JPH05211340A - Enhancement-mode field-effect transistor - Google Patents

Abstract

PURPOSE:To realize an ultrahigh-speed and low-power consumption enhancement- mode field-effect transistor in an AlGaSb/InAs/AlGaSb quantum well system. CONSTITUTION:In an electron storage mechanism in an AlGaSb/InAs/AlGaSb quantum well system, an electron supply source in an AlGaSb barrier layer 1 is donors in a direct manner, but the positional relation between a deep acceptor level in the layer 1 and a quantum level in an InAs quantum well layer 2 plays an important role. A structure in which the quantum level in the layer 2 is higher than the deep acceptor level 4 in the AlGaSb barrier layer 1 was fabricated and the normally OFF-state was confirmed. In this structure, the layer 1 on the side of the surface of the layer 1 is sufficiently thick and the supply of electrons from the surface level in the layer 1 is inhibited.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high speed, low power consumption, low noise transistor using a 3-5 group compound semiconductor.

[0002]

2. Description of the Related Art In the field of semiconductor devices, further speedup, lower power consumption, and lower noise are being promoted in the field of semiconductor devices by constructing materials and improving the device structure in order to build ultrahigh-speed computers and ultrahigh-speed communication systems. I am trying to do.

The approach from the material system in the device using the group III-V compound semiconductor heterojunction is as follows.
Toward s, the direction of using a material having a high electron mobility / saturation speed is being pursued. It is important for the heterojunction system to have lattice matching, and for the GaAs system, (AlGaA
s / GaAs / GaAs substrate) structure, and InGaAs-based (AlInAs / InGaAs / InP substrate) structure. On the other hand, in the case of InAs, there is no barrier layer that satisfies the lattice matching, but the lattice mismatch with the AlGaSb system is comparatively small at 0.7 to 1.3%, and is within a few hundred angstroms (hereinafter referred to as A). For example, strain-free growth due to strain is possible, and (AlGaSb / InAs
/ AlGaSb / GaSb substrate) and the like are generally used. Field effect transistor (FET)
Among them, enhancement type is expected to have higher speed, but AlGaSb / InAs / AlGaSb
It has not been realized yet in the system.

[0004]

In order to improve the high speed and low power consumption of the FET, it is desirable that the threshold voltage is small and the barrier layer is thin. AlGaAs / Ga
In the case of As series, the threshold voltage tended to decrease as the AlGaAs barrier layer thickness decreased. However, Al
In the case of GaSb / InAs / AlGaSb system, it increases conversely. This is AlGaSb rather than the conduction band of InAs.
Since the surface pinning level of is about 300 meV, electrons are supplied from the surface level to the InAs channel. Therefore, unlike the case of the AlGaAs / GaAs system, thinning the barrier layer and reducing the threshold voltage conflict with each other.

On the other hand, even when the surface AlGaSb layer is made sufficiently thick to suppress the surface effect, electron accumulation in InAs is observed. The cause of this has not been clarified. As the easiest method for reducing the concentration of accumulated electrons, p-type doping in the AlGaSb barrier layer is conceivable, but in reality, holes accumulate at the AlGaSb / InAs interface and the device does not function as a device. I couldn't beat it. Further, although an FET having an InAs gate structure has been devised, there still remains a problem to be solved, such as the need for a dry process that has not yet been established in this system.

[0006]

[Means for Solving the Problems] (1) We use AlGa
The presence of a donor and a deep acceptor, which are considered to be due to an intrinsic defect in Sb, of which the deep acceptor traps the electron of the donor,
It has been found that it serves as a source of electrons to the nAs layer.

In order to remove the electron storage effect from the acceptor level, the thickness of the InAs well layer of the AlGaSb / InAs / AlGaSb quantum well enhancement type FET is reduced to reduce the first quantum level to AlGaSb / InAs / AlGaSb.
It is characterized in that it is higher than the deep acceptor level in Sb.

(2) In improving the FET characteristics, the process of making the barrier layer thin is inevitable. As the barrier layer becomes thinner, the electron accumulation from the surface level becomes remarkable. To avoid this, AlGaSb / InAs / AlGaS
b Quantum well type enhancement type FET, surface side A
It is characterized in that the outermost surface of the lGaSb layer has a high concentration of a few tens of amperes of a p-type layer.

[0009]

[Operation] The operation relating to (1) will be described.

In the case of this InAs / AlGaSb system, I
The energy difference between the quantum level in the nAs well layer and the electron trap level in the AlGaSb layer is small, and is at most 150 me.
It is about V. This is because the electron trap level is the acceptor level and is at a lower energy position than the general donor level. The value of 150 meV corresponds to the quantum level of a relatively thick well of about 100 A for InAs. In the GaAs system, the quantum level is 1
A rise of as much as 50 meV will have a fatal effect on the electrical characteristics. However, in the case of InAs, the valley energy difference is as large as 780 meV, and InAs / AlGaSb
Due to the large conduction band discontinuity between them, the quantum level can be raised without adversely affecting the electrical characteristics.

Therefore, in a system in which the effect of the surface level is not exerted (that is, a system in which the surface AlGaSb layer is thick), electron accumulation can be performed while maintaining good electric characteristics by only slightly increasing the quantum level. It can be prevented.

The operation relating to (2) will be described.

Due to the high surface concentration doping, most of the electrostatic potential due to the Schottky contact is eliminated near the surface. Since the doped layer is completely empty, there is no carrier conduction and parallel conduction can be suppressed. Also,
In this structure, when the gate voltage is swung to the negative side, most of the electric field is consumed by the surface-doped layer, but when swung to the positive side (the direction in which electrons are accumulated), the electron concentration in the channel is directly increased. Because you can control
There is no problem in using it as an enhancement type FET.

[0014]

EXAMPLE A first example will be described with reference to FIG.

[0015] _{Al 0. 5 Ga 0. 5} Sb / InAs / A
For _{l 0. 5 Ga 0. 5} Sb quantum well structure, Al
_{0. 5} Ga _{0. 5} deep acceptor level in the Sb is on about 140meV top of the valence band. This is InA
It means that the energy is higher than the bottom of the conduction band of s by about 100 meV. Therefore, in order to suppress the electron supply from the barrier layer to the well layer, the quantum level of the InAs well may be set to 100 meV or more, but the band parameter in this system includes a considerable uncertainty. In consideration of this, the thickness of the InAs well layer 2 is designed to be 100 angstrom (A) so that the quantum level is actually 150 meV. This structural diagram and its band diagram are shown in FIG. Surface AlGa so that the effect of the surface can be ignored
The thickness of the Sb barrier layer 1 was 1000A. At this time, the system showed high resistance at 77 k, and the electron concentration could not be measured. In other words, normally-off InAs
The channel was realized. This is because, as described above, the quantum level in the well is higher than the deep acceptor level 4 trapping electrons.

Next, a second embodiment will be described. FIG. 2 shows a sample structure diagram and a hand diagram thereof when the barrier layer surface is heavily p-doped. Surface Al _{0. 5} Ga _{0. 5}
The thickness of the Sb barrier layer 11 is set to 250 A, and the outermost surface 50 A is doped with 1 × 19 cm ⁻³ of p-type dopant such as Be. The thickness of the InAs well layer 12 was 100A. In this case, most of the approximately 400 meV Schottky barrier is eliminated by the surface-doped layer 13, the electron concentration in InAs is reduced by one digit or more as compared with the case without the high-doped layer, and a normally-off state is realized. Dope layer thickness is 5
Since it is as thin as 0 A, it can be applied to a structure having a barrier layer thinner than 250 A. Therefore, according to the present invention, an enhancement type FET having good characteristics can be realized.

[0017]

According to the present invention, a high-speed enhancement type FET using the AlGaSb / InAs / AlGaSb quantum well structure which has been considered to be more difficult than the conventional one.
Can be realized.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment of the present invention,
(A) is a diagram showing a structure and (B) is a band diagram.

FIG. 2 is a diagram for explaining a second embodiment of the present invention,
(A) is a diagram showing a structure and (B) is a band diagram.

[Explanation of symbols]

 1 Undoped AlGaSb (barrier layer) 2 InAs (quantum well layer) 3 Surface pinning level 4 Acceptor level 11 Undoped AlGaSb (barrier layer) 12 InAs (quantum well layer) 13 High-concentration p-type doping layer 14 Surface pinning level 15 Acceptor level

Claims (2)

[Claims] 1. AlGaSb / InAs / AlGaSb
An enhancement-type field effect transistor having a quantum well structure, wherein the quantum level of an InAs well layer is Al.
An enhancement type field effect transistor characterized by being higher than an acceptor level in a GaSb layer. 2. AlGaSb / InAs / AlGaSb
An enhancement-type field-effect transistor having a quantum well structure, characterized in that a region of several tens of angstroms on the surface of an AlGaSb layer is heavily doped with p-type.