JP2728481B2 - Ultra-high frequency high electron mobility transistor - Google Patents

Description

It relates DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention plurality of high electron mobility transistors for very high frequency high power with two-dimensional electron gas channel (HEMT), eliminating the drain current I _Ds dependence of the cutoff frequency f _T With the purpose of growing on an insulating or semi-insulating substrate and on said substrate
A high electron mobility transistor comprising first and second two-dimensional electron gas channel layers made of GaAs and an AlGaAs electron supply layer formed between the first and second two-dimensional electron gas channel layers. The thickness t (Å) of the electron supply layer and the concentration N _{D of} the impurity doped into the electron supply layer (1 × 10 ¹⁷ cm
^-3) is configured to satisfy _{t ≦ 380 × (N D)} -0,79.

[Industrial applications]

The present invention relates to an ultra-high frequency high power high electron mobility transistor (HEMT) having a plurality of two-dimensional electron gas channels.

HEMT (High Electron Mobility Transistor) is used in satellite communications and radio astronomy astronomical radio observation in the analog field, and in ultra-high-speed computers and high-speed signal processors in the digital field, and is expected to be a new-generation high-speed LSI. Has been sent.

HEMTs are a type of field-effect transistor (FET), but their operating principles and structure are quite different from ordinary FETs. FIG. 7 shows the basic structure of the HEMT. As shown, this is an undoped GaAs layer on a semi-insulating GaAs substrate, n-type AlGaAs
Layers are sequentially grown by epitaxy, n-type source and drain regions are formed in these layers, and a source electrode S, a drain electrode D, and a gate electrode G are attached. The electron affinity (4.07 eV) of GaAs is Al
Since it is larger than that of xGa (1-X) As (x = 0.3) (3.75 eV), the electrons supplied from the n-AlGaAs donor are GaAs.
Then, a two-dimensional electron gas channel 2DEG having a thickness of about 10 nm is formed at a heterojunction interface between undoped GaAs and n-AlGaAs. In this case, very high electron mobility can be realized because impurity scattering can be ignored. HE
The basic principle of MT is to control the two-dimensional electron gas by the electric field effect of the gate voltage.

[Conventional technology]

As shown in FIG. 8, the substrate structure of the HEMT has an i-GaAs layer.
24 with the n-AlGaAs layer 25 and the i-AlGaAs layer 23 interposed therebetween.
2DEG (T _W0 Dimentional Electron Gas) ones and the 2 heterozygous layer also. This is because the type shown in FIG. 7 has a shortage of drain current, and is intended to solve this problem and increase _IDS to achieve higher output. In this FIG.
Reference numeral 10 denotes a semi-insulating GaAs substrate on which an i-AlGaAs layer 21 and n-Al
GaAs layer 22, i-AlGaAs layer 23, i-GaAs layer 24, n-AlGaAs
Layer 25, graded n-AlGaAs layer 26, and n-GaAs layer 20 are sequentially grown. The example of the thickness of each layer is 0.1
μm, 7 nm, 8 nm, 20 nm, 10 nm, 30 nm, and 200 nm. The n-type impurity concentration is 2 × 10 ¹⁷ cm ^{−3 in} the layer 20 and 2 × 10 ¹⁷ cm ^{−3 in} the layer 25.
¹⁸ cm ⁻³ and 2 × 10 ¹⁸ cm ⁻³ for layer 22.

In FIG. 8, electrons are supplied to the 2DEG layer 24 from both sides thereof with the n-AlGaAs layers 22 and 25 interposed therebetween. To achieve higher output, there is also a type in which a plurality of 2DEG layers are provided and electrons are supplied to both sides of each layer.

FIG. 9 shows typical drain voltage V _D and drain current _ID characteristics of the HEMT. The V _D −I _D characteristic curve is the gate voltage of the HEMT,
(Gate-source voltage) V _GS is used as a parameter. The gate voltage V _GS is set to ABCDE around a certain bias point.
When the sine change is made, V _D and I _D move on the load line l, and if V _GS has a small amplitude, V _D and I _D also change sine as shown in the figure. The output power P ₀ of the relationship of input power P _i at this time is shown in Figure 10.

As shown in FIG. 10, the P _i −P ₀ characteristic is linear in a small input range, but as the input P _i increases, the output P ₀
Begins to saturate. Input P ₁ in the figure is in the linear range, the input
Output P ₂ P ₀ is the maximum value P _0max of the linear region, the output P ₀ the input P ₃ become saturated output _P 0sat. Saturation depends on the gate voltage V
_This is due to the fact that the amplitude of _GS becomes too large and exceeds the points P 'and Q' on the load line.

One of the important characteristics required for a high-output HEMT is linearity of input / output characteristics, and it is desired that the linear range is wide.
For this purpose, I _Fmax and V _{(BR) DS} are large (curve V _GS = + V _bi
Is longer and the horizontal portion of V _GS = −V _bj is longer), and the uniformity of the current change with respect to the gate voltage V _GS, that is, G
equal over the _m in a wide range of V _GS (when changing V _GS at equal intervals each I _D -V _D characteristic so that changes at regular intervals from each other) it is necessary.

The substrate of the ultra-high frequency high power HEMT is one of the two shown in FIG.
For a type having an active layer with a two-dimensional electron gas channel (two heterojunctions), the CV measurement result is
As shown in FIG. The horizontal axis is the distance ([mu] m), the vertical axis and the carrier concentration ^{(10 i cm -3, i =} 16~19). As is clear from the figure, a carrier profile having an extremely sharp peak indicating the formation of the secondary electron gas channel is formed.

Prototype HEMT using this epitaxial wafer,
The results of actual measurement of the 1mm gate width per transconductance g _m (mS / mm) indicated by a curve C ₁ in Figure 12. Gate length Lg is 0.6
μm. Curve C ₁ has a peak per V _GS = -1.5V. As shown in FIG. 13, the input / output characteristics of such a high-output HEMT are as shown in FIG. 13 where the ideal input / output characteristics are A, and the input / output characteristics saturate with much smaller input power.
This is because V _GS = −1.5V and V _GS further away
G _m is small at the point, due to the gain decreases correspondingly.

As a method of solving such a problem, a method of providing a plurality of two-dimensional electron gas channels (multi-channel) has been considered. This is aimed to be a broad flat portion of the peaks of the curve C ₁ in a multi-channelization.

FIG. 1 shows an epi structure of a multi-channel high-power HEMT substrate. This is an i-GaAs layer 11 on a semi-insulating GaAs substrate 10,
i-GaAlAs layer 12, n-AlGaAs layer 13, i-GaAs layer 14, n-
AlGaAs15, i-GaAs layer 16, n-AlGaAs17, i-GaAs layer 1
8, an n-AlGaAs layer 18, an n-AlGaAs layer 19, and an n-GaAs layer 20 are sequentially grown by epitaxy. x = 0.25, n = 1 × 10 ¹⁸ cm ⁻³ , and the thickness of each layer is 0.5 μm for 11; 0.2 μm for 12;
nm, 14 and 16 and 18-20 are 30 nm, 15 and 17 are 15 nm, the spacer is
5 nm. i-Ga surrounded by n-AlGaAs layers 19, 17, 15, and 13
The As layers 18, 16, and 14 become two-dimensional electron gas channels. That is, this substrate has a multi- (3) channel structure.

Using this epi wafer, about 800 mm from the wafer surface
Make a Schottky gate at the place
HEMT was prototyped and CV measurement was performed. Second result
Shown in the figure. Two large peaks and four small peaks are observed in this carrier profile. Among these peaks, P ₁ and P ₂ corresponds to the second 2DEG channel 16, P ₃ and P ₄ corresponding to the third of the 2DEG channel 14.

In this example, since there are three layers of two-dimensional electron gas (2DEG) channels, it is considered that three large peaks may be observed. However, only two are actually observed. This is the first two-dimensional electron gas (1st 2DE
G) This is because the channel 18 is formed in the channel 18 and the channel 18 is depleted.

Shows the transconductance g _m of the HEMT curve C ₂ in Figure 12. The gate length Lg is also 0.6 μm. Gate voltage compared to curve C ₁ (using two heterojunction type substrates in FIG. 8)
To exhibit g _m high values over a wide range of V _GS, it is characteristic that the peak of g _m is in two. These two peaks correspond to the two large peaks in FIG.

In FIG. 3, cut-off frequency f _T which is measured in this HEMT (GHz)
The drain current I _DS (mA) dependence of Curve C ₂ is first
FIG substrate using the HEMT, the curve C ₁ is the substrate used in Figure 8 HEMT
It is the characteristic of. Since the drain current I _DS is changed by changing the gate voltage V _GS , the horizontal axis may be considered as the gate voltage V _GS . The curve C1 has _one peak, and the curve C2 has _two peaks. This unimodal transconductance g _m of Figure 12, correspond to the double-humped _{(f T = g m / 2πC} gs).

The input / output characteristics of such a multi-channel high-output HEMT having two peaks at g _m and f _T are as shown by a curve C in FIG. That is, a small place in the output P ₀ is reduced in the input P _i (this portion corresponds to the recessed portion of the g _m, f _T), the input P _i is Ho and that of HEMT gain single channel structure as becomes largeヾ They become the same, and finally, the input / output characteristics show linearity up to the part with higher output.

[Problems to be solved by the invention]

However, the input / output characteristics as shown by the curve C in FIG.
The gain is small at small input and large input, the gain is large only at the middle part,
Therefore, the distortion of the amplifier is significantly increased.

To improve small gain at the input P _i is small, eliminate the bimodal characteristic of the cutoff frequency f _T, transconductance g _m
Eliminate the V _GS-dependent (to flat relative to the g _m V _GS)
It is necessary.

That is, the present invention is intended to be an f _T -I _DS curve C ₂ bimodal characteristic of FIG. 4 thus flat dotted curve C _3, eliminating the drain current I _DS dependence of the cutoff frequency f _T It is.

[Means for solving the problem]

The high electron mobility transistor (HEMT) of the present invention has an active layer epitaxially grown on an insulating or semi-insulating substrate, and the active layer has three or more heterojunctions and a plurality of two-dimensional electron gases. A substrate with channels is used.

The layer sandwiched between the plurality of two-dimensional electron gas channels is made of a material having a smaller electron affinity than the material constituting the two-dimensional electron gas channel. In the present invention, the thickness t of the layer of this material (unit is: Å), the following relationship is given to the concentration N _D (unit: 1 × 10 ¹⁷ cm ⁻³ ) of the impurity doped in the material.

t380 × (N _D) ^-0.79 [action] Figure 2, recessed transconductance g _m shown in FIG. 3, recessed cut-off frequency f _T, and the curve C in FIG. 13 in which these characteristics are reflected The distortion is the 2nd2DEG channel 1 in FIG.
Entered into between 6 and 3rd2DEG channel 14, the impurity concentration N _D and the thickness t of the n-AlGaAs layer 15 serving as the electron supply layer is due to the fact that not optimized. That is, if the impurity concentration of the n-AlGaAs layer 15 is too high or the thickness is too thick, residual carriers are present in the layer 15 and the above-described phenomenon (dent or distortion) occurs. That is, if there are residual carriers in the electron supply layer 15, this layer also contributes to the drain current, but the electron mobility in the layer 15 is low. g _m and f _T are related to the electron mobility, and the lower this is, the worse the g _m and f _T are. The electron supply layer 15 is 2D
Electrons are supplied to the EG, but it is better not to be a current path.

6A and 6B show a hand diaphragm when the thickness t of the n-AlGaAs layer 15 is changed. FIG. 6A shows a case where a neutral region (electrons collect in this portion) is sufficient, and FIG. and if the neutral region has just disappear, the difference between the bottom of (c) is the Fermi level E _F and conductive layer to further reduce the thickness 3kT
This is the case. the impurity concentration N _D is too high or the thickness t of the n-AlGaAs layer 15 with or too thick there is a residual carrier in the layer, a state of FIG. 6 (a) or (b). As will reduce the residual carriers in the layer shown in Figure No. 6 (c), it is desirable to base E _c and the Fermi level E _F of the conduction band are separated by more than 3 kT, in the layer In this way The residual carrier is the concentration of impurities in the layer (n-AlGaAs).
Of 5% or less of N _D. That is, the residual carriers in if N _D 10 ¹⁸ the order said layer becomes 10 ¹⁶ order. FIG. 6 (b)
Then it is in the order of 10 ¹⁷ .

When the relationship between _ND and t is obtained under such conditions, FIG. 5 is obtained. That is, if N _D is 2 × 10 ¹⁸ t is 36Å or less, 1 × 10
If it is ^18, the condition shown in FIG. 6 (c) is obtained at 62 ° or less and 5 × 10 ^{17 at} 107 ° or less. That N _D and t is if as in the inside of the curve C of FIG. 5 (shown hatched),
The depression of g _m and f _T and the non-linearity of the input / output characteristics almost disappear. Curve C can be approximated by the following equation, t = 380 × N _D ^-0.79 therefore necessary t, the relationship N _D may be a t380 × (N _D) ^-0.79. However, the unit of t is Å, the unit of the N _D 10
¹⁷ cm ^-3 .

〔Example〕

The substrate structure of the HEMT of the present invention is, for example, three 2DEGs shown in FIG.
It may have layers 14, 16, 18. Assuming that the thicknesses of the layers 11 to 20 of the substrate shown in FIG.
The thickness 15 nm, since the N _D is 1 × 10 ¹⁸ cm ^-3, on one side 75Å
6 (b). 6 (c), the thickness of one side should be 62 ° or less from FIG. 5, and the thickness of the layer 15 should be 12.4 nm or less.

If the 1st2DEG layer 18 is not depleted even when the gate of the recess structure is attached by thickening the uppermost n-GaAs layer 20 or the like, the peaks are reduced to three in FIGS. 2 and 3. Become
More extensive and ho Isuzu flat g _m, f _T characteristics. To have a relationship t380 × (N _D) ^-0.79 also in this case n-AlGaAs17. Note t380 × (N _D) ^-0.79 is inside of the curve C of FIG. 5, an approximation equation showing the, thus the form of the formula can be modified. In short, the electron supply layers 15, 17, surrounded by 2DEG layers ...
... it is is (to僅小) having no residual charge as to determine the thickness t and the impurity concentration N _D.

A gate electrode is attached using the substrate of FIG.
Drain regions are formed, and source / drain electrodes are attached to these regions. The procedure is the same as in FIG. The source / drain regions are formed from the surface to around the layer 12.

〔The invention's effect〕

As described above, according to the present invention, it is possible to flatten the g _m and f _T characteristics in a multi-channel type high-output HEMT and expand the linear range of input / output characteristics from low to high input.

[Brief description of the drawings]

FIG. 1 is an explanatory view of the substrate structure of the HEMT of the present invention, FIG. 2 is a graph showing an example of characteristics of the substrate of FIG. 1 by CV measurement, and FIG. 3 is a diagram of the HEMT using the substrate of FIG. graph showing f _T characteristics graph Figure 4 is for explaining a desirable f _T characteristics, Figure 5 is desirable t, graphs showing the relationship between N _D, band diagram when FIG. 6 is obtained by changing the thickness of the AlGaAs layer , FIG. 7 is an explanatory diagram of the structure of the HEMT, FIG. 8 is an explanatory diagram of an example of the substrate of the HEMT, FIG. 9 is a diagram of the I _D and V _D characteristics of the HEMT, and FIG. graph, Fig. 11 is a graph showing a characteristic example by C-V measurement of the substrate of FIG. 8, FIG. 12 is a graph showing the g _m characteristic example, FIG. 13 is a graph showing an input-output characteristic example. In FIG. 1, 10 is an insulating or semi-insulating substrate, (1 to 20 are epitaxially grown layers, 14, 16, and 18 are layers forming a two-dimensional electron gas channel, and 13, 15, 17, and 19 are materials of the layers. This is a layer of a material having a smaller electron affinity than that of the material.

Claims (1)

(57) [Claims] An insulating or semi-insulating substrate; first and second two-dimensional electron gas channels made of GaAs grown on the substrate; and first and second two-dimensional electron gas channels. In a high electron mobility transistor having an AlGaAs electron supply layer formed between layers, the electron supply layer has a thickness t (Å) and an impurity concentration N _D (1 × 10 ¹⁷ ) doped into the electron supply layer. cm ⁻³ ) satisfying t ≦ 380 × (N _D ) ^−0.79 .