RU2539754C1 - Modulation-doped field-effect transistor - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to electronic engineering. A modulation-doped field-effect transistor comprises a flange, a pedestal, a heteroepitaxial structure, a buffer layer, a source, a gate, a drain and ohmic contacts. The pedestal is made of a heat-conducting layer of polycrystalline diamond. A transistor chip is placed on top of the pedestal, said chip comprising series-arranged base substrate made of GaAs, buffer layers, heteroepitaxial heterostructure based on AlGaAs/GaAs, and on the surface of the heteroepitaxial structure, between the source, the gate and the drain, there is a dielectric coating layer and two buffer layers arranged in series, said layers being made of hafnium dioxide and a metal oxide, wherein the buffer layers have total thickness of 1.0-4.0 nm; in the gate region, the buffer layers are placed under the gate, directly on an epitaxial structure in the form of a gradient layer of n-type GaAs.

EFFECT: reduced effect of DX centres on device characteristics, high mobility of majority charge carriers, providing minimum leakage of gate current, improved heat removal from the pedestal, achieving the lowest noise coefficient in the GHz range, high efficiency and reliability of high-power field-effect transistors.

8 cl, 3 tbl

Description

The invention relates to electronic equipment, namely to semiconductor devices designed to amplify microwave electromagnetic waves.

Heterostructured field-effect transistors with modulated alloying (PTML, MODFET) based on compounds of semiconductor materials of groups A ^III B ^V are currently the fastest field-effect transistors, allowing at the same time to achieve the lowest noise coefficients in the GHz frequency range. High speed is achieved due to the effect of an increase in the drift velocity of electrons forming a two-dimensional electron gas at the interface of a modulated-doped heterostructure (MLHS).

From the "prior art" known microwave field-effect transistor containing a substrate on which is formed a buffer layer of wide-gap semiconductor, which is an active layer of narrow-gap semiconductor with electrodes of the source, drain and gate. In addition, the active layer under the gate electrode is made unevenly doped. In this case, the concentration of the dopant in the direction of the source-drain electrode increases monotonically from a value corresponding to the concentration of residual impurities to a value corresponding to the concentration of impurities in the buffer layer, and the concentration of impurities in the buffer layer is 4-5 orders of magnitude higher than the concentration of residual impurities in the active layer (see A.S. USSR No. 1118245, publ. 06/19/1995).

The disadvantages of the known device are the low value of the microwave power, the low value of the heat sink from the active part of the transistor and the presence of low-frequency noise. In addition, a field-effect transistor based on gallium and aluminum nitrides is known, the structure of which consistently includes: a substrate, a GaN layer, a barrier layer made of two sublayers: Al _0.2 Ga _0.8 N, GaN on it. The contacts are made on the structure: drain, source, and gate with corresponding gaps between them; then a dielectric coating is made of MgO, Sc ₂ O ₃ or SiN _x . Between the contacts, the dielectric coating is located on the barrier layer and serves to protect the exposed surfaces of the barrier layer from external influences, see B. Luo et al. The role of cleaning conditions and epitaxial layer structure on reliability of Sc ₂ O ₃ and MgO passivation on AlGaN / GaN HEMTS, Solid-State Electronics, 46, pp. 2185-2190, 2002.

The disadvantages of the known device are the low mobility of the main charge carriers, a high level of degradation due to the low value of the heat sink from the active part of the transistor.

The objective of the present invention is to remedy all of the above disadvantages.

The technical result consists in reducing the influence of DX centers on instrument characteristics, in increasing the mobility of the main charge carriers, in ensuring minimal shutter current leakage, in increasing the heat sink from the pedestal, in achieving the lowest noise figure in the GHz frequency range, and also in increasing the efficiency and reliability powerful field effect transistors.

The technical result is ensured by the fact that the modulated-doped field-effect transistor contains a flange, a pedestal, a heteroepitaxial structure, a buffer layer, a source, a gate, a drain, and ohmic contacts. The pedestal is made of a thermally conductive CVD layer of polycrystalline diamond. A transistor crystal is placed on top of the pedestal, containing sequentially placed GaAs base substrate, buffer layers, AlGaAs / GaAs heteroepitaxial heterostructure, and on the surface of the heteroepitaxial structure, between the source, gate, and drain, a dielectric coating layer and two barrier layers made of hafnium dioxide and metal oxide, while the barrier layers are made with a total thickness of 1.0-4.0 nm, in addition, in the gate region, the barrier layers are placed under the gate, directly on the epitaxial structure of a graded layer of GaAs n-type conductivity.

In accordance with special cases, the transistor has the following design features.

The dielectric coating may be made of SiO ₃ , or of silicon nitride, or of MgO, or of Sc ₂ O ₃ .

The second barrier layer may be made of ZrO ₂ , or of La ₂ O ₃ , or of Y ₂ O ₃ .

The essence of the present invention is illustrated by the following illustrations:

figure 1 displays the crystal of the transistor;

figure 2 is a schematic illustration of the energy zones of a modulated-doped heterojunction n-AlGaAs / GaAs;

figure 3 shows the dependence of the proportion of DX centers in the total number of donors introduced on the doping level of the AlXGa _1-X As: Si layer in AlGaAs / GaAs MLHS;

figure 4 shows the current-voltage characteristics of a powerful microwave transistor without additional layers on the crystal surface of the transistor;

figure 5 shows the current-voltage characteristics of a powerful microwave transistor with additional layers.

Figure 1 displays the following structural elements:

1 - base substrate of semi-insulating GaAs;

2 - the first buffer layer;

3 - second buffer layer of GaAs;

4 - superlattice of Al _X Ga _1-X As / GaAs;

5 - the third buffer layer;

6 - heavily doped n-Al _X Ga _1-X As;

7 - spacer Al _X Ga _1-X As;

8 - smoothing layer;

9 - channel GaAs;

10 - smoothing layer;

11 - spacer Al _X Ga _1-X As;

12 - heavily doped n-type layer;

13 - a barrier layer;

14 —n-GaAs barrier layer;

15 - gradient layer;

16 - contact layer;

17 - source;

18 - shutter;

19 - stock;

20 - ohmic contacts;

21 - a dielectric coating of SiO ₂ / Si ₃ N ₄ with a thickness of 1.6 μm (1.5 μm and 0.1 μm, respectively);

22 - additional barrier layer of hafnium dioxide;

23 - an additional barrier layer of metal dioxide.

The transistor crystal is mounted on an AuGe sublayer, which is placed on a pedestal of a heat-conducting CVD layer of polydiamond, annealed after implantation of Ni into its surface layers. An AuSn solder layer is placed on the other side of the pedestal. Then the whole structure is placed on the flange.

The microwave transistor is based on the AlGaAs / GaAs epitaxial heterostructure. The design of the heterostructure is shown in table No. 1, and table No. 2 presents the main electrophysical parameters of the heterostructures.

Table 1 Layer of transistor heterostructure, Purpose Composition x, y Thickness Doping level cm-3 semi-insulating GaAs base 625 ± 25 μm first buffer layer 200 nm Unalloyed second GaAs buffer layer 200 nm Unalloyed superlattice from Al _X Ga _1-X As / GaAs 0.22 / 0.00 (2 nm / 2 nm) × 10 Unalloyed third buffer layer 0.22 100 nm Unalloyed heavily doped n-Al _X Ga _1-X As 0.22 4.5 nm 3 × 10 ¹⁸ spacer Al _X Ga _1-X As 0.22 2 nm Unalloyed smoothing layer 3 nm Unalloyed GaAs channel 0.16-0.18 12 nm Unalloyed smoothing layer 1.5 nm Unalloyed spacer Al _X Ga _1-X As 0.22 2 nm Unalloyed heavily doped n-type layer 0.22 16 nm 3 × 10 ¹⁸ n-type barrier layer 0.22 13 nm 5 × 10 ¹⁶ n-GaAs barrier layer 15 nm 5 × 10 ¹⁶ n-type gradient layer 20 nm 5 × 10 ¹⁶ -4 × 10 ¹⁸ n-type contact layer 50 nm 4 × 10 ¹⁸

Table 2 presents the main electrophysical parameters of heterostructures.

table 2 Temperature, K Layer concentration of charge carriers in the channel not less than, cm ^-2 The mobility of charge carriers in the channel is not less than cm ² / (V * s) 77 3.0 × 10 ¹² 1.3 * 10 ⁴ 300 3.0 × 10 ¹² 5.0 * 10 ³

Table 3 shows the main electrical parameters of transistors.

Table 3 Name of parameter, unit of measure Measurement mode Measurement result Output power P _OUT , W, not less f = 4 GHz, fifteen P _BX ≤1,6 W Power gain K _ur , dB, not less f = 4 GHz, 9 P _BX ≤1,6 W Coefficient, power gain in the linear region of the amplitude characteristic. To _{ur. L} , dB, not less f = 4 GHz 10 Coefficient of performance, η%,%, not less f = 4 Hz, P _BX ≤1,6 W twenty Gate leakage current I _{Z. UT} , mA, no more U _ZI = -3 V 0.4

The fabrication of multilayer nanoheterostructures of AlGaAs / GaAs solid solutions was carried out by molecular beam epitaxy.

The present device is manufactured as follows.

A solder layer of AuSn composition with a thickness of 25 μm is placed on a flange with a thickness of 1600 μm, then a layer of heat-conducting CVD polycrystalline diamond ~ 150-170 μm thick, prepared as a pedestal, is placed in both surface regions of which nickel is preliminarily implanted and annealed. Then, after the polycrystalline diamond is placed on the surface of the heat-conducting CVD layer of CVD, an AuGe sublayer with a Ge content of up to 12% and a thickness of ~ 25 μm is placed. Then, a transistor crystal is mounted on the surface of the AuGe sublayer (Fig. 1). The transistor crystal contains a sequentially placed base substrate 1, consisting of semi-insulating GaAs, 625 μm thick, the first undoped buffer layer 2, 200 nm thick, the second undoped buffer layer 3 of GaAs, 200 nm thick, 4 Al _X Gai _1-X As / superlattice GaAs undoped, thickness (2 nm / 2 nm) * 10, third undoped buffer layer 5, 100 nm thick, heavily doped layer 6 Al _X Ga _1-X As n-type, concentration 3 * 10 cm ^-3 , thickness 4.5 nm, spacer 7 Al _x Ga _1-X As, undoped, 2 nm thick, undoped smoothing layer 8, 3 nm thick, channel 9 of Ga As, undoped, 12 nm thick, undoped smoothing layer 10, 15 nm thick, spacer 11 Al _x Ga _1-X As, undoped, 2 nm thick, heavily doped layer 12 of Al _X Ga _1-X As, 16 nm thick, barrier layer 13, 13 nm thick, n-type GaAs barrier layer 14, 15 nm thick, n-type GaAs gradient layer 15, 20 nm thick, low-resistance local n-type contact layer 16, 50 nm thick, source 17, gate 18, drain 19, ohmic contacts 20. In addition, the device is provided with additional layers located between source 17, gate 18 and drain 19. Additional layers of Nena a dielectric coating (e.g., SiO ₂ / Si3N4, 1,6 microns) 21, additional barrier layers of hafnium dioxide 22 and the metal oxide 23, as which may be used Al ₂ O _3, or ZrO _2, or La ₂ O ₃ , or Y ₂ O ₃ . In this case, the barrier layers 22 and 23 are made with a total thickness of about 4.0 nm. In the region of the gate 18, the barrier layers 22 and 23 are located under the gate 18, directly on the epitaxial structure in the form of a gradient layer 15 of n-type GaAs conductivity.

The modulated doping technology, first used in GaAs / AlGaAs: Si superlattices, limits the region of impurity introduction by wide-gap material (AlGaAs), while the narrow-gap material (GaAs) adjacent to it remains unalloyed. The donor level in AlGaAs lies much higher than the bottom of the GaAs conduction band, which leads to the transfer of part of the electrons to the narrow-gap material. The created built-in field terminates this process and also leads to strong bending of the zones. As a result, a quasitriangular potential well localizing electrons is formed at the heterointerface (Fig. 3)

The mobility in MLSs is mainly limited by scattering by phonons at high (≥150 K) temperatures and by the Coulomb potential of ionized donors in a wide-gap barrier at low (≤100 K) temperatures. The introduction of an unalloyed AlGaAs layer, a spacer, between the doping region and the channel makes it possible to reduce the influence of the last of these factors. Thanks to the further separation of electrons and donors, the mobility increases even more. At present, mobilities of more than 100,000 cm ² / Bc at 77 K and more than 1,000,000 cm ² / Vs at liquid helium temperature have been achieved. At room temperature, typical values are 6000-8000 cm ² / Sun.

At present, HEMTs allow one to obtain the smallest noise figure at the highest concomitant gain, as well as the highest output power and maximum efficiency, in cm and mm wavelength ranges, in comparison with other types of microwave transistors (based on gallium or silicon arsenide). Excellent instrument characteristics are achieved primarily due to the special design of the semiconductor transistor heterostructure, which contains a set of ultrathin semiconductor layers with a specific composition and doping profile.

To create field effect transistors with high electron mobility, only epitaxial technologies can be applied that allow controlling the doping and composition profiles, controlling the growth of layers with a thickness of less than 10 nm, and obtaining sharp and defect-free heteroboundaries. Molecular beam epitaxy (MBE) satisfies all these requirements.

MPE, in accordance with its specific features, allows to solve the following most important problems: 1) obtaining crystalline films of high purity due to ultra-high vacuum and purity of the starting materials; 2) obtaining sharp profiles of composition and alloying - due to the short response time of the dampers (<< growth time of one monolayer) and relatively low growth temperatures that impede diffusion; 3) obtaining ultrathin layers of controlled thickness as a result of precise flow control at typical growth rates.

An important advantage of MPE over other epitaxial technologies is the possibility of using a variety of control methods directly in the process of growing, and above all, diffraction of fast electrons by reflection. An analysis of the observed diffraction patterns allows us to judge the state of the surface of the epitaxial layer directly during growth. The observed phenomenon of oscillations in the intensity of a specularly reflected electron beam associated with a periodic change in the surface roughness during layer-by-layer growth allows one to reliably determine both the growth rate and the composition of the compounds.

Among A3B5 compounds, the GaAs / AlGaAs material system for a long time remained the only well-studied and practically used. First of all, this is due to the possibility of achieving agreement with the GaAs substrate over the entire composition range.

In MLSs based on the n-AlGaAs / GaAs system, an increase in the electron density and mobility in the channel is achieved by the optimal choice of independent structural parameters: the concentration of the dopant in the barrier material is 1 ÷ 3 × 10 cm ^-3 , and the thickness of the undoped spacer is W _SP ≈2- 3 nm and the molar fraction of AlAs in the barrier is more than 0.35.

An increase in the density and mobility of electrons in the channel is achieved by choosing independent structural parameters:

- dopant concentration in the barrier material, with a value of 1 ÷ 3 × 10 ¹⁸ cm ^-3 , due to limited self-compensation of donor impurity atoms, starting from ~ 5 × 10 ¹⁸ cm ^-3 , and an increase in the gate leakage current, due to the formation of a parallel conductivity along the doped layer;

- the thickness of the undoped spacer with an optimal value of W _SP ≈3 nm, which allows one to obtain a mobility of ~ 6000 cm / Sun at concentrations of about 1-2 × 10 ¹² cm ^-2 at room temperature;

- the molar fraction of AlAs in the barrier, with a jump in the energy of the bottom of the conduction band at the heteroboundary equal to from 0.6 eV to 1.2 eV, which is achieved by choosing a modulated-dopant and layer thickness.

The doping level cannot be too high. On the one hand, it is limited by the self-compensation of atoms of the donor impurity (silicon), which begins to show at doping levels of ~ 5 × 10 ¹⁸ cm ^-3 , and on the other, by an increase in the gate leakage current due to the formation of parallel conductivity along the doped layer. Typical values are 1 ÷ 2 × 10 ¹⁸ cm ^-3 . For this reason, high-power microwave transistors use structures with two-sided doping, i.e. the lower (from the substrate side) doped layer is introduced into the structure. This allows you to increase the electron concentration in the channel several times without a significant decrease in carrier mobility. Another way to increase carrier concentration is to use multichannel structures.

The introduction of a spacer reduces the density of carriers in the channel, because the capacitance of the capacitor, which serves as the depleted AlGaAs layer, decreases without a corresponding contribution to the space charge. At the same time, electron mobility is increasing as a result of a greater separation of electrons and donors. In instrument structures Al _0.3 Ga _0.7 As / GaAs, the optimum is achieved at W _SP ≈3 nm, allowing one to obtain a mobility of ~ 6000 cm ² / Vs at concentrations of about 1 × 10 ¹² cm ^-2 at room temperature.

The n-type impurities introduced into AlGaAs create two types of donors: ordinary small ones with an ionization energy of ~ 7 meV, and deep electron traps, the so-called DX centers. At low temperatures, the DX center has a very small capture cross section, which allows the electrons released from it (for example, white light) to remain free for a long time, contributing to the conductivity. This phenomenon is called the stable photoconductivity (UVP) effect. Since UVP is associated with the emptying of DX centers, their concentration can be found as the difference between the concentration of carriers measured at 77 K after illumination and in the dark. This dependence is presented in figure 2. It can be seen that, for compositions x> 0.3, the DX centers predominate, while for x <0.2 the opposite situation takes place. The share of DX centers in the total number of donors does not depend on the growing conditions, but is determined by the composition, and also by doping in the transition region.

Based on the indicated gap in the conduction band at the AlGaAs / GaAs heteroboundary and the peculiarities of doping AlGaAs with silicon impurities associated with the formation of DX centers, in the manufacture of transistor structures, Al _X Ga _1-X As layers are usually used as barriers, where the x value is in the range 0.2-0.3.

The density of electrons, ceteris paribus, the greater, the higher the gap in the conduction band at the heterointerface (ΔE _C ). For the AlGaAs / GaAs system, the latter reaches a maximum at x values corresponding to the region of transition from the direct to the indirect zone (x ~ 0.43).

An increase in the density is achieved due to an increase in the discontinuity of the conduction band at the heterointerface (ΔE _C ) as a result of an increase in the molar fraction of AlAs over ≈0.3–0.35 in wide-gap AlGaAs solid solution. However, if the molar fraction of AlAs increases above ≈0.3–0.35, the increase in the ΔE _C gap will be suppressed by the greater depth of the donor level in AlGaAs.

To suppress the suppression of the increase, the size of the conduction band gap at the heterointerface (ΔE _C ) is greater than the depth of the donor level in the wide-gap AlGaAs solid solution, with an increase in the molar fraction of AlAs over 0.3-0.35, an impurity is initially used as the dopant in the AlGaAs solid solution with a reduced value of the donor level.

Thus, the improvement of the parameters is the optimization of the details of the proposed design of the microwave field-effect transistor in the form of:

- the use of an additional buffer layer in the form of a short-period AlGaAs / GaAs superlattice, which can significantly reduce the growth defects density and improve the electrical insulation between the transistor channel and the substrate;

- a relatively small total thickness of the GaAs buffer layers, which allows to improve the carrier restriction in the channel of the short-circuit transistor;

- the presence of an additional thin layer of undoped GaAs between the channel and the AlGaAs spacer, which improves the structural quality of the interface (smoothing layer);

- the presence of an additional gate layer of n-GaAs, which provides the formation of a Schottky barrier on chemically more stable material compared to AlGaAs;

- a relatively large thickness of the heavily doped gradient and contact layers of n + GaAs, respectively 20 nm and 50 nm thick, selected taking into account the characteristic thickness of the alloy layer during the formation of ohmic contacts of the source and drain.

The advantages of this device are:

1) reducing the density of growth defects and improving electrical insulation between the channel of the transistor and the substrate due to the implementation of an additional buffer layer in the form of a short-period superlattice Al _X Ga _1-X As / GaAs;

2) improvement of carrier limitation in the channel of the short-circuit transistor;

3) ensuring the optimization of heat removal from the heterostructure of the transistor crystal and minimizing gate leakage;

4) the absence of labor-intensive operations in the manufacture;

5) an increase in the breakdown voltage of the transistor;

6) an increase in the density and mobility of charge carriers;

7) virtually no current “collapse”.

Figures 4 and 5 show the current-voltage characteristics: Fig. 4 - without a layer of an insulating heat-conducting layer (for example polycrystalline diamond), on the pedestal of a microwave transistor, under the substrate of the heterostructure and additional barrier layers under the gate; 5) - with layers of an insulating heat-conducting layer (for example, polycrystalline diamond) on the transistor's crystal pedestal, as well as additional layers of hafnium dioxide and aluminum oxide under the gate of the transistor and on top of the insulating, dielectric coating.

Placing a pedestal from a heat-conducting CVD layer of polycrystalline diamond, and simultaneously placing a barrier layer of hafnium dioxide and aluminum oxide under the gate, reduce the thermal resistance and leakage currents of the transistor structure and thereby increase the breakdown voltage, which improves the efficiency of the proposed invention.

All of the above advantages allow you to create solid-state microwave blocks and modules with improved parameters, designed for phased array antennas and other electronic systems.

Claims (8)

1. A modulated-doped field-effect transistor containing a flange, a pedestal, a heteroepitaxial structure, a buffer layer, a source, a gate, a drain and ohmic contacts, characterized in that the pedestal is made of a heat-conducting CVD layer of polycrystalline diamond, a transistor crystal is placed on top of the pedestal, containing sequentially placed transistor GaAs base substrate, buffer layers, AlGaAs / GaAs heteroepitaxial heterostructure, and on the surface of the heteroepitaxial structure, between the source, gate, and drain, after The dielectric coating layer and two barrier layers made of hafnium dioxide and metal oxide are carefully placed, while the barrier layers are made with a total thickness of 1.0-4.0 nm, in addition, in the gate region, the barrier layers are placed under the gate, directly on the epitaxial structure in the form of a gradient layer of GaAs n-type conductivity. 2. The transistor according to claim 1, characterized in that the dielectric coating is made of SiO ₂ . 3. The transistor according to claim 1, characterized in that the dielectric coating is made of silicon nitride. 4. The transistor according to claim 1, characterized in that the dielectric coating is made of MgO. 5. The transistor according to claim 1, characterized in that the dielectric coating is made of 8 Sc ₂ O ₃ . 6. The transistor according to any one of claims 1 to 5, characterized in that the second barrier layer is made of ZrO ₂ . 7. The transistor according to any one of claims 1 to 5, characterized in that the second barrier layer is made of La ₂ O ₃ . 8. The transistor according to any one of claims 1 to 5, characterized in that the second barrier layer is made of Y ₂ O ₃ .

Images