JPH067554B2 - Varistable heterojunction bipolar transistor - Google Patents

Description

The present invention relates to a heterojunction bipolar transistor, and more particularly to a ballistic heterojunction bipolar transistor having a wide bandgap structure.

[Conventional technology]

The bipolar transistor has an excellent feature that it has a larger current driving capability than the field effect transistor. Therefore, in recent years, research and development of bipolar transistors using not only Si but also compound semiconductors such as GaAs have been actively conducted. In particular, in a bipolar transistor using a compound semiconductor, it is possible to easily introduce a heterojunction into the emitter / base junction by using MBE or MOCVD technology, and it is possible to maintain a large emitter injection efficiency even if the concentration of the base layer is increased. Is big.

In order to increase the frequency and speed of such a compound semiconductor bipolar transistor, for example, in the case of an NPN transistor, it is important to shorten the electron transit time in the base region.

A conventional heterojunction bipolar transistor (hereinafter referred to as HBT) intended to shorten the base transit time will be described.

FIG. 3 is an energy band diagram of the first conventional example.

This conventional example is an application of an abrupt heterojunction between the emitter and the base, and a conduction band bottom discontinuity δEc is generated at the emitter-base interface due to the difference in electron affinity.
Electron 1 at the moment of being injected from the emitter region to the base region
Has a kinetic energy of δEc. This electron 1 flies ballistically only for the mean free path λ (up to the point 3 in FIG. 3), and thereafter travels in the base region by diffusion. Since the ballistic flight is included, the traveling time is shortened as compared with the case where the entire base area is driven by diffusion. However, when the base region is made of P ⁺ -type GaAs, for example, λ is several tens of nanometers, and therefore the HBT having a base layer thickness of 100 nm or more, which is usually used, cannot significantly improve the traveling time.

FIG. 4 is an energy band diagram of the second conventional example.

This conventional example is a so-called graded band gap base (graded band-gap base) in which the bottom of the conduction band of the base region is inclined.
gap base) structure HBT. This structure is realized by setting x from 0.15 to 0 from the emitter side to the collector side when the base region is formed of, for example, Al _x Ga _1-x As. In this structure, electrons are accelerated by the electric field created in the base region, and the base transit time is shortened. However, in this structure, the above Al _x Ga _1-x As (x: 0.15 → 0)
In the case of the base structure, when the thickness of the base layer is 150 nm, the built-in electric field is about 10 kV / cm, and the valley scattering of the conduction band increases, so that the electron velocity cannot be greatly increased. Although valley scattering also exists in the structure of FIG. 3, its value is the value immediately before the ballistic electrons are injected from the emitter, that is, the value when the electric field is almost zero (small electron velocity), and therefore the graded Smaller than the base structure.

[Problems to be solved by the invention]

In order to speed up the HBT, it is necessary to increase the electron velocity in the base region as much as possible. For this reason, a graded base structure with large valley scattering is not suitable, and it is necessary to devise so that the electrons fly ballistically over the entire region of the base. To achieve this, the thickness of the base region may be set to λ or less, but this makes the thickness of the base region several tens of nanometers or less, increasing the base resistance, and overall Instead, it deteriorates.

It is an object of the present invention to provide a ballistic heterojunction bipolar transistor which can operate at high speed even when the thickness of the base region is larger than the mean free path of minority carriers.

[Means for solving problems]

In the ballistic heterojunction bipolar transistor of the present invention, the P (or N) type base region having an electron affinity (or the sum of the forbidden band width and the electron affinity) larger than that of the N (or P) type wide bandgap emitter region is an electron. (Or holes) are divided into a plurality of sections having a section length of about the mean free path, and within each section, the electron affinity (or forbidden band width and electron affinity) which is constant but is closer to the emitter region is smaller. It is made of a semiconductor material having

[Action]

According to the heterojunction bipolar transistor of the present invention as described above, the average free path λ of the minority carriers injected from the emitter causes a ballistic flight to rapidly decrease the speed, but immediately the conduction band bottom between the next section and Since the ballistic flight is restarted after passing through the discontinuity, the minority carriers can fly ballistically in most of the base region even if the thickness of the base region is larger than λ. Further, because of the bipolar transistor structure, even a part of minority carriers whose ballistic flight is temporarily stopped in the base region can reach the collector region by diffusion and contribute to the amplification operation.

〔Example〕

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

FIG. 1 is a sectional view of a semiconductor chip showing a main part of an embodiment of the present invention.

This embodiment is larger than the N-type wide bandgap emitter region formed by the emitter layer 16 made of N-Al _0.3 Ga _0.7 As (dopant: Si, concentration: 1 × 10 ¹⁹ cm ⁻³ ) having a thickness of 200 nm. A first base layer 17 in which the base region having an electron affinity has a section length of about the mean free path of electrons,
The second base layer 18 is divided into two sections and is made of a semiconductor material having a constant electron affinity in each of the above-mentioned sections, but having a smaller electron affinity in a section closer to the emitter. To be more specific, the thickness is 5 by MBE method on the semi-insulating GaAs substrate 21.
00 nm of n ⁺ -GaAs (dopant: Si, concentration: 5 × 1
0 ¹⁸ cm ⁻³ ) collector contact layer 20, thickness 5
00 nm n ^-- GaAs (dopant: Si, concentration: 3 × 1)
0 ¹⁶ cm ^-3 ), collector layer 19 of 50 nm thick p
⁺ −GaAs (dopant: Be, concentration: 1 × 10 ¹⁹ cm ⁻³ ), second base layer 18, p ⁺ −Al _{0.15 with} a thickness of 50 nm
First base layer 17 made of Ga _0.85 As (dopant: Be, concentration: 1 × 10 ¹⁹ cm ⁻³ ), n-Al _0.3 G with a thickness of 200 nm
a _0.7 As (dopant: Si, concentration: 1 × 10 ¹⁹ cm ^-3) emitter layer 16 made of a thickness of 200 nm ⁿ + -GaAs (dopant: Si, concentration: 5 × 10 ¹⁸ cm ^-3) made of a cap Layer 5 is being grown. The base electrode 12 is provided on the exposed surface of the first base layer 17, and the collector electrode 13 is provided on the exposed surface of the collector contact layer 20. 14 is an isolation layer in which protons are ion-implanted.

Next, the operation of this embodiment will be described.

FIG. 2 is an energy band diagram of the embodiment shown in FIG.

In FIG. 2, an emitter layer 16 made of n-Al _0.3 Ga _0.7 As and a first base layer 1 made of p ⁺ -Al _0.15 Ga _0.85 As
7, second base layer 18 made of p ⁺ -GaAs, n ⁻ −Ga
Conduction band bottom 4 when the collector layer 19 made of As is joined
And the state of the valence band upper limit 5 is shown. In the figure, the conduction band bottom discontinuity δEc due to the emitter-base ablupto heterojunction is about 0.1 eV. The ballistic electron 1 injected via this δEc flies by the mean free path λ (position 3 in the figure) and loses kinetic energy.
After a slight diffusion running, p ⁺ -Al _0.15 Ga _0.85 As and p ⁺
The ballistic flight is restarted by the conduction band bottom discontinuity δEc '(about 0.1 eV in this case) at the interface of the abrupt heterojunction with -GaAs, and the average free path λ'is reached to reach the collector region.

〔The invention's effect〕

As described above, the present invention divides the base region into a plurality of sections, and increases the electron affinity (or the sum of the forbidden band width and the electron affinity) in the section away from the emitter,
Even if the thickness of the base region is set to be larger than the mean free path of the minority carriers, the minority carriers can ballistically fly over almost the entire region of the base region. Therefore, even when the base region is thickened and the base resistance is lowered, the base transit time can be greatly reduced, and the HBT can operate at high frequency and high speed. It should be noted that even a part of minority carriers whose ballistic flight is stopped in the base region can reach the collector region by diffusion and contribute to the amplification operation.

In the above-described embodiment, the base area is divided into two sections, but the number of sections is not limited to two and may be any number. Furthermore, the semiconductor materials are AlGaAs and GaAs, which are lattice-matched to each other.
However, the material is not limited to this, and any material having a difference in electron affinity (or a sum of band gap and electron affinity) may be used. The heterojunction used in the present invention is not limited to the lattice-matching system, but may be a lattice-mismatching system.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor chip showing a main part of an embodiment of the present invention, FIG. 2 is an energy band diagram of the embodiment of FIG. 1, and FIGS. 3 and 4 are first and second figures, respectively. FIG. 7 is an energy band diagram of a conventional example of FIG. 1, 2 ... Electron, 3 ... Point from ballistic flight to diffusion run, 4 ... Conduction band bottom, 5 ... Valence band upper limit, 6, 7,
8 ... Fermi level, 11 ... Emitter electrode, 12 ... Base electrode, 13 ... Collector electrode, 14 ... Isolation layer, 15 ... Cap layer, 16 ... Emitter, 17 ... First base layer, 18 ... Second base layer, 19 ... Collector layer, 20 ...
Collector contact layer, 21 ... Semi-insulating GaAs substrate.

Claims (1)

[Claims] 1. A P (or N) type base region having an electron affinity (or a sum of forbidden band width and electron affinity) larger than an N (or P) type wide bandgap emitter region is an electron (or hole) region. It is divided into a plurality of sections having a section length of about the mean free path, and is made of a semiconductor material which has a constant electron affinity in each section but has a smaller electron affinity (or band gap and electron affinity) in sections closer to the emitter region. A ballistic heterojunction bipolar transistor characterized in that