(19)
(11)EP 4 027 393 A1

(12)EUROPEAN PATENT APPLICATION

(43)Date of publication:
13.07.2022 Bulletin 2022/28

(21)Application number: 21305017.2

(22)Date of filing:  07.01.2021
(51)International Patent Classification (IPC): 
H01L 29/778(2006.01)
H01L 29/20(2006.01)
H01L 29/10(2006.01)
(52)Cooperative Patent Classification (CPC):
H01L 29/7786; H01L 29/1066; H01L 29/2003
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(71)Applicant: Commissariat à l'énergie atomique et aux énergies alternatives
75015 Paris (FR)

(72)Inventor:
  • ATMACA, Gokhan
    38054 GRENOBLE CEDEX 09 (FR)

(74)Representative: Brevalex 
95, rue d'Amsterdam
75378 Paris Cedex 8
75378 Paris Cedex 8 (FR)

  


(54)NORMALLY-OFF HEMT


(57) A HEMT GaN transistor with a conductive gate (110) comprising:
- an upper metal region (111),
- a lower semi-conductor region (112) provided to lower current gate leakage, the lower semiconductor region being formed of:
- a first sub-region (112a) that is P-doped and in contact with said metal region,
- a second sub-region (112c) that is P-doped and in contact with said second layer,
- an intermediate sub-region (112b) arranged between said first sub-region (112a) and said second sub-region (112c), said third sub-region being un-doped or unintentionally doped or doped with a low concentration of dopant compared to that of said first sub-region and second sub-region respectively (figure 2).




Description

TECHNICAL FIELD



[0001] The present invention pertains to the field of transistors having a channel structure that is formed of at least two layers of different bandgaps and which therefore form a hetero-structure.

[0002] It relates more particularly to a GaN-based High Electron Mobility Transistor (HEMT) with a reduced gate leakage current.

PRIOR ART



[0003] A HEMT transistor, also known as hetero-structure field effect transistors (HFET), is a transistor in which semi-conductor layers with different bandgaps, for example InAlAs/InGaAs/InP or AlGaN/GaN, creating a hetero-interface are provided.

[0004] Such hetero-structure enables generation, in an electronically controllable way, of a so-called 2-dimensional gas (2DEG). HEMT transistors are characterized by the possibility of operating at high frequencies, as well as presenting high breakdown voltages.

[0005] The operating principle of the HEMT is based on the modulation of the conductance between two ohmic source and drain contacts, by the electrostatic action of a gate electrode which can control the carrier density in the two-dimensional gas. The variation of this conductance is proportional to the number of free carriers in the channel and therefore to the current between source and drain. Typically, the charge carriers are electrons, the two-dimensional gas being a 2D electron gas.

[0006] The use of an AlGaN/GaN heterojunction in such transistor is advantageous because of the high density of electrons and the high mobility of these electrons that can be obtained in the two-dimensional electron gas.

[0007] Figure 1A shows a HEMT transistor on a substrate 2 and that is provided with such type of heterojunction. It comprises a semiconductor body 1, which in turn includes a GaN bottom layer 4 and an AIGaN top layer 6. To control the channel and therefore the current in the GaN layer 4, a gate 10 comprising a metal region 11 lying on a semiconductor region 12 and forming a Schottky contact is provided.

[0008] The semi-conductor region 12 can be a p-doped GaN portion as described for example in document: "Forward Bias Gate Breakdown Mechanism in Enhancement-Mode p-GaN Gate AlGaN/GaN High-Electron Mobility Transistors", Wu et al., IEEE Electron Device Letters, 2015. The disclosed transistor is a normally OFF transistor also called enhancement transistor.

[0009] Under high forward gate bias, this transistor with a p-doped GaN gate has a tendency to have electrons injected into the semiconductor region 12. It leads to increase the ON-state leakage current. Such a structure may further undergo avalanche breakdown under high forward gate bias.

[0010] In document "E-mode p-n Junction/AlGaN/GaN (PNJ) HEMTs", from Wang et al., IEEE Electron Device Letters, 2020, another gate structure 10' is provided in order to reduce the gate leakage current, Figure 1B. In this gate structure 10', a thin N-type GaN layer 14 is inserted between metal region 11 and a p-GaN layer 16 that is in contact with the AIGaN top layer 6.

[0011] Due to the thin N-type GaN layer, gate metal region 11 is chosen so as to form an ohmic contact. It leads to low electric field below the gate. Due to PN junction formed by N-type layer 14 and P-type layer 16, low hole injection from gate can be obtained. It results in a low gate leakage current. However, such a structure shows a degradation on the OFF-state leakage current.

[0012] The problem then arises of finding a new heterojunction transistor structure that is preferably improved with respect to the aforementioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION



[0013] One aim of the present invention is to propose a transistor with a heterojunction, that is having a reduced gate leakage current and that does not have the drawbacks of the transistors according to the prior art, that is in particular capable of maintaining a good level of drain current with a reduced OFF-state leakage current and that is not subjected to avalanche breakdown under high forward gate bias.

[0014] For this, according to an embodiment, the present invention provides: A field effect transistor, in particular of HEMT type, including:
  • a source electrode,
  • a drain electrode,
  • a channel region formed in a semiconductor block, the semiconductor block being provided with a hetero-structure and comprising at least a first semi-conductor layer of a first semi-conductor material having a first band gap and a second semi-conductor layer of a second semi-conductor material having a second band gap,
  • a conductive gate for controlling a current flow between the source electrode and the drain electrode, the conductive gate being composed of an upper region comprising metal contacting a lower semi-conductor region,
the lower semiconductor region being formed of:
  • a first sub-region that is P-type and in contact with said upper region,
  • a second sub-region that is P-type and in contact with said second layer,
said lower semiconductor region of said conductive gate further comprising: an intermediate sub-region arranged between said first sub-region and said second sub-region, said intermediate sub-region being un-doped, or unintentionally doped, or P-doped with a lower concentration of dopant compared to that of said first sub-region and second sub-region respectively.

[0015] Advantageously, the field effect transistor is a normally off type transistor.

[0016] According to a possible implementation, the first layer may be a GaN layer. The second layer may be an AIGaN layer. Thus, the heterojunction may be a GaN/AlGaN heterojunction.

[0017] According to a possibility of implementation, the lower semiconductor region of the gate may be a GaN region, the first sub-region thus being p-GaN, the second sub-region being p-GaN, the intermediate being un-doped or unintentionally doped or lightly P-doped GaN.

[0018] In the case the intermediate sub-region is P-doped with a lower concentration of dopant compared to that of said first sub-region and second sub-region respectively, intermediate sub-region is preferably lightly doped with Mg as dopant, the concentration of Mg being lower than 51017 cm-3 in this intermediate sub-region.

[0019] Intermediate sub-region has a thickness preferably comprised between 10 nm and 30 nm and more preferably before 20 nm and 30 nm.

[0020] According to a possible embodiment, first sub-region and second sub-region contain Mg as dopant, the concentration of Mg in the first sub-region and the second sub-region is higher than the Mg concentration of 5.0x1018 cm-3, the concentration of Mg in the first sub-region and in the second sub-region being typically comprised between 5.0x1018 cm-3 and 1x1019 cm-3.

[0021] The Mg concentration in said intermediate sub-region may be at least lower 100 times lower than the concentration of Mg in said first sub-region and said second sub-region, said intermediate sub-region being preferably unintentionally doped.

[0022] Advantageously, the upper region is provided with a metal having a high work function, in particular equal or higher than 4.8 eV. With a high work-function metal such as Nickel, this device still keeps the hole injection phenomena that helps increase of drain current at high gate voltages.

BRIEF DESCRIPTION OF THE DRAWINGS



[0023] The present invention will be better understood upon reading the description of example embodiments, provided purely for information and in a nonlimiting way, done in reference to the appended drawings, in which:

Figures 1A, 1B illustrate HEMT transistor structures according to prior art°;

Figure 2 illustrate a HEMT transistor with a particular gate structure according to an embodiment of the invention";

Figures 3A, 3B, 3C give different "ON-state" electrical characteristics of a HEMT transistor according to the invention and of transistors according to prior art°;

Figure 4 give electric field estimates at different points in a structure according to an embodiment of the invention and in structures according to prior art°;

Figures 5A, 5B give different "OFF-state" electrical characteristics of a HEMT transistor according to the invention and of transistors according to prior art°;

Figures 6A, 6B give different current characteristics of a HEMT transistor according to an embodiment of the invention for different compositions of its metal gate contact";

Figures 7, 8 give different current characteristics of a HEMT transistor according to an embodiment of the invention for different Mg concentrations of the P-doped regions in the semi-conductor part of its gate°;

Figures 9A, 9B show different current characteristics of a HEMT transistor according to an embodiment of the invention for different Mg concentrations of a low-doped region in the semi-conductor part of its gate°;

Figure 10 gives barrier potential evolution in a transistor structure according to the invention for different Mg concentrations";

Figures 11A, 11B give gate leakage current characteristics respectively for a transistor according to prior art and for a HEMT transistor according to an embodiment of the invention, this for different operating temperatures";

Figures 12A, 12B show different current characteristics of a HEMT transistor according to an embodiment of the invention for different thicknesses of an un-doped region provided in the semi-conductor part of its gate°;



[0024] Identical, similar or equivalent parts of the various figures bear the same numerical references so as to facilitate the passage from one figure to the next.

[0025] The different parts shown in the figures are not necessarily shown using a uniform scale, to make the figures more legible.

[0026] Furthermore, in the description below, terms that depend on the orientation of the structure such as "on", "above", "vertical", "lateral", "upper", "lower", apply considering that the structure is oriented in the manner illustrated in the figures.

DETAILED DISCLOSURE OF SPECIFIC EMBODIMENTS



[0027] Reference is now made to figure 2, which corresponds to a schematic cross-sectional view of a transistor, here of the HEMT type, comprising an heterojunction and an improved gate arrangement according to a first embodiment.

[0028] The transistor is a "Normally off" (or "n-off") also called enrichment transistor, that is to say with a positive threshold voltage and which is blocked ("Off") when its gate source voltage is zero.

[0029] The transistor is made from a semiconductor substrate 102, for example silicon-based, on which a semi-conductor block comprising the heterojunction is arranged. The substrate 102 may also comprise SiC or even Al2O3 or sapphire.

[0030] The heterojunction is made in a stack comprising a first layer 104 of a III-N semi-conductor material having a first bandgap and a second layer 106 of a III-N semi-conductor material having a second bandgap, larger than said first bandgap.

[0031] Preferably, the transistor is a GaN based transistor. Thus, the first layer 104, also called "buffer layer", may be a GaN layer, with a thickness typically chosen according to the breakdown voltage desired for the transistor. The first layer's thickness may be for example of the order of 4 µm.

[0032] The second layer 106, also called "barrier layer" may advantageously be an AIGaN layer. The second layer's thickness may be between 15 and 20 nm, for example of the order of 15 nm. The aluminium concentration of Al in the second layer 106 may be between 15% and 25%. A particular embodiment provides a layer 106 of Al0.25Ga0.75N.

[0033] Although not visible in figure 2, one or several transition layers used for the growth of the materials of the heterojunction may be positioned between the substrate 102 and the first layer 104. Among these transition layers, a nucleation layer for example that is GaN based, may be positioned on and in contact with the substrate 102 in order to provide an adaptation of the crystal lattice parameter. A back barrier layer for example AIGaN based and several micrometres thick may further be positioned on and in contact with the nucleation layer. An additional buffer layer, for example GaN based and several micrometres thick, may further be provided on said back barrier layer or said nucleation layer.

[0034] The transistor further includes source 107 and drain 108 electric contacts, which are arranged on and in contact with regions of the second layer 106. Each of the electric contacts 107 and 108 can be a metallic layer or a stack of metal layers. The metal of source and drain electrode is provided preferably so as to form ohmic type contacts with the second layer 106. The source and drain contacts 107, 108 may be formed from one or more of the following metals: Ti, Al, Ni, Au. According to a particular embodiment, the source drain contacts 107 and 108 may be composite electrodes made of plural metals. A particular example provides source and drain contacts 107 and 108 made of Ti/Al/Ti/Au metal stack.

[0035] A two-dimensional electron gas (not represented) can be formed in a channel region situated in the first layer 104, typically under the interface between the second layer 106 and the first layer 104.

[0036] The transistor further comprises a gate electrode 110 that is arranged in contact and here on a portion of the second layer 106 to control the two-dimensional electron gas. The gate electrode 110 is made of an upper region 111 that is metal-based and that is contacting a lower region 112 made of semi-conductor.

[0037] The semi-conductor region 112 has a particular arrangement that is provided to reduce gate current leakage.

[0038] This lower region 112, that may be for example with a thickness of 70 nm is made of a first sub-region 112a that is in contact with said metal region, said first sub-region 112a being arranged on and in contact with an intermediate sub-region 112b of different composition, the intermediate sub-region 112b being arranged on and in contact with a second sub-region 112c that is in contact with said second semi-conductor layer 106, said second sub-region 112b having a different composition than that of said intermediate sub-region 112b.

[0039] The metal region and the first sub-region 112a are provided preferably so that the difference between the work function of their respective materials is such that the metal region and the first semi-conductor sub-region 112a form a Schottky type contact also called "rectifying contact". The metal region 111 is advantageously formed of a metallic material or a metal with a high work function, for example around 5.2 eV, for example such as Nickel. The use of such type of metal makes it possible to keep a satisfactory drain current level. Alternatively, other types of metals such as Ohmic contact to p-GaN, Pt, TiN or W may be used to form metal region 111 to reduce gate leakage current.

[0040] As will be seen later, the composition of the semiconductor region 112 allows the use, for metal region 111, of a metal having a high work function without degrading gate leakage current, even for higher gate voltages, for example gate voltages of 6 volts.

[0041] The HEMT transistor is typically a N-type channel transistor. Thus, the first sub-region 112a is typically a P-type semi-conductor region and may be made of P-doped GaN (hereafter noted "p-GaN"). P-type doping can be obtained for example using Mg as a dopant. The Mg concentration in said first sub-region 112a is preferably between 5x1018 and 1.0x1019 cm-3, and more preferably between 7.5x1018 and 1.0x1019 cm-3, for example 1.0x1019 cm-3. The thickness of the first sub-region 112a may be between approximately 20 nm and 30 nm, for example 20 nm.

[0042] The second sub-region 112c that is in contact with the second layer 106, is also typically a P-doped semi-conductor region and may be made of GaN as well. The concentration of dopant and thickness in said second sub-region 112c may be the same as that of sub-region 112a.

[0043] A particular feature of the gate electrode 110 is that the intermediate sub-region 112b arranged between first sub-region 112a and second sub-region 112c are P-type doped regions, is un-doped or unintentionally doped semi-conductor. The intermediate sub-region 112b may be made of GaN as well, the un-doped GaN region being in this case noted "u-GaN".

[0044] It may also be P-doped but with a low concentration of dopant, this concentration being lower than that of said first sub-region 112a and said second sub-region 112c. The dopant concentration in the intermediate region 112b is in this case typically provided at least 10 times lower than that in said first and second sub-regions 112a, 112c and preferably at least 100 times lower than that in said first and second sub-regions 112a. The intermediate region 112b that is most preferably un-doped or unintentionally doped or that may be lightly P-doped, is provided so as to increase potential barrier at p-GaN/AlGaN interface for preventing electron high injection from channel to gate metal, especially at high voltages while maintaining a sufficient drain current.

[0045] By "low concentration" in said intermediate region 112b, it is meant a concentration of dopant that is preferably lower than 51017 cm-3.

[0046] By "unintentionally doped", it is meant that this region 112b may be formed without any doping step, for example by epitaxy without in situ doping or subsequent implant, and the dopant concentration being then typically lower than 11016 cm-3.

[0047] The thickness of the intermediate sub-region 112b may be provided between approximately 10 nm and 30 nm, preferably between 20 nm and 30 nm, for example 30 nm.

[0048] As an alternative to GaN, one may use AlGaN to form the semi-conductor region 112 of the gate. Thus, the sub-regions 112a, 112c may be P-type AlGaN, whereas the intermediate region 112b is undoped or unintentionally doped, or low doped AIGaN.

[0049] Regarding its dimensions, the gate 110 may be provided with a width WG (smallest dimension measured in a plane parallel to the [O; x; y] plane given in figure 2) comprised between 1 and 5µm, for example 2 µm.

[0050] Preferably, the gate electrode 110 is decentered from a median between the source 107 electrode and the drain 108 electrode, so that the gate 110 is located closer to the source electrode 107 than to the drain electrode 108. The distance LGS between the source electrode 107 and the gate electrode 110 may for example be of the order of 2 µm, while the distance LGD between the gate 110 and the drain electrode 108 can be of the order of 13.5 µm.

[0051] Figures 3A, 3B, 3C give different electrical characteristics of:
  • a HEMT transistor similar to that above-described;
  • a transistor, called "first conventional transistor", provided with a gate structure according to prior art and as disclosed in relation with figure 1A, i.e. with a P-doped GaN gate;
  • a transistor, called "second conventional transistor", provided with a structure as disclosed in relation with figure 1B and having a NP junction under the gate contact.


[0052] In this particular example, the transistor implemented according to an embodiment of the invention has a first layer 104 that is a 4 µm GaN layer, a second layer 106 that is a 15 nm Al0.25Ga0.75N, a gate upper region 111 made of a Nickel and contacting a 70 nm semi-conductor semiconductor region 112. This region 112 is made of a stack comprising a P-doped GaN sub-region 112a, an un-doped GaN intermediate sub-region 112b, a P-doped GaN sub-region 112c, LGS = 2 µm, LGD = 13.5 µm, WG = 2 µm, Mg doping of sub-regions 112a, 112c being such that the dopant concentration is 11019 cm-3. The first conventional transistor and the second conventional transistor are provided with the same above given dimensions (LGS = 2 µm, LGD = 13.5 µm, WG = 2 µm), same dopant concentration (11019 cm-3) for the P-doped region and same semi-conductor thickness (70 nm) under the gate metal.

[0053] In figure 3A, representing the drain current as a function of the gate source voltage when the drain source voltage is 1 volt, one can note that first conventional transistor (curve C31) has a threshold voltage of 1.45 V and a drain current at VGS = 8 V that is 91.3 mA/mm, whereas the transistor according to the invention (curve C32) has a similar threshold voltage and a drain current at VGS = 8 V of approximately 89.5 mA/mm which is close to that of first conventional transistor. The second conventional transistor (curve C33) has a higher threshold voltage to 1.62 V.

[0054] In figure 3B, giving the gate current evolution as a function of the gate source voltage, one can note that the gate current leakage for the transistor according to the invention (curve C35) is much reduced compared to that of first conventional transistor (curve C34) and is the same as for the second conventional transistor (curve C36) when VGS is less than 4 volts. The gate structure with p-GaN/u-GaN/p-GaN stack enables to reduce gate leakage current from 1.8x10-7 A/mm to 8.6x10-12 A/mm at VGS=+8.0 V compared to a p-GaN/n-GaN stack. In the conventional first structure depletion width between metal/p-GaN increases, which means lower hole injection from gate and electron injection from channel. In the structure according to the invention potential barrier at p-GaN/AIGaN interface is raising due to p-i-p-n diode biasing. This higher potential barrier leads to lower electron injection from channel. It thus results in a reduced gate current.

[0055] In figure 3C giving the maximum transconductance gm as a function of the gate source voltage, one can note that the maximum transconductance gm of the second conventional transistor (curve C39) is much reduced compared to that (curve C38) of the transistor according to the invention and close to that of the first conventional transistor (curve C37).

[0056] In Figure 4, the evolution of the electric field as a function of the positioning in the structure, is given for the transistor according to the invention respectively when the source gate voltage is 0 Volt (curve C43) and when it is 8 Volt (curve C44). Same type of distribution is given for the first conventional transistor (curves C41, C42) and for the second conventional transistor (curves C45, C46). One can note that the second conventional structure has a very high electric field when the gate source voltage is equal to 8 volts (curve C46). This may degrade its gate reliability. The electric field below the gate of the transistor according to the invention and provided with a p-GaN/u-GaN/p-GaN stack is, at VGS=8.0 V, lower than that of conventional first structure. The junction electric fields at p-GaN/u-GaN and p-GaN/AIGaN interfaces are lower than conventional first structure. This results in higher gate reliability.

[0057] Figures 5A-5B, give a comparison of the OFF state currents as a function of the drain source voltage for the 3 above-mentioned transistors.

[0058] In figure 5A, curves C51, C52, C53 are representative of the OFF-state drain current, respectively for said first conventional transistor, for the transistor according to the particular embodiment of the invention, and for said second conventional transistor.

[0059] In figure 5B, one can note that second conventional transistor (curve C56) has an OFF-state gate current that is increased when increasing the drain-source voltage and that is much higher than that of transistor (curve C55) according to the particular embodiment of the invention.

[0060] In figures 6A and 6B drain current and gate leakage current of the transistor structure according to said particular embodiment are now respectively represented both as a function of the gate source voltage for a drain source voltage of 1 Volt, this for different compositions of the upper region of the gate, the upper region 111 being either made of Nickel (curves C61, C65), or made of TiN (curves C62, C66), or made of W (curves C63, C67). It shows that if lower work-function metals such as W and TiN are used, there is no increase in gate leakage current even in higher gate voltages. Thus, the upper region 111 allows to use high metal work-function metals such as Nickel so that the drain current level can be maintained.

[0061] In Figures 7 and 8, drain current and gate leakage current of a transistor having a structure similar to that according to said particular embodiment, is represented for different concentrations of dopant (Mg) in the sub-regions 112a, 112c, C71 and C81, corresponding to an Mg concentration of 11019 cm-3, C72 and C82 corresponding to an Mg concentration of 7.51018 m-3, C73 and C83 corresponding to an Mg concentration of 51018 cm-3, C74 and C84 to 11018 cm-3, C75 and C85 to 51017 cm-3, C96 and C106 to 11017 cm-3. Sub-regions 112a, 112c are thus provided preferably with an Mg concentration below or 1x1019 cm-3 in order to keep low gate current leakage current even for Vgs values higher than 6 volts.

[0062] In Figures 9A and 9B drain current and gate leakage current of a transistor having a structure similar to that according to said particular embodiment, are represented for different concentrations of dopant (Mg) in the intermediate region 112b, C91 and C901, corresponding respectively to an Mg concentration of 0, C92 and C902, to 11015,m-3, C93 and C903 to 11016 cm-3, C94 and C904 to 11017 cm-3, C95 and C905 to 51017 cm-3, C96 and C906 to 11018 cm-3, C97 and C907 to 51018 cm-3. Figures 10 gives the potential barrier when Vgs= 8 Volt, for the same transistor and the same concentrations of Mg (curves C101, C102, C103, C104, C105, C106, C107). An intermediate region 112b with low Mg concentrations still allows to obtain low gate leakage current, (i.e. below than 1x10-10 A/mm) in this case until an Mg concentration of 5x1017 cm-3.

[0063] Figures 11A and 11B give the evolution of the gate leakage current for different operating temperatures and a comparison between a transistor with the first prior art structure (curves C'110, C'111 C'112, C'113) having a structure similar to that according to said particular embodiment (curves C110, C111, C112, C113).

[0064] With a transistor according to the invention, a gate leakage current as low as 6x10-9 A/mm at VGS=+8 V may be obtained even with temperatures up to 200°C. Even with such temperature, the leakage current remains lower than that of the transistor of conventional first structure having a semi-conductor part of the gate that is only made of p-GaN.

[0065] Figures 12A and 12B respectively show drain current and gate leakage current of the transistor according to said particular embodiment for different thicknesses (resp. 10 nm, 20 nm, 30 nm) of the intermediate region 112b of the gate structure that is un-doped or unintentionally doped. Curve C127 corresponding to an intermediate region 112b of 30 nm confirms that gate leakage is reduced as the thickness of the intermediate region 112b is increased due to larger depletion width and higher potential barrier at the interface between second sub-region 112 of the gate and second layer 106.

[0066] An example of a method for fabricating a HEMT transistor as described above is now given.

[0067] The first layer 104 is typically made by epitaxial growth of GaN on the substrate 102.

[0068] The second layer 106 of AIGaN can then formed via epitaxy on the first layer 104. A first passivation dielectric layer may then deposited on the second layer 106. This first passivation dielectric layer is for example a silicon nitride layer.

[0069] Etching of the first passivation dielectric layer may then be implemented in order to form first and second openings through the first passivation dielectric layer, these openings forming accesses to the second semi-conductor layer 106. Source and drain contacts 107, 108 may be fabricated in the openings by depositing at least a metal layer that is subsequently etched.

[0070] A second passivation dielectric layer is then deposited and covers the source and drain contacts 107, 108 and the first passivation dielectric layer.

[0071] A portion of the second passivation dielectric layer and the first passivation dielectric layer is etched in order to form a third opening and to provide an access to the second semi-conductor layer 106.

[0072] Then, the gate can be formed in said third opening.

[0073] The fabrication of the gate comprises forming a p-doped GaN layer on a region of said second layer 106 that is not covered by passivation and exposed by said third opening. The p-doped GaN layer may be obtained via epitaxial growth. The doping may be conducted during epitaxy growth by in situ doping using Mg as a dopant.

[0074] On the doped GaN layer, an un-doped or unintentionally doped is then formed, typically by epitaxy growth. Alternatively, a light doping is conducted with an Mg concentration lower than 51017 cm-3.

[0075] Another p-doped GaN layer on the un-doped or low-doped GaN layer is then formed, typically via epitaxial growth on a region exposed by said third opening. The doping may be conducted during epitaxy growth by in situ doping.

[0076] Then the metallic upper region 111 of the gate is formed by depositing a metal layer such as Ni and etching.


Claims

1. A normally off field effect transistor, in particular of HEMT type, including a source electrode (107), a drain electrode (108), a channel region formed in a semiconductor block, said semiconductor block being provided with an hetero-structure and comprising at least a first semi-conductor layer (104) of a first semi-conductor material having a first band gap and a second semi-conductor layer (106) of a second semi-conductor material having a second band gap, the transistor being provided with a conductive gate (110) for controlling a current flow between the source electrode and the drain electrode, the conductive gate (110) being composed of an upper region (111) comprising metal contacting a lower semi-conductor region (112),
said lower semiconductor (112) region being formed of:

- a first sub-region (112a) that is P-type and in contact with said upper region (111),

- a second sub-region (112c) that is P-type and in contact with said second layer (106),

said lower semiconductor (112) region of said conductive gate (110) further comprising: an intermediate sub-region (112b) arranged between said first sub-region (112a) and said second sub-region (112c), said intermediate sub-region (112c) being un-doped, or unintentionally doped, or P-doped with a lower concentration of dopant compared to that of said first sub-region and second sub-region respectively.
 
2. The transistor according to claim 1, wherein said first layer (104) is a GaN layer.
 
3. The transistor according to claims 1 or 2, wherein said second layer (106) is an AIGaN layer.
 
4. The transistor according to any of the claims 1 to 3, said lower semiconductor (112) region being a GaN region.
 
5. The transistor according to any of the claims 1 to 4, wherein said intermediate sub-region (112b) contains Mg as dopant, the concentration of Mg being lower than 51017 cm-3.
 
6. The transistor according to any of the claims 1 to 5, wherein said first sub-region (112a) and said second sub-region (112c) contain Mg as dopant, the Mg concentration in said intermediate sub-region (112b) being at least 100 times lower than the concentration of Mg in said first sub-region (112a) and said second sub-region (112c), said intermediate sub-region (112b) being preferably unintentionally doped, the Mg concentration in said first sub-region (112a) and said second sub-region (112c) being comprised between 5x1018 and 1x1019 cm-3.
 
7. The transistor according to any of the claims 1 to 6, wherein said intermediate sub-region (112b) has a thickness comprised between 10 and 30 nm and more preferably between 20 and 30 nm.
 
8. The transistor according to any of the claims 1 to 5, wherein said upper region (111), is provided with a metal, in particular a metal having a high work function, preferably around 5.2 eV such as Nickel.
 




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Search report




Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Non-patent literature cited in the description