JP2015005671A - Diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diode in which initial voltage VF and ON resistance are reduced.SOLUTION: A diode 100 includes: a substrate 10; a first nitride semiconductor layer 122B; a second nitride semiconductor layer 122A formed on the first nitride semiconductor layer 122B and having a large band gap as compared with the first nitride semiconductor layer 122B; an anode electrode 131 and a cathode electrode 132; and a first p-type nitride semiconductor layer 123 formed on the second nitride semiconductor layer 122A between the anode electrode 131 and the cathode electrode 132 and connected to the anode electrode 131. In the diode 100, when voltage allowing a current to flow between the anode electrode 131 and the cathode electrode 132 is applied to the anode electrode 131, voltage on the anode side of a 2DEG channel formed in the first nitride semiconductor layer 122B just under the first p-type nitride semiconductor layer 123 is set lower than anode voltage.

Description

  The present invention relates to a diode using a nitride semiconductor, and more particularly to a diode applicable to a power device used in a power supply circuit or the like.

In recent years, in order to overcome the material limitation and reduce conduction loss, introduction of a semiconductor device using a group III nitride semiconductor represented by GaN or a wide gap semiconductor such as silicon carbide (SiC) has been studied. In particular, nitride semiconductors exhibit various multi-element mixed crystals represented by the general formula Al _x Ga _y In _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). Form. Therefore, a heterostructure can be easily formed by using multi-element mixed crystals having different band gaps.

For example, in the heterostructure of aluminum gallium nitride (AlGaN) and gallium nitride (GaN), charges are generated at the heterointerface on the (0001) plane due to spontaneous polarization and piezopolarization, and 1 × 10 ¹³ even when undoped. A sheet carrier concentration of cm ⁻² or more is obtained. Therefore, by using a two-dimensional electron gas (2DEG: 2 Dimensional Electron Gas) at the heterointerface as a channel, a diode having a high current density and low on-resistance, and a heterojunction field effect transistor (HFET: Hetero-junction) Field Effect Transistor) can be realized. For this reason, AlGaN / GaN diodes and heterojunction field effect transistors (HFETs) are expected as power switching elements that realize low on-resistance and high breakdown voltage.

  In the structure of an AlGaN / GaN diode, a Schottky barrier diode (SBD) that uses a Schottky electrode as an anode electrode is generally used.

  It is desirable that the SBD has a forward rising voltage (VF) as small as possible. However, since GaN or AlGaN has a large band gap, the height of the Schottky barrier cannot be lowered sufficiently even if the electrode material is changed, and the rising voltage (VF) cannot be reduced as compared with SBD made of Si or GaAs.

  Therefore, a transistor type diode has been proposed that includes two electrodes, an electrode having an anode electrode in ohmic contact with the channel and an electrode that depletes the channel (see, for example, Patent Document 1).

  Further, the electrode for depleting the channel is not limited to the Schottky electrode, and a configuration of a p-type semiconductor and an ohmic electrode formed thereon is also proposed (for example, see Patent Document 2).

JP 2008-108870 A JP 2012-23268 A

  However, in the transistor type diode, the channel resistance component becomes large under the electrode that has depleted the channel in the anode electrode after the voltage applied to the anode electrode exceeds the threshold voltage. As a result, there is a problem that the on-resistance of the diode is large.

  In view of the above problems, an object of the present invention is to provide a diode with a small rising voltage VF and on-resistance.

  In order to solve the above problems, a diode according to the present disclosure is formed on a substrate, a first nitride semiconductor layer formed on the substrate, the first nitride semiconductor layer, and A second nitride semiconductor layer having a larger band gap than the first nitride semiconductor layer, and a cathode electrode and an anode electrode formed on the second nitride semiconductor layer at a distance from each other; A first p-type nitride semiconductor layer formed on the second nitride semiconductor layer between the anode electrode and the cathode electrode and connected to the anode electrode, the anode electrode The first nitride immediately below the first p-type nitride semiconductor layer when a voltage is applied between the cathode electrode and the cathode electrode so that a current in a direction from the anode electrode toward the cathode electrode flows. In the semiconductor layer It made the 2DEG (2 Dimensional Electron Gas) voltage of the anode side of the channel is set lower than the anode voltage.

  Thereby, the diode according to the present disclosure can increase the concentration of electrons present at the anode side end of the channel formed in the first nitride semiconductor layer immediately below the first p-type nitride semiconductor layer. As a result, the channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced, and the on-resistance can be reduced.

  The anode electrode and the first p-type nitride semiconductor layer may not be in contact with each other on the surface of the second nitride semiconductor layer.

  Thereby, the diode according to the present disclosure can take a distance between the anode electrode and the first p-type nitride semiconductor layer, and the first nitride immediately below the first p-type nitride semiconductor layer. When a resistance (Ra) is generated between the anode side end of the channel formed in the semiconductor layer and the anode electrode, and the forward current flows, the channel anode side voltage V1 becomes smaller than the anode voltage Va. As a result, the channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced, and the on-resistance can be reduced.

  In addition, a second p-type nitride semiconductor layer may be provided on the second nitride semiconductor layer between the anode electrode and the first p-type nitride semiconductor layer.

  As a result, the diode according to the present disclosure has a 2DEG concentration under the second p-type nitride semiconductor layer that decreases and Ra increases, so that when the forward current flows, the anode side voltage V1 of the channel is higher than the anode voltage Va. And the channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced. As a result, the on-resistance can be reduced.

  The second nitride semiconductor layer has a recess portion that does not reach the first nitride semiconductor layer, and the first p-type nitride semiconductor layer fills the recess portion. It may be formed.

  As a result, the diode according to the present disclosure controls VF directly under the recess portion, and the p-type nitride semiconductor layer in the non-recess portion does not affect VF.

  The length of the first p-type nitride semiconductor layer on the non-recessed portion of the second nitride semiconductor layer is longer on the anode electrode side than on the cathode electrode side.

  As a result, the diode according to the present disclosure increases the resistance of the channel under the non-recessed portion of the first p-type nitride semiconductor layer and increases Ra, so that when the forward current flows, the anode of the channel is greater than the anode voltage Va. The side voltage V1 is reduced, and the channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced. As a result, the on-resistance can be reduced.

  Further, the anode electrode may be formed in a recess portion that reaches the first nitride semiconductor layer.

  Thereby, in the diode according to the present disclosure, the contact resistance between the anode electrode and the channel is reduced, and as a result, the on-resistance can be reduced.

  The second nitride semiconductor layer may contact the bottom surface of the anode electrode.

  Accordingly, in the diode according to the present disclosure, the contact resistance between the anode electrode and the channel is increased, and Ra is increased, whereby the anode side voltage V1 of the channel is smaller than the anode voltage Va when the forward current flows. The channel resistance Rch immediately below the p-type nitride semiconductor layer 1 can be reduced. As a result, the on-resistance can be reduced.

  The second nitride semiconductor layer may include a third nitride semiconductor layer having a band gap different from that of the second nitride semiconductor layer at a contact portion with the anode electrode.

  The third nitride semiconductor may have a composition different from that of the second nitride semiconductor layer.

  The third nitride semiconductor may be an ion-implanted high resistance layer.

  Thereby, in the diode according to the present disclosure, the contact resistance between the anode electrode and the channel is greatly increased and Ra is increased, so that the anode side voltage V1 of the channel becomes smaller than the anode voltage Va when the forward current flows. The channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced. As a result, the on-resistance can be reduced.

  According to the diode according to the present disclosure, it is possible to provide a diode with a small rising voltage VF and on-resistance.

It is sectional drawing of the diode concerning 1st Embodiment. It is a figure which shows the relationship between on-resistance (= Ra + Rch + Rc) and anode side resistance Ra. It is sectional drawing of the diode concerning 2nd Embodiment. It is sectional drawing of the diode concerning 3rd Embodiment. It is sectional drawing of the diode concerning 4th Embodiment. It is sectional drawing of the diode concerning 5th Embodiment. It is sectional drawing of the diode concerning 6th Embodiment. It is sectional drawing of the diode concerning the basic technology of this indication. It is sectional drawing of the diode concerning the basic technology of this indication.

(Knowledge that became the basis of the present invention)
In the structure of an AlGaN / GaN diode, a Schottky Barrier Diode (SBD) using a Schottky electrode as an anode electrode is generally used, and development and commercialization are actively performed in many research institutions and companies. ing.

  In SBD, the forward rising voltage (VF) is determined by the height of the Schottky barrier generated by the contact between the Schottky electrode and the semiconductor. The VF is preferably as small as possible because energy loss occurs when a forward current is passed through the diode. However, since GaN and AlGaN have a large band gap, the height of the Schottky barrier cannot be lowered sufficiently even if the electrode material is changed. In other words, there is a problem that VF cannot be reduced as compared with SBD made of Si or GaAs.

  As one method for solving the problem, for example, as disclosed in Patent Document 1, a diode including an electrode in which an anode electrode is in ohmic contact with a channel and an electrode in which a channel is depleted is provided. Proposed. The structure of this diode is the same as the structure in which a gate electrode and a source electrode are short-circuited and regarded as an anode electrode and a drain electrode is regarded as a cathode electrode in a transistor composed of a source electrode, a gate electrode, and a drain electrode. It will be called a transistor type diode.

  Since the VF in the transistor type diode is equal to the threshold voltage of the transistor constituting the transistor type diode, it is possible to design the device so that the VF is smaller than the SBD made of the same semiconductor material. As an electrode for depleting a channel, as disclosed in Patent Document 2, not only a Schottky electrode but also a configuration of a p-type semiconductor and an ohmic electrode formed thereon has been proposed.

  A transistor type diode using the p-type semiconductor and an ohmic electrode formed thereon will be described with reference to FIG.

  A transistor type diode 1000 shown in FIG. 8 includes a substrate 1101, a buffer layer 1120, a semiconductor layer stack 1102, and a cathode electrode 1103 and an anode electrode 1104 formed on the semiconductor layer stack 1102 at intervals. And a protective film 1106 covering the top of the semiconductor layer stack 1102.

  The semiconductor layer stack 1102 includes a first nitride semiconductor layer 1121 made of GaN and a second nitride semiconductor layer 1122 made of AlGaN having a larger band gap than the first semiconductor layer 1121. 2DEG (not shown) is generated at the interface between the first nitride semiconductor layer 1121 and the second nitride semiconductor layer 1122 to form a channel.

  The anode electrode 1104 includes a p-type third nitride semiconductor layer 1143 made of p-GaN formed on the semiconductor layer stack 1102, and a Ni / O film that makes ohmic contact with the third nitride semiconductor layer 1143. A first metal layer 1141 made of Au and a second metal layer 1142 made of Ti / Al that is in contact with the first metal layer 1141 and in ohmic contact with the channel are provided. At this time, VF is determined by the impurity concentration of the p-type third nitride semiconductor layer 1143, the film thickness and the band gap of the second nitride semiconductor layer 1122 immediately below the third nitride semiconductor layer 1143, and the like.

  For example, if the thickness of the second nitride semiconductor layer 1122 immediately below the third nitride semiconductor layer 1143 is increased, the threshold voltage is lowered because the 2DEG concentration increases. By appropriately designing the thickness of the second nitride semiconductor layer 1122 within a range where the threshold voltage does not become negative, the value of VF can be made lower than SBD.

  However, in the transistor-type diode, the channel resistance component increases in the lower part of the electrode that has depleted the channel in the anode electrode after the voltage applied to the anode electrode exceeds the threshold voltage. There was a problem that the on-resistance of the diode was large.

  As the cause of the above problem, the present inventors have found the following mechanism. This will be described with reference to FIG. A transistor type diode 1000 shown in FIG. 9 is a conventional transistor type diode, and 2DEG (not shown) is generated at the interface between the first nitride semiconductor layer 1121 and the second nitride semiconductor layer 1122. Yes.

  The on-resistance component of the transistor type diode 1000 is decomposed into three resistance components. That is, the channel resistance immediately below the p-type nitride semiconductor layer 1143 is Rch, the resistance component existing on the second metal layer 1142 side is Ra, and the resistance on the cathode electrode 1103 side is Rc.

  The potential of the channel formed in the first nitride semiconductor layer 1121 immediately below the p-type nitride semiconductor layer 1143 on the second metal layer 1142 side is V1, and the potential of the second metal layer 1142 is Va. In that case, when a threshold voltage, that is, a voltage equal to or higher than VF in the transistor type diode is applied to the second metal layer 1142, the current starts to flow, but immediately after starting to flow, the potential of V1 is raised by almost the same as Va. Therefore, even when a voltage of VF or higher is applied to the anode electrode, a part of the lower channel layer on the second metal layer 1142 side of the p-type nitride semiconductor layer 1143 is depleted as shown in FIG. There is. That is, the field effect transistor operation is in the same state as the saturation region operation in which carriers are depleted (pinched off) at the gate end. As a result, it has been found that the channel resistance Rch under the p-type nitride semiconductor layer increases, and there is a structural problem that the on-resistance expressed by Ra + Rch + Rc increases.

  Similarly, when a Schottky electrode is used instead of the p-type nitride semiconductor layer, the channel layer under the anode electrode end of the Schottky electrode remains depleted, and the channel resistance under the Schottky electrode increases, and the ON There is a structural problem that increases the resistance.

  Therefore, for example, if an insulating film is provided between the portion of the anode electrode that is in ohmic contact with the channel and the p-type nitride semiconductor layer, the distance between the anode electrode and the p-type nitride semiconductor layer can be increased. . Thereby, a resistance (Ra) is created between the anode side end of the channel formed in the first nitride semiconductor layer immediately below the p-type nitride semiconductor layer and the anode electrode, and when a forward current flows, The anode voltage V1 of the channel is smaller than the anode voltage Va.

  Therefore, it is possible to increase the concentration of electrons present at the anode side end of the channel formed in the first nitride semiconductor layer immediately below the p-type nitride semiconductor layer (first p-type nitride semiconductor layer). As a result, the channel resistance Rch immediately below the first p-type nitride semiconductor layer can be reduced, and the on-resistance can be reduced.

  Hereinafter, in each embodiment, the nitride semiconductor diode according to the present disclosure will be described in detail.

(First embodiment)
Hereinafter, a first embodiment will be described with reference to the drawings. FIG. 1 shows a cross-sectional configuration of a diode 100 according to the first embodiment.

  As shown in FIG. 1, in the diode 100, a semiconductor layer stacked body 122 is formed on a substrate 10 with a buffer layer 21 interposed therebetween.

  The substrate 10 is made of silicon, sapphire, silicon carbide, gallium nitride, or the like. The buffer layer 21 has a film thickness of about 2 μm. The semiconductor layer stack 122 includes an undoped GaN layer 122B having a thickness of 2 μm and an undoped AlGaN layer 122A having a thickness of 25 nm. The undoped GaN layer 122B corresponds to the first nitride semiconductor layer in this embodiment, and the first AlGaN layer 122A corresponds to the second nitride semiconductor layer in this embodiment.

  An anode electrode 131 and a cathode electrode 132 are formed on the semiconductor layer stack 122 with a space therebetween.

  The anode electrode 131 and the cathode electrode 132 are formed in a portion recessed from the interface between the undoped GaN layer 122B and the first AlGaN layer 122A. That is, the anode electrode 131 and the cathode electrode 132 are formed in a recess portion (concave portion) reaching the undoped GaN layer 122B. Accordingly, the anode electrode 131 and the cathode electrode 132 are formed so as to be in contact with the side surface of the recess portion (recessed portion) in which the first AlGaN layer 122A is dug and the bottom surface and side surface of the undoped GaN layer 122B. The anode electrode 131 and the cathode electrode 132 are in ohmic contact with a channel formed of 2DEG generated at the interface between the undoped GaN layer 122B and the first AlGaN layer 122A, respectively. The anode electrode 131 and the cathode electrode 132 may be formed of a metal composed of, for example, titanium / aluminum (Ti / Al).

  The first p-type nitride semiconductor layer 123 is formed on the first AlGaN layer 122A, and is made of, for example, p-type AlGaN. Between the first p-type nitride semiconductor layer 123 and the anode electrode 131, an insulating film 125 is formed on the first AlGaN layer 122A.

  The insulating film 125 is made of, for example, SiN, SiO, AlN, or the like. The p-type ohmic electrode 133 may be a metal that is in ohmic contact with the first p-type nitride semiconductor layer 123, and may be a metal composed of Ti, TiN, Pd, or Ni, for example. The first p-type nitride semiconductor layer 123 and the anode electrode 131 are connected via the p-type ohmic electrode 133.

  The diode 100 according to the present embodiment includes a channel (point a) immediately below the anode electrode side of the first p-type nitride semiconductor layer 123 by the insulating film 125 and a point (point b) where the anode electrode 131 is in contact with 2DEG. Can take distance. The resistance component corresponding to this distance causes a difference between the potential below the anode electrode end of the first p-type nitride semiconductor layer 123 and the potential of the anode electrode 131 when the diode 100 is turned on, and the first p-type nitriding is performed. The channel resistance Rch directly under the physical semiconductor layer 123 can be reduced. Thereby, the on-resistance of the diode 100 can be reduced.

  The inventor of the present application derived the relationship between the anode-side resistance Ra and the on-resistance of the transistor type diode as follows. The 2DEG concentration generated under the first p-type nitride semiconductor layer 123 is expressed as exp (q (V−Vth) / nkT near V slightly exceeding Vth, where V is the forward voltage and Vth is the threshold voltage. ). Here, q is an electron charge, n is an ideal coefficient, k is a Boltzmann constant, and T is an absolute temperature.

  In a transistor type diode, V = Vth when current starts to flow, and approximately V = Vth + IfRa after starting to flow. However, If is a current flowing through this transistor type diode. Therefore, assuming that the channel resistance Rch immediately below the first p-type nitride semiconductor layer 123 after the current starts flowing is inversely proportional to the 2DEG concentration, Rch∝1 / exp (q (V−Vth) / nkT) = It is expressed as 1 / exp (qIfRa / nkT).

  Here, when R0 is set as Rch when If = 0, it can be expressed as Rch = R0 / exp (qIfRa / nkT). Therefore, when If is constant, it can be said that the channel resistance Rch immediately below the first p-type nitride semiconductor layer 123 is a function of the anode-side resistance Ra.

  Here, FIG. 2 shows a plot of the relationship between the on-resistance (= Ra + Rch + Rc) and the anode-side resistance Ra of the transistor-type diode. In FIG. 2, R0 is 20 Ωmm and Rc is 10 Ωmm. Moreover, q / kT was calculated as 25 mV generally used as a value at room temperature.

  Initially, the ON resistance was expected to increase as the value of the anode-side resistance Ra of the transistor type diode increased. However, the result of FIG. 2 shows that the ON-resistance decreases when the anode-side resistance Ra is low. It shows a tendency to increase after showing a minimum value at a certain anode side resistance Ra.

  Considering this result, when the value of the anode side resistance Ra is about 0 Ωmm to about 5 Ωmm, the channel resistance immediately below the first p-type nitride semiconductor layer 123 with respect to the linear increase amount of the anode side resistance Ra. It shows that the amount of reduction in the reciprocal of the exponential function of Rch is greater.

  Thereafter, when the value of the anode-side resistance Ra exceeds about 5 Ωmm, the rate of decrease of the channel resistance Rch immediately below the first p-type nitride semiconductor layer 123 decreases, and the value immediately below the first p-type nitride semiconductor layer 123 decreases. It shows that the increase amount of the anode side resistance Ra exceeds the decrease amount of the channel resistance Rch. That is, the on-resistance has a minimum value with respect to the anode-side resistance Ra, depending on the ideal coefficient n and the absolute temperature T. Accordingly, the on-resistance can be minimized when the anode-side resistance Ra is about 5 Ωmm.

  Since such a phenomenon does not occur in SBD, it is a mechanism that has been confirmed for transistor type diodes for the first time.

  As described above, according to the diode 100 according to the present embodiment, the first AlGaN layer (which is formed on the undoped GaN layer 122B which is the first nitride semiconductor layer) and has a larger band gap than the undoped GaN layer 122B ( (Second nitride semiconductor layer) 122A, cathode electrode 132 and anode electrode 131 formed on first AlGaN layer 122A at a distance from each other, and first electrode between anode electrode 131 and cathode electrode 132. The first p-type nitride semiconductor layer 123 is formed on the AlGaN layer 122A and connected to the anode electrode 131. Between the anode electrode 131 and the cathode electrode 132, the anode electrode 131 to the cathode electrode are provided. The first p-type nitride semiconductor layer 1 is applied when a voltage is applied so that a current in a direction toward 132 flows. 2DEG the voltage on the anode side of the channels formed in the undoped GaN layer 122B just below the 3 is set lower than the anode voltage.

  As a result, the diode 100 can increase the concentration of electrons present at the anode side end of the channel formed in the undoped GaN layer 122B immediately below the first p-type nitride semiconductor layer 123. As a result, the channel resistance Rch immediately below the first p-type nitride semiconductor layer 123 can be reduced, and the on-resistance can be reduced.

  In the first embodiment, the first AlGaN layer 122A has a recess (see FIG. 3) dug up to the vicinity of the interface of the undoped GaN layer 122B, and the first p-type nitride is filled so as to fill the recess. A semiconductor layer (see the first p-type nitride semiconductor layer 223 in FIG. 3) may be formed.

(Second Embodiment)
Hereinafter, a second embodiment will be described with reference to the drawings. FIG. 3 shows a cross-sectional configuration of the diode according to the second embodiment. In FIG. 3, the same components as those in FIG.

  The diode 200 according to the present embodiment is different from the diode 100 according to the first embodiment in that the diode 200 is in contact with the first AlGaN layer in the insulating film between the first p-type nitride semiconductor layer and the anode electrode. As described above, the second p-type nitride semiconductor layer is formed.

  As shown in FIG. 3, in the diode 200, the first AlGaN layer 222A has a recessed portion (concave portion) dug up to the vicinity of the interface of the GaN layer 222B, and the first p-type nitriding is performed so as to fill the recessed portion. A physical semiconductor layer 223 is formed. For example, the depth of the recess portion may be 25 nm, the AlGaN layer thickness may be 25 nm, and the AlGaN layer thickness of the non-recess portion may be 50 nm. The GaN layer 222B corresponds to the first nitride semiconductor layer in this embodiment, and the first AlGaN layer 222A corresponds to the second nitride semiconductor layer in this embodiment.

  A second p-type nitride semiconductor layer 224 is formed in the insulating film 225 between the first p-type nitride semiconductor layer 223 and the anode electrode 231 so as to be in contact with the first AlGaN layer 222A. Yes. Since the second p-type nitride semiconductor layer 224 can be formed simultaneously with the first p-type nitride semiconductor layer 223, the number of process steps does not increase.

  In the diode 200, the 2DEG concentration under the second p-type nitride semiconductor layer 224 is decreased and the anode-side resistance Ra is increased, so that the diode 200 is turned on and immediately below the first p-type nitride semiconductor layer 223. The channel resistance Rch can be reduced. Therefore, the on-resistance of the diode 200 can be reduced.

  As described above, according to the diode 200 of the present embodiment, the second AlGaN layer 222A is in contact with the first AlGaN layer 222A in the insulating film 225 between the first p-type nitride semiconductor layer 223 and the anode electrode 231. Since the p-type nitride semiconductor layer 224 is formed, the on-resistance of the diode 200 can be reduced.

(Third embodiment)
The third embodiment will be described below with reference to the drawings. FIG. 4 shows a cross-sectional configuration of the diode according to the third embodiment. In FIG. 4, the same components as those of FIG.

  The diode 300 according to this embodiment is different from the diode 100 according to the first embodiment in that the non-recessed portion of the first p-type nitride semiconductor layer 323 is longer on the anode electrode 331 side than on the cathode electrode 332 side. It is the point formed in.

  As shown in FIG. 4, in the diode 300, the first AlGaN layer 322A has a concave portion dug up to the vicinity of the interface of the GaN layer 322B, and the first p-type nitride semiconductor formed so as to fill the concave portion. The non-recessed portion of the layer 323 is formed so that the anode electrode 331 side is longer than the cathode electrode 332 side. Here, the non-recessed portion refers to a portion of the first p-type nitride semiconductor layer 323 formed on the upper surface of the first AlGaN layer 322A other than the recess. The GaN layer 322B corresponds to the first nitride semiconductor layer in the present embodiment, and the first AlGaN layer 322A corresponds to the second nitride semiconductor layer in the present embodiment.

  As a result, in the diode 300, the resistance of the channel below the non-recessed portion of the first p-type nitride semiconductor layer 323 increases, and immediately below the lowermost portion of the first p-type nitride semiconductor layer 323 when the diode 300 is turned on. A difference can be generated between the potential on the anode side of the channel formed in the first nitride semiconductor layer (GaN layer) 322B and the potential of the anode electrode 331. As a result, the channel resistance Rch immediately below the first p-type nitride semiconductor layer 323 can be reduced. Therefore, the on-resistance of the diode 300 can be reduced.

  As described above, according to the diode 300 according to the present embodiment, the non-recessed portion of the first p-type nitride semiconductor layer 323 is formed so that the anode electrode 331 side is longer than the cathode electrode 332 side. The on-resistance can be reduced.

(Fourth embodiment)
Hereinafter, a fourth embodiment will be described with reference to the drawings. FIG. 5 shows a cross-sectional configuration of the diode according to the fourth embodiment. In FIG. 5, the same components as those in FIG.

  The diode 400 according to the present embodiment is different from the diode 100 according to the first embodiment in that the anode electrode is not formed in the recess portion (concave portion) provided in the first AlGaN layer and the GaN layer. The first AlGaN layer is in contact with the bottom surface.

  As shown in FIG. 5, in the diode 400, the anode electrode 431 is formed without forming a recess portion with respect to the first AlGaN layer 422 </ b> A and the GaN layer 422 </ b> B. That is, the first AlGaN layer 422A is in contact with the bottom surface of the anode electrode 431. The GaN layer 422B corresponds to the first nitride semiconductor layer in this embodiment, and the first AlGaN layer 422A corresponds to the second nitride semiconductor layer in this embodiment.

  According to this configuration, the contact resistance between the anode electrode 431 and the channel is increased, the anode-side resistance Ra is increased, and the channel resistance Rch immediately below the first p-type nitride semiconductor layer 423 is reduced when the diode 400 is turned on. be able to. Therefore, the on-resistance of the diode 400 can be reduced.

  In the fourth embodiment, the diode 400 may be formed so shallow that the recess depth on the anode electrode side does not reach the interface between the first AlGaN layer 422A and the GaN layer 422B.

  In the fourth embodiment, the first AlGaN layer 422A has a recessed portion (concave portion) dug to the vicinity of the interface of the GaN layer 422B, and the first p-type nitride semiconductor layer is filled so as to fill the concave portion. 423 may be formed.

  As described above, according to the diode 400 according to the present embodiment, the anode electrode 431 is not formed in the recesses (recesses) of the first AlGaN layer 422A and the GaN layer 422B, and the first AlGaN layer is formed on the bottom surface of the anode electrode 431. Since 422A is in contact, the on-resistance of the diode 400 can be reduced.

(Fifth embodiment)
Hereinafter, a fifth embodiment will be described with reference to the drawings. FIG. 6 shows a cross-sectional configuration of the diode according to the fifth embodiment. In FIG. 6, the same components as those in FIG.

  The diode 500 according to this embodiment is different from the diode 100 according to the first embodiment in that a second portion having an Al composition different from that of the first AlGaN layer 522A is formed in a lower portion of the anode electrode 531 of the AlGaN layer 522A. An AlGaN layer 522C is formed. Note that the second AlGaN layer 522C corresponds to the third nitride semiconductor layer in the present embodiment.

  As shown in FIG. 6, in the diode 500, the anode electrode 531 is formed without recessing the first AlGaN layer 522A or the GaN layer 522B, and the lower part of the anode electrode 531 of the AlGaN layer 522A is formed. The second AlGaN layer 522C having an Al composition different from that of the first AlGaN layer 522A is formed. For example, the Al composition of the first AlGaN layer 522A may be 25%, and the Al composition of the second AlGaN layer 522C may be 20%.

  According to this configuration, the diffusion of metal from the anode electrode 531 into the nitride semiconductor layer 522 is prevented at the interface between the first AlGaN layer 522A and the second AlGaN layer 522C, so that the anode electrode 531 and the channel The contact resistance between the first p-type nitride semiconductor layer 523 and the potential of the anode electrode 531 is different from that of the first p-type nitride semiconductor layer 523 when the diode 500 is turned on. The channel resistance Rch immediately below the type nitride semiconductor layer 523 can be reduced. Therefore, the on-resistance of the diode 500 can be reduced.

  As described above, according to the diode 500 according to the present embodiment, the second AlGaN layer 522C having an Al composition different from that of the first AlGaN layer 522A is formed below the anode electrode 531 of the AlGaN layer 522A. The on-resistance of the diode 500 can be reduced.

  In the fifth embodiment, the recess depth on the anode electrode side may be shallow enough not to reach the channel.

  In the fifth embodiment, the second AlGaN layer 522C may be formed inside the first AlGaN layer 522A other than the surface layer of the first AlGaN layer 522A. In the fifth embodiment, the second AlGaN layer 522C may be formed to reach the cathode electrode 532 as long as the second AlGaN layer 522C is formed below the anode electrode of the AlGaN layer 522A.

  Further, in the fifth embodiment, the AlGaN layer 522A has a recess dug up to the vicinity of the interface of the GaN layer 522B, and the first p-type nitride semiconductor layer 523 is formed so as to fill the recess. Also good.

(Sixth embodiment)
The sixth embodiment will be described below with reference to the drawings. FIG. 7 shows a cross-sectional configuration of the diode according to the sixth embodiment. In FIG. 7, the same components as those in FIG.

  The diode 600 according to the present embodiment is different from the diode 100 according to the first embodiment in that an ion-implanted high resistance layer 626 is formed in the first AlGaN layer and the GaN layer under the anode electrode. is there.

  As shown in FIG. 7, in the diode 600, the anode electrode 631 is formed without recessing the first AlGaN layer 622A and the GaN layer 622B. In the first AlGaN layer 622A and the GaN layer 622B under the anode electrode 631, an ion-implanted high resistance layer 626 in which boron (B) is implanted until reaching the channel is formed.

  According to this configuration, the electron concentration of the channel is reduced by boron implantation, the contact resistance between the anode electrode 631 and the channel is appropriately increased, and the anode electrode 631 below the end of the first p-type nitride semiconductor layer 623 is turned on at the time of ON. And the potential of the anode electrode 631, the channel resistance Rch immediately below the first p-type nitride semiconductor layer 623 can be reduced, and the on-resistance can be reduced.

  As described above, according to the diode 600 according to the present embodiment, since the ion-implanted high resistance layer 626 is formed in the first AlGaN layer and the GaN layer under the anode electrode, the on-resistance of the diode 600 can be reduced. it can.

  In the sixth embodiment, the recess depth on the anode electrode side may be shallow enough not to reach the channel.

  In the sixth embodiment, if the implanted element of the ion-implanted high resistance layer 626 is an element that can increase the resistance of the channel, for example, iron (Fe), hydrogen (H), fluorine (F), oxygen You may form with elements other than boron, such as (O).

  In the sixth embodiment, the AlGaN layer 622A has a recess dug to the vicinity of the interface of the GaN layer 622B, and the first p-type nitride semiconductor layer 623 is formed so as to fill the recess. Good.

  In the diode described above, below the first p-type nitride semiconductor layer, the second nitride semiconductor layer has a recess dug up to the vicinity of the first nitride semiconductor interface, and fills the recess. Thus, the first p-type nitride semiconductor layer may be formed.

  In the above diode, the semiconductor substrate is preferably made of silicon, sapphire, silicon carbide, or gallium nitride.

  Although the diode according to the present invention has been described based on the above-described embodiments and modifications, the present invention is not limited to these embodiments and modifications. The present invention can be realized by arbitrarily combining the embodiments obtained by subjecting various embodiments and modifications to various modifications conceived by those skilled in the art and the components in the embodiments and modifications without departing from the gist of the present invention. Forms are also included in the present invention.

  The nitride semiconductor diode of the present invention can realize both reduction of VF and on-resistance, and is useful as a diode used in a power supply circuit or the like.

10, 1101 Substrate 21, 1120 Buffer layer 122, 222, 322, 422, 522, 622, 1102 Semiconductor layer stack 122A, 222A, 322A, 422A, 522A, 622A First AlGaN layer (second nitride semiconductor layer) )
122B, 222B, 322B, 422B, 522B, 622B GaN layer (first nitride semiconductor layer)
522C Second AlGaN layer (third nitride semiconductor layer)
123, 223, 323, 423, 523, 623 First p-type nitride semiconductor layer 224 Second p-type nitride semiconductor layer 125, 225 Insulating film 626 Ion-implanted high resistance layer 131, 231, 331, 431, 531 631, 1142 Anode electrode 132, 232, 332, 432, 532, 632, 1103 Cathode electrode 133, 233, 333, 433, 533, 533, 633 P-type ohmic electrode

Claims (10)

A substrate,
A first nitride semiconductor layer formed on the substrate;
A second nitride semiconductor layer formed on the first nitride semiconductor layer and having a larger band gap than the first nitride semiconductor layer;
A cathode electrode and an anode electrode formed on the second nitride semiconductor layer and spaced apart from each other;
A first p-type nitride semiconductor layer formed on the second nitride semiconductor layer between the anode electrode and the cathode electrode and connected to the anode electrode;
When a voltage is applied between the anode electrode and the cathode electrode such that a current flows in a direction from the anode electrode toward the cathode electrode, the first p-type nitride semiconductor layer immediately below the first p-type nitride semiconductor layer is applied. A diode in which a voltage on the anode side of a 2DEG (2 Dimensional Electron Gas) channel formed in the nitride semiconductor layer is set lower than the anode voltage. 2. The diode according to claim 1, wherein the anode electrode and the first p-type nitride semiconductor layer are not in contact with each other on the surface of the second nitride semiconductor layer. 3. The diode according to claim 2, further comprising a second p-type nitride semiconductor layer on the second nitride semiconductor layer between the anode electrode and the first p-type nitride semiconductor layer. The second nitride semiconductor layer has a recess portion that does not reach the first nitride semiconductor layer, and the first p-type nitride semiconductor layer is formed so as to fill the recess portion. The diode according to any one of claims 1 to 3. The diode according to claim 4, wherein a length of the first p-type nitride semiconductor layer on the non-recessed portion of the second nitride semiconductor layer is longer on the anode electrode side than on the cathode electrode side. The diode according to any one of claims 1 to 5, wherein the anode electrode is formed in a recess portion reaching the first nitride semiconductor layer. The diode according to claim 1, wherein the second nitride semiconductor layer is in contact with a bottom surface of the anode electrode. 2. The diode according to claim 1, wherein the second nitride semiconductor layer includes a third nitride semiconductor layer having a band gap different from that of the second nitride semiconductor layer at a contact portion with the anode electrode. The diode according to claim 8, wherein the third nitride semiconductor has a composition different from that of the second nitride semiconductor layer. The diode according to claim 8, wherein the third nitride semiconductor is an ion-implanted high resistance layer.

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