JPH0496334A - Hetero-junction bipolar transistor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a heterojunction bipolar transistor using a compound semiconductor.

(b) Conventional technology: heterojunction bipolar transistor (hereinafter referred to as I-IBT)
(abbreviated)) are attracting attention as next-generation ultra-high-speed devices because they have high driving capacity and excellent high-frequency characteristics. In order to increase the speed of I-IBT, shortening the base running time is effective.

Therefore, recently, so-called composition-graded AlGaAs, in which the composition in the thickness direction is graded, are being used as the base layer (7).
An attempt has been made to shorten the base transit time by creating an internal i-field in the base layer and accelerating electrons (G1/- dead base structure AIGλA
"S/G a A S Heterojunction Bipolar Transistor", Applied Electronics Materials Subcommittee Research Report No. 410. I
P-'/4, Japan Society of Applied Physics (1985)) 8 is rice grading A I G
The structure of aAs/GλAs HB”r is shown in FIG.

As shown in the figure, on a semi-insulating GaAs substrate 21, n'
-GaAs collector contact l-111122, nGa
Asco I/Cuta layer 23, p A I IIG a +
-r, AS (x = O-0, 1) base J124 (
composition graded), n A 1 nG a +,,
gA s (x = 0.1-(・0.3) emitter transition 11125, n A l mG a, +-++A
s (X = 063) emitter layer 26 and n −G^
As emitter contacts 27 are sequentially formed.

However, 12. The direction (-) of composition grading is shown in the direction of Evitagi soyal growth. An emitter electrode 28 is formed on the emitter contact layer 27, and base and collector electrodes 29 and 210 are formed on the base layer 24 and collector contact layer 22 exposed by mesa etching, respectively.

FIG. 3 shows an energy band diagram of such a graded base HBT. Electrons injected from the emitter to the base have an internal electric field Ebi (=ΔEg/Wb, JE
g: energy gap difference between the emitter end and the core tough end of the base layer, Wb: base layer thickness), and is accelerated and travels through the base layer. In the structure shown in FIG. 2, when the thickness of the base layer 24 is 1100 nm, Ebi is approximately 12 kV.
/a@ becomes.

(c) Problem to be solved by the invention In order to shorten the base transit time of electrons and make the HBT faster, in the HBT as shown in Fig. 2, AlG1A
It is conceivable to increase the A1 composition on the emitter end side of the s-base layer 24 to further increase the internal electric field.

However, as the A1 composition of AIGaAs increases, the base layer sheet resistance increases due to crystallinity deterioration and the base contact resistance increases due to barrier hydride to the base ohmic electrode. Adversely affect. Furthermore, the energy barrier to injection of holes from the base layer to the emitter layer becomes low, leading to a decrease in the efficiency of electron injection into the base layer, which tends to deteriorate the current gain.

Therefore, as the At composition increases at the emitter end of the A I GaAs base layer, the A of the AlGaAs emitter layer increases.
In principle, it is possible to prevent the drop in injection efficiency by increasing the t composition, but in this case, the number of recombination centers such as DX centers will increase significantly in the AlGaAs base layer, which will cause electron This causes the inconvenience of decreasing injection efficiency.

The present invention was made in view of these conventional problems, and it is possible to increase the internal electric field of the base without increasing the base resistance or decreasing the emitter injection efficiency.
The purpose is to provide an HBT with improved speed.

(d) Means for Solving the Problems In order to achieve the above objects, the present invention provides an emitter layer in a heterojunction bipolar transistor using AI Ga
While it is composed of A s layer, the base layer! nAIGλA
It is characterized by being composed of a compositionally graded layer consisting of s.

The heterojunction bipolar transitus in the present invention is
Basically, n-G
It can be constructed by forming a collector layer such as aAs, and arranging the base layer and emitter layer on top of this, and each collector layer, base layer, and emitter layer has a collector electrode, a base electrode, and an emitter layer. The electrodes are connected to each other. Here, between the base layer and the collector layer and between the base layer and the emitter layer, there is a composition transition layer (having a composition similar to that of the adjacent layer on the negative side) to alleviate the composition mismatch between each layer. However, a composition gradient layer whose composition is similar to that of the adjacent layer may be provided on the other side.

Further, it is preferable that the collector electrode and the emitter electrode are respectively connected to the collector layer and the emitter layer through a low resistance contact layer such as nG1As.

The base layer of the present invention is composed of a compositionally graded r n AI Ga As layer whose composition is graded in the thickness direction. Such a base layer is doped with p-type impurities (npnH
BT) or n-type impurity doping (pnpHBT). In terms of device characteristics, the composition gradient is preferably such that the In content of InAlG1As decreases and the Al content increases from the substrate side to the emitter side of the base layer. Here, the compositional gradient may be a continuous (linear) gradient or a stepwise gradient. Such an InAlG1As layer can be obtained by continuously or intermittently changing the supply amount of In source gas, Al source gas, etc. using a known method such as vapor phase epitaxial or MBE.

On the other hand, the emitter layer used in the present invention is composed of an AIGλA$ layer doped with appropriate impurities, and the layer itself is a well-known AIGλA$ layer.
A layer of lGaAs can be applied.

(E) Function Since the In and Al compositions of the XnAIGzAs base shoulder used in the present invention can be changed, there is a higher degree of freedom in designing the base structure compared to the conventional AlGaAs graded base. Here, the most practical and effective examples will be selected and the present invention will be explained in detail.

Figures 4(a) and (b) show the conventional AI
-xA s graded base and In yA L mG +Lo of an example of the present invention, showing the band structures of sA S graded base, respectively.Axm component X is a straight line from O to 0.1 from the collector side to the emitter side. It is changing. InyA l of the present invention
In mG aO,9A s graded base, In
1L composition y is changed linearly from 0.1 to 0 in the same direction.

It can be seen from FIG. 4 that although the base layer of the present invention is made of AlGaAs with the same A1 composition as the conventional base layer at the emitter end, it has a JE larger by about 0.1@V than that of the conventional structure. When the base layer thickness is 1100n in the II/ structure in Figures 4(a) and (b), the internal electric field is 1, respectively.
2kV/am, 23kV/c+a to FIF. Mame, ;, hex F91 te+7)1 & child transfer a1
Assuming that 1 degree 81200cm'/V=see, the drift speed is 1,4xio'''cyn/5ecs, respectively.
2.4X l O'am/sea. As described above, in the case of the base layer of the present invention, the base transit time of electrons can be shortened without increasing the A1 composition by 17 times compared to the conventional one, that is, without increasing the base resistance.
It is possible to improve the speed of T.

(f) Example The embodiments of the present invention will be described below.

i-! of the present invention! B T is an n GaAs substrate 1 on a semi-insulating GaAs (100) substrate 11 as shown in Fig.
Noktacontact l-11912, n-G a, A
s:] Retaku Fl113, n-1nGaAs co1nocta transition layer 14, composition graded p-InAlGaAs
Base width 15, n AlGaAs emitter transition [16
, n-AlGaAs emitter l1117, n-Ga
, As emitter contacts 91B are actively inverted.

Such HBT has a mesa structure, and at the top of each mesa: X-mitter, base, collector 11EIfi19.20
21 are formed respectively. In addition, in the above-mentioned bitwise structure, the collector transition a and the emitter transition 714.16 are for intensifying the spike of the conduction 1h at each interface. To explain the production of such H B T, first, L E C semi-insulating (1)>10'
n-GaAs (Si doped, 5X
1 O"am-", 500 nm)=il nocta contact layer 12, n-GaAs (St doped, 5X 10 l
llam-', 500 nm): ff177 layer 13, n-
I nxGat-+aAs (x=Q-eo, l, S
t doped, 5XlO"am-", 20nm) collector transition $814, p InmAlo, t-1lG&a
, 5As(X””0.1□0, Be-doped, lXl0”
am-' 100nrn) base layer 15, n A
I XG a i-, A s (x = O, l→0.3
, St doped, 5 X 10 "es-", 20
nm): T-mitter transition mts, n-Ale, 5G
ao, tAS (x=0°3, St doped, 5x 1
0”cm-”l OOnm) Emitter s17, n-
GaAs (St doped, 5XIO1lem-'1
50 nm) Emitter contact) MlB is sequentially grown in layers. Next, a mesa structure is formed by wet etching, and after M, emitter, base, and collector transitions are vaporized at the top of each mesa, and further alloying heat treatment is performed to complete the device.

Emitter and: +1 nocta' i-pole are Au-Ge based,
・-S [A Δu-Z n-based ohmic metal was used for the pole.

A conventional graded base HBT was fabricated using the same process for characteristic comparisons f and -h. The shrimp structure consists of a base layer (p-
AImGa+-nAs, , X=0→0.1% Be-doped, 1 x l O"ca+-' 100 nrn)
and the absence of a collector transition layer (therefore,
The embodiments of the present invention are the same except that the conventional HBT has a thinner collector layer. As a result of RF measurement, the cutoff frequency Ft = 71 GHz (Jc = 8 x
On the other hand, in the conventional graded base HBT, F t = 60 G In4 Z under the same emitter size and measurement conditions, thus demonstrating the effectiveness of the present invention. Ta.

Note that the base layer of this example has A I G at the emitter end.
a A s composition, In Ga A at the collector end
It has a structure with s composition, but other structures (for example, I
til,AlxGaxAs,y=o,,2-”0,x
= o -0°1, z = 0.8-0.9).

Further, although the present embodiment uses an npn type HBT, a pnp type may also be used.

(g) Effects of the invention As described above, according to the present invention, I
By using the nAIGaAs graded base, the base transit time of electrons can be shortened by increasing the base resistance, making it possible to further increase the speed of HBT.

[Brief explanation of drawings]

FIG. 1 is a configuration explanatory diagram showing an example of the graded base HBT of the present invention, and FIG. 2 is a conventional graded base HBT.
BT configuration explanatory diagram, Figure 3 is graded base HB
The band diagrams of T, FIGS. 4(a) and (b) are respectively the conventional A I I I G a 1-i A! ! Band structure of graded base and In+iA1 of the present invention
It is an explanatory diagram showing the band structure of ol-*G ao, eAs graded base. 11. 21...G a As semi-insulating substrate, 1
2.22...n GaAs collector contact layer 13.23-n-GaAs collector layer, 14-=-n
-InAIGaAs collector transition layer, 15-p-1nA
I GaAs base layer, 24----”p Al
GaAs base layer, 16.25-n -AIGaAs emitter, 17.26...n -A I G
a A s D - Mitter layer・18.27・・・・・・n
"GaAs emitter contact, 19.28...Emitter electrode, 110.29...Base electrode, 111.210
...Collector electrode. 1st defense q *3 Figure scales 2!1ly (1mi, ivy) (base) (kolkuta) Alx Gap-xAs GaAs Iny Alx Ga0.9AS 1 n O,lGa0,9A5

Claims (1)

[Claims] 1. In a heterojunction bipolar transistor in which the base layer is made of a semiconductor material with a smaller band gap than the emitter layer, the emitter layer is made of an AlGaAs layer,
InAlGaA whose base layer has a gradient composition in the thickness direction
A heterojunction bipolar transistor comprising an S layer.