEMT in the second regions associated with gate fingers that are arranged in a central portion of the HEMT is lower than that in the second regions associated with gate fingers that are arranged in an outer portion of the HEMT.

12. The HEMT according to claim 2, wherein the HEMT is a GaN-based HEMT, wherein the channel layer comprises an Al.sub.xGa.sub.1-xN layer and the donor layer an Al.sub.yGa.sub.1-yN layer, wherein x < y, wherein 0 <= x <= 0.10 and 0.10 <= y <= 0.50.

13. The HEMT according to claim 3, wherein the third thickness and the fourth thickness has a ratio that lies in a range between 0.5 and 0.9.

14. The HEMT according to claim 10, wherein the one or more fieldplates comprise a plurality of fieldplates, wherein the second region associated with the gate finger among the plurality of gate fingers comprises a plurality of different segments, each segment corresponding to a different number of fieldplates that extend above it, and wherein the 2DEG concentration in absence of any voltage applied to the HEMT differs among the plurality of different segments that have at least one fieldplate extending above them.

15. The HEMT according to claim 10, wherein the 2DEG concentration in absence of any voltage applied to the HEMT is not substantially lowered in a remaining part of the second region associated with the gate finger among the plurality of gate fingers.

16. An electronic device comprising the HEMT as defined in claim 1, wherein the electronic device is a device out of the group consisting of GaN HEMTs configured as a stand-alone device or in cascode with a low-voltage switch and operating in either enhancement-mode or depletion-mode.

17. A method for manufacturing a multi-finger high-electron mobility transistor (HEMT), comprising the steps of: providing a semiconductor body comprising a channel layer configured to hold a two-dimensional electron gas (2DEG), and a donor layer configured to supply electrons for the purpose of forming the 2DEG; forming a plurality of drain fingers; forming a plurality of gate fingers; wherein each gate finger is associated with a respective first region of the channel layer that is arranged underneath that gate finger, and a respective second region of the channel layer that is arranged in between that first region and a drain finger among the plurality of drain fingers that is associated with the gate finger; lowering a concentration of the 2DEG in absence of any voltage applied to the HEMT in a part of the second region associated with a gate finger among the plurality of gate fingers by means of ion-implanting the donor layer and/or by etching the donor layer, relative to a different part of the second region associated with the gate finger, respectively.

18. The method according to claim 17, wherein the ion-implanting is performed prior to forming the plurality of drain fingers; and/or wherein the etching of the donor layer is performed prior to forming the plurality of gate fingers.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0043] So that the manner in which the features of the present disclosure can be understood in detail, a more particular description is made with reference to embodiments, some of which are illustrated in the appended figures. It is to be noted, however, that the appended figures illustrate only typical embodiments and are therefore not to be considered limiting of its scope. The figures are for facilitating an understanding of the disclosure and thus are not necessarily drawn to scale. Advantages of the subject matter claimed will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying figures, in which like reference numerals have been used to designate like elements, and in which:

[0044] FIG. 1 illustrates a schematic top view of a known HEMT.

[0045] FIG. 2 illustrates a cross-sectional view corresponding to the HEMT of FIG. 1.

[0046] FIG. 3 illustrates a first embodiment of a HEMT in accordance with an aspect of the present disclosure.

[0047] FIG. 4 illustrates a second embodiment of a HEMT in accordance with an aspect of the present disclosure.

[0048] FIGS. 5A, 5B and 5C illustrate various third embodiments of a HEMT in accordance with an aspect of the present disclosure.

[0049] FIGS. 6A and 6B illustrate various fourth embodiments of a HEMT in accordance with an aspect of the present disclosure.

[0050] FIG. 7 illustrates a method of manufacturing a HEMT in accordance with an aspect of the present disclosure.

[0051] FIG. 8 illustrates a diode in which the 2DEG concentration is modified similar to the HEMTs of FIGS. 3-7.

DETAILED DESCRIPTION

[0052] FIG. 3 illustrates a first embodiment of a HEMT 100 in accordance with an aspect of the present disclosure. Here, donor layer 7 is subjected to an ion-implantation process as a result of which ions have been implanted into donor layer 7. In this case, Ar+ ions have been implanted. The dose used for this implantation is indicated using the dashed line. Nd0 may represent a background level and may correspond to 0 #/cm2, thereby corresponding to no ion implantation at all. As shown, in the first region directly underneath gate contact layer 2Ac, a higher implantation dose has been used. As a result of this implantation, the 2DEG concentration has been lowered and the threshold voltage is shifted to lower values.

[0053] FIGS. 4 and 5 illustrate different embodiments of a HEMT in accordance with an aspect of the present disclosure. In HEMT 200 shown in FIG. 4, the first region has essentially not been subjected to ion implantation, or with a background level Nd0, while the second region has been subjected to different ion implantations. More in particular, the first segment of the second region, arranged closest to the first region, and above which fieldplates 9A, 9B and metal layer M1 of gate finger 2A extend, is subjected to the highest implantation dose Nd1. The adjacent segment, above which only fieldplates 9A, 9B extend, is subjected to a lower implantation dose Nd2, and the segment adjacent to that to an ever lower implantation dose Nd3.

[0054] The present disclosure is not limited to a configuration in which NdO<Nd3<Nd2<Nd1. Furthermore, the different doses may be applied to the entire second region for each gate finger or it may be applied to only a part of the second region, for example to stripes in the second region. In this manner, there will exist a variation in the 2DEG concentration in a direction parallel to the gate finger. In addition, the entire first region or parts thereof may also have been subjected to an ion implantation process.

[0055] The output capacitance, Coss, being equal to the sum of the drain-source capacitance and the gate-drain capacitance, depends on the applied drain-source voltage. Each step in Coss represents a change in the dV/dt gradient during switching.

[0056] A well-designed HEMT will have several steps in Coss, which correspond to the individual contributions made by the retracting 2DEG. The biggest change in Coss always occurs around the threshold voltage of the HEMT and the Coss steps diminish in magnitude as the drain-source voltage is increased due the growing fieldplate-to-2DEG separation.

[0057] The sudden changes in Coss lead to unwanted oscillations and overshoots during switching. According to an aspect of the present disclosure, this problem can be mitigated by partially or fully deactivating the 2DEG in the second region to actively manage the contributions to Coss and hence smooth out the overall Coss response.

[0058] In the HEMTs shown in FIGS. 5A-5C, of which schematic top views are illustrated, the first region under gate finger contact 2Ac is divided into segments 21-27. Each segment is subjected to a different ion-implantation dose, not excluding embodiments in which one segment is not subjected to ion implantation while others are.

[0059] FIG. 5A illustrates an embodiment in which the first region is divided in 7 segments 21-27. Among these 7 segments, at least one segment is subject to ion implantation. Moreover, at least two segments among the 7 segments differ with respect to the ion-implantation dose that was used for that segment.

[0060] FIG. 5B illustrates an embodiment in which the first region is divided in 2 segments 21, 27. Among these segments, at least one segment is subject to ion implantation. Moreover, segments 21, 27 differ with respect to the ion-implantation dose that was used for that segment, not excluding embodiments in which one segment is not subjected to ion implantation while the other is. In this ‘2-step’ example, a lower 2DEG concentration is realized at the edge of the first region to reduce charge trapping and minimize dynamic on-resistance effects.

[0061] FIG. 5C illustrates an embodiment in which the first region is subjected to a graded ion-implantation. More in particular, line 29 indicates a line in the first region in which the same ion implantation dose has been used. Arrow 30 indicates that a direction in which the ion-implantation dose increases or decreases. More in particular, other lines parallel to line 29 can be drawn that correspond to different ion-implantation doses. In this ‘graded’ example, the 2DEG concentration is lowered at an angle to create regions with a gradual transition from fully on to fully off.

[0062] In the embodiments shown in FIGS. 5A-5C, the second region has not been subjected to ion implantation. As such, the effect of the ion implantation is that the threshold voltage is changed locally. This means that part of a gate finger can be associated with a different threshold voltage than another part of the gate finger. In this manner, the transconductance of the overall HEMT can be controlled.

[0063] In HEMT 300A shown in FIG. 6A, the second regions associated with different gate fingers 2A have been subjected to different ion implantation doses. For example, the second regions corresponding to the gate fingers in an outer section of HEMT 300A have been subjected to an implantation dose Nd1, the second regions corresponding to the gate fingers in a center section of HEMT 300A to an implantation dose Nd3, and the remaining gate fingers to an implantation dose Nd2. Here, Nd3>Nd2>Nd1.

[0064] As a result of this implantation, the local saturated current will be lower for the gate fingers in the center section than for the gate fingers in the outer sections. Furthermore, current flow into the center gate fingers is suppressed while it is promoted for gate fingers in the outer sections.

[0065] For HEMTs having a constant distribution of the saturation current, a considerable increase in temperature in the center section may be observed during operation compared to the temperature in the outer sections. This imbalance and associated risks for reduced reliability are mitigated using the abovementioned ion-implantation approach.

[0066] It should be noted that the ion implantation need not be applied to the entire second regions of the gate fingers. For example, the ion implantation may be applied in a stripe pattern as shown for HEMT 300B in FIG. 6B.

[0067] FIG. 7, left, illustrates a semiconductor body to be used for a GaN-based insulated-gate HEMT in accordance with an aspect of the present disclosure. It comprises a silicon substrate 5, a GaN buffer layer 6, and a donor layer 7. Here, donor layer 7 comprises a separate GaN cap layer 7B in addition to an AlGaN layer 7A that provides the electrons for forming the 2DEG. The sheet resistance, Rsh, associated with the 2DEG is roughly between 200 and 1000 Ohm/square.

[0068] FIG. 7, center, illustrates how ion implantation can be performed in a window 10. The step of performing ion implantation is preferably performed after providing the insulation layer that is required underneath the gate contact layer and before providing the drain and source contact layers.

[0069] An experimentally verified method of selectively controlling the 2DEG density is by an Ar+ implant directly into the 2DEG region, thus reducing the 2DEG electron concentration, which in turn leads to an increase in 2DEG sheet resistance, Rsh, and a reduction in the local threshold voltage.

[0070] As a result of implanting the Ar+ ions, some of the Al atoms in the AlGaN layer are removed, leading to AlGaN barrier relaxation and consequently reducing the strain at the GaN/AlGaN interface, giving lower 2DEG concentration in the implanted region.

[0071] The degree of AlGaN barrier relaxation, and hence the reduction in 2DEG concentration, depends on the implant dose and can be changed by at least 2 orders of magnitude with moderate implant doses (1012 cm-2) and at moderate implant energies of around 200 keV for typical GaN HEMTs.

[0072] In this implementation, the Ar+ implant is additional to the so-called isolation implant, whose function is to destroy the 2DEG concentration for the purpose of component isolation. Typically, the Ar+ dose would be in the region of 5-10 ×1012 cm-2 at 180 kV, which is well within the normal capability of commercial implanters used in semiconductor fabs. By applying the ion implantation in window 10, the sheet resistance inside window 10 increases up to 6-16 kOhm/square.

[0073] FIG. 7, right, illustrates that a reduction of the 2DEG concentration in the first region corresponding to window 10 can also be obtained by reducing the thickness of donor layer 7. As such, instead of using ion implantation in the embodiments discussed in connection with FIGS. 3-6, a reduction of the donor layer thickness could additionally or alternatively have been used.

[0074] For GaN-based HEMTs, removing part of the AlGaN barrier by Cl2-based dry-etching will lead to a less strained AlGaN layer, which will reduce electron concentration in the 2DEG quantum well. For these HEMTs, the insulation layer that is required underneath the gate contact layer is typically applied after performing the abovementioned etching for reducing a thickness of the donor layer.

[0075] FIG. 8 illustrates a diode 400 in which the 2DEG concentration is modified similar to the HEMTs of FIGS. 3-7. Furthermore, the semiconductor body of diode 400 is substantially identical to that of the HEMTs of FIGS. 3-7.

[0076] Diode 400 comprises an anode contact layer 2Ac, which forms a Schottky contact with underlying donor layer 7. Cathode contact layer 3Ac forms an Ohmic contact with underlying donor 7. Furthermore, fieldplates 9A, 9B, 9C are connected to cathode contact layer 3Ac. As show, the region of the channel layer in between anode contact layer 2Ac and cathode contact layer 3Ac can be divided in several sections s1-s4. The 2DEG concentration in each of these sections may be modified, either fully or partially, using the abovementioned ion-implantation process and/or the donor layer etching process. More in particular, sections s1-s4 may be modified similar to the different sections in FIG. 4.

[0077] In the above, the present disclosure has been described using detailed embodiments thereof. However, the present disclosure is not limited to these embodiments. Instead, various modifications are possible without departing from the scope of the present disclosure which is defined by the appended claims and their equivalents.

[0078] For example, the embodiments shown in the figures are mostly based on GaN. However, the present disclosure is not limited to this technology. Aspects of the present disclosure may equally be used in other material systems such as GaAs, Ga2O3, or other III-V semiconductor material systems. For HEMTs, both enhancement and depletion devices may benefit from the aspects of the present disclosure.

[0079] Particular and preferred aspects of the disclosure are set out in the accompanying independent claims. Combinations of features from the dependent and/or independent claims may be combined as appropriate and not merely as set out in the claims.

[0080] The scope of the present disclosure includes any novel feature or combination of features disclosed therein either explicitly or implicitly or any generalization thereof irrespective of whether or not it relates to the claimed disclosure or mitigate against any or all of the problems addressed by the present disclosure. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in specific combinations enumerated in the claims.

[0081] Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub combination.

[0082] The term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality. Reference signs in the claims shall not be construed as limiting the scope of the claims.