JP2538872B2 - Semiconductor device - Google Patents

Description

The present invention relates to a semiconductor device having a channel of a two-dimensional electron gas supplied from a gallium arsenide / aluminum arsenide superlattice structure, in which an ohmic contact electrode is in contact with an indium gallium arsenide layer. By arranging them, good contact can be obtained without performing alloying heat treatment, and thermal deterioration such as decrease in electron density can be prevented.

[Industrial applications]

The present invention relates to a semiconductor device, and more particularly, to improvement of a semiconductor device such as a high electron mobility field effect transistor (HEMT) which uses a carrier having high mobility by space separation doping and interface quantization as a channel.

For example, HEMT forms two-dimensional electrons in the vicinity of a heterojunction interface such as a non-doped gallium arsenide (GaAs) layer and an n-type aluminum gallium arsenide (AlGaAs) layer, and an impurity doping region and a carrier moving region are formed. These are spatially separated to increase carrier mobility, and high expectations are placed on them as high-speed devices.

[Conventional technology]

2A and 2B are schematic side sectional views of an example of a HEMT as an example of a semiconductor device that realizes high mobility by interfacial quantization of carriers in space separation doping.

In the conventional HEMT example shown in FIG. 1A, a non-doped i-type GaAs layer 12 and a donor impurity having a lower electron affinity, for example, a concentration of about 2 × 10 ¹⁸ cm ⁻³ are provided on a semi-insulating GaAs substrate 11. A doped n-type Al _x Ga _1-x As layer 13 and an n-type GaAs layer 14 doped with a donor impurity to the same extent or more are provided to convert the n-type AlGaAs layer 13 to the i-type GaAs layer 12. A two-dimensional electron gas 12e is formed near the heterojunction interface by the transitioned electrons. This two-dimensional electron gas 12e has almost no mobility decrease due to impurity scattering, and lattice scattering decreases, for example, 77
Highest mobility is obtained at low temperature below K.

For example, gold germanium / gold (AuGe / Au) is used as the source / drain electrode 15 on the n-type GaAs layer 14 of this semiconductor substrate, and the gate electrode 16 is made of, for example, titanium / platinum / gold (AuGe / Au) on the n-type AlGaAs electron supply layer 13. Ti / Pt / Au) is used, and the Schottky depletion layer by the gate electrode 16 controls the areal density of the two-dimensional electron gas 12e to perform the transistor operation. 15A
Is an alloyed region formed between the source / drain electrodes 15 and the semiconductor substrate.

If the energy level difference of the conduction band between the n-type Al _x Ga _1-x As electron supply material 13 and the i-type GaAs channel layer 12 of this structure is small, the surface density Ns of the two-dimensional electron gas 12e becomes small. The normal conduction band level difference is about 0.25 eV,
It is preferable that the Al composition ratio x of the Al _x Ga _1-x As electron supply layer 13 is about 0.3 or more.

On the other hand, however, when the Al composition ratio x of the Al _x Ga _1-x As mixed crystal is larger than about 0.25, the donor level of doped silicon (Si) or the like suddenly deepens. In this deep donor level, electrons are excited to the conduction band when infrared rays are incident at about 200 K or less, and conduction electrons do not fall to the donor level even if light irradiation is stopped. PPC (persistent photo conductivit)
y) and other phenomena are called DX centers, but due to this, even if the doping amount is increased, a high carrier concentration cannot be obtained, and in particular, the two-dimensional electron gas areal density Ns decreases at low temperatures, and therefore the transfer conductance g _m Has caused a decline in.
In addition, when light enters or hot electrons jump from the channel into the AlGaAs layer, the phenomenon that the two-dimensional electron gas surface density Ns, electron mobility μ _n, threshold voltage V _th, etc. fluctuates appears, causing low frequency noise. It is also accompanied.

There is a conventional example shown in FIG. 2 (b) for dealing with the problem caused by the deep donor level in the AlGaAs mixed crystal.
In this conventional example, instead of the n-type Al _x Ga _1-x As layer 13 of the conventional example, the electron supply layer has a lattice structure 23 composed of a GaAs layer 23a and an AlAs layer 23b, and the GaAs layer 23a has, for example, a concentration of 5 ×. The donor impurity is doped to about 10 ¹⁸ cm ⁻³ , and the AlAs layer 23b is undoped.

With this improvement, HE that uses AlGaAs mixed crystal layer as electron supply layer
MT has an electron mobility μ _n ≈1 × 10 ^{5 at a} temperature of 77 K, for example.
cm ² / Vs, two-dimensional electron gas areal density Ns ≈ 1.2 × 10 ¹¹ cm ^-2 , while the equivalent HEMT using a GaAs / AlAs superlattice structure as an electron supply layer has a two-dimensional electron gas surface Density Ns≈3.0 × 1
It is reported that about 0 ¹¹ cm ^-2 is obtained with an equivalent electron mobility μ _n , and problems such as fluctuations in the threshold voltage V _th are improved.

[Problems to be solved by the invention]

As mentioned above, the GaAs / AlAs superlattice structure improves the problem caused by the deep donor level in the AlGaAs mixed crystal.
When the HEMT device is actually completed, the two-dimensional electron gas areal density is higher than that of the semiconductor substrate immediately after epitaxial growth.
Deterioration such as a decrease in Ns and therefore an increase in sheet resistance often occurs.

The largest contributor to this degradation during the manufacturing process is heat treatment. That is, as the ohmic contact electrode material, for example, gold germanium alloy (AuGe) with a thickness of 20 mm and gold (Au) with a thickness of 280 mm are laminated and deposited on the n-type GaAs layer 14 and patterned, and the temperature is 450 ° C. for about 1 minute. Source and drain electrodes are formed by alloying the electrode material and semiconductor substrate by heat treatment.
Although the ohmic contact resistance of 15 is reduced, the superlattice structure is damaged by the diffusion of Si doped in the GaAs layer 23a during the heat treatment.

Increasing the number of carriers in a semiconductor device that uses interface quantized carriers such as HEMT is an important issue, and elimination of factors that impair its effect is strongly desired.

[Means for solving problems]

The problem is that a channel layer made of a gallium arsenide layer and an electron supply layer having a superlattice structure in which a non-doped aluminum arsenide layer and an n-type gallium arsenide layer formed alternately are formed on the channel layer. A gate electrode provided on the electron supply layer, and source and drain electrodes provided on both sides of the gate electrode on the electron supply layer via an n-type indium gallium arsenide layer, respectively. The source electrode, the drain electrode and the gate electrode can be solved by the semiconductor device according to the present invention, which is made of the same electrode material and formed of non-alloy in the same process.

[Work]

According to the present invention, electrons transition from a superlattice structure composed of a GaAs layer containing a donor impurity and a non-doped AlAs layer, and a two-dimensional electron gas is formed in the vicinity of the interface between the non-doped GaAs layer and the superlattice structure. In a semiconductor device to be used, its ohmic contact electrode is made of In _x Ga containing a donor impurity.
_{It is} placed in contact with the _1-x As layer.

The energy band gap Eg of In _x Ga _1-x As mixed crystal is narrower than that of GaAs crystal, and for example, In _0.5 G for Eg ≈ 1.42 eV.
Since a _0.5 As is Eg≈0.76 eV, a low resistance ohmic contact can be obtained without performing alloying heat treatment.

Chromium / gold (Cr / Au), for example, is suitable as this electrode material. With this electrode material, the source and drain electrodes that make ohmic contact with the In _x Ga _1-x As mixed crystal layer, and GaAs / AlA
s It is also possible to simultaneously form a gate electrode that is in Schottky contact with the superlattice structure.

〔Example〕

 Hereinafter, the present invention will be described specifically with reference to Examples.

FIG. 1 is a schematic side sectional view showing an embodiment of the present invention related to HEMT.

The semiconductor substrate of this embodiment is formed by a molecular beam epitaxial growth method (MBE method) or the like on a semi-insulating GaAs substrate 1, a non-doped i-type GaAs layer 2 having a thickness of, for example, about 1 μm, for example, a concentration of 5 × 10 ¹⁸ cm ^−3. N with a thickness of about 5 mm after being doped with Si
I-type AlA with a thickness of, for example, about 2 mm, which is doped with the ton-type GaAs layer 3a
GaAs / AlAs superlattice structure 3 consisting of 7 cycles with s layer 3b, for example, the composition ratio of indium (In) is gradually increased from GaAs to In _0.5 Ga _0.5 As in the thickness range of about 5 nm, and the total thickness is 100 to 200 nm. N-type InGa doped with Si at a concentration of about 1 × 10 ¹⁹ cm ^-3
As layer 4 is growing in sequence. I-type GaAs of this semiconductor substrate
A two-dimensional electron gas 2e is formed near the interface of the layer 2 with the GaAs / AlAs superlattice structure 3. The gate formation region of this semiconductor substrate was recess-etched to form an electrode material layer of about 50 m Cr, A
A source / drain electrode 5 and a gate electrode 6 are simultaneously formed by laminating about 300 nm. The heat treatment for forming the alloyed region is not performed.

In this example, immediately after the epitaxial growth, for example, the temperature
At 77 K, electron mobility μ _n ≈ 1 × 10 ⁵ cm ² / Vs, using a semiconductor substrate with a two-dimensional electron gas surface density Ns ≈ 3.0 × 10 ¹¹ cm ^-2 , there is a significant difference at the same temperature after the HEMT device is completed. A two-dimensional electron gas areal density Ns is obtained, and AuGe / Au is used as the ohmic electrode material on the same semiconductor substrate at a temperature of 450
The comparative sample, which has been subjected to alloying heat treatment at about 1 ° C for 1 minute,
The effect of the present invention is proved in comparison with the two-dimensional electron gas areal density Ns≈1.2 × 10 ¹¹ cm ⁻² .

The application of the present invention is not limited to HEMTs. For example, when a two-dimensional electron gas is used for the base layer of a hot electron transistor or a resonant hot electron transistor to increase the speed, the present invention is applied to the base resistance. The present invention can be applied to all semiconductor devices using high mobility carriers due to space separation doping and interface quantization.

〔The invention's effect〕

As described above, according to the present invention, HE that utilizes high mobility carriers by space separation doping and interface quantization
In a semiconductor device such as MT, the electron supply layer of the GaAs / AlAs superlattice structure is provided with good heat resistance during the manufacturing process without causing thermal damage that causes deterioration such as a decrease in two-dimensional electron gas surface density and an increase in sheet resistance. A characteristic semiconductor device is realized.

[Brief description of drawings]

FIG. 1 is a schematic side sectional view of an embodiment of the present invention related to HEMT, and FIG. 2 is a schematic side sectional view of a conventional example of HEMT. In the figure, 1 is a semi-insulating GaAs substrate, 2 is a non-doped GaAs layer, 2e is a two-dimensional electron gas, 3 is a GaAs / AlAs superlattice structure, 3a is an n-type GaAs layer, 3b is a non-doped AlAs layer, and 4 is n-type In _x Ga _1-x As layer, 5 is a Cr / Au source / drain electrode, and 6 is a Cr / Au gate electrode.

Claims (2)

(57) [Claims] 1. A channel layer made of a gallium arsenide layer, and an electron supply layer having a superlattice structure formed on the channel layer, in which a non-doped aluminum arsenide layer and an n-type gallium arsenide layer are alternately laminated. A gate electrode provided on the electron supply layer, and source and drain electrodes provided on both sides of the gate electrode on the electron supply layer via an n-type indium gallium arsenide layer, respectively. A semiconductor device, wherein the source electrode, the drain electrode, and the gate electrode are made of the same electrode material formed in a non-alloy by the same process. 2. The semiconductor device according to claim 1, wherein the source electrode, the drain electrode and the gate electrode are made of chromium and gold.