JPH0541388A - Hetero bipolar transistor - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] The purpose of the present invention is to provide a hetero-bipolar transistor with excellent withstand voltage characteristics by forming a collector layer that suppresses avalanche breakdown due to impact ionization during high collector current injection. To do. A first conductivity type emitter layer 1, a second conductivity type base layer 2 having a band gap smaller than that of the emitter layer 1, a first conductivity type first collector layer 4 and a second conductivity type base layer 2. In the hetero-bipolar transistor including the collector layer 3, an impurity exhibiting the first conductivity type is added to the second collector layer 3 at a concentration distribution that compensates for space charges at the time of injecting a collector current of 10 ⁵ A / cm ² or more. It is characterized by

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a compound semiconductor heterobipolar transistor (hereinafter referred to as HBT) suitable for high speed operation.

[0002]

2. Description of the Related Art As shown in FIGS. 7 and 8, a conventional HBT structure and an impurity distribution have a first conductivity type or a first conductivity type having a low impurity concentration between a base layer 6 and a collector layer 8 of the first conductivity type. It has a non-conductive second collector layer 7, and the depletion layer thickness of the collector layer is determined by the impurity concentration, the layer thickness or the bias state.

One problem when injecting a high collector current is that space charges are accumulated in the second collector layer 7, and electrostatic energy caused by this causes band bending of the collector as shown in FIG. This is to cause an increase in electric field strength at the edges. One of the mechanisms that determines the collector breakdown voltage of the HBT is impact ionization of holes (electrons in the conduction band) in the valence band due to electrons (holes) accelerated to a high energy state. The ionization rate, which strongly depends on the local electric field strength in the inside, and which generally indicates the frequency of impact ionization, shows an exponential increase with respect to this increase in the electric field strength. Therefore, the electric field concentration effect at the rear end of the collector due to the space charge promotes avalanche breakdown due to impact ionization, and the withstand voltage characteristic of the HBT deteriorates.
Such a problem becomes remarkable when a collector current of 10 ⁵ A / cm ² or more is injected.

In summary, it can be said that the conventional application of the collector layer having a low impurity concentration to the HBT structure is not intended to suppress the impact ionization at the time of injecting a high collector current.

[0005]

SUMMARY OF THE INVENTION The present invention provides a hetero-bipolar transistor having excellent withstand voltage characteristics by forming a collector layer for suppressing the avalanche breakdown phenomenon due to impact ionization at the time of high collector current injection as described above. The purpose is to provide.

[0006]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a first conductivity type emitter layer, a second conductivity type base layer having a bandgap smaller than that of the emitter layer, and a first conductivity type emitter layer. In a hetero-bipolar transistor consisting of a first collector layer and a second collector layer of conductivity type, the concentration distribution for compensating the space charge at the time of injecting a collector current of 10 ⁵ A / cm ² or more into the second collector layer is It exists in a hetero bipolar transistor characterized by being doped with an impurity exhibiting the first conductivity type.

[0007]

For example, when the HBT structure is an Npn type (Pnp type), space charges in the collector layer are formed by electrons (holes), and therefore a donor (acceptor) is added. .. 1 and 2 are HBs showing the concept of the present invention.
It is an example of a T structure and shows an Npn type HBT. In general, since space charges are accumulated in the rear half of the second collector layer 3 where the electron velocity decreases, the structure is such that the donor is concentrated in the rear half, and as shown in FIG. It is designed to prevent the band bending of the collector even during injection.

After all, according to the present invention, since the impurities are intentionally added to the second collector layer 3 according to a predetermined concentration distribution, the space charge in the second collector layer 3 is electrically compensated. However, it is possible to eliminate the electric field concentration effect due to band bending that promotes impact ionization.

In the present invention, specific numerical examples will be described. Generally, in the present invention, the concentration of impurities to be added to the second collector layer 3 is low, and the second collector layer is not formed during the normal operation bias of the HBT. It is depleted as in the conventional structure.

[0010]

EXAMPLES The present invention will be described in detail below with reference to examples.

In FIG. 1, indium phosphorus (hereinafter referred to as InP) is used for the emitter layer 1, and indium gallium arsenide (hereinafter referred to as InGaAs) is used for the base layer 2, the first collector layer 4, and the second collector layer 3. An Npn type HBT structure will be described as an example. The space charge distribution in the second collector layer 3 is obtained by the following formula based on the current continuation condition that the current distribution in the second collector layer 3 is constant.

N = J _C / (qv) where J _C is the collector current density, q is the charge, and v is the electron velocity. Therefore, the second collector layer 3
Will be uniquely determined by the charge distribution at.
The electrons injected from the base layer 2 into the second collector layer 3 are accelerated by the reverse voltage applied between the base and the collector, and the electron velocity exceeds the saturation velocity. This is generally called the velocity overshoot effect. However, when the kinetic energy begins to relax due to lattice scattering and the like, the electron velocity decreases, and as a result, approaches the saturation velocity. FIG. 4 shows the above situation. Here, the velocity distribution of FIG. 4 is not precisely obtained, but the velocity distribution is approximated as shown in FIG. 5 and considered phenomenologically. The overshoot speed V _O , the saturation speed V _S , and the speed overshoot region W _O shown in FIG. 5 are parameters depending on the base-collector voltage and the collector layer thickness W _C , but here, V _O and V _S are respectively 5 x 1
It is assumed that it is 0 ⁷ , 1 × 10 ⁷ cm / s. In order to know the exact electron velocity distribution, it is necessary to use numerical analysis such as Monte Carlo simulation. Given the above assumptions,
For example, the space charge distribution when J _C = 10 ⁵ A / cm ² is determined by the above equation as follows.

[0013] ^{n = 1.3 × 10 16 cm -3} (0 <W <W O) = 6.5 × 10 16 cm -3 (W O <W <W C) Therefore, for the space-charge compensation Donor ions such as Si may be added to the second collector layer 3 according to the distribution of the above formula. At this time, the second collector layer thickness is 3000
If it is about Å, the second collector layer 3 is completely depleted in a general operating bias state, so that it is not necessary to consider the increase of the collector electric field due to the reduction of the depletion layer thickness.

FIG. 6 shows that W _O is 1500 Å and W _C is 3000 Å
The frequency of impact ionization in the high collector current region is compared between the HBT structure according to the present invention and the conventional HBT structure manufactured as above, and the collector injection current density dependency of the collector current increase rate due to impact ionization is shown. Here, the collector current increase rate is 1 which is the current injected from the base to the collector.
Shows the increase in current due to impact ionization.
Here, the bias between the base and the collector is set to 1.5V. It can be seen that in the conventional structure, an increase in collector current due to impact ionization is clearly observed with an increase in collector injection current, whereas in the structure according to the present invention, dependence on the collector injection current is relatively small. Here, the collector layer was designed using the approximate electron velocity distribution shown in FIG. 5, but by obtaining a precise velocity distribution by Monte Carlo simulation or the like and achieving an optimum donor addition distribution, a higher collector current injection can be achieved. It is possible to improve the collector breakdown voltage depending on the time.

In this embodiment, the effect of the present invention has been shown by taking the InGaAs collector as an example, but it goes without saying that other materials also have similar effects.

[0016]

According to the present invention, that is, by forming a collector layer that suppresses avalanche breakdown due to impact ionization during high collector current injection, it is possible to provide a hetero-bipolar transistor having a high collector breakdown voltage. ..

[Brief description of drawings]

FIG. 1 is an HBT structure diagram showing the concept of the present invention.

FIG. 2 is a conceptual diagram (Npn-type HBT) showing an example of the impurity distribution of the HBT structure according to the present invention.

FIG. 3 is a conceptual diagram showing a band structure of the HBT according to the present invention during high and low collector current operation.

FIG. 4 is a graph showing an electron velocity distribution in the second collector layer.

FIG. 5 is a graph showing an approximate example of an electron velocity distribution in the second collector layer (FIG. 4).

FIG. 6 is a graph showing a collector current increase rate with respect to a collector injection current density.

FIG. 7 is a conceptual diagram showing a conventional HBT structure.

FIG. 8 is a conceptual diagram (Npn-type HBT) showing an example of an impurity distribution of a conventional HBT structure.

FIG. 9 is a conceptual diagram showing a band structure of a conventional HBT during high and low collector current operation.

[Explanation of symbols]

1 emitter layer of the first conductivity type, 2 base layer of the second conductivity type, 3 second collector layer in which an impurity exhibiting the first conductivity type is added to a predetermined concentration distribution, 4 first Conductivity type first collector layer, 5 first conductivity type emitter layer, 6 second conductivity type base layer, 7 first conductivity type second collector layer with low impurity concentration, 8 first A conductive first collector layer.

Claims (1)

[Claims] 1. An emitter layer of a first conductivity type, a base layer of a second conductivity type having a bandgap smaller than that of the emitter layer, and a first collector layer and a second collector layer of the first conductivity type. In the hetero-bipolar transistor, the impurity of the first conductivity type is added to the second collector layer at a concentration distribution of 10 ⁵ A / cm ² or more for compensating the space charge at the time of injecting the collector current. Characteristic hetero-bipolar transistor.