JP3952815B2 - Heterojunction bipolar transistor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor (HBT), and more particularly to an improvement in technology for improving its β (current amplification factor) and device reliability.
[0002]
[Prior art]
A heterojunction bipolar transistor (HBT) using a heterojunction as an emitter-base junction can increase the emitter injection efficiency because the band gap of the emitter layer is wider than the band gap of the base layer. It is expected to be used as a high-power device. In particular, HBTs made of AlGaAs / GaAs are highly developed as high-speed electronic devices for optical communication because they are excellent in high-speed performance and high-current drive capability.
[0003]
FIG. 4 shows the structure of a conventional HBT. The conventional HBT structure will be described from the bottom layer. On the semi-insulating GaAs substrate 11, an n ⁺ -GaAs layer (carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) of the subcollector layer 12 for reducing the collector resistance. And n ⁻ −GaAs layer (carrier concentration 3 × 10 ¹⁶ cm ⁻³ ) of the collector layer 13, p ⁺ −GaAs layer (carrier concentration 4 × 10 ¹⁹ cm ⁻³ ) of the base layer 14, and the emitter layer 15. The n-AlGaAs graded layer 15a, n-AlGaAs layer 15b, and n-AlGaAs graded layer 15c (carrier concentration of each emitter layer 5 × 10 ¹⁷ cm ⁻³ ) and the ohmic contact resistance of the emitter layer are reduced. n ⁺ -GaAs layer 16a constituting the emitter cap layer 16 for (carrier concentration ^{5 × 10 18 cm -3),} n + -InGaAs graded layer 16b (carrier Degrees 2 × 10 ¹⁹ cm ^-3), and n ⁺ -InGaAs layer 16c (carrier concentration 2 × 10 ¹⁹ cm ^-3), and has a configuration in which sequentially stacked.
[0004]
Subcollector layer 12, collector layer 13, the n ⁺ -GaAs layer 16a in the emitter layer 15 and emitter cap layer 16, is a Si (silicon) as the donor, i.e. n-type dopant is doped, also the emitter cap layer 16 n ⁺ The InGaAs graded layer 16b and the n ⁺ -InGaAs layer 16c are doped with Se (selenium) as an n-type dopant, and are doped with C (carbon) as an acceptor of the base layer 14, that is, a p-type dopant.
[0005]
[Problems to be solved by the invention]
By the way, it is important for the HBT to improve the current amplification factor β and to improve the reliability of the element. For this reason, much research has been conducted, and the characteristics of the HBT layers have been improved.
[0006]
However, the current amplification factor β and the reliability are still low, and further improvement is necessary. In particular, the reliability of the element is poor, and it has been a problem that the current amplification factor β is significantly lowered as the energization time is increased.
[0007]
FIG. 5 shows the fluctuation of the energization time and current amplification factor β of the conventional HBT. As shown in FIG. 5, in the conventional HBT, as the energization time increased, the current amplification factor β decreased significantly, and the energization time of 600 hours was 70% of the initial value.
[0008]
Accordingly, an object of the present invention is to solve the above problems and improve and stabilize the current amplification factor β of the heterojunction bipolar transistor and improve the reliability of the element.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as follows.
[0010]
The present invention provides an n ⁺ -type GaAs subcollector layer for lowering the collector resistance on a GaAs (gallium arsenide) substrate, an n ⁻ -type GaAs collector layer doped with Si, and a p doped with C. ^{A +} -type GaAs base layer, an n-type AlGaAs (aluminum gallium arsenide) emitter layer, and an n ⁺ -type GaAs layer for reducing ohmic contact resistance of the emitter layer or an emitter cap layer composed of this and an InGaAs layer In the heterojunction bipolar transistor thus configured, the GaAs collector layer is doped with C (carbon) at a concentration lower than that of the n-type dopant Si, and the doping concentration of C is 5 × 10 ¹⁵ cm ⁻³ to It is characterized by being 2 × 10 ¹⁶ cm ⁻³ .
[0015]
<Key points of the invention>
The collector layer made of n-type GaAs is usually doped with, for example, Si as a donor. In the present invention, as a means for solving the above problem, the acceptor C (carbon) is further doped into the collector layer doped with Si. However, the doping amount is set so that the acceptor concentration is smaller than the donor concentration (donor concentration> acceptor concentration).
[0016]
As a result, the current amplification factor β increases, and even if the energization time is increased, the current amplification factor β is not significantly reduced and stabilized, and the reliability of the HBT element is improved.
[0017]
The C concentration doped in the collector layer is in the range of 5 × 10 ¹⁵ cm ⁻³ to 2 × 10 ¹⁶ cm ⁻³ . The increase in current amplification factor β is remarkably increased from the C concentration of 5 × 10 ¹⁵ cm ⁻³ , and thereafter the current amplification factor β gradually increases. However, if it exceeds 2 × 10 ¹⁶ cm ⁻³ , it cannot be measured. is there.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on the illustrated embodiments.
[0019]
FIG. 1 shows the structure of an epitaxial wafer of the HBT of the present invention.
[0020]
In FIG. 1, reference numeral 1 denotes a semi-insulating GaAs substrate on which a Si-doped n ⁺ -type GaAs subcollector layer 2 (thickness 500 nm, carrier concentration 5 × 10 ¹⁸ cm ⁻³⁾ for reducing collector resistance. ), A Si and C-doped n ⁻ -type GaAs collector layer 3 (thickness 1000 nm, carrier concentration 3 × 10 ¹⁶ cm ⁻³ ), and a C-doped p ⁺ -type GaAs base layer 4 (carrier) that controls the flow of electrons. And a Si-doped n-type emitter layer 5 that forms a heterojunction with the base layer, injects electrons into the base layer, and suppresses injection of holes from the base layer, and a concentration of 4 × 10 ¹⁹ cm ⁻³ In this structure, an Se-doped n-type emitter cap layer 6 for sequentially reducing the ohmic contact resistance of the emitter layer and forming an ohmic contact with the metal electrode is laminated.
[0021]
The reason why carbon C is used for the p-type impurity of the base layer is that C (carbon) has a smaller diffusion constant than zinc (Zn), beryllium (Be), etc., and is steep at the boundary between the collector layer and the emitter layer. This is because it is easy to form a p-type impurity layer having a high concentration gradient.
[0022]
The emitter layer 5 is an n-type Al _x Ga _1-x As (x = 0 → 0.3) graded layer 5a (AlAs mixed crystal ratio x is gradually increased from 0 to 0.3 and is grown. About 20 nm, Si-doped carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) and n-type Al _x Ga _1-x As (x = 0.3) layer 5b (thickness about 80 nm, Si-doped carrier concentration 5 × 10) ¹⁷ cm ⁻³ ) and n-type Al _x Ga _1-x As (x = 0.3 → 0) graded layer 5c (AlAs mixed crystal ratio x was gradually reduced from 0.3 to 0, and was grown. And a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ ) by Si doping.
[0023]
The emitter cap layer 6 includes an n ⁺ type GaAs layer 6a (thickness 100 nm, carrier concentration 5 × 10 ¹⁸ cm ⁻³ ), an n ⁺ type In _y Ga _1-y As graded layer 6 b (InAs mixed crystal ratio y). Grown by gradually increasing from 0 to 0.5, thickness about 50 nm, carrier concentration by Se doping 2 × 10 ¹⁹ cm ⁻³ ), n ⁺ -type In _y Ga _1-y As layer 6c (y = 0. 5 and a thickness of about 50 nm and a carrier concentration of 2 × 10 ¹⁹ cm ⁻³ ).
[0024]
The n-type GaAs collector layer 3 is doped with two types of impurities, one each of donor Si and acceptor C, in such a relationship that the acceptor concentration is lower than the donor concentration. Here, the donor Si having a carrier concentration of 1 × 10 ¹⁶ cm ⁻³ is doped, and the collector layer 3 is doped with an acceptor C having a carrier concentration of 2 × 10 ¹⁶ cm ⁻³ . The total carrier concentration by doping of these two kinds of impurities was set to 3 × 10 ¹⁶ cm ⁻³ .
[0025]
The HBT epitaxial wafer was fabricated using metal organic vapor phase epitaxy (MOVPE). At that time, as the type of dopant gas, CBr ₄ to C (carbon tetrabromide), in S i using Si ₂ H ₆ (disilane).
[0026]
Using the above-described epitaxial wafer for HBT, metal electrodes were provided on the subcollector layer, base layer, and emitter layer, respectively, to produce an HBT.
[0027]
When the current amplification factor β was compared between the HBT having the conventional structure and the HBT having the structure of the present embodiment, the current amplification factor β was 120 in the case of the HBT having the conventional structure. The amplification factor β was 170, and the current amplification factor β was greatly improved.
[0028]
Therefore, a reliability test of the HBT according to this embodiment was performed. FIG. 2 shows the result of this reliability test (variation in current amplification factor β with the passage of energization time of the HBT). As shown in the figure, the current amplification factor β of the HBT according to the present embodiment was greatly improved by 98% of the initial value at the energization time of 600 hours and 94% of the initial value at 2000 hours.
[0029]
Next, the degree to which the C concentration (acceptor concentration) of the collector layer 3 gives to the current amplification factor β was examined.
[0030]
FIG. 3 shows the relationship between the C concentration of the collector layer 3 and the current amplification factor β. The scale of C concentration on the horizontal axis means, for example, 5E + 15 and 5 × 10 ¹⁵ cm ⁻³ .
[0031]
C concentration is 1 × 10 ¹⁴ cm ⁻³ , 5 × 10 ¹⁴ cm ⁻³ , 1 × 10 ¹⁵ cm ⁻³ , 5 × 10 ¹⁵ cm ⁻³ , 1 × 10 ¹⁶ cm ⁻³ , 2 × 10 ¹⁶ cm ⁻ Experimented with ³ . Note that for C concentrations of 2 × 10 ¹⁶ cm ⁻³ or more, it is impossible to adjust the n-type carrier concentration (3 × 10 ¹⁵ cm ⁻³ ), so experiments cannot be performed (the same concentration of donor and acceptor). The collector layer becomes depleted or p-type).
[0032]
As a result of the experiment, as shown in FIG. 3, it was found that the current amplification factor β gradually increased as the C concentration increased from 5 × 10 ¹⁵ cm ⁻³ or more. That is, the current amplification factor β can be increased by setting the C concentration as an acceptor of the collector layer 3 in the range of 5 × 10 ¹⁵ cm ⁻³ to 2 × 10 ¹⁶ cm ⁻³ . The reason why the lower limit is set to 5 × 10 ¹⁵ cm ⁻³ is that an increase in the current amplification factor β is remarkably recognized from the C concentration of this value, and the upper limit is set to 2 × 10 ¹⁶ cm ⁻³ . This is because of the restriction that the maximum value can be actually measured for the above reason.
[0033]
From this experimental result, it has been found that doping the collector layer 3 with C as an acceptor improves the film quality of the collector layer. In addition, this is considered to improve the current amplification factor β and the reliability of the element.
[0034]
In the above embodiment, CBr ₄ is used as the dopant gas for C, but another usable material is BrCCl ₃ (bromotrichloromethane).
[0035]
If the collector layer is grown using TMG (trimethylgallium arsenide: chemical formula (CH ₃ ) ₃ Ga) as a source gas, the C concentration can be easily adjusted by adjusting the flow rate of arsine without using a dopant gas. Can be adjusted (also called auto-doping).
[0036]
【The invention's effect】
As described above, according to the present invention, an n-type GaAs subcollector layer, an n-type GaAs collector layer, a p-type GaAs base layer, and an n-type AlGaAs for lowering the collector resistance are formed on a GaAs substrate. In the heterojunction bipolar transistor configured by stacking an emitter layer and a GaAs layer exhibiting n-type conduction for reducing ohmic contact resistance of the emitter layer or an emitter cap layer made of an InGaAs layer, the GaAs collector In the layer, one kind of each of a donor and an acceptor, for example, Si as a donor and C as an acceptor, a total of two kinds of impurities, having a specific C concentration because the acceptor C concentration is lower than the Si concentration of the donor . it is obtained by the doping.
[0037]
As a result, the current amplification factor β is increased, and the reliability of the HBT element is stabilized because the current amplification factor β is not significantly reduced as the energization time increases. Therefore, according to the present invention, it is possible to realize a current amplification factor β and a highly reliable HBT.
[Brief description of the drawings]
FIG. 1 is a view showing the structure of an HBT epitaxial wafer according to an embodiment of the present invention.
FIG. 2 is a diagram showing test results of reliability (relationship between energization time and current amplification factor β) of the HBT of the present invention.
FIG. 3 is a diagram showing a relationship between a collector layer C concentration (acceptor concentration) and a current amplification factor β;
FIG. 4 is a view showing the structure of a conventional HBT epitaxial wafer.
FIG. 5 is a diagram showing the reliability (relationship between energization time and current amplification factor β) of a conventional HBT.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 GaAs substrate 2 Subcollector layer 3 Collector layer 4 Base layer 5 Emitter layer 5a AlGaAs graded layer 5b AlGaAs layer 5c AlGaAs graded layer 6 Emitter cap layer 6a GaAs layer 6b InGaAs graded layer 6c InGaAs layer

Claims (1)

An n ⁺ -type GaAs subcollector layer for lowering the collector resistance on the GaAs substrate, an n ⁻ -type GaAs collector layer doped with Si , a p ⁺ -type GaAs base layer doped with C , In a heterojunction bipolar transistor composed of an n-type AlGaAs emitter layer and an n ⁺ -type GaAs layer for reducing the ohmic contact resistance of the emitter layer or an emitter cap layer made of this and an InGaAs layer,
The GaAs collector layer is doped with C (carbon) at a concentration lower than that of the n-type dopant Si, and the doping concentration of C is set to 5 × 10 ¹⁵ cm ⁻³ to 2 × 10 ¹⁶ cm ⁻³ . A heterojunction bipolar transistor characterized by the above.

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