High electron mobility transistor

Abstract

To provide a technique of suppressing an influence of current collapse due to a defect on a surface of a group III nitride in HEMT using the group III nitride.SOLUTION: A high electron mobility transistor includes: a group III nitride multilayer body having a substrate, a channel layer provided on the substrate and composed of a first group III nitride, and a barrier layer provided on the channel layer and composed of a second group III nitride having a larger band gap than a band gap of the first group III nitride; a gate electrode, a source electrode, and a drain electrode provided on a surface of the group III nitride multilayer body; and a protection film provided so as to be in contact with at least a part of outer parts of the gate electrode, the source electrode, and the drain electrode in a plan view on the surface of the group III nitride multilayer body and composed of a p-type organic semiconductor containing a thiophene ring.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a high electron mobility transistor.

Background Art

[0002] In a high electron mobility transistor (HEMT) using group III nitride, current collapse, which is a phenomenon in which the drain current decreases due to the application of a high voltage, is known. Various techniques for suppressing the influence of current collapse have been proposed (see, for example, Patent Document 1). As one of the causes of current collapse, as described later, those caused by defects on the surface of group III nitride can be mentioned.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] One object of the present invention is to provide a technique for suppressing the influence of current collapse caused by defects on the surface of group III nitride in a HEMT using group III nitride.

Means for Solving the Problems

[0005] According to one aspect of the present invention, a substrate, a channel layer provided on the substrate and composed of a first group III nitride, and a barrier layer provided on the channel layer and composed of a second group III nitride having a bandgap larger than that of the first group III nitride, a group III nitride laminate having, a gate electrode, a source electrode, and a drain electrode provided on the surface of the group III nitride laminate, and A protective film made of a p-type organic semiconductor containing a thiophene ring, which is provided so as to be in contact with at least a part of the outer portions of the gate electrode, the source electrode, and the drain electrode in a plan view of the surface of the group III nitride laminate. A high electron mobility transistor comprising is provided.

Effects of the Invention

[0006] A technique for suppressing the influence of current collapse caused by defects on the surface of a group III nitride in a HEMT using a group III nitride is provided.

Brief Description of the Drawings

[0007]

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DETAILED DESCRIPTION OF THE INVENTION

[0008] First, preliminary knowledge regarding a high electron mobility transistor (HEMT) according to an embodiment of the present invention will be outlined. The inventor of the present application has studied a technique for suppressing the influence of current collapse in a HEMT using a group III nitride. Although several causes are conceivable for current collapse, among them, in particular, a technique for suppressing the influence of current collapse caused by defects on the surface of the group III nitride on the electrode side where the gate electrode and the like are disposed has been studied.

[0009] Specifically, an attempt was made to suppress the influence of current collapse by using an organic semiconductor as the material of the protective film formed on the surface of the group III nitride. As the organic semiconductor, a p-type one that does not contain a thiophene ring (material A described later), an n-type one that contains a thiophene ring (material B described later), and a p-type one that contains a thiophene ring (material C described later) were compared and studied. As a result, a new finding was obtained that by forming the protective film with an organic semiconductor containing a thiophene ring in the p-type, the time constant of recovery from current collapse can be reduced. The HEMT according to the embodiment of the present invention is obtained based on such findings.

[0010] With reference to FIG. 1, the HEMT 100 according to the embodiment of the present invention will be described. FIG. 1 is a cross-sectional view schematically exemplifying the HEMT 100. The HEMT 100 includes a substrate 10, a group III nitride laminate 20, an electrode 30, and a protective film 40.

[0011] The substrate 10 is a base substrate for epitaxially growing the group-III nitride laminate 20. As the substrate 10, a heterogeneous substrate such as a silicon carbide (SiC) substrate or a silicon (Si) substrate may be used, or a homogeneous substrate composed of a group-III nitride such as a gallium nitride (GaN) substrate may be used. In this example, an SiC substrate is used as the substrate 10.

[0012] The group-III nitride laminate 20 is provided on the substrate 10 and has a buffer layer 21, a buffer / channel layer 22, a barrier layer 23, and a cap layer 24, each of which is composed of a group-III nitride. The buffer layer 21 and the cap layer 24 are each provided as necessary and may be omitted.

[0013] The buffer layer 21 is appropriately formed between the substrate 10 and the buffer / channel layer 22 according to the material of the substrate 10 and the like. In this example, as the buffer layer 21, for example, an aluminum gallium nitride (AlGaN) layer having a thickness of less than 100 nm is formed.

[0014] The buffer / channel layer 22 (also referred to as the channel layer 22) is provided on the substrate 10 (via the buffer layer 21). The lower portion of the channel layer 22 functions as a buffer layer for improving the crystallinity of the upper portion of the channel layer 22. The upper portion of the channel layer 22 is a portion where a two-dimensional electron gas (2DEG) is formed near the interface with the barrier layer 23 and functions as a channel layer through which electrons travel during the operation of the HEMT 100. In this example, as the channel layer 22, for example, a GaN layer having a thickness of 400 nm is formed.

[0015] The barrier layer 23 is provided on the channel layer 22 and is composed of a group III nitride having a bandgap larger than that of the group III nitride (GaN in this example) constituting the channel layer 22. The barrier layer 23 functions as a barrier layer that forms a 2DEG in the channel layer 22 and spatially confines the 2DEG in the channel layer 22. In this example, as the barrier layer 23, for example, an AlGaN layer with a thickness of 22 nm (Al composition is 0.23) is formed.

[0016] The cap layer 24 is appropriately formed on the barrier layer 23. In this example, as the cap layer 24, for example, a GaN layer with a thickness of 2 nm is formed. In this example, the surface 20s of the group III nitride laminate 20 (on the side of the electrode 30) becomes the upper surface of the cap layer 24. Note that when the cap layer 24 is omitted, the surface 20s of the group III nitride laminate 20 (on the side of the electrode 30) becomes the upper surface of the barrier layer 23.

[0017] The electrode 30 is provided on the surface 20s of the group III nitride laminate 20 and has a gate electrode 31, a source electrode 32, and a drain electrode 33. In this example, the electrode 30 is formed such that the gate length is 5 μm, the gate width is 100 μm, the source electrode - gate electrode distance is 5 μm, and the gate electrode - drain electrode distance is 5 μm.

[0018] The protective film 40 is provided so as to be in contact with at least a part of the outer portions of the gate electrode 31, the source electrode 32, and the drain electrode 33 in a plan view of the surface 20s of the group III nitride laminate 20 (preferably, in contact with the surface 20s of the group III nitride laminate 20 between the gate electrode 31 and the drain electrode 33). This is to suppress the influence of current collapse caused by defects on the surface 20s of the group III nitride laminate 20. The protective film 40 is preferably in contact with the drain electrode 33 and is preferably in contact with the gate electrode 31.

[0019] The protective film 40 is more preferably provided so as to be in contact with at least a part of the upper surface of the gate electrode 31 and the upper surface of the drain electrode 33. By forming the protective film 40 in this way, the contact between the protective film 40 and the gate electrode 31 can be made more reliable, and the contact between the protective film 40 and the drain electrode 33 can also be made more reliable. Note that FIG. 1 illustrates a cross section of a portion where the protective film 40 covers the entire upper surfaces of the gate electrode 31, the source electrode 32, and the drain electrode 33. However, in the connection regions between each of the gate electrode 31, the source electrode 32, and the drain electrode 33 and the wiring, the protective film 40 has appropriate openings, and the electrodes 31 to 33 are each connected to the wiring at the openings of the protective film 40. Note that an insulating layer or a semiconductor layer may be further formed on the protective film 40.

[0020] In the HEMT 100 of the present embodiment, the protective film 40 is characterized by being composed of a p-type organic semiconductor 41 containing a thiophene ring. Note that in the present embodiment, the fact that the organic semiconductor 41 contains a thiophene ring means that the organic semiconductor 41 contains a thiophene ring in its mother skeleton or main chain. The thiophene ring may be included as a structural unit alone, or may be included as a structural unit condensed with a heterocyclic ring such as a benzene ring or another thiophene ring. These structural units may have a substituent. Examples of the substituent include an alkyl group, a cycloalkyl group, an alkoxy group, a cycloalkoxy group, an alkylthio group, a cycloalkylthio group, an aryl group, a monovalent heterocyclic group, a halogen atom, a silyl group, an amino group, an alkenyl group, a cycloalkenyl group, an alkynyl group, a hydroxyl group, a nitro group, a cyano group, a carboxyl group, an alkylcarbonyl group, a cycloalkylcarbonyl group, an alkoxycarbonyl group, and a cycloalkoxycarbonyl group.

[0021] By forming the protective film 40 with a p-type organic semiconductor 41 containing a thiophene ring, as will be described in detail later, the time constant of recovery from current collapse of the HEMT 100 can be reduced. Preferably, the organic semiconductor 41 has a length of 0.1 cm 2It has a hole mobility of Vs or more. Thereby, the amount of decrease in the drain current during current collapse can be reduced. Further, preferably, the organic semiconductor 41 is a polymer organic semiconductor. Thereby, the heat resistance of the protective film 40 can be increased to, for example, 200 ° C or more.

[0022] The HEMT 100 further has an element isolation structure 50 for isolating it from other adjacent HEMTs. In this example, as the element isolation structure 50, for example, an element isolation groove having a depth of 180 nm (a depth that reaches the middle of the thickness of the channel layer 22 and divides the 2DEG) is formed. Note that the element isolation structure 50 may be formed by, for example, ion implantation.

[0023] Note that the group III nitride laminate 20 may have another structure. For example, a structure in which the buffer layer 21 and the channel layer 22 are made of indium gallium nitride (InGaN), and the barrier layer 23 is made of aluminum indium gallium nitride (AlInGaN) or scandium aluminum gallium nitride (ScAlGaN) is exemplified.

[0024] Known methods may be appropriately used to fabricate the HEMT 100. The growth of the group III nitride laminate 20 on the substrate 10 is performed by, for example, metalorganic vapor phase epitaxy (MOVPE) or the like. The formation of the electrode 30 is performed by, for example, sputtering and lift-off.

[0025] The formation of the protective film 40 is performed, for example, by applying a solution in which an organic semiconductor 41 or its precursor is dissolved or dispersed onto the surface 20s of the group III nitride laminate 20 on which the electrode 30 is formed. Thereafter, annealing of the protective film 40 may be performed as necessary. As the coating method, slit coating method, knife coating method, spin coating method, casting method, microgravure coating method, gravure coating method, bar coating method, roll coating method, wire bar coating method, dip coating method, spray coating method, screen printing method, gravure printing method, flexographic printing method, offset printing method, inkjet printing method, dispenser printing method, nozzle coating method, capillary coating method, etc. can be used, and the slit coating method, capillary coating method, gravure coating method, microgravure coating method, bar coating method, knife coating method, nozzle coating method, inkjet printing method, spin coating method are preferable.

[0026] The solvent used for film formation from the solution may be any solvent that can dissolve the organic semiconductor 41 or its precursor. Examples of the solvent include unsaturated hydrocarbon solvents such as toluene, xylene, mesitylene, tetralin, decalin, bicyclohexyl, n-butylbenzene, sec-butylbenzene, tert-butylbenzene, halogenated saturated hydrocarbon solvents such as carbon tetrachloride, chloroform, dichloromethane, dichloroethane, chlorobutane, bromobutane, chloropentane, bromopentane, chlorohexane, bromohexane, chlorocyclohexane, bromocyclohexane, halogenated unsaturated hydrocarbon solvents such as chlorobenzene, dichlorobenzene, trichlorobenzene, and ether solvents such as tetrahydrofuran and tetrahydropyran.

[0027] From the viewpoint of forming a good thin film during thin film production, the solution used for film formation preferably contains 0.1% by weight (hereinafter sometimes referred to as "wt%") or more of the organic semiconductor 41, and more preferably contains 0.4 wt% or more.

[0028] From the perspective of fully exerting the effect of suppressing the influence of current collapse, the thickness of the protective film 40 is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more. Also, from the perspective of suppressing leakage current, it is preferably 300 nm or less, and more preferably 100 nm or less.

[0029] Next, an experimental example of comparative study on the organic semiconductor used for the protective film 40 will be described. First, the organic semiconductor used in this experimental example will be described. In this experimental example, as the organic semiconductor, a p-type one without a thiophene ring (also referred to as Material A), an n-type one with a thiophene ring (also referred to as Material B), and a p-type one with a thiophene ring (also referred to as Material C) were comparatively studied.

[0030] As the p-type organic semiconductor without a thiophene ring (Material A), specifically, the following were used. Material A was synthesized by mixing the monomer 9,9-dioctylfluorene-2,7-bis(dimethylborate) and N,N-bis(4-bromophenyl)-N-(4-sec-butylphenyl) in a ratio of 50:50 by the method described in Patent No. 4375820. The weight-average molecular weight in terms of polystyrene was 351,000, and the number-average molecular weight was 85,000. The chemical structural formula of Material A is shown in Fig. 7(c).

[0031] As the n-type organic semiconductor with a thiophene ring (Material B), specifically, N2200 manufactured by Sigma-Aldrich (number-average molecular weight is 50,000 - 100,000) was used. The chemical structural formula of Material B (N2200) is shown in Fig. 8(c).

[0032] As the p-type organic semiconductor with a thiophene ring (Material C), specifically, the following Materials C1 to C4 were used. As Material C1, PDPP2T-TT-ОD manufactured by Sigma-Aldrich (weight-average molecular weight 40,000 - 60,000) was used. The chemical structural formula of Material C1 is shown in Fig. 3(c).

[0033] As Material C2, PDPP3T (weight-average molecular weight > 30,000) manufactured by Luminescence Technology (Lumtec) was used. The chemical structural formula of Material C2 is shown in Fig. 4(c).

[0034] As Material C3, the following was used. Material C3 was synthesized according to the method of International Publication No. 2016 / 013460. The weight-average molecular weight in terms of polystyrene was 129,000, and the number-average molecular weight was 63,000. The chemical structural formula of Material C3 is shown in Fig. 5(c).

[0035] As Material C4, the following was used. Material C4 was synthesized according to the method of International Publication No. 2011 / 052709. The weight-average molecular weight in terms of polystyrene was 106,000. The chemical structural formula of Material C4 is shown in Fig. 6(c).

[0036] The mobilities of each of Materials C1 to C4 of Material C are as follows. The hole mobility of Material C1 (PDPP2T-TT-OD) is 0.1 - 0.3 cm 2 / Vs. Note that Material C1 (PDPP2T-TT-OD) has an electron mobility of 0.01 - 0.03 cm 2 / Vs and is an ambipolar organic semiconductor. The hole mobility of Material C2 (PDPP3T) is 0.01 - 0.03 cm 2 / Vs. Note that Material C2 (PDPP3T) has an electron mobility of 0.001 - 0.003 cm 2 / Vs and is an ambipolar organic semiconductor. The hole mobility of Material C3 is 0.5 - 2.5 cm 2 / Vs. The hole mobility of Material C4 is 0.2 - 2.0 cm 2 / Vs. Materials C3 and C4 are unipolar organic semiconductors.

[0037] Here, the mobility of the organic semiconductor is measured as follows. A gate electrode and a gate insulating film are sequentially formed on a substrate, and a source electrode and a drain electrode are further formed thereon. An organic semiconductor is applied thereon. In the case of a low-molecular-weight organic semiconductor, vapor deposition may be used. Then, annealing is performed as necessary. In this way, a field-effect transistor (FET) using the organic semiconductor is formed. Then, for the FET, the transfer characteristics are measured at room temperature (for example, 25°C), and the mobility is calculated from the slope of the √Id-Vg characteristics in the saturation region. Note that the substrate, the gate electrode, and the gate insulating film may be substituted with a Si substrate with an oxide film (SiO2 film). The surface of the insulating film may be appropriately treated with a silane coupling agent or the like. The surface of the electrode may be treated with a thiol agent or the like from the viewpoint of promoting charge injection. Also, the lamination order of the organic semiconductor, the gate insulating film, and the electrode may be different. For example, a source electrode, a drain electrode, an organic semiconductor, a gate insulating film, and a gate electrode may be laminated on the substrate in this order.

[0038] The value of the mobility can vary to some extent depending on conditions such as the structure (positional relationship of the electrodes) of the organic semiconductor transistor element used for the mobility measurement, the gate insulating film, the surface treatment agent, and annealing. The mobility values of each of the above-described materials C1 to C4 are values measured in an element structure in which the mobility of each material is considered to be maximized by adjusting such conditions. That is, the mobility value measured as described above indicates a value that serves as a measure of the upper limit of the mobility of each organic semiconductor material at room temperature.

[0039] In this experimental example, HEMTs having the same structure as the HEMT100 exemplified in the above-described embodiment and having protective films 40 made of materials A, B, and C (C1 to C4) respectively were fabricated, and various characteristics of the HEMTs were measured. Note that HEMTs in which the protective film 40 was omitted were also fabricated and various characteristics were measured. A sample without a protective film may also be referred to as D.

[0040] The protective films 40 made of Material A and C (C1 to C4) were each formed by using a solution dissolved in toluene at a concentration of 0.5 wt%, spin-coating at 1000 rpm, and then annealing on a hot plate at 150 °C for 30 minutes. The protective film 40 made of Material B was formed by using a solution dissolved in orthodichlorobenzene at a concentration of 0.5 wt%, spin-coating at 1000 rpm, and then annealing on a hot plate at 110 °C for 30 minutes.

[0041] The first experimental example will be described. In the first experimental example, it was investigated how the time constant of the recovery of the drain current Id after stress addition (i.e., from current collapse) changes depending on Materials A, B, and C (C1 to C4) that form the protective film. Additionally, measurements were also performed on Sample D without a protective film.

[0042] Referring to FIG. 11(a), the measurement method in the first experimental example will be described. FIG. 11(a) is a timing chart showing changes in the gate voltage Vg, drain voltage Vd, and drain current Id in the first experimental example.

[0043] At time t1, -4V is applied as the gate voltage Vg to turn off the HEMT. At time t2, while keeping the HEMT in the off state (with the gate voltage Vg at -4V), 40V is applied as the drain voltage Vd to apply stress to the HEMT. The interval from time t1 to time t2 is 10 milliseconds.

[0044] The 40V drain voltage Vd is applied until time t3. The interval from time t2 to time t3, that is, the period during which stress is applied, is 40 seconds. The application of this stress causes current collapse in the HEMT.

[0045] At time t3, the drain voltage Vd is decreased from 40V for applying stress to 1V for turning on the HEMT. However, at time t3, -4V for turning off the HEMT is still applied as the gate voltage Vg, and the HEMT does not turn on.

[0046] At time t4, the gate voltage Vg is increased from -4V for turning off the HEMT to 0V for turning on the HEMT. As a result, the HEMT turns on at time t4, and the increase in the drain current Id starts from time t4. The interval from time t3 to time t4 is 2 milliseconds.

[0047] The drain current Id immediately after time t4 is lower than the normal drain current Id when there is no current collapse effect due to the current collapse effect. As time passes from time t4, the influence of the current collapse decreases, so the drain current Id recovers (increases) to approach the normal value.

[0048] After time t4, the drain current Id is measured at predetermined time intervals. Specifically, from time t4 to 4 seconds later, the drain current Id is measured at 2 - millisecond intervals, and from 4 seconds after time t4 to 300 seconds, the drain current Id is measured at 100 - millisecond intervals. Then, the normalized drain current is calculated by normalizing the drain current Id so that the magnitude of the drain current Id at 300 seconds is 1.

[0049] Figure 2 is a graph showing the time - variation of the normalized drain current in the HEMT in which the protective film is composed of each of materials A, B, and C (C1 - C4) in the first experimental example. Figure 2 also shows the results of sample D without a protective film. The horizontal axis of Figure 2 indicates the elapsed time from time t4, and the vertical axis indicates the magnitude of the normalized drain current. The measurement in the first experimental example was performed by controlling Keithley's 2657A and 2636B with ACS (Automated Characterization Suite).

[0050] In the first experimental example, the elapsed time (from time t4) until the normalized drain current recovers to 0.5 is referred to as the time constant. It is evaluated that the larger the time constant, the slower the recovery from current collapse, and the smaller the time constant, the faster the recovery from current collapse. Hereinafter, the time constant of a HEMT in which a protective film is formed of a certain material may be simply referred to as the time constant of this material.

[0051] The time constants of the respective materials are 4 seconds for Material A, 5 seconds for Material B (N2200), 4×10 -2 seconds for Material C1 (PDPP2T-TT-ОD), 1×10 -1 seconds for Material C2 (PDPP3T), 2×10 -2 seconds for Material C3, and less than 2×10 -3 seconds for Material C4. Note that the time constant of Material C4 has been obtained as a result interpreted to be smaller than the measurement interval, and here it is evaluated to be less than 2×10 -3 seconds, which is the measurement interval. The time constant of the sample D without a protective film is 30 seconds, which is significantly larger than the time constants of Material A and Material B.

[0052] Compared with the time constants (on the order of several seconds) of Material A, which is a p-type organic semiconductor not containing a thiophene ring, and Material B, which is an n-type organic semiconductor containing a thiophene ring, the time constants of Material C, which is a p-type organic semiconductor containing a thiophene ring, are less than one-tenth.

[0053] More specifically, even for Material C2 (PDPP3T), which has the largest time constant (1×10 -1 seconds) among Materials C, the ratio of the time constant is 1 / 40 with respect to Material A, which has a smaller time constant (4 seconds) among Materials A and B.

[0054] Thus, it has been found that, as an organic semiconductor constituting the protective film of a HEMT using a group III nitride, by using a p-type one not containing a thiophene ring (Material A) and an n-type one containing a thiophene ring (Material B), compared with the case of using a p-type one containing a thiophene ring (Material C), the time constant of the recovery from current collapse can be significantly reduced.

[0055] Material C can be said to be an organic semiconductor having a time constant of 1 / 10 or less (preferably 1 / 20 or less, more preferably 1 / 30 or less) as compared with the time constant of Material A and the time constant of Material B (N2200) (in the measurement method in the first experimental example). Further, Material C can be said to be an organic semiconductor having a time constant of 2 seconds or less (preferably 1 second or less, more preferably 0.5 second or less) (in the measurement method in the first experimental example).

[0056] When comparing each of Materials C1 to C4 of Material C, the time constants of Material C1 (PDPP2T-TT-OD), Material C3, and Material C4 are smaller than the time constant of Material C2 (PDPP3T), the time constants of Material C3 and Material C4 are smaller than the time constant of Material C1 (PDPP2T-TT-OD), and the time constant of Material C4 is smaller than the time constant of Material C3.

[0057] Next, a second experimental example will be described. In the second experimental example, for each of Materials A, B, and C (C1 to C4), the dependence of the drain current Id on the gate voltage Vg (Id-Vg characteristics) was compared before and after the application of stress. In addition, measurements were also performed on sample D without a protective film. Incidentally, the dependence of the leakage current Ig on the gate voltage Vg was also measured.

[0058] The measurement method in the second experimental example will be described. First, the Id-Vg characteristics before the application of stress are measured. In the measurement before the application of stress, the drain voltage Vd is set to 1 V, the gate voltage Vg is changed from -1.5 V to -3.5 V (in 0.02 V steps), and the drain current Id (and leakage current Ig) at each gate voltage Vg is measured. In the measurement of the drain current Id (and leakage current Ig), the integration time is 20 msec, the hold time is 100 msec, and the delay time is 10 msec.

[0059] After measuring the Id-Vg characteristics before stress application, stress is applied to the HEMT. For stress application, a stress of 200 seconds is applied with the drain voltage Vd set to 50 V and the gate voltage Vg set to -4 V. After stress application, the Id-Vg characteristics after stress application are measured. In the measurement after stress application, the drain current Id (and leakage current Ig) is measured by changing the gate voltage Vg under the same conditions as the measurement before stress application.

[0060] Hereinafter, the Id-Vg characteristics of a HEMT with a protective film formed of a certain material may be simply referred to as the Id-Vg characteristics of this material. Fig. 3(a) shows the Id-Vg characteristics of material C1 (PDPP2T-TT-OD) before and after stress application, Fig. 4(a) shows those of material C2 (PDPP3T), Fig. 5(a) shows those of material C3, Fig. 6(a) shows those of material C4, Fig. 7(a) shows those of material A, Fig. 8(a) shows those of material B (N2200), and Fig. 9(a) shows those of sample D without a protective film before and after stress application. The characteristics before stress application are shown by solid lines, and the characteristics after stress application are shown by dashed lines. The horizontal axis in Figs. 3(a) to 9(a) represents the gate voltage Vg, and the left and right vertical axes represent the drain current Id and leakage current Ig, respectively. The measurement in the second experimental example was performed by controlling the 4156C manufactured by Agilent with Keysight's EasyEXPERT.

[0061] In all samples, the Id-Vg characteristics after stress application tend to shift downward from the Id-Vg characteristics before stress application, that is, the drain current Id at the same gate voltage Vg tends to decrease after stress application compared to before stress application.

[0062] According to the findings of the inventors of the present application, the magnitude of the decrease in the drain current Id after stress application on the side where the gate voltage Vg is close to zero (the right end side of the graph), that is, the on-state side, reflects the magnitude of current collapse caused by surface defects in the group III nitride layer.

[0063] The decrease amount of the drain current Id after stress application on the on-state side is large in the sample D without the protective film (Fig. 9(a)), relatively large also in the materials A (Fig. 7(a)) and B (Fig. 8(a)), and significantly suppressed in the material C (C1 - C4, Figs. 3(a) - 6(a)) compared to the materials A and B.

[0064] Next, a third experimental example will be described. In the third experimental example, for each of the materials A, B, and C (C1 - C4), the recovery of the drain current Id after stress application (i.e., from current collapse) was measured, and a comparison of the drain current Id before and after stress application was made. Additionally, measurements were also made for the sample D without the protective film.

[0065] Referring to Fig. 11(b), the measurement method in the third experimental example will be described. Fig. 11(b) is a timing chart showing the changes in the gate voltage Vg, drain voltage Vd, and drain current Id in the third experimental example. In the third experimental example, drain current measurement before stress application (up to time t1), stress application (from time t2 to t3), and drain current measurement after stress application (from time t4 onwards) are performed. Note that the intervals from the end of drain current measurement before stress application to the start of stress application (the interval from time t1 to time t2), and from the end of stress application to the start of drain current measurement after stress application (the interval from time t3 to time t4) are each about 2.6 seconds.

[0066] Up to time t1, the gate voltage Vg is set to 0V for turning on the HEMT, and the drain voltage Vd is set to 1V for turning on the HEMT, so as to turn on the HEMT and measure the drain current Id before stress application. The measurement of the drain current Id is performed for 20 seconds at 1 - second intervals, and the average of the values in the latter half of 10 seconds is taken as the drain current Id before stress application.

[0067] After the measurement of the drain current before stress application is completed, from time t2 to time t3, -4V is applied as the gate voltage Vg to turn off the HEMT, and 50V is applied as the drain voltage Vd to apply stress to the HEMT. The interval from time t2 to time t3, that is, the period during which stress is applied, is 200 seconds. Due to the application of this stress, current collapse occurs in the HEMT.

[0068] After the application of the drain voltage of 50V for stress application is completed, at time t4, the gate voltage Vg is set to 0V to turn on the HEMT, and the drain voltage Vd is set to 1V to turn on the HEMT. As a result, the HEMT becomes on at time t4, and the increase in the drain current Id starts from time t4.

[0069] The drain current Id immediately after time t4 is lower than the normal drain current Id in the case without the influence of current collapse due to the influence of current collapse. As time passes from time t4, the influence of current collapse decreases, so the drain current Id recovers (increases) so as to approach the normal value. After time t4, the drain current Id is measured at predetermined time intervals. Specifically, the drain current Id after stress application is measured by measuring the drain current Id from 0 seconds to 1000 seconds at 2-second intervals from time t4.

[0070] Figure 3(b) shows the drain current Id of material C1 (PDPP2T-TT-OD) before and after stress application, Figure 4(b) shows that of material C2 (PDPP3T), Figure 5(b) shows that of material C3, Figure 6(b) shows that of material C4, Figure 7(b) shows that of material A, Figure 8(b) shows that of material B (N2200), and Figure 9(b) shows that of sample D without a protective film before and after stress application. The characteristics before stress application are shown by a solid line, and those after stress application are shown by a dashed line. The horizontal axis in Figures 3(b) to 9(b) represents the elapsed time from time t3, and the vertical axis represents the magnitude of the drain current Id. Since the horizontal axis represents the elapsed time from time t3, the first measurement point (the measurement point 0 seconds after time t4) is shown at the 2.6-second position on the horizontal axis. The measurements in the third experimental example were performed by controlling an Agilent 4156C with Keysight's EasyEXPERT.

[0071] The drain current Id after stress application shows the time variation (increase) associated with the recovery from current collapse after time t4. For the sake of illustration, the drain current Id before stress application is shown as a constant value after time t4. The drain current Id after stress application gradually recovers after time t4 (i.e., immediately after stress application) and approaches the value of the drain current Id before stress application.

[0072] The amount of decrease in the drain current Id immediately after stress application with respect to the drain current Id before stress application may be simply referred to as the "decrease amount" hereinafter. Also hereinafter, the decrease amount in a HEMT with a protective film formed of a certain material may be simply referred to as the "decrease amount of this material". The decrease amount is large for sample D without a protective film (Figure 9(b)), relatively large for materials A (Figure 7(b)) and B (Figure 8(b)), and significantly suppressed for materials C (C1 to C4, Figures 3(b) to 6(b)) compared to materials A and B.

[0073] Among materials C (C1 to C4, Figures 3(b) to 6(b)), the decrease amount of material C2 (PDPP3T, Figure 4(b)) tends to be relatively large compared to the decrease amounts of material C1 (PDPP2T-TT-OD, Figure 3(b)), material C3 (Figure 5(b)), and material C4 (Figure 6(b)).

[0074] As described above, the hole mobility of material C1 (PDPP2T-TT-OD) is 0.1 to 0.3 cm 2 / Vs, the hole mobility of material C2 (PDPP3T) is 0.01 to 0.03 cm 2 / Vs, the hole mobility of material C3 is 0.5 to 2.5 cm 2 / Vs, and the hole mobility of material C4 is 0.2 to 2.0 cm 2 / Vs.

[0075] Material C2 (PDPP3T) has a significantly lower (one to two orders of magnitude lower) hole mobility compared to material C1 (PDPP2T-TT-OD), material C3, and material C4. Specifically, while materials C1, C3, and C4 have a hole mobility on the order of 0.1 cm 2 / Vs or more, material C2 has a hole mobility on the order of the lower half (less than 0.05 cm 2 / Vs) of 0.01 cm 2 / Vs.

[0076] Thus, since the hole mobility of material C2 (PDPP3T) is significantly lower than that of material C1 (PDPP2T-TT-OD), material C3, and material C4, it is speculated that the decrease amount of material C2 may be larger than that of materials C1, C3, and C4. That is, from the viewpoint of reducing the decrease amount of the drain current Id during current collapse, the hole mobility of the organic semiconductor constituting the protective film of the HEMT using group III nitrides should not be excessively low, specifically, it is preferably 0.1 cm 2 / Vs or more.

[0077] The degree of decrease in the drain current Id of each material due to stress is more specifically estimated as follows. Here, immediately after stress application is defined as 2 seconds after time t4 (4.6 seconds after time t3). The ratio (Id2 / Id1) of the drain current Id2 immediately after stress application to the drain current Id1 before stress application is referred to as the decrease ratio.

[0078] The reduction ratios are 0.93 for Material C1 (PDPP2T-TT-OD), 0.81 for Material C2 (PDPP3T), 0.995 for Material C3, and 0.97 for Material C4. The reduction ratio of Material A is 0.15, the reduction ratio of Material B is 0.22, and the reduction ratio of Sample D is 0.0024. The reduction ratio is preferably 0.8 or more, more preferably 0.9 or more, and even more preferably 0.95 or more.

[0079] Regarding the time constant of Material C in the first experimental example (Figure 2), there is generally a tendency that the larger the hole mobility, the smaller the time constant. For example, Materials C3 and C4 have shorter time constants compared to Materials C1 and C2. From this perspective, the hole mobility of Material C (the upper limit achievable at room temperature) is preferably more than 0.5 cm 2 / Vs, more preferably more than 1.0 cm 2 / Vs, and even more preferably more than 1.5 cm 2 / Vs.

[0080] Materials C3 and C4 are, as described above, unipolar organic semiconductors. Being a unipolar organic semiconductor is preferable from the perspective of suppressing electron injection from the electrode and injecting holes onto the surface of the group III nitride laminate when a drain voltage is applied, and is also preferable from the perspective of suppressing leakage.

[0081] All of the organic semiconductors examined in the above first to third experimental examples are polymer organic semiconductors. From the perspective of improving heat resistance (for example, increasing it to 200 °C or higher), the organic semiconductor used for the protective film of the HEMT is preferably a polymer organic semiconductor. In this embodiment, when the organic semiconductor is a polymer, it means that the number average molecular weight (Mn) in terms of polystyrene measured by gel permeation chromatography is 1 × 10 3 ~1 × 10 8 and the weight average molecular weight (Mw) in terms of polystyrene is 1 × 10 3 ~2 × 10 8 of the compound. From the perspective of forming a good thin film during thin film fabrication, the number average molecular weight is 1 × 10 3The above is preferable, and the weight average molecular weight is 1×10 3 or more is preferable. From the viewpoints of solubility and film-forming property, the number average molecular weight is preferably 1×10 6 or less, and the weight average molecular weight is preferably 1×10 6 or less.

[0082] In addition, as the p-type polymer organic semiconductor containing a thiophene ring (in the mother skeleton or main chain), in addition to the above-mentioned materials C1 to C4, for example, a polymer organic semiconductor having a chemical structural formula as shown in FIGS. 12(a) to 12(h) may be used. Here, R is the above-mentioned substituent.

[0083] In addition, regarding the organic semiconductor (material A) that is p-type and does not contain a thiophene ring, a low-molecular-weight one (also referred to as material a) was also studied. As the low-molecular-weight organic semiconductor (material a) that is p-type and does not contain a thiophene ring, specifically, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) manufactured by Sigma-Aldrich was used. The chemical structural formula of material a (TIPS pentacene) is shown in FIG. 10(c).

[0084] An HEMT having the same structure as HEMT100 exemplified in the above embodiment and having a protective film 40 made of material a was fabricated, and the same measurements as in the above second experimental example and third experimental example were performed. The protective film 40 made of material a was formed by using a solution dissolved in toluene at a concentration of 0.5 wt%, spin-coating at 1000 rpm, and then annealing at 80° C. for 10 minutes on a hot plate.

[0085] FIG. 10(a) is a graph showing the Id-Vg characteristics of material a (TIPS pentacene) before and after stress application (showing the results of the second experimental example). FIG. 10(b) is a graph showing the drain current Id of material a (TIPS pentacene) before and after stress application (showing the results of the third experimental example).

[0086] In Material A, as shown in Fig. 10(b), it was found that the amount of decrease in the drain current Id was relatively large. More specifically, the decrease ratio of Material A was 0.55, which was found to be less than 0.8.

[0087] <Preferred Embodiment of the Present Invention> Hereinafter, the preferred embodiments of the present invention will be appended.

[0088] (Appendix 1) A substrate, A channel layer provided on the substrate and composed of a first group III nitride, and A barrier layer provided on the channel layer and composed of a second group III nitride having a band gap larger than that of the first group III nitride, A group III nitride laminate having the same, A gate electrode, a source electrode, and a drain electrode provided on the surface of the group III nitride laminate, and A protective film composed of a p-type organic semiconductor containing a thiophene ring (in the main skeleton or main chain) and provided so as to be in contact with at least a part of the outer portions of the gate electrode, the source electrode, and the drain electrode in a plan view of the surface of the group III nitride laminate (preferably, in contact with the surface of the group III nitride laminate between the gate electrode and the drain electrode), A high electron mobility transistor comprising the same.

[0089] (Appendix 2) The high electron mobility transistor according to Appendix 1, wherein the hole mobility of the organic semiconductor is 0.1 cm 2 / Vs or more.

[0090] (Appendix 3) The high electron mobility transistor according to Appendix 1 or 2, wherein the organic semiconductor is a polymer organic semiconductor.

[0091] (Appendix 4) The high electron mobility transistor according to Appendix 1 or 2, wherein the organic semiconductor is a unipolar organic semiconductor.

[0092] (Supplementary Note 5) The high electron mobility transistor according to Supplementary Note 1 or 2, wherein the protective film is provided so as to be in contact with at least a part of the upper surface of the gate electrode and the upper surface of the drain electrode.

Explanation of Reference Numerals

[0093] 10... Substrate, 20... Group III nitride laminate, 20s... Surface (of the group III nitride laminate), 21... Buffer layer, 22... Buffer / channel layer (channel layer), 23... Barrier layer, 24... Cap layer, 30... Electrode, 31... Gate electrode, 32... Source electrode, 33... Drain electrode, 40... Protective film, 41... Organic semiconductor, 50... Element isolation structure, 100... HEMT

Claims

1. A substrate, A channel layer provided on the substrate and composed of a first group-III nitride, and A barrier layer provided on the channel layer and composed of a second group-III nitride having a bandgap larger than that of the first group-III nitride, A group-III nitride laminate having the same, A gate electrode, a source electrode, and a drain electrode provided on the surface of the group-III nitride laminate, and A protective film provided so as to be in contact with at least a part of the outer portions of the gate electrode, the source electrode, and the drain electrode in a plan view of the surface of the group-III nitride laminate, and composed of a p-type organic semiconductor containing a thiophene ring, A high electron mobility transistor comprising the same.

2. The hole mobility of the organic semiconductor is 0.1 cm ２ / Vs or more, and the high electron mobility transistor according to claim 1.

3. The high electron mobility transistor according to claim 1 or 2, wherein the organic semiconductor is a polymer organic semiconductor.

4. The high electron mobility transistor according to claim 1 or 2, wherein the organic semiconductor is a unipolar organic semiconductor.

5. The high electron mobility transistor according to claim 1 or 2, wherein the protective film is provided so as to be in contact with at least a part of the upper surface of the gate electrode and the upper surface of the drain electrode.

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