JPH0714057B2 - Field effect transistor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a field effect transistor using a heterojunction.

2. Description of the Related Art Conventionally, as a field effect transistor using a heterojunction, a high electron mobility transistor (High Electron Mobility Transistor) using a high mobility two-dimensional electron gas accumulated at the heterojunction interface between N-type AlGaAs and non-doped GaAs has been used. HE
MT) is well known.

Problems to be Solved by the Invention The drawbacks of the HEMT are that the saturation concentration of the two-dimensional electron gas accumulating at the hetero interface is low (up to 1 × 10 ¹² / cm ² ), and the conductivity of the channel is at room temperature. Compared with this, it is about the same or less, and because of the deep level called DX center in N-type AlGaAs, the change in device characteristics is large and unstable with respect to light and temperature. further,
Discontinuity of N-type AlGaAs and GaAs conduction band,
Since ΔEc is small, the two-dimensional electron gas in a high electric field is AlGa
There is also a problem that AlGaAs is overflowed from GaAs beyond the As barrier.

Means for Solving the Problems The present invention provides a heterojunction field effect transistor having a new structure that solves the above conventional problems. N-type channel layer to increase electron concentration
The structure is composed of a GaAs layer and non-doped In _Z Ga _1-Z As, and the channel layer is sandwiched between AlGaAs layers that serve as barriers to electrons. Al on both sides of the channel layer
The GaAs layer may be undoped or N-type, but when N-type AlGaAs is used, the adverse effect of the DX center can be eliminated by setting the AlAs composition in AlGaAs to 0.2 or less.

Function By using the structure of the present invention, the characteristics of the field effect transistor can be improved. In order to further increase the concentration of electrons accumulated in the channel layer, N-type impurities may be added to AlGaAs on both sides or one side of the channel layer. In this case, AlAs of AlGaAs should be removed in order to eliminate the adverse effect of the DX center. It is necessary to make the composition ratio 0.2 or less.

Example 1 A first example of the present invention is shown in FIG. In the sectional structure of FIG. 1 (a), 1 is a semi-insulating GaAs substrate, 2 is 0.1 μm.
Non-doped GaAs layer with a thickness of 3 μm, a non-doped Al _0.3 Ga _0.7 As layer with a thickness of 0.2 μm, and a donor concentration of 4 × 10 4.
¹⁸ / cm ³ N-type GaAs layer with a film thickness of 50 Å, 5 is undoped In
_Z Ga _1-Z As layer, 7 is undoped Al _0.3 Ga with a film thickness of 200Å
_{It is the 0.7} As layer. In _Z Ga _1-Z As layer thickness is 100Å, z is 0.25
And N-type GaAs layers (4 and 6) and In _0.25 Ga _0.75 As layer 5
The channel layer is formed by the
It showed a very high value of 10 ¹² / cm ² . Al _0.3 Ga _0.7 As layer (3
And 7) are barrier layers for confining in the channel layer. MIS (Metal-Insulator-Semiconductor) 8 is a source electrode, 9 is a drain electrode, and 10 is a gate electrode.
Type field effect transistor is constructed. Fig. 1 (b)
Shows a band diagram near the channel layer.

N-type GaAs channel layer is also serves as its own conductive layer also acts as an electron supply layer of the In _Z Ga _1-Z As layer.
That is, as shown in the band diagram of FIG. 1 (b), N-AlG in HEMT constituted by an In _Z Ga _1-Z electrons from N type GaAs layer into the quantum well of As are supplied N-AlGaAs / GaAs
It will play the same role as aAs. The advantage of using N-type GaAs as the electron supply layer is that it has a high donor concentration (6-8 × 10
¹⁸ / cm ³ ) can be obtained, so the desired electron concentration
A field-effect transistor that has a high mutual conductance and stable element characteristics, because the N-type GaAs layer for accumulating the _Z Ga _1-Z As layer can be thinned and the DX center does not have a bad effect unlike N-AlGaAs. Is suitable for obtaining Further, by the sandwiched channel layer of AlGaAs layers, In viewed from _Z Ga _1-Z As the accumulated electrons can sufficiently increase the height of the barrier, the hot electrons in the high electric field, In _Z Even if the Ga _1-Z As quantum well overflows, the barrier of the AlGaAs layer increases the probability of electrons staying in the channel layer.
Channel layer and GaAs, which is configured by In _Z Ga _1-Z As layer, electrons, as long stay in the channel layer, so that high saturation velocity is maintained. The electronic majority of the channel layer, since the accumulated non-doped In _Z Ga _1-Z As layer, the electron mobility and saturated velocity becomes higher than the channel layer of GaAs alone.

A second embodiment of the present invention is shown in FIG. FIG. 2 shows the first
The structure of the channel layer in FIG.
And 100Å undoped In _0.25 Ga _0.75 A formed on it
s, the N-type GaAs film thickness was 100Å, and the donor concentration was 4 × 10 ¹⁸ / cm ³ . Also in this case, the electron concentration accumulated in the channel was 4 × 10 ¹² / cm ² .

In the first and second embodiments, the undoped
Replace the Al _0.3 Ga _0.7 As layer with the Al _0.2 Ga _0.8 As layer, and
One part of the _0.2 Ga _0.8 As layer was doped with N-type impurities at 1 × 10 ¹⁸ / cm ³ and the electron concentration was 5-6 × in both cases.
A value of 10 ¹² cm ² was obtained.

In the field effect transistor in the above-described examples, no change in the threshold voltage due to temperature or light and no drastic change in FET characteristics were observed, and the characteristics were stable as compared with the conventional N-AlGaAs / GaAs HEMT.

Effect of the Invention As described above, in the heterojunction field effect transistor of the present invention, N-type Al _X Ga _1-X As (x to 0.
3) it is possible not only to eliminate the DX center by instability of the characteristics in, the channel layer, because of the use of In _Z Ga _1-Z As accumulate the N-type GaAs and an electron supplying high electron density A high electron concentration with high mobility can be realized in the channel. Also, since electrons mainly travel in the In _Z Ga _1-Z As quantum well, the Al of the AlGaAs barrier layer outside the channel is
Even if the composition ratio is 0.2 or less, it acts as a sufficiently high barrier against electrons, reducing the probability that electrons will overflow from the channel in a high electric field. Therefore, by using the structure of the present invention, a heterojunction field effect transistor having high transconductance and stable characteristics with respect to temperature and light can be realized. Incidentally, the thickness of the In _Z Ga _1-Z As layer, 50A～150A is appropriate, as the value of z is 0.25 or less, is suitable as a condition for not enter dislocation in the film.

[Brief description of drawings]

1 (a) and 2 are sectional views showing the element structure of the field effect transistor as the first and second embodiments of the present invention, and FIG. 1 (b) is the same as the first embodiment. It is a conduction band energy band diagram near a channel layer. 1 ... Semi-insulating GaAs substrate, 2 ... Non-doped GaAs layer, 3,
7 ... Non-doped Al _0.3 Ga _0.7 As layer, 4, 6 ... N-type GaAs layer,
5 ... Non-doped In _Z Ga _1-Z As layer, 8 ... Source electrode, 9
...... Drain electrode, 10 …… Gate electrode.

Claims (2)

[Claims] 1. A channel is sandwiched between AlGaAs barrier layers, said channel layer being an N-type GaAs electron supply layer and non-doped InGa.
A field-effect transistor using a heterojunction structure composed of As quantum well layers. 2. The AlAs composition ratio of the AlGaAs barrier layer is greater than 0 and 0.2 or less, and a part of the AlGaAs barrier layer is doped with N-type impurities.
A field effect transistor according to item.