JPH0620070B2 - Heterojunction bipolar transistor - Google Patents

Description

The present invention relates to a heterojunction bipolar transistor.

[Prior art]

In the heterojunction bipolar transistor, the forbidden band width of the emitter layer can be set larger than that of the base layer. Therefore, even if the base layer is heavily doped to reduce the base resistance, the emitter injection efficiency does not decrease. . For this reason, with the recent progress of molecular beam epitaxial (MBE) technology and metal organic chemical vapor deposition (MOCVD) technology, research and development of heterojunction bipolar transistors using compound semiconductors have been actively conducted.

In order to further increase the frequency and speed of a compound semiconductor heterojunction bipolar transistor having such excellent characteristics, for example, in the case of an npn transistor, not only the electron transit time in the base layer is shortened but also the collector layer is shortened. It is also important to shorten the electron transit time in the depletion layer part of the.

FIG. 3 is a band configuration diagram of an example of a conventional heterojunction bipolar transistor.

In this conventional example, a p-type base layer 5'and an n-type emitter layer 6 'having a smaller electron affinity and a wider band gap than the base layer 5'are sequentially provided on an n-type collector layer 3'. It has a structure and is injected from the emitter layer 6'and the base layer 5 '.
Electrons 9 ', which have reached the depletion layer of the collector layer 3'after passing through, are accelerated by the high electric field in the depletion layer and drift as shown by an arrow 10'.

[Problems to be solved by the invention]

In the above-mentioned conventional heterojunction bipolar transistor, the electric field strength in the depletion layer in the portion in contact with the base layer of the collector layer is 10 to 50 kV / cm, so electrons in the conduction band are subjected to valley scattering and the average drift velocity is reduced. However, it has the drawback of impairing high-speed and high-frequency performance.

The maximum electron velocity υsat due to such drift is
For example, even in the case of GaAs, it is at most 1 to 1.5 × 10 ⁷ cm / sec. If the thickness of the depletion layer is 1000 Å, the transit time of drift electrons in the depletion layer is 0.7 to 1 psec, which is the same as the transit time of the base layer. Is a value that cannot be ignored.

An object of the present invention is to provide a heterojunction bipolar transistor excellent in high-speed and high-frequency performance capable of ultra-high-speed operation in which the transit time in the depletion layer is significantly reduced by causing electrons to fly so-called paristically in the depletion layer of the collector layer. To provide.

[Means for solving problems]

The heterojunction bipolar transistor of the present invention comprises a heterojunction bipolar transistor having a base layer of opposite conductivity type formed on a collector layer of one conductivity type and an emitter layer formed on the base layer and having a band gap wider than the base layer. In the transistor, at least one impurity layer of one conductivity type having a predetermined thickness is provided at the interface between the collector layer and the base layer,
The electron affinity decreases in the order of the collector layer, the impurity layer, and the base layer.

[Action]

According to such a heterojunction bipolar transistor of the present invention, the electrons that have passed through the base layer are
Only the discontinuity of the conduction band energy existing at the collector interface is received as kinetic energy and injected into the collector side. The injected electrons then fly ballistically over the mean free path to lose most of the kinetic energy, at which time they reach the energy discontinuity in the next conduction band and receive kinetic energy again. Conducts by ballistic flight. As described above, since electrons pass through the depletion layer of the collector layer while repeating ballistic flight, it is possible to prevent a decrease in velocity due to valley scattering and to significantly reduce the transit time in the depletion layer.

〔Example〕

 Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention.

In this embodiment, a semi-insulating substrate 1 is partitioned by an insulating region 1a formed by ion implantation of protons, Si is used as a dopant, an impurity concentration is 5 × 10 ¹⁸ atom / cm ³ , and a thickness of 5000 Å n ^+. -MB for the high concentration layer 2 composed of GaAs layer
It is formed by the E method or the MOCVD method, and the dopant concentration is Si on the high concentration layer 2 and the impurity concentration is 5 × 10 ¹⁶ at
The collector layer 3 is an n ^-- GaAs layer having a thickness of 3000 Å at om / cm ³ , and the dopant is Si and the impurity concentration is 5 × 10 ¹⁶ at
Collector layer consisting of n ⁻ −Al _0.075 Ga _0.925 As layer having a thickness of om / cm ³ and 500Å 4, a dopant of Be, an impurity concentration of 2 × 10 ¹⁹ atom / cm ³ and a thickness of 1500 Å p ⁺ −Al _0.15
Base layer 5 consisting of Ga _0.85 As layer 5, Si as dopant, n with impurity concentration of 3 × 10 ¹⁷ atom / cm ³ and thickness of 500Å
An emitter layer 6a made of an Al _x Ga _1-x As layer (x: 0.15 → 0.3), the dopant concentration is Si, and the impurity concentration is 3 × 10 ¹⁷
n-Al _0.3 GAl _0.7 A with atom / cm ³ and thickness of 200Å
n ⁺ with an impurity concentration of 5 × 10 ¹⁸ atom / cm ³ and a thickness of 1000Å with Si as the emitter layer 6b composed of the s layer and a dopant
-High concentration layer 7 made of GaAs layer is processed by MBE method or MOCV
It has a structure in which it is sequentially formed in a predetermined pattern by the D method or the like, and further, collectors and emitters and base electrodes 8c, 8e and 8b are provided on the high concentration layers 2 and 7 and the base layer 5, respectively.

FIG. 2 is a band configuration diagram of an embodiment of the present invention.

In this embodiment, the electrons 9 injected from the emitter layers 6b and 6a and passing through the base layer 5 firstly have energy discontinuity δEc (about 0.05) due to the difference in electron affinity between the base layer 5 and the collector layer 4. eV) is the initial kinetic energy, and the mean free path (about 500 Å) of electrons in the depletion layer causes so-called halistic flight to lose kinetic energy, and then the difference in electron affinity between collector layer 4 and collector layer 3 Based on the energy discontinuity δEc ′ (about 0.05 eV) as the initial kinetic energy, the ballistic flight in the depletion layer is performed again, and the kinetic energy is lost when traveling on the mean free path (about 500 Å) and the collector layers 4 and 3 are depleted. Go through the layers. As described above, the electrons are ballistic-flyed and conducted in the depletion layer of the collector layer, so that the conduction velocity is prevented from lowering due to the valley scattering, and the transit time of the electrons in the depletion layer can be shortened.

In the embodiment of the present invention, the depletion layer portion of the collector layer is divided into two sections by pushing the collector layer 4 into the interface between the collector 3 and the base layer 5, but the number of sections is two.
The number is not limited to one and may be increased.

In the embodiment of the present invention, the semiconductor material is a combination of AlGaAs and GaAs which are lattice-matched with each other, but the material is not limited to this, and a lattice-mismatched material may be used as long as the material has a difference in electron affinity. .

〔The invention's effect〕

As described above, in the present invention, at least one other collector layer made of an impurity layer having the same conductivity type as that of the collector layer is provided at the interface between the collector layer and the base layer, and the electron affinity is set to the collector layer, the impurity layer, and the base layer. Since the electrons injected from the emitter layer and passing through the base layer travel in the collector layer and the depletion layer by so-called ballistic flight, the movement speed of the valley scattering is reduced. There is an effect that it is possible to provide a heterojunction bipolar transistor which is excellent in high-speed and high-frequency characteristics in which the depletion layer portion has a short transit time by preventing the deterioration.

In the heterojunction bipolar transistor using such ballistic flight, the average velocity of electrons in the depletion layer portion of the collector layer reaches 5 × 10 ⁷ cm / sec or more, and the transit time of the depletion layer portion is 0.3 psec or less. Therefore, the traveling time is 1/3 or less as compared with the conventional one.

[Brief description of drawings]

1 and 2 are a sectional view and a band structure diagram of an embodiment of the present invention, and FIG. 3 is a band structure diagram of an example of a conventional heterojunction bipolar transistor. 1 ... Semi-insulating substrate, 1a ... Insulating region, 2 ... High concentration layer, 3, 3 ', 4 ... Collector layer, 5, 5' ... Base layer, 6a, 6b, 6 '... Emitter Layer, 7 ... High-concentration layer, 8b ... Base electrode, 8c ... Collector electrode, 8e
...... Emitter electrode, 11, 11 ', 12, 12', 13, 13 '......
Fermi level.

Claims (1)

[Claims] 1. A heterojunction bipolar transistor having a base layer of opposite conductivity type formed on a collector layer of one conductivity type and an emitter layer formed on the base layer and having a forbidden band width wider than that of the base layer. At least one layer of one conductivity type impurity layer having a predetermined thickness is provided at the interface between the collector layer and the base layer, and the electron affinity is sequentially reduced to the collector layer, the impurity layer, and the base layer. Junction bipolar transistor.