JPH0927615A - Field effect transistor crystal and field effect transistor - Google Patents

Abstract

(57) Abstract: A two-dimensional electron gas concentration of a HEMT semiconductor crystal is increased. SOLUTION: The GaAs buffer layer 2 immediately below the undoped GaAs carrier transit layer 3 is doped with Si, Se, or S to form the n-type impurity GaAs buffer layer 101 on the carrier transit layer 3 side of the GaAs buffer layer 2. As a result, the electric field of the channel layer formed in the carrier transit layer 3 is relaxed, and the area of the triangular potential is increased.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a field effect transistor crystal having a HEMT structure and a field effect transistor.

[0002]

2. Description of the Related Art Conventionally, molecular beam epitaxy (MBE)
HEMT (High Ele) as an example of a high-speed field-effect transistor using a semiconductor crystal epitaxially grown by a sputtering method or a metal organic vapor phase epitaxy (MOVPE) method.
The example of ctron Mobility Transistor) is well known.
This HEMT realizes a high speed device by using a two-dimensional electron gas generated at the interface of the heterojunction as a channel.

FIG. 5 shows the structure of an HEMT using a GaAs layer as a carrier transit layer and its energy band structure diagram. In FIG. 5A, an undoped GaAs buffer layer 2, 10000A is formed on a semi-insulating GaAs substrate 1.
Undoped GaA with a thickness of (angstrom)
s Carrier transit layer 3, undoped AlGaAs spacer layer 4, n-type Al _0.3 Ga _0.7 with high concentration of Si added
The As carrier supply layer 5, the AlGaAs cap layer 6, and the GaAs cap layer 7 are sequentially stacked.

In FIG. 5B, 9 is an electron conduction band level, 10 is a two-dimensional electron gas forming a channel at the heterojunction interface, and 20 is a Fermi level. The concentration of the two-dimensional electron gas 10 is determined by the difference in electron affinity between the AlGaAs and GaAs semiconductors forming the heterojunction.

[0005]

In the field effect transistor having the above structure, n having a small electron affinity is used.
Electrons in the type Al _0.3 Ga _0.7 As layer 5 flow out to the undoped GaAs carrier transit layer 3 having a high electron affinity by diffusion, and a two-dimensional electron gas is accumulated at the heterojunction interface between these layers.

As shown in FIG. 5B, the two-dimensional electron gas 10 is accumulated in the acute-angled triangular potential (hatched portion) formed by the heterojunction interface, so that the conductivity of the channel is increased. In order to do so, it is necessary to apply a positive bias to the gate electrode to make the angle of the triangular potential more acute and deepen it to increase the two-dimensional electron gas concentration.

However, if the angle of the triangular potential is made more acute, it becomes difficult for electrons to be accumulated, so that it becomes difficult to increase the channel conductivity, and conversely, the transconductance starts to decrease.

In order to solve these drawbacks, a quantum well type (pseudomorphic type) in which an InGaAs carrier transit layer formed by adding In to an undoped GaAs layer 3 having a high electron affinity and mixed to form a well potential is used. (Type) HEMT structures have been proposed.

FIG. 6 shows a structure of a pseudomorphic HEMT using an InGaAs layer as a carrier transit layer and its energy band structure diagram. In FIG. 6A, undoped GaA is formed on the semi-insulating GaAs substrate 1.
s buffer layer 2, undoped InGaAs carrier transit layer 8 having an extremely thin thickness of several tens of atomic layers, undoped AlGaAs spacer layer 4, n-type Al _0.3 Ga _0.7 As carrier supply layer 5 containing Si at a high concentration, AlGa
The As cap layer 6 and the GaAs cap layer 7 are sequentially stacked. Since the difference in electron affinity between AlGaAs and InGaAs is larger than that in the normal HEMT structure, the concentration of the two-dimensional electron gas 10 is increased as shown in FIG. 6 (b).

However, the InGaAs layer has a critical film thickness, and the two-dimensional electron gas concentration is determined from this film thickness. Also, the well-type potential is also affected by the potential gradient, and when the angle of the triangular potential is made more acute, it becomes difficult for electrons to accumulate, so it becomes difficult to increase the channel conductivity, and conversely the mutual conductance begins to decrease. There are drawbacks.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a field effect transistor crystal capable of increasing the concentration of a two-dimensional electron gas.

It is another object of the present invention to provide a field effect transistor capable of suppressing the amount of change in transconductance with respect to a gate voltage and manufacturing a device having a good amplification factor linearity.

[0013]

A field effect transistor crystal of the present invention is a field effect transistor crystal having at least a buffer layer, a carrier transit layer, and an n-type carrier supply layer in this order on a substrate. A layer doped with an n-type impurity capable of maintaining a depleted state is formed on the side.

As described above, by forming a layer doped with an n-type impurity capable of maintaining a depleted state on the carrier transit layer side of the buffer layer, the electric field in the channel layer that can be the carrier transit layer is relaxed, and the channel is formed. The amount of the two-dimensional electron gas accumulated in the layer can be increased.

Further, the field effect transistor of the present invention is
In a field effect transistor having at least a buffer layer, a carrier transit layer, an n-type carrier supply layer, a gate, a source, and a drain on a substrate, an n-type impurity capable of maintaining a depleted state on the carrier transit layer side of the buffer layer is doped. It is formed by forming a layer.

Here, the n-type impurities include Si, Se, and S.
and so on. The crystal structure is classified into a normal type having a GaAs layer as a carrier transit layer and a pseudomorphic type having an InGaAs layer as a carrier layer.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of a semiconductor crystal of a conventional HEMT structure using a GaAs layer as a carrier transit layer according to the first embodiment. Semi-insulating G by MBE device
undoped GaA on aAs substrate 1 at a substrate temperature of 470 ° C.
The s buffer layer 2 is grown to a thickness of 500 nm, and Si is doped thereon (doping concentration 1 × 10 ¹⁶ cm ⁻³ to 1 × 10 ¹⁸ cm
^-3 ) The formed n-type GaAs buffer layer 101 from 1 nm to 10
It was grown to 0 nm. At this time, the Si doping concentration is
The n-type GaAs buffer layer 101 is depleted when the gate bias is 0 volt or a negative voltage is applied.

Subsequently, the undoped GaAs layer 3 is used as a carrier transit layer for 10000 A, undoped Al _0.3 Ga.
4 nm with _0.7 As layer 4 as a spacer layer, Si-doped (doping concentration 3 × 10 ¹⁸ cm ⁻³ ) n-type Al _0.3 Ga _0.7 A
20 nm was grown using the s layer 5 as a carrier supply layer. Then, an undoped Al _0.3 Ga _0.7 As layer and an undoped GaAs layer are sequentially used as cap layers each having a thickness of 7 nm,
It was epitaxially grown to 5 nm.

Thus, Si is formed on the carrier transit layer 3 side of the GaAs buffer layer 2 immediately below the GaAs carrier transit layer 3.
As shown in FIG. 1B, since the electric field near the depleted Si-doped GaAs buffer layer is relaxed and the electron conduction band level becomes low, the area of the triangular potential becomes large. Therefore, the two-dimensional electrons 10 accumulated in the triangular potential increase in proportion to this increase.

The HEMT of this embodiment formed as described above
Electrical characteristics of a semiconductor crystal having a structure, and a conventional HE in which a buffer layer is formed of only undoped GaAs for comparison
The electrical characteristics of the MT structure semiconductor crystal were measured by the Hall effect. As a result, the electron mobility is 7100 cm of the conventional example.
In contrast to / V · s, the present embodiment is 6,900 cm / V · s, which is slightly small due to the influence of the n-type GaAs buffer layer 101 of the underlying layer, but the sheet carrier concentration of this embodiment is the same as that of the conventional example. It was possible to significantly increase it to 1.8 × 10 ¹² cm ⁻² with respect to 1.0 × 10 ¹² cm ⁻² .

FIG. 2 shows that, on the surface of the semiconductor crystal having the HEMT structure, the AI gate electrode 31 and AuGe / Ni are further formed.
A source electrode 32 and a drain electrode 33 of / Au are formed to form a HEMT transistor.

FIG. 3 is a pseudomorphic HE using an InGaAs layer as a carrier transit layer according to the second embodiment.
It is a schematic sectional drawing of the semiconductor crystal of MT structure.

As in the case of FIG. 1, an undoped GaAs buffer layer 2 was grown to a thickness of 500 nm on a semi-insulating GaAs substrate 1 at a substrate temperature of 470 ° C. using an MBE apparatus, and Si was doped thereon (doping concentration 1 × 10 ^16). cm ^-3 to 1 x
10 ¹⁸ cm ^-3 ) of n-type GaAs buffer layer 101 with a thickness of 1 nm
Was grown to 100 nm.

Subsequently, the undoped In _0.15 Ga _0.85 As layer 8 was used as a carrier transit layer for 12 nm, and then the undoped A
l _0.3 Ga _0.7 As layer 4 as spacer layer 4 nm, Si
Doped (doping concentration 3 × 10 ¹⁸ cm ^-3 ) n-type Al
20 nm of _0.3 Ga _0.7 As layer 5, undoped Al _0.3 G
In the second embodiment in which a _0.7 As is 7 nm, an undoped GaAs layer is 5 nm, and a cap layer is sequentially epitaxially grown also in the second embodiment, Si is formed on the carrier transit layer 8 side of the GaAs buffer layer 2 immediately below the GaAs carrier transit layer 8.
3B, the area of the triangular potential is increased and the two-dimensional electrons 10 accumulated in the triangular potential are increased, as shown in FIG. 3B.

The HEM of this embodiment formed in this way
Electrical characteristics of a T-structure semiconductor crystal, and a conventional H in which a buffer layer is formed of only undoped GaAs for comparison.
The electrical characteristics of the semiconductor crystal having the EMT structure were measured by the Hall effect. As a result, the electron mobility was 6800 of the conventional example.
In the present embodiment, the sheet carrier concentration is 6,200 cm / V · s, which is slightly smaller than that of the conventional example. 1.5
× could be increased relative to ^{10 12 cm -2 2.5 × 10 12} cm -2 and significantly.

FIG. 4 shows this pseudomorphic HEM.
A gate electrode 31 of AI, a source electrode 32 of AuGe / Ni / Au, and a drain electrode 33 are further formed on the surface of a T-structure semiconductor crystal to form a HEMT transistor.

Although the HEMT structure formed on the GaAs substrate has been described in the above embodiment, the same applies to the case where the substrate is another III-V group crystal or the HEMT structure which is the Si substrate. The carrier concentration could be increased.

[0029]

According to the field effect transistor crystal of the present invention, by forming a layer doped with n-type impurities on the carrier transit layer side of the buffer layer, the electric field in the channel layer which can be the carrier transit layer is relaxed. , The concentration of the two-dimensional electron gas can be increased.

Further, according to the field effect transistor of the present invention, the concentration of the two-dimensional electron gas can be increased by forming the layer doped with the n-type impurity on the channel layer side of the buffer layer, so that the gate voltage can be increased. The amount of change in mutual conductance with respect to can be suppressed, and a device having a good amplification factor linearity can be manufactured.

[Brief description of drawings]

1A and 1B are diagrams illustrating a semiconductor crystal having a HEMT structure according to a first embodiment of the present invention, in which FIG. 1A is a sectional view showing a crystal structure and FIG. 1B is an energy band structure diagram.

FIG. 2 is a sectional view showing a HEMT structure according to a first embodiment.

3A and 3B are diagrams illustrating a semiconductor crystal having a pseudomorphic HEMT structure according to a second embodiment, in which FIG. 3A is a sectional view showing a crystal structure and FIG. 3B is an energy band structure diagram.

FIG. 4 is a sectional view showing a HEMT structure of a second embodiment.

5A and 5B are diagrams illustrating a semiconductor crystal having a HEMT structure of a conventional example, FIG. 5A is a diagram showing a crystal structure, and FIG. 5B is an energy band structure diagram.

6A and 6B are diagrams illustrating a semiconductor crystal having a pseudomorphic HEMT structure of a conventional example, FIG. 6A is a diagram showing a crystal structure, and FIG. 6B is an energy band structure diagram.

[Explanation of symbols]

 1 semi-insulating GaAs substrate 2 undoped GaAs buffer layer 3 undoped GaAs carrier transit layer 4 AlGaAs spacer layer 5 Si-doped n-type AlGaAs carrier supply layer 6 AlGaAs cap layer 7 GaAs cap layer 8 InGaAs carrier transit layer 9 electron conduction band level 10 Two-dimensional electron gas 20 Fermi level 101 Si-doped n-type GaAs buffer layer

Claims (7)

[Claims] 1. A field-effect transistor crystal having at least a buffer layer, a carrier transit layer, and an n-type carrier supply layer on a substrate in this order, and an n-type impurity capable of maintaining a depleted state on the carrier transit layer side of the buffer layer. A field-effect transistor crystal characterized by forming a doped layer. 2. The field effect transistor crystal according to claim 1, wherein the thickness of the doping layer is 1 nm to 100 nm. 3. The field effect transistor crystal according to claim 1, wherein the doping layer has a carrier concentration of 1 × 10 ¹⁶ cm ⁻³ to 1 × 10 ¹⁸ cm ^−3. Transistor crystal. 4. A field effect transistor crystal according to claim 1, wherein the dopant species of the doping layer is Si. 5. The field effect transistor crystal according to claim 1, wherein the dopant species of the doping layer is Se. 6. The field-effect transistor crystal according to claim 1, wherein the doping species of the doping layer is S. 7. The field-effect transistor crystal according to claim 1, further comprising a gate, a source,
A field effect transistor characterized in that a drain is formed.