WO2014009863A1

WO2014009863A1 - Planar semiconductor heterojunction diode

Info

Publication number: WO2014009863A1
Application number: PCT/IB2013/055566
Authority: WO
Inventors: Gregory Bunin; David Rozman; Tamara Baksht
Original assignee: VISIC Tech Ltd
Current assignee: VISIC Tech Ltd
Priority date: 2012-07-09
Filing date: 2013-07-08
Publication date: 2014-01-16
Anticipated expiration: 2015-01-09

Abstract

A semiconductor device comprising: a stack having a plurality of semiconductor layers; a first conducting terminal having electrical contact with layers in the stack; and a second terminal comprising first and second metallic components that interface with layers in the stack at first and second Schottky junctions and have work functions ϕ₁and ϕ₂ respectively that satisfy a relationship ϕ₁> ϕ₂; wherein the metal components are spatially configured so that when the Schottky junctions are back biased, a depletion region generated at the first Schottky junction enhances isolation of the second metal component from mobile current carriers in the device.

Description

PLANAR SEMICONDUCTOR HETEROJUNCTION DIODE

RELATED APPLICATIONS

[0001] The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application 61/669, 138 filed on July 9, 2012, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] Embodiments of the invention relate to planar heterojunction diodes.

BACKGROUND

[0003] Many different products and systems, including, radar systems, electric motor controllers, uninterrupted power supply (UPS) systems, and consumer products such as TVs and electric powered vehicles, require relatively large amounts of electric power, typically provided by a high voltage power supply. Various types of semiconductor field effect transistors (FETs) are generally used as power switches in semiconductor circuits that provide switching functions required by the circuits to connect the power supply to the products and systems. Often, the FET power switches in the circuits are integrated with semiconductor diodes that rectify currents that pass through the FETs. Often, it is advantageous to simultaneously produce the diodes and FETs in a same manufacturing process.

[0004] The FET switches are advantageously characterized by relatively high breakdown voltages when they are OFF, high "ON currents" between source and drain when they are ON, and relatively low gate and drain leakage currents. Diodes integrated with the FET switches advantageously pass relatively large currents when forward biased with relatively low voltages and pass relatively small currents when back biased or not forward biased, that is, when the diode is "OFF".

[0005] For example, it may be advantageous for a FET power switch used in an electric vehicle, UPS, or photovoltaic inverter to have a breakdown voltage equal to or greater than about 600 V and drain leakage currents less than about ΙΟΟμΑ per mm (millimeter) of gate periphery when OFF. When ON, it is advantageous that the switch have a relatively small ON resistance that is less than or equal to about 10 Ohm per mm and be capable of safely supporting a drain current greater than or equal to about 50 A (amps). Advantageously, a semiconductor diode used with the FET power switch supports the at least 50A drain current at a relatively small forward bias voltage , of, for example, between IV to 5 V, and when back biased with a voltage between about 600 V (volts) and 1200 V, passes relatively small current.

[0006] Nitride based semiconductor materials, such as GaN (Gallium Nitride) and A1N (Aluminum Nitride) are characterized by relatively large band gaps of 3.4 eV and 6.2 eV respectively. The large band gaps of the materials are advantageous for providing semiconductor devices with relatively large breakdown voltages and low reverse bias currents. The materials have been used to produce high power, planar power FETs that exhibit fast switching times, relatively large breakdown voltages, and support large source drain currents. The materials have also been used to produce diodes that pass large forward currents at relatively small forward voltage biases.

[0007] The FETs and diodes are typically produced having a nitride semiconductor layer structure, referred to as a "heterostructure", comprising a small band gap semiconductor layer adjacent a large band gap semiconductor layer. The semiconductor layers produce a relatively high concentration of high mobility electrons characterized by a high saturation drift velocity at a junction, referred to as a "heterojunction", of the layers. The high mobility electrons accumulate in a narrow substantially triangular shaped potential well at the heterojunction and form a relatively thin, sheet-like electron concentration, referred to as a two dimensional electron gas (2DEG). Because of the geometrical construction and location of the 2DEG, electrons in the 2DEG generally evidence very low donor impurity scattering, and as a result, relatively high electron mobility. Electron concentrations in a 2DEG may be as high as 1 x 10¹³/cm² and mobilities that support saturation drift velocities as high as 1.5 x 10^ cm/s.

[0008] The high electron concentrations, mobilities, and saturation drift velocities enable the FETs and diodes to support and rapidly switch ON and OFF relatively large currents. Semiconductor devices such as FETs and diodes that use 2DEGs to carry current may be referred to as HEM (high electron mobility) devices.

SUMMARY

[0009] An aspect of an embodiment of the invention relates to providing a high electron mobility (HEM) semiconductor device having an anode structure that provides the device with improved operating parameters. In an embodiment of the invention, the device is a HEM diode for which currents between a cathode and an anode comprised in the diode are carried by at least one 2DEG.

[0010] In an embodiment of the invention, the anode comprises first and second metal components that form first and second Schottky junctions respectively with semiconductor material in the device. The first metal component is characterized by a work function φ\ that is greater than a work function ψ2 of the second metal component. An anode comprising first and second metal components having different work functions, in accordance with an embodiment of the invention, may be referred to hereinafter as a "compound anode".

[0011] The metal components in the compound anode may be spatially configured and seat in a recess of the diode so that for a condition in which there is substantially no voltage difference between the cathode and anode the diode is in an OFF state, and the first metal component of the anode generates a depletion region in the semiconductor material that enhances isolation of the second metal component from mobile electrons in the device. Optionally, the recess and/or structure of the anode is formed so that a portion of the anode overhangs a semiconductor region of the diode through which the at least one 2DEG extends and carries current between the cathode and anode when the diode is forward biased to turn ON the diode. A depletion region extending from the overhang operates to increase electrical isolation of the at least one 2DEG from the compound anode when the diode is OFF. As a result, in the OFF state, the at least one 2DEG that supports current through the diode in an ON state, is blocked, distanced from the anode, and substantially prevented from carrying leakage current through the diode.

[0012] Because φι > ( 2, a breakdown voltage at the Schottky junction between the first metal component of the anode and semiconductor material in the device is expected to be greater than that at the Schottky junction between the second metal component and the semiconductor material. However, the enhanced isolation of the second metal component provided by the first metal component and/or the overhang structure of the anode increases the breakdown voltage between the second metal component and the semiconductor material relative to the breakdown voltage that might be predicted from the work function ψ2 of the second metal component. The device therefore has an improved breakdown voltage, which is greater than might be expected from the work function c 2-

[0013] Whereas the high work function of the first metal component provides the device with a relatively high breakdown voltage, forward electric current flowing from the anode through the device is dominated by current from the relatively low work function ψ2 of the second metal component. As a result, the relatively low work function ψ2 that characterizes the second metal component provides the device with a relatively low positive forward bias voltage, hereinafter also referred to as a turn-on voltage, at which the diode turns on and conducts current. [0014] In an embodiment of the invention, the anode may be formed so that it is characterized by a single work function and the overhang structure of the anode enhances breakdown voltage and electrical isolation of the anode from the cathode when the diode is OFF.

[0015] There is therefore provided in accordance with an embodiment of the invention, a semiconductor device comprising: a stack having a plurality of semiconductor layers; a first conducting terminal having electrical contact with layers in the stack; and a second terminal comprising first and second metallic components that interface with layers in the stack at first and second Schottky junctions and have work functions φ \ and ψ2 respectively that satisfy a relationship φι > ψ2; wherein the metal components are spatially configured so that when the

Schottky junctions are back biased, a depletion region generated at the first Schottky junction enhances isolation of the second metal component from mobile current carriers in the device.

[0016] Optionally, the electrical contact of the first conducting terminal with the layers in the stack is substantially ohmic. Additionally or alternatively the stack comprises at least one heterojunction at which a 2DEG is formed. Optionally, the at least one he teroj unction comprises a plurality of heterojunctions.

[0017] In an embodiment of the invention, the plurality of semiconductor layers comprises III-V layers. In an embodiment of the invention, the device comprises a diode and the first terminal is a cathode of the diode and the second terminal is an anode of the diode.

[0018] There is further provided in accordance with an embodiment of the invention, a semiconductor device comprising: a stack having a plurality of semiconductor layers; a first conducting terminal having electrical contact with layers in the stack; and a second terminal comprising first and second metallic components that interface with layers in the stack at first and second Schottky junctions; wherein the metal components are spatially configured so that when the Schottky junctions are back biased, a depletion region generated at the first Schottky junction enhances isolation of the second metal component from mobile current carriers in the device.

[0019] Optionally, the stack comprises at least one heterojunction at which a 2DEG is formed.

Optionally, the at least one heterojunction comprises a plurality of heterojunctions. In an embodiment of the invention, the device comprises a diode and the first terminal is a cathode of the diode and the second terminal is an anode of the diode. [0020] In the discussion unless otherwise stated, adjectives, such as "substantially" and "about", modifying a condition or relationship characteristic of a feature or features of an embodiment of the invention, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended.

[0021] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF FIGURES

[0022] Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale.

[0023] Fig. 1 schematically shows a cross section view of a normally OFF HEM semiconductor diode comprising a compound anode, in accordance with an embodiment of the invention;

[0024] Fig. 2 schematically shows a cross section view of the diode shown in Fig. 1 when the diode is biased ON by a voltage, in accordance with an embodiment of the invention;

[0025] Fig. 3 shows a schematic curve of current verses bias voltage for the diode shown in Figs.

1 and 2, in accordance with an embodiment of the invention; and

[0026] Fig. 4 schematically shows a cross section view of another normally OFF HEM semiconductor diode, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0027] Fig. 1 schematically shows a cross section view of a HEM diode 20 comprising a stack 30 having a plurality of optionally III-V semiconductor layers, coupled to a cathode 21 and a compound anode 22, in accordance with an embodiment of the invention. In Fig. 1 , HEM 20 is assumed to have no voltage difference between cathode 21 and anode 22 and to be in an OFF state.

[0028] In an embodiment of the invention, stack 30 comprises a bottom, optionally high resistance substrate layer 31, on which overlaying III-V semiconductor layers 32-40 are epitaxially formed, optionally by a metal organic chemical vapor deposition (MOCVD) growth process. In some embodiments of the invention, the III-V layers are grown by a molecular beam epitaxy (MBE) growth process. Substrate 31 may comprise by way of example a single crystal Si, AI2O3 (Sapphire), A1N, or a single crystal polytype of SiC (silicon carbide, carborundum) such as 4H-SiC, 6H-SiC, or 3C-SiC.

[0029] In Figs. 1 and 2 composition of substrate layer 31 is assumed by way of example to be 4H-SiC, which is shown in parentheses in the substrate layer. A composition of each of semiconductor layers 32 - 40 in accordance with an embodiment of the invention is given in the following list, in which numerals labeling the layers in Figs. 1 and 2 are followed by their respective compositions in parentheses: 32 AlGaN; 33 (GaN); 34 (AlGaN); 35(GaN); 36 (InGaN); 37(GaN); 38 (A1N); 39(AlGaN); and 40 (GaN). The optional compositions given in the list are shown in Figs. 1 and 2 in parentheses in their respective layers or associated with the numerals labeling the layers.

[0030] Stack 30 has two heterojunctions 51 and 52, at which 2DEGs comprising high mobility electrons are formed. The 2DEGs located at heterojunctions 51 and 52 are schematically represented by dashed lines 61 and 62, respectively. Heterojunction 51 is an interface between narrow GaN layer 33 and wide band gap AlGaN layer 34 and 2DEG 61 is formed in narrow band gap GaN layer 33. Heterojunction 52 is an interface between narrow band gap GaN layer 37 and wide band gap A1N layer 38 and 2DEG 62 is formed in narrow band gap GaN layer 37. (In an embodiment of the invention A1N layer may be absent, and 2DEG 62 formed in narrow band gap GaN layer 37 at a heterojunction of the GaN layer and AlGaN layer 39.) Layers 33 and 37 may be referred to as 2DEG layers 33 and 37.

[0031] Compound anode 22 comprises first and second metal components 23 and 24 respectively. Metal component 23 has a work function 923 and metal component 24 has a work function 924. In an embodiment of the invention, the work functions satisfy a relationship 923 >

Φ24- Optionally, metal components 23 and 24 are electrically connected so that they form a substantially equipotential body. By way of example, first metal component 23 may be formed from nickel, Ni, having a work function equal to about 5.2 ev (electron volts) and second metal component 24 may be formed from silver, Ag, having a work function 4.7 ev.

[0032] Stack 30 is recessed to expose a surface 131 in GaN layer 37 and, optionally, a surface 132 in GaN layer 33. Metal component 23 optionally seats on surface 131. Metal component 24 optionally seats on surface 131 and on surface 132. Anode metal component 23 contacts layer 37 at surface 131 along an interface schematically represented by a bold line 71 below surface 131 and is formed having a Schottky junction with layer 37 at the interface. Anode metal component 24 contacts layer 37 at surface 131 along an interface schematically represented by a bold line 72 below surface 131 and is formed so that the metal component forms a Schottky junction with layer 40 at the interface. Anode component 23 also optionally forms a Schottky junction with layer 40 at an interface with the layer represented by a bold line 73 along a surface 133, and Schottky junctions at interfaces represented by a bold line 74 with layers 37-40 along a surface 134. Anode component 24 also forms Schottky junctions optionally with each of layers 33-37 along a surface 135 at interfaces represented by a bold line 75 of the anode component with the layers. Interfaces 71, 72 and 73 overhang regions of semiconductor layers in stack 30 located relatively near to and along surfaces 134 and 135. Depletion regions, which are substantially devoid of mobile electrons, extend from interfaces 71 and 72 and 73 into semiconductor layers of diode 20 that respectively contact metal components 23 and 24 at interfaces 74 and 75. A depletion region extending from interface 71 is schematically shown as a stippled area 81. A depletion region extending from interface 72 is schematically shown as a stippled area 82. A depletion region extending from interface 73 is schematically shown as a stippled area 83. Cathode 21 is formed having ohmic contacts with layers 33-40.

[0033] Because interfaces 71 and 72 of anode metal component 23 and anode metal component 24 with layer 37 lie over layers 33-36, depletion regions 81 and 82 extend from the interfaces into layers 33-36 and in particular into layers 33 and 37. Depletion regions 81 and 82 therefore overlay depletion regions (not indicated) that extend from interface 75 and enlarge and/or enhance a region of diode 20 that is substantially devoid of mobile electrons and extends from interface 75 of metal component 24 and into layers 33-37. In particular, depletion regions 81 and 82 increase distances of mobile electrons in 2DEG 61 in layer 33 beyond distances effected by the depletion region extending into layer 33 from the Schottky junction at interface 75, and 2DEG 61 is schematically shown extending from cathode 21 to, but not into, depletion region 81. Similarly because interface 73 of anode metal component 23 with layer 40 overlays GaN layer 37, depletion region 83 increases distances of mobile electrons in 2DEG 62 in layer 37 beyond distances effected by a depletion region extending into layer 37 from the Schottky junction at interface 74. 2DEG 62 is therefore schematically shown extending from cathode 21 to, but not into, depletion region 83.

[0034] As a result of the enlarged region of diode 20 that is substantially devoid of mobile electrons, at zero voltage between cathode 21 and compound anode 22, electrical insulation of double anode 22 from cathode 21 is enhanced and breakdown voltage increased and 2DEGs 61 and 62 are substantially blocked from carrying leakage current between the cathode and the anode. In addition, the back bias breakdown voltage of diode 20 is increased relative to a back bias breakdown voltage that would characterize the diode were anode 22 formed only from a metal from which metal component 24 is formed.

[0035] Breakdown voltage between a metal and a semiconductor at a Schottky junction and a voltage at which the Schottky junction conducts forward current generally increase with increasing work function of the metal. Were diode 20 to have an anode formed only from metal comprised in metal component 24, because work function ψ2 i^s l^{ess trian} work function 923, the diode would generally have a breakdown voltage less than that were the anode formed only from metal comprised in anode metal component 23. However, because work function 924 is less than work function 923, the diode would also generally have a lower turn-on voltage than were the anode formed only from metal comprised in anode component 23. And for a given forward current, the diode would have a smaller forward voltage drop than it would have were the anode formed only from metal in anode metal component 23.

[0036] Because, in accordance with an embodiment of the invention, metal component 23 of compound anode 22 generates depletion region 81 that increases isolation of metal component 24 of the anode, when diode 20 is back biased the diode 20 has a breakdown voltage greater than that which would be predicted from work function 924. In addition, when diode 20 is forward biased, the lower work function 924 of metal component 24 determines a lower resistance current path for current through the diode from cathode 21 to compound anode 22. Metal component 24 dominates current flow through diode 20 when the diode is forward biased and provides the diode with a lower turn-on voltage and lower forward voltage for a given current than the diode would be expected to have in view of work function 923.

[0037] Fig. 2 schematically shows diode 20 when a positive voltage forward biases the diode. In response to the forward voltage, depletion regions 81 and 82 substantially disappear, and 2DEGs 61 and 62 extend all the way from cathode 21 to compound anode 22 and carry current between the cathode and the anode. A greater portion of current that flows through diode 20 flows along a current path provided by 2DEGs 61 between cathode 21 and metal component 24 of compound anode 22 because metal component 24 has a lower work function than metal compound 23.

[0038] Fig. 3 shows schematic curves labeled "923", "Φ24 "_> ^and "diode 20", of current "I" through diode 20 verses voltage "V" applied to the diode were the diode to respectively have an anode formed only from metal having a work function 923, an anode formed only from a metal having work function 924, and a compound anode 22 as shown in Fig. 1 formed from metals having work functions 923 and 924. Respective breakdown voltages for the configurations of diode 20 corresponding to curves 923, c 24, and diode 20 are indicated by witness lines respectively labeled VB23, VB24, and VB20 Respective turn-on voltages for the configurations of diode 20 corresponding to curves 923, c 24, and diode 20 are indicated by witness lines labeled VO23, VO24, and VO20 respectively.

[0039] Fig. 4 schematically shows a cross section of another HEM diode 220, in accordance with an embodiment of the invention. Diode 220 is similar to diode 20, but instead of comprising a compound anode, such as anode 22 (Figs. 1 and 2), comprises a metal anode 222 having uniform composition. For example, anode 222 may be formed from Ni or from Ag.

[0040] In an embodiment of the invention, anode 222 has an external shape that is substantially the same as the external shape of compound anode 22 and seats on surfaces 131 and 132 formed in semiconductor layer stack 30. An overhang region 241 of anode 222 contacts surface 131 along an interface schematically represented by a bold line 242 below surface 131 and is formed so that the anode 222 forms a Schottky junction at the interface. Schottky junction 242 contributes to generating a relatively large depletion region 252 that provides diode 222 with a relatively large back bias breakdown voltage and low leakage currents. Similarly an overhang region 245 of anode 222 contacts surface 133 along an interface schematically represented by a bold line 246 below surface 133 and is formed so that anode 222 forms a Schottky junction at the interface. The Schottky junctions at interfaces 242 and 246 contribute to generating a relatively enhanced depletion region 253, schematically shown as a stippled region, which provides diode 222 with a relatively large back bias breakdown voltage and low leakage currents.

[0041] In the description and claims of the present application, each of the verbs, "comprise" "include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb.

[0042] Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.

Claims

1. A semiconductor device comprising:

a stack having a plurality of semiconductor layers;

a first conducting terminal having electrical contact with layers in the stack; and a second terminal comprising first and second metallic components that interface with layers in the stack at first and second Schottky junctions and have work functions φι and ψ2 respectively that satisfy a relationship φ\ >

wherein the metal components are spatially configured so that when the Schottky junctions are back biased, a depletion region generated at the first Schottky junction enhances isolation of the second metal component from mobile current carriers in the device.

2. A semiconductor device according to claim 1 wherein the electrical contact of the first conducting terminal with the layers in the stack is substantially ohmic.

3. A semiconductor device according to claim 1 or claim 2 wherein the stack comprises at least one heterojunction at which a 2DEG is formed.

4. A semiconductor device according to claim 3 wherein the at least one heterojunction comprises a plurality of heterojunctions.

5. A semiconductor device according to any of the preceding claims wherein the plurality of semiconductor layers comprises III-V layers.

6. A semiconductor device according to any of the preceding claims wherein the device comprises a diode and the first terminal is a cathode of the diode and the second terminal is an anode of the diode.

7. A semiconductor device comprising:

a stack having a plurality of semiconductor layers;

a first conducting terminal having electrical contact with layers in the stack; and a second terminal comprising first and second metallic components that interface with layers in the stack at first and second Schottky junctions; wherein the metal components are spatially configured so that when the Schottky junctions are back biased, a depletion region generated at the first Schottky junction enhances isolation of the second metal component from mobile current carriers in the device.

8. A semiconductor device according to claim 7 wherein the stack comprises at least one heterojunction at which a 2DEG is formed.

9. A semiconductor device according to claim 8 wherein the at least one heterojunction comprises a plurality of heterojunctions.

10. A semiconductor device according to any of claims 7-9 wherein the device comprises a diode and the first terminal is a cathode of the diode and the second terminal is an anode of the diode.