JPH0831593B2 - High speed transistor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor device capable of high-speed operation, and more particularly to a hot electron transistor.

(Prior Art) One of active semiconductor devices considered to be capable of high-speed operation
One hot electron transistor (THETA, HET)
There is. For example, M. Heiblum by Solid State Elec
tronics, Vol. 24, p343, 1981). This device injects high-energy electrons from the emitter into the base, so that the electrons pass through the base at high speed and the base transit time is short. Therefore, it is suitable for high-speed operation.

FIG. 3 shows a schematic sectional view of a conventional HET. Third
In the figure, 1 is a substrate, 2 is a collector layer made of an n-type semiconductor, 3 is a collector barrier layer made of an undoped semiconductor having a conduction band energy higher than that of the collector layer, 4
Is a base layer made of the same kind of semiconductor as the collector layer, 5
Is a thin emitter barrier layer made of an undoped semiconductor having a conduction band energy higher than that of the base layer and capable of tunneling electrons, 6 is an emitter layer made of the same kind of semiconductor as the collector layer, 7 is a collector electrode, 8 is a base electrode, 9 is an emitter electrode.

The operation of this conventional structure is performed by using the semi-insulating GaAs as the semiconductor substrate 1, the collector layer 2, the base layer 4, and the emitter layer 6.
As n-GaAs having a donor concentration of about 1 × 10 ¹⁸ cm ⁻³ , Al _0.2 Ga _0.8 As as the collector barrier layer 3 and Al _0.3 Ga _0.7 As as the emitter barrier layer 5, the band structure is shown in FIG. It demonstrates using.

FIG. 4 shows a schematic band structure extending from the emitter layer 6 to the collector layer 2 in FIG. In FIG. 4, 10 is the bottom of the first conduction band and Ef is the Fermi level.

As shown in Fig. 4, the potential difference between the emitter and the base is 0.
If the Fermi level of the base layer is set lower than that of the emitter layer at about 3V, some of the electrons in the emitter layer 6 tunnel through the emitter barrier layer 5 and move to the base layer 4.
The electrons obtain energy (about 0.3 eV) due to the potential difference between the emitter and the base, and become hot electrons, most of which pass through the base and reach the collector barrier layer 3. Since the energy difference at the bottom of the first conduction band between the GaAs base layer and the Al _0.2 Ga _0.8 As collector barrier layer is about 0.15 eV, the Fermi energy of the collector layer 10 is set to the base layer 4
If the temperature is set lower than that, hot electrons enter the collector barrier layer and are accelerated by the electric field of the collector barrier layer,
Move to the collector layer.

Thus, in this device, the electrons injected from the emitter to the base become hot electrons and pass through the base layer 4 and the collector barrier layer 3 at high speed,
High speed operation is possible.

(Problems to be solved by the invention) However, although the conventional HET has the above-mentioned advantages, the current gain ([collector current] / [base current]) is small and it cannot operate at a large current. Sufficient speedup has not been achieved.

Since electrons are injected from the emitter to the base by tunneling out the emitter barrier layer, it is difficult to obtain a large current. Also, some of the electrons injected from the emitter to the base lose energy due to scattering in the high impurity concentration base layer and do not enter the collector barrier layer, resulting in a base current. The frequency of plasmon scattering coupled with vibration is very high. Therefore, the number of electrons that lose energy due to scattering and are reflected by the collector barrier layer and the number of electrons that reach the collector layer are almost the same, and the current gain cannot be increased.

An object of the present invention is to eliminate the drawbacks of the conventional HET and provide a transistor capable of operating at an ultrahigh speed.

(Means for Solving the Problems) A transistor of the present invention has a first conduction band and a second conduction band having a symmetry different from the first conduction band, and has a bottom of the second conduction band. An emitter layer made of an n-type first semiconductor whose energy is lower than the bottom of the first conduction band, a bottom of a second conduction band of energy lower than that of the second conduction band of the emitter layer, and A base layer made of an n-type second semiconductor having a bottom of a first conduction band having an energy lower than that of the second conduction band, and an energy lower than that of a bottom of the second conduction band of the emitter layer and A low impurity concentration second having a bottom of a second conduction band having a higher energy than the bottom of the first conduction band of the base layer and a first conduction band of a higher energy than the bottom of the first conduction band of the base layer. And a collector barrier layer made of a semiconductor of No. 3 and a collector layer made of an n-type fourth semiconductor It is characterized by having a structure in which and are laminated.

(Operation) In the transistor of the present invention, the injection of electrons from the emitter to the base is performed by thermal excitation, so that a large current operation is possible, and the electrons injected into the base layer travel in the second conduction band. Since it rarely falls into the first conduction band and becomes a base current, a high current gain is obtained and an ultrahigh speed operation is possible.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings illustrating an example.

FIG. 1 is a schematic sectional view showing an embodiment of the present invention.
In FIG. 1, those having the same numbers as those in FIG. 3 are the same as those in FIG. 3 and perform the same functions. 14 is an n-type having a first conduction band and a second conduction band having a symmetry different from that of the first conduction band and having an energy at the bottom of the second conduction band lower than that of the first conduction band. The first semiconductor emitter layer 13 has a second conduction band bottom having a lower energy than the second conduction band bottom of the emitter layer 14 and a first energy having a lower energy than the second conduction band bottom. A base layer made of an n-type second semiconductor having a bottom of the first conduction band of the emitter layer 14 and having a lower energy than the bottom of the second conduction band of the emitter layer 14 and a bottom of the first conduction band of the base layer 13. A collector barrier layer composed of a third semiconductor of low impurity concentration having a second conduction band with a higher energy than that and a first conduction band with a higher energy than the bottom of the first conduction band of the base layer 13, Reference numeral 11 is a collector layer made of an n-type fourth semiconductor.

FIG. 2 is a band diagram for explaining the operation of the embodiment of the present invention. In FIG. 2, those having the same numbers as those in FIGS. 1 and 4 are the same as those in FIGS. 1 and 4 and have the same functions. Reference numeral 15 is the bottom of the second conduction band having a symmetry different from that of the first conduction band.

In the thermal equilibrium state, the depletion layer extends to the emitter side due to the difference in energy between the bottom of the second conduction band of the emitter layer and the bottom of the first conduction band of the base layer, which prevents movement of electrons between the emitter and the base. Has been. Now, as shown in FIG. 2, when a voltage is applied between the emitter and the base, the depletion layer is reduced and the barrier against electrons is lowered, so that all of the electrons that are thermally excited and have energy higher than the barrier height are emitters. Is injected into the base. At this time, electrons are not injected into the first conduction band of the base layer due to the difference in symmetry of the conduction band.
The electrons injected into the second conduction band gain energy due to the energy difference at the bottom of the second conduction band between the emitter and the base, and become hot electrons, most of which pass through the base to the collector barrier layer. Reach In the base layer, like the conventional HET, some of the injected electrons lose energy due to plasmon scattering coupled to the lattice vibration.
It is rarely lower than the bottom of the second conduction band.
This is because the probability of scattering from the second conduction band having a large density of states to the first conduction band having a small state density is generally small.
Therefore, it is very small that the electrons injected into the base fall to the bottom of the first conduction band and contribute to the base current. The electrons that have passed through the base layer in the second conduction band enter the collector barrier layer as in the case of the conventional HET, are accelerated by the electric field of the collector barrier layer, and move to the collector layer.

As described above, according to the structure of the present invention, the number of electrons injected from the emitter to the base is large, a large current can be easily obtained, and the electrons injected from the emitter to the base generate energy in the base layer. Since it hardly contributes to the lost base current, a high current gain can be obtained. As a result, ultra-high speed operation becomes possible.

Next, the manufacturing method, structure, and characteristics of the device prepared according to this embodiment of the present invention will be described. MBE (Molecular Beam Epitaxy) is used as the crystal growth method, and the thickness is 0.5 μm and the donor concentration is 1 on the semi-insulating GaAs substrate 1.
× 10 ¹⁸ cm ^-3 n-GaAs collector layer 11, 3000 Å thickness, undoped Al _0.2 Ga _0.8 As collector barrier layer 12, 500 Å thickness
In n-GaAs base layer 13 of donor concentration 1 × 10 ¹⁸ cm ^-3, the donor concentration in the thickness 0.5μm of ^{1 × 10 18 cm -3 n-} Al 0.4 Ga
_{A 0.6} As emitter layer 14 was sequentially grown. The base layer and collector layer were exposed by etching for forming electrodes. The collector electrode 7, the base electrode 8 and the emitter electrode 9 were formed by depositing and alloying AuGe / Au. In this structure, the first conduction band Γ point and the second
The conduction band of is at point L. In this HET, a collector current of 1 × 10 ⁵ A / cm ² or more was obtained, and a current gain of 10 was obtained.

In the above embodiments, the energy at the bottom of the second conduction band of the collector barrier layer was slightly higher than that at the bottom of the second conduction band of the base layer, but the purpose of the present invention is to increase the current gain. Therefore, it is desirable that there is almost no energy difference or the opposite. Although only AlGaAs type semiconductors have been shown as semiconductor materials, it is obvious that the present invention can be applied to semiconductors such as InGaAlAs / InP type and AlGaSb / InAs type and other various semiconductors. Further, the materials shown above are combinations in which the lattice constants are substantially the same, but materials having different lattice constants and strain may also be used. Further, although only the Γ point and the L point are shown as the combination of the second conduction band of the first conduction band, the combination of the Γ point and the X point, the L point and the X point, or the like may be used.

(Effects of the Invention) In the transistor of the present invention, since the scattering of electrons in the base layer is suppressed, a large current can be obtained, the current gain is significantly improved, and an ultrahigh-speed operation is possible.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an embodiment of the present invention, FIG. 2 is a band structure diagram thereof, FIG. 3 is a schematic sectional view of a conventional HET, and FIG. 4 is a band diagram thereof. 1 ... Substrate, 2 ... Collector layer, 3 ... Collector barrier layer, 4 ... Base layer, 5 ... Emitter barrier layer, 6 ...
Emitter layer, 7 ... collector electrode, 8 ... base electrode,
9 ... Emitter electrode, 10 ... Bottom of first conduction band, 11 ...
Collector layer, 12 ... Collector barrier layer, 13 ... Base layer, 14 ... Emitter layer, 15 ... Bottom of second conduction band, Ef ...
... Fermi level.

Claims (1)

[Claims] 1. The energy of the bottom of the second conduction band is lower than that of the bottom of the first conduction band, which has a first conduction band and a second conduction band having a symmetry different from that of the first conduction band. an emitter layer made of an n-type first semiconductor, a bottom of the second conduction band of lower energy than the bottom of the second conduction band of the emitter layer, and a lower energy of lower energy than the bottom of the second conduction band of the emitter layer; A base layer made of an n-type second semiconductor having a bottom of a first conduction band, and having a lower energy than the bottom of the second conduction band of the emitter layer and a bottom of the first conduction band of the base layer. A high-energy second conduction band bottom and a base layer first conduction band higher conduction energy first conduction band bottom collector collector layer comprising a third semiconductor having a lower impurity concentration, and n. A fourth type semiconductor layer and a collector layer made of a fourth semiconductor are laminated. High-speed transistor.