CN111863962B - A new type of AlGaN-based multi-channel field effect transistor - Google Patents

Abstract

The invention provides a novel AlGaN-based multi-channel field effect transistor, which comprises: the GaN-based solar cell comprises a substrate, an AlN buffer layer, a back barrier layer, a multiple quantum well structure, a GaN cap layer, a source electrode, a drain electrode and a gate electrode, wherein the multiple quantum well structure comprises an N-layer AlGaN channel layer with gradually changed aluminum components and an AlN quantum barrier layer arranged between every two AlGaN channel layers with gradually changed aluminum components, and N is more than or equal to 2. The invention utilizes polarization induced doping to realize the combination of high-density two-dimensional electron gas or two-dimensional hole gas at the heterojunction interface and three-dimensional electron gas or three-dimensional hole gas in the AlGaN channel layer with gradually changed aluminum components, thereby being beneficial to the high-frequency and heavy-current application of the device; the back potential barrier is utilized to realize more effective carrier limiting effect, effectively reduce the electric leakage of the device, and realize better off-state breakdown characteristic and current saturation under on-state large bias.

Description

A new type of AlGaN-based multi-channel field effect transistor

Technical Field

The invention relates to the technical field of semiconductor devices, and in particular to a novel AlGaN-based multi-channel field effect transistor.

Background Art

Field effect transistors (FETs) based on aluminum gallium nitride (AlGaN) alloys can usually be doped in two ways, namely impurity doping and polarization induced doping. Impurity doping of AlGaN materials generally uses silicon (Si) as a donor impurity to generate electrons through ionization; polarization induced doping is achieved by using the unique polarization characteristics of group III nitrides, using polarized charges to induce body region electrons to form a conductive channel.

The existing problems of doping technology and device structure are as follows: 1. In AlGaN materials with high aluminum components, the impurity doping efficiency is significantly reduced and the carrier concentration is limited. In addition, the carriers will be subject to greater impurity scattering, and the mobility will be significantly reduced; 2. Under high frequency and high voltage conditions, the concentration of two-dimensional electron gas (2DEG) at the interface of AlGaN/GaN heterojunction is too high, which suppresses electron mobility and saturation rate, that is, there is a compromise between carrier concentration and mobility. The decrease in electron mobility and saturation rate will cause a significant decrease in the transconductance of the transistor, resulting in nonlinear transmission characteristics, signal distortion and other problems; 3. For the problem described in 2, one solution is to use an AlGaN channel layer with a gradient composition, and improve the high-frequency linearity of the transistor through a three-dimensional electron gas or a three-dimensional hole gas formed by polarization-induced doping. However, the surface density of the three-dimensional electron gas or three-dimensional hole gas is small, the series resistance of the source and drain paths is large, and the alloy scattering in the channel is serious, the carrier mobility is limited, and the current density of the device is limited; 4. Most of the commonly used HEMT devices are based on the metal polarity AlGaN/GaN heterojunction structure. This type of device has the problem of high surface Al component and the 2DEG conductive channel is far away from the device surface, resulting in large contact resistance and series resistance, which inhibits the current density and power density of the device; 5. Usually, there is no special structural design under the conductive channel in AlGaN-based field effect transistors, and there is a common phenomenon of poor carrier confinement ability and large device leakage, which is not conducive to the off-state breakdown characteristics of the device and current saturation under large on-state bias.

Summary of the invention

1. Technical issues to be resolved

The above five problems exist in existing doping technology and device structure.

(II) Technical solution

In order to solve the above problems, the present invention provides a novel AlGaN-based multi-channel field effect transistor, the transistor comprising: a substrate 1; an AlN buffer layer 2 and a back barrier layer 3 sequentially arranged on the substrate 1; a multi-quantum well structure 4 arranged on the back barrier layer 3, the multi-quantum well structure 4 being N layers of AlGaN channel layers 401 with a gradient aluminum component and an AlN quantum barrier layer 402 arranged between every two layers of AlGaN channel layers 401 with a gradient aluminum component, N≥2; a GaN cap layer 5 arranged on the multi-quantum well structure; a source electrode 6 and a drain electrode 7 extending from both ends of the GaN cap layer 5 to a preset depth toward the multi-quantum well structure 4, and; a gate 8 arranged on the GaN cap layer 5 and between the source electrode 6 and the drain electrode 7.

Optionally, the aluminum component in the AlGaN aluminum component gradient doping layer 401 gradiently varies between k1 and k2 along the growth direction of the AlGaN channel layer 401 with the aluminum component gradient, wherein 0≤k1＜1, 0≤k2＜1, and there is no specific requirement for the relationship between k1 and k2, that is, the aluminum component can gradiently vary in an increasing manner or in a decreasing manner.

Optionally, the starting value and the ending value of the aluminum composition gradient in each AlGaN channel layer 401 with the aluminum composition gradient are the same.

Optionally, the starting value and the ending value of the aluminum composition gradient in each AlGaN channel layer 401 with the aluminum composition gradient are different.

Optionally, in all the AlGaN channel layers 401 with a gradient aluminum composition, when N>3, at least two of the AlGaN channel layers 401 with a gradient aluminum composition have the same starting value and the same ending value of the gradient aluminum composition.

Optionally, the AlGaN channel layer 401 with a graded aluminum composition in the multiple quantum well structure may also be a _{BxAlyGa1 -xyN graded} aluminum composition channel layer or _{an InzAlfGa1 -zfN} graded aluminum composition channel layer.

Optionally, the aluminum component in the _{BxAlyGa1 -xyN} aluminum component gradient layer or the _{InzAlfGa1 -zfN} aluminum component gradient layer gradually changes between k1 and k2 along the growth direction of _{the BxAlyGa1 -xyN} aluminum component gradient layer or _{the InzAlfGa1 -zfN} aluminum component gradient layer, where 0≤k1＜ ₁ , 0≤k2＜1, and there is no specific requirement for the relationship between k1 and k2, that _is , the aluminum component can gradually change in an increasing manner or in a decreasing manner.

Optionally, the back barrier layer 3 is one of Al _m Ga _1-m N, B _x Al _y Ga _1-xy N or In _z Al _f Ga _1-zf N, and the bandgap width of the back barrier layer 3 is greater than the bandgap width of the AlGaN alloy corresponding to the gradient starting point of the first layer of the AlGaN channel layer 401 with a gradient aluminum component in the multi-quantum well structure 4. For example, the aluminum component in the first layer (401) changes gradually from 0.1 to 0.2, and the gradient starting point is Al _0.1 Ga _0.9 N, then the bandgap width of the back barrier is greater than the bandgap width corresponding to Al _0.1 Ga _0.9 N.

Optionally, the substrate 1 has a thickness of 0.1-10 μm; the back barrier layer 3 has a thickness of 0.1-5 μm, the GaN cap layer 5 has a thickness of 1-5 nm; the AlGaN aluminum component graded doping layer 401 has a thickness of 1-100 nm, and the AlN quantum barrier layer (402) has a thickness of 1-10 nm.

Optionally, the source electrode 6 and the drain electrode 7 are made of a Ti/Al/Ni/Au ohmic contact metal stack layer, and the gate 8 is made of a Ni/Au metal stack layer.

(III) Beneficial effects

The present invention has at least the following beneficial effects:

The embodiment of the present invention utilizes the polarization characteristics of group III nitrides, and realizes a three-dimensional electron gas or three-dimensional hole gas conductive channel without an impurity doping source by regulating the gradient of the aluminum component in the AlGaN channel layer 401 with a gradient aluminum component. By regulating the doping curve of the three-dimensional carriers in the channel, the linearity of the transistor under high-frequency conditions can be effectively regulated, which is beneficial to reducing signal distortion and gain attenuation in high-frequency electronic circuits; in addition, at the heterojunction interface formed by the AlGaN channel layer with a gradient aluminum component and the AlN quantum barrier, there is a high-surface-density two-dimensional electron gas or two-dimensional hole gas formed by polarization-induced doping, which is beneficial to improving the output power density of the device. At the same time, combined with the special energy band structure of the AlGaN/AlN multiple quantum well, a more effective carrier confinement effect is achieved, the device leakage is effectively reduced, and better off-state breakdown characteristics and current saturation under a large bias voltage in the on-state can be achieved. The AlN quantum barrier as an insertion layer can reduce alloy scattering at the channel and further improve the carrier mobility. Therefore, the transistor with a multi-quantum well structure utilizes the polarization effect to simultaneously realize three-dimensional electron/hole gas and two-dimensional electron/hole gas conductive channels, combining good output power density and good high-frequency linearity; the transistor described in the present invention can be based on metal polarity or nitrogen polarity Group III nitrides, among which the N-type field effect transistor based on nitrogen polarity AlGaN material is conducive to forming an ohmic contact with low contact resistance due to the lower aluminum component content on the device surface, thereby improving the current density of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a novel AlGaN-based multi-channel field effect transistor provided by an embodiment of the present invention;

FIG2 is a schematic diagram of the principle of a multi-quantum well structure in a novel AlGaN-based multi-channel field effect transistor provided by an embodiment of the present invention;

FIG3A is a schematic diagram of an AlGaN channel layer with a gradient aluminum composition in a metal polarity N-type field effect transistor provided by an embodiment of the present invention; wherein the aluminum composition is incrementally gradient;

FIG3B is an energy band diagram of the metal polarity N-type field effect transistor provided in FIG3A of the present invention;

FIG3C is a diagram showing the electron concentration distribution of the metal polarity N-type field effect transistor provided in FIG3A of the present invention;

FIG4A is a schematic diagram of an AlGaN channel layer with a gradient aluminum component in a metal polarity P-type field effect transistor provided by another embodiment of the present invention; wherein the aluminum component is a decreasing gradient;

FIG4B is an energy band diagram of the metal polarity P-type field effect transistor provided in FIG4A of the present invention;

FIG4C is a diagram showing the electron concentration distribution of the metal polarity P-type field effect transistor provided in FIG4A of the present invention;

FIG5A is a schematic diagram of an AlGaN channel layer with a gradient aluminum component in a metal polarity N-type field effect transistor provided by another embodiment of the present invention; wherein the aluminum component is gradually increased;

FIG5B is an energy band diagram of the metal polarity N-type field effect transistor provided in FIG5A of the present invention;

FIG5C is a diagram showing the electron concentration distribution of the metal polarity N-type field effect transistor provided in FIG5A of the present invention;

FIG. 6 is a flow chart of a method for preparing a multi-quantum well structure provided in an embodiment of the present invention.

DETAILED DESCRIPTION

Below, embodiments of the present invention will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present invention. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of embodiments of the present invention. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of concepts of the present invention.

The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and/or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

An embodiment of the present invention provides a novel AlGaN-based multi-channel field effect transistor, referring to FIG1 , wherein the transistor comprises: a substrate 1; an AlN buffer layer 2 and a back barrier layer 3 sequentially arranged on the substrate 1; a multi-quantum well structure 4 arranged on the back barrier layer 3, wherein the multi-quantum well structure 4 is an N-layer AlGaN channel layer 401 with a gradient aluminum component and an AlN quantum barrier layer 402 arranged between every two layers of the AlGaN channel layer 401 with a gradient aluminum component, N≥2; a GaN cap layer 5 arranged on the multi-quantum well structure 4; a source electrode 6 and a drain electrode 7 extending from both ends of the GaN cap layer 5 to a preset depth toward the multi-quantum well structure 4, and; a gate 8 arranged on the GaN cap layer 5 and between the source electrode 6 and the drain electrode 7.

First of all, it should be noted that metal polarity and nitrogen polarity III-nitrides: Group III nitrides are composed of metal elements (Al, Ga, In) and nitrogen (N) elements, and exist stably in a wurtzite structure at room temperature. Taking GaN as an example, Ga atoms and N atoms are combined in a tetrahedral form, and at the same time form Ga atomic layers and N atomic layers arranged alternately along the c-axis. The different arrangements of the Ga-N double atomic layers determine the different polarities of the GaN material: if the Ga (metal) atomic layer is on top, the material is Ga polarity (metal polarity); if the N (nitrogen) atomic layer is on top, the material is N (nitrogen) polarity. The new AlGaN-based multi-channel field effect transistor provided by the present invention can be a field effect transistor based on metal polarity AlGaN material, or it can be a field effect transistor based on nitrogen polarity AlGaN material. By adjusting the aluminum component gradient in the AlGaN channel layer with a gradient aluminum component, through polarization-induced doping, a high-concentration three-dimensional electron gas or three-dimensional hole gas conductive channel can be formed without introducing doping impurities. Among them, the nitrogen-polarity-based N-type field effect transistor has a lower aluminum component in the device, which is conducive to forming a source-drain ohmic contact with low contact resistance, thereby improving the current density and power density of the device.

Therefore, in the embodiment of the present invention, epitaxially growing the AlN buffer layer 2 on the substrate 1 can ensure the crystal quality of the subsequent epitaxy; epitaxially growing an intrinsically doped back barrier layer on the AlN buffer layer 2 can be used to suppress leakage caused by electrons drifting toward the back barrier in the channel under high voltage. In the multi-quantum well structure 4, the AlGaN channel layer 401 with a gradient aluminum component serves as a quantum well, and the AlN quantum barrier layer 402 serves as a quantum barrier, so that the device has multiple AlGaN/AlN heterojunction structures. Referring to FIG. 2 , the III-group nitride material has spontaneous polarization and piezoelectric polarization. In the embodiment of the present invention, the lattice constants of AlN and GaN are respectively and By calculating the residual polarization charge at each heterojunction interface, it can be known that: (1) in a device based on metal polar AlGaN material, when the AlN quantum barrier is epitaxially grown on the AlGaN channel layer with a gradient aluminum composition, there is a polarization positive charge at the interface between the two; when the AlGaN channel layer with a gradient aluminum composition is epitaxially grown on the AlN quantum barrier, a polarization negative charge is generated at the interface between the two. (2) in a device based on nitrogen polar AlGaN material, when the AlN quantum barrier is epitaxially grown on the AlGaN channel layer with a gradient aluminum composition, there is a polarization negative charge at the interface; when the AlGaN channel layer with a gradient aluminum composition is epitaxially grown on the AlN quantum barrier, a polarization positive charge is generated at the interface. The polarization charge generated at the above heterojunction interface will cause the bending of the energy band and realize polarization-induced doping to form a conductive channel.

At the same time, the AlGaN channel layer with a gradient aluminum component can be equivalent to countless tiny mutant heterojunctions, each of which has a polarization electric field. Referring to Figure 3, in the AlGaN channel with a gradient aluminum component of metal polarity, the polarization charge expands in the three-dimensional direction, which can form a high concentration of fixed polarization positive charge in the entire gradient layer, and finally use the polarization induced effect to form a three-dimensional electron gas for conduction. Similarly, by adjusting the polarity (metal polarity, nitrogen polarity) and gradient (increasing, increasing) of the transistor, four types of three-dimensional carrier doping can be achieved: metal polarity three-dimensional electron gas doping; metal polarity three-dimensional hole gas doping; nitrogen polarity three-dimensional electron gas doping; nitrogen polarity three-dimensional hole gas doping.

It can be concluded that the embodiment of the present invention utilizes the polarization characteristics of group III nitrides to realize the combination of two-dimensional electrons or hole gases at the heterojunction and three-dimensional electron gases or three-dimensional hole gases in the AlGaN channel with a gradual aluminum component change. The concentration of the three-dimensional electron/hole gas is determined only by the gradual gradient of the aluminum component. This doping curve is conducive to the device to achieve constant transconductance, and from the perspective of device design, it realizes linear transmission characteristics at high frequencies, which is conducive to reducing gain attenuation and signal distortion in signal transmission; the two-dimensional electron/hole gas at the heterojunction interface has a higher carrier density and mobility, which is conducive to the output current and power density of the device; and the AlN quantum barrier can effectively reduce alloy scattering and improve carrier mobility; at the same time, combined with the special band structure of AlGaN/AlN multiple quantum wells, a more effective carrier confinement effect is achieved, which can effectively reduce device leakage, and can achieve better off-state breakdown characteristics of the device and current saturation under large on-state bias.

Among them, it should be noted that the polarization effect of the III-nitrides mentioned above (i.e., the polarization characteristics of the III-nitrides) is as follows: the centers of positive and negative charges in the c-axis direction of the wurtzite-structured III-nitrides do not coincide, resulting in a macroscopic polarization effect, which is called spontaneous polarization; at the same time, the pressure inside the material can also cause the separation of the positive and negative charge centers inside the III-nitride material, and this effect is called piezoelectric polarization. The polarization effect has an important influence on the performance of the III-nitride materials and devices.

In addition, the N layers of AlGaN channel layers 401 with a gradient aluminum composition and the AlN quantum barrier layers 402 arranged between every two layers of AlGaN channel layers 401 with a gradient aluminum composition, N≥2, mentioned above, refer to: a first layer of AlGaN channel layer 401 with a gradient aluminum composition is arranged on the back barrier layer 3, a first layer of AlN quantum barrier layer 402 is epitaxially grown on the first layer of AlGaN channel layer 401 with a gradient aluminum composition, and then a second layer of AlGaN channel layer 401 with a gradient aluminum composition is epitaxially grown on the first layer of AlN quantum barrier layer 402, and a second layer of AlN quantum barrier layer 402 is epitaxially grown on the second layer of AlGaN channel layer 401 with a gradient aluminum composition, and then by analogy, a third layer of AlGaN channel layer 401 with a gradient aluminum composition, a third layer of AlN quantum barrier layer 402, ..., an Nth layer of AlGaN channel layer 401 with a gradient aluminum composition is epitaxially grown on the second layer of AlN quantum barrier layer 402. Then, a GaN cap layer 5 can be disposed on the Nth AlGaN channel layer 401 with a graded aluminum composition.

In a feasible manner of the embodiment of the present invention, the aluminum component in the AlGaN channel layer 401 with a gradient aluminum component gradually changes between k1 and k2 along the growth direction of the AlGaN channel layer 401 with a gradient aluminum component, wherein 0≤k1＜1, 0≤k2＜1. The gradient specifically means that the value of x in the molecular formula _{AlxGa1 -xN} can gradually change. For example, the Al component x in the AlGaN film increases linearly from k1＝0.1 to k2＝0.3, and x decreases linearly from k1＝0.6 to k2＝0.2, etc. The value of x is any number from 0 to 1, for example, it can also be a gradient from Al _0.001 Ga _0.999 N to Al _0.231 Ga _0.769 N, etc. It should be noted that, under normal circumstances, the growth direction of the AlGaN conductive layer 401 with a gradient aluminum component is from bottom to top.

It should be noted that, for field effect transistors based on metal polarity and nitrogen polarity AlGaN materials, the AlGaN channel layer 401 with a gradient aluminum composition can have the following growth and corresponding doping schemes:

(1) Metal polarity:

The aluminum component in the AlGaN channel layer 401 with a gradient aluminum component gradually increases along the growth direction of the AlGaN channel layer 401 with a gradient aluminum component, forming a three-dimensional electron gas channel, and the transistor is of N-type. For example: first, epitaxially grow graded layer 1, and the Al component gradually changes from _a1 to _a2 , that is, Al _a1 Ga _1-a1 N-Al _a2 Ga _1-a2 N ( _a1 ＜ _a2 ＜1); epitaxially grow the first AlN quantum barrier on graded layer 1; then, epitaxially grow graded layer 2, and the Al component gradually changes from _b1 to _b2 , that is, Al _b1 Ga _1-b1 N-Al _b2 Ga _1-b2 N ( _b1 ＜ _b2 ＜1); epitaxially grow the second AlN quantum barrier on the epitaxial layer; and so on, until the last epitaxial graded layer n (n≥2) is grown, and the Al component gradually changes from _n1 to _n2 , that is, Al _n1 Ga _1-n1 N-Al _n2 Ga _1-n2 N ( _n1 ＜ _n2 ＜1); epitaxially grow a GaN cap layer on graded layer n to increase the effective barrier height on the device surface.

The aluminum component in the AlGaN channel layer 401 with a gradient composition gradually decreases along the growth direction of the AlGaN channel layer 401 with a gradient composition, forming a three-dimensional hole gas channel, and the transistor is of P type. For example: first, epitaxially grow graded layer 1, and the Al component gradually changes from _a1 to _a2 , that is, Al _a1 Ga _1-a1 N-Al _a2 Ga _1-a2 N (1＞ _a1 ＞ _a2 ); epitaxially grow the first AlN quantum barrier on graded layer 1; then, epitaxially grow graded layer 2, and the Al component gradually changes from _b1 to _b2 , that is, Al _b1 Ga _1-b1 N-Al _b2 Ga _1-b2 N (1＞ _b1 ＞ _b2 ); epitaxially grow the second AlN quantum barrier on the epitaxial layer; and so on, until the last epitaxial graded layer n (n≥2) is grown, and the Al component gradually changes from _n1 to _n2 , that is, Al _n1 Ga _1-n1 N-Al _n2 Ga _1-n2 N (1＞ _n1 ＞ _n2 ); epitaxially grow a GaN cap layer on graded layer n to increase the effective barrier height on the device surface.

(2) Nitrogen polarity: the aluminum component in the AlGaN aluminum component graded doping layer 401 gradually decreases along the growth direction of the AlGaN aluminum component graded doping layer 401, forming a three-dimensional electron gas channel, and the transistor is N-type. For example: first, epitaxially grow graded layer 1, and the Al component gradually decreases from _a1 to _a2 along the growth direction, that is, _{AlalGa1 -a1N} - _{Ala2Ga1 -} a2N( _a1 ＞ _a2 ＞1); epitaxially grow the first AlN quantum barrier on graded layer 1; then, epitaxially grow graded layer 2, and the Al component gradually decreases from _b1 to _b2 along the growth direction, that is, _{Alb1Ga1 -b1N} - _{Alb2Ga1 -b2N} ( _b1 ＞ _b2 ＞1); epitaxially grow the second AlN quantum barrier on the epitaxial layer; and so on, until the last epitaxial graded layer n(n≥2), the Al component gradually decreases from _n1 to _n2 along the growth direction, that is, _{AlnlGa1 -n1N} - _{Aln2Ga1 -n2N} ( _n1 ＞ _n2 >1); a GaN cap layer is epitaxially grown on the graded layer n to increase the effective barrier height on the device surface.

The aluminum component in the AlGaN channel layer 401 with a gradient aluminum component gradually increases along the growth direction of the AlGaN channel layer 401 with a gradient aluminum component, forming a three-dimensional electron gas channel, and the transistor is of P type. For example: first, epitaxially grow graded layer 1, and the Al component gradually changes from _a1 to _a2 , that is, Al _a1 Ga _1-a1 N-Al _a2 Ga _1-a2 N ( _a1 ＜ _a2 ＜1); epitaxially grow the first AlN quantum barrier on graded layer 1; then, epitaxially grow graded layer 2, and the Al component gradually changes from _b1 to _b2 , that is, Al _b1 Ga _1-b1 N-Al _b2 Ga _1-b2 N ( _b1 ＜ _b2 ＜1); epitaxially grow the second AlN quantum barrier on the epitaxial layer; and so on, until the last epitaxial graded layer n (n≥2) is grown, and the Al component gradually changes from _n1 to _n2 , that is, Al _n1 Ga _1-n1 N-Al _n2 Ga _1-n2 N ( _n1 ＜ _n2 ＜1); epitaxially grow a GaN cap layer on graded layer n to increase the effective barrier height on the device surface.

In a feasible manner of the embodiment of the present invention, the starting value of the aluminum component gradient in each layer of the AlGaN aluminum component gradient doping layer 401 is the same, and the ending value is the same. For example, there are 3 layers of AlGaN aluminum component gradient doping layers, and the first layer of AlGaN aluminum component gradient doping layer, the second layer of AlGaN aluminum component gradient doping layer, and the third layer of AlGaN aluminum component gradient doping layer are all AlGaN film Al component r linearly increases from 0.1 to 0.3 or r linearly decreases from 0.6 to 0.2.

In a feasible manner of the embodiment of the present invention, the starting value and the ending value of the aluminum composition gradient in each AlGaN channel layer 401 with the aluminum composition gradient are different. For example, there are three AlGaN aluminum composition gradient doping layers, the Al composition in the first AlGaN channel layer with aluminum composition gradient changes from _a1 to _a2 , that is, Al _a1 Ga _1-a1 N changes gradually to Al _a2 Ga _1-a2 N ( _0≤a1 ＜ _a2 ＜1), the Al composition in the second AlGaN channel layer with aluminum composition gradient changes gradually from _b1 to _b2 , that is, Al _b1 Ga _1-b1 N changes gradually to Al _b2 Ga _1-b2 N ( _0≤b1 ＜ _b2 ＜1); and the Al composition in the third AlGaN channel layer with aluminum composition gradient changes gradually from _n1 to _n2 , that is, Al _n1 Ga _1-nl N-Al _n2 Ga _1-n2 N ( _0≤n1 ＜ _n2 ＜1). At this time, _a1 , _b1 and _n1 are not equal, and _a2 , _b2 and _n2 are not equal. Here, the linear increase of Al component is taken as an example for explanation, and those skilled in the art can understand that the Al component can also decrease.

In another feasible mode of the present invention, when N>3, among all the AlGaN channel layers 401 with aluminum component gradient, at least two layers of the AlGaN channel layers 401 with aluminum component gradient have the same starting value and the same ending value of the aluminum component gradient. That is, the starting value and the ending value of the gradient of some layers can be equal, and those of the remaining layers can be different.

In addition, the aluminum composition gradient channel layer 401 in the multi-quantum well structure 4 may also be a _{BxAlyGa1 -xyN aluminum} composition gradient channel layer or an _{InzAlfGa1 -zfN} aluminum composition gradient channel layer. The aluminum composition in _{the BxAlyGa1 -xyN} aluminum composition gradient doping layer or _{the InzAlfGa1 -} zfN aluminum composition gradient doping layer varies gradually between k1 and k2 along the thickness of the _{BxAlyGa1 - xyN} aluminum composition gradient channel layer or _{the InzAlfGa1 -zfN aluminum} composition gradient channel layer, wherein 0≤k1＜1, 0≤k2＜1, and there is no specific requirement for the relationship between k1 and k2, that is, the aluminum composition may vary gradually in increasing order or in decreasing order.

The back barrier layer 3 is one of Al _m Ga _1-m N, B _x Al _y Ga _1-xy N or In _z Al _f Ga _1-zf N, wherein 0≤m＜1, 0≤x＜1, 0≤y＜1, 0≤z＜1, 0≤f＜1. Here, in order to achieve sufficient carrier confinement and prevent leakage current increase and device breakdown under high voltage working conditions, it is necessary to ensure that the bandgap width of the back barrier layer 3 is greater than the bandgap width of the alloy corresponding to the gradient starting point of the first aluminum component gradient channel layer 401 in the multi-quantum well structure 4. For example, the aluminum component in the first layer (401) gradually changes from 0.1 to 0.2, and the gradient starting point is Al _0.1 Ga _0.9 N, then the bandgap width of the back barrier is greater than the bandgap width corresponding to Al _0.1 Ga _0.9 N.

Please refer to FIGS. 3A-3C , 4A-4C , and 5A-5C , which respectively provide energy band diagrams and carrier doping distributions of field effect transistors containing multi-quantum well structures described in three patents as typical examples.

FIG3A is an N-type field effect transistor based on metal polarity AlGaN material, in which the aluminum component in the AlGaN channel layer 401 with a gradient aluminum component in the channel increases linearly along the growth direction, and the transistor comprises three AlGaN channel layers 401 with a gradient aluminum component and two AlN quantum barriers 402. Its energy band diagram is shown in FIG3B , where it can be seen that a very narrow conductive channel is formed in the heterojunction interface formed by the two AlGaN channel layers 401 with a gradient aluminum component and the AlN quantum barriers 402; at the same time, a gentle conduction band below the Fermi level is formed in the AlGaN channel layer 1 with a gradient aluminum component. The electron doping distribution of the transistor structure is shown in Figure 3C. In the heterojunction interface formed by the two AlGaN channel layers 401 with gradient aluminum composition and the AlN quantum barrier 402, there are high-density two-dimensional electron gases with electron concentrations of 2.9×10 ¹⁹ cm ^-3 and 4.3×10 ¹⁹ cm ^-3 , respectively. At the same time, a three-dimensional electron gas conductive channel with a density of about 1×10 ¹⁹ cm ^-3 is formed in the AlGaN channel layer 1 with gradient composition.

FIG4A is a P-type field effect transistor based on metal polarity AlGaN material, in which the aluminum component in the AlGaN channel layer 401 with a gradient aluminum component in the channel decreases linearly along the growth direction, and the transistor comprises three AlGaN channel layers 401 with a gradient aluminum component and two AlN quantum barriers 402. Its energy band diagram is shown in FIG4B , where it can be seen that a very narrow conductive channel is formed in the heterojunction interface formed by the two AlGaN channel layers 401 with a gradient aluminum component and the AlN quantum barriers 402; at the same time, a flat valence band above the Fermi level is formed in the AlGaN channel layer 1 with a gradient aluminum component. The electron doping distribution of the transistor structure is shown in Figure 4C. In the heterojunction interface formed by the two AlGaN channel layers 401 with gradient aluminum composition and the AlN quantum barrier 402, there are high-density two-dimensional hole gases with hole concentrations of 2.3×10 ¹⁹ cm ^-3 and 5.6×10 ¹⁹ cm ^-3 , respectively. At the same time, a three-dimensional hole gas conductive channel with a density of about 1×10 ¹⁹ cm ^-3 is formed in the AlGaN channel layer 1 with gradient composition.

FIG5A is an N-type field effect transistor based on metal polarity AlGaN material, in which the aluminum component in the AlGaN channel layer 401 with a gradient aluminum component in the channel decreases linearly along the growth direction, and the transistor includes two AlGaN channel layers 401 with a gradient aluminum component and an AlN quantum barrier 402. Its energy band diagram is shown in FIG5B , where it can be seen that a very narrow conductive channel is formed in the heterojunction interface formed by the AlGaN channel layer 401 with a gradient aluminum component and the AlN quantum barrier 402; at the same time, a gentle conduction band below the Fermi level is formed in both the AlGaN channel layers 1 and 2 with a gradient aluminum component. The electron doping distribution of the transistor structure is shown in FIG5C . In the heterojunction interface formed by the AlGaN channel layer with a gradient aluminum composition and the AlN quantum barrier 402, there is a high-density two-dimensional electron gas with an electron concentration of 4×10 ¹⁹ cm ^-3 . At the same time, a three-dimensional hole gas conductive channel with a density of about 1×10 ¹⁹ cm ^-3 is formed in the AlGaN channel layer 1 with a gradient composition, and an electron doping with a density of about 4×10 ¹⁹ cm ^-3 is formed in the AlGaN channel layer 2 with a gradient composition. The electron doping shape formed by the AlGaN channel layer 2 with a gradient composition is similar to the delta doping technology in impurity doping, which is a feasible solution to improve the high-frequency linearity of the transistor.

The substrate 1 is a sapphire substrate, and the thickness of the substrate 1 is 0.1-10 μm, preferably 2 μm; the thickness of the back barrier layer 3 is 0.1-5 μm, preferably 3 μm; the thickness of the GaN cap layer 5 is 1-5 nm; the thickness of the aluminum composition gradient channel layer 401 is 1-100 nm, and the thickness of the AlN quantum barrier layer 402 is 1-10 nm.

The source electrode 6 and the drain electrode 7 are made of Ti/Al/Ni/Au ohmic contact metal stacking layers, and the gate 8 is made of Ni/Au metal stacking layers. Before preparing the source and drain electrodes, the electrode pattern area can be etched, and the etching depth is not specifically limited in the examples of the present invention. The length and width dimensions of the gate, source and drain electrodes, as well as the distances between the three electrodes are not specifically limited in the examples of the present invention.

In addition, the source electrode 6 and the drain electrode 7 described above extend from both ends of the GaN cap layer 5 to the multi-quantum well structure 4 at a preset depth. The embodiment of the present invention does not impose any specific limitation on the preset depth, which may extend to part of the multi-quantum well structure 4, for example, to the second aluminum composition gradient channel layer 401, or to the entire multi-quantum well structure 4.

Please refer to Figure 3. The preparation method of the field effect transistor in the embodiment of the present invention can be: step S1, growing an AlN buffer layer on a sapphire substrate, step S2, epitaxially growing a back barrier layer on the AlN buffer layer, step S3, alternately growing an aluminum component gradient channel layer 401 and an AlN quantum barrier layer 402 on the back barrier layer N times to obtain a multi-quantum well structure, step S4, epitaxially growing a GaN cap layer on the multi-quantum well structure, and step S5, preparing a source electrode, a drain electrode and a gate 8 on the product obtained by steps S1-S4.

The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A novel AlGaN-based multi-channel field effect transistor, characterized in that the transistor comprises: Substrate (1); An AlN buffer layer (2) and a back barrier layer (3) are sequentially arranged on the substrate (1); A multi-quantum well structure (4) disposed on the back barrier layer (3), the multi-quantum well structure (4) comprising N layers of AlGaN channel layers (401) with a gradient aluminum component and an AlN quantum barrier layer (402) disposed between every two layers of AlGaN channel layers (401) with a gradient aluminum component, N≥2; A GaN cap layer (5) disposed on the multi-quantum well structure (4); A source electrode (6) and a drain electrode (7) are respectively extended to a preset depth from both ends of the GaN cap layer (5) toward the multi-quantum well structure (4), and; A gate electrode (8) is disposed on the GaN cap layer (5) and between the source electrode (6) and the drain electrode (7). 2. The field effect transistor according to claim 1 is characterized in that the aluminum component in the AlGaN channel layer (401) with a gradient aluminum component gradually changes between k1 and k2 along the growth direction of the AlGaN channel layer (401) with a gradient aluminum component, wherein 0≤k1＜1, 0≤k2＜1. 3. The field effect transistor according to claim 2, characterized in that the starting value and the ending value of the aluminum composition gradient in each AlGaN channel layer (401) with the aluminum composition gradient are the same. 4. The field effect transistor according to claim 2, characterized in that the starting value and the ending value of the aluminum composition gradient in each AlGaN channel layer (401) with the aluminum composition gradient are different. 5. The field effect transistor according to claim 2, characterized in that when N>3, among all the AlGaN channel layers (401) with aluminum composition gradient, at least two layers of the AlGaN channel layers (401) with aluminum composition gradient have the same starting value and the same ending value of the aluminum composition gradient. 6. The field effect transistor according to claim 1, characterized in that the AlGaN channel layer (401) with aluminum composition gradient in the multi-quantum well structure (4) can also be a _{BxAlyGa1 -xyN aluminum} composition gradient channel layer or _{an InzAlfGa1 -zfN} aluminum composition gradient channel layer. 7. The field effect transistor according to claim 6, characterized in that the aluminum component in _{the BxAlyGa1 -xyN} aluminum composition gradient layer or _{the InzAlfGa1 -zfN} aluminum composition gradient layer gradually changes between k1 and k2 along the growth direction of the _{BxAlyGa1 -xyN} aluminum composition gradient layer or _{the InzAlfGa1 -zfN} aluminum composition gradient layer, wherein 0≤k1＜1 _, 0≤k2＜1. 8. The field effect transistor according to claim 2 _or 7, characterized in that the back barrier layer (3) is one of _{AlmGa1 -mN} , _{BxAlyGa1 -xyN} or _{InzAlfGa1 -zfN} , _and the bandgap width of the back barrier layer (3) is greater than the bandgap width of the AlGaN alloy corresponding to the gradient starting point of the first AlGaN channel layer (401) with gradient aluminum composition in the multi-quantum well structure (4). 9. The field effect transistor according to claim 1, characterized in that the thickness of the substrate (1) is 0.1 to 10 μm; the thickness of the back barrier layer (3) is 0.1 to 5 μm, the thickness of the GaN cap layer (5) is 1 to 5 nm; the thickness of the AlGaN aluminum component graded doping layer (401) is 1 to 100 nm, and the thickness of the AlN quantum barrier layer (402) is 1 to 10 nm. 10. The field effect transistor according to claim 1, characterized in that the source electrode (6) and the drain electrode (7) are made of Ti/Al/Ni/Au ohmic contact metal stack layers, and the gate (8) is made of Ni/Au metal stack layers.