JPS614279A - Bipolar transistor - Google Patents

Abstract

PURPOSE:To decrease depletion-layer running time tauc of a collector to a large extent, by forming a base region and collector regions by different kinds of compound semiconductors, and forming a characteristic potential distribution in the depletion of the collector. CONSTITUTION:An N-P-N heterojunction bipolar transistor is formed by collector regions 301-305, a base 306 and emitters 307-308. The emitter junction forms a so-called wide-gap emitter comprising In1-xGaxAsyP1-y(y 0.5)/ In0.53Ga0.47As. Owing to the low emitter junction capacity, excellent high frequency characteristics are shown. In the collector-junction depletion layer, a characteristic potential distribution structure, which can realize a high drifting speed, is provided. The collector depletion-layer running time of the electrons from the base to the collector is conspicuously shortened. The high frequency characteristics are markedly improved in comparison with the conventional structure.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a bipolar transistor having a novel structure with excellent ultra-high frequency and ultra-high speed operating characteristics.

(Prior art and its problems) is given below. Therefore, in order to realize fT as large as possible, it is necessary to reduce these three time constants as much as possible.

τ8 is called the emitter depletion layer charging time, where the emitter current is IB, the emitter junction capacitance and other parasitic capacitances are Oe, c, and 0. As T τB = 2, Oe+Oc〇, ) = (2
)? It will be given at Here, 4 is Boltzmann's constant, ? is the electron charge, and T is the temperature: Since the emitter junction is forward biased, the emitter junction capacitance Ce is large;
The challenge is to reduce it.

τ is called the base layer charging time and is given by . Here, WB is the thickness of the base layer, DB is the diffusion constant of minority carriers in the base region, and η is a constant that accounts for the contribution of the electric field effect to the impurity ion distribution in the base region, and is a constant for uniform impurity ion distribution. In this case, η=2.

Therefore, in order to shorten the base layer charging time τ6, it is necessary to reduce the base layer thickness WB, select a material with a large diffusion constant, and increase η by an accelerating electric field in the base region.

τ0 is called the collector depletion layer transit time and is given by. Here, Wc is the thickness of the collector junction depletion layer, and Vd is the drift velocity of carriers traveling through the layer. Therefore, in order to reduce the collector depletion layer transit time τ,
It is necessary to reduce the depletion layer thickness Wc and increase the drift velocity Vj as much as possible.

Recently, the base layer thickness W has been reduced due to advances in semiconductor crystal growth technology and process technology such as impurity introduction, and the element structure has been made extremely fine through microfabrication, and wide-gap emitters have been improved due to advances in heterojunction technology.
The emitter junction capacitance has been reduced by realizing the r structure, etc., and τ8. and τ are being reduced. However, since reducing the collector junction depletion layer thickness W lowers the collector breakdown voltage of the transistor, the reduction is limited, and in the depletion layer where a high electric field is applied, the carrier drift velocity is saturated and its upper limit is regulated. collector depletion layer transit time τ. The reduction in fT is gradually becoming a factor that limits the fT of transistors.

(Object of the Invention) The object of the present invention is to realize a large drift speed that greatly exceeds the saturation drift speed, thereby reducing the collector depletion layer transit time τ. The object of the present invention is to provide a bipolar transistor with a novel structure that can significantly reduce the

(Principle of the Invention) As described in detail below, the present invention has a base region and a collector region each made of a different type of compound semiconductor,
The essence is that a characteristic potential distribution is formed in the collector depletion layer.

Velocity saturation of conduction electrons in a compound semiconductor means that electrons accelerated to high energy by a high electric field in the semiconductor layer move from a high-mobility F valley located at the center of wave number space to a low-mobility position located at a high energy. It is caused by a transition to the L or X valley of the degree. - Considering gallium arsenide as an example, the energy difference between the F valley and the L valley is about 0.3 eV, and there is a probability that electrons in the F valley will transition to a valley when they are accelerated more than this energy difference. The relaxation time of energy loss accompanying this transition is ~1013N! During this time, conduction electrons exist within the F valley, and a transiently high drift speed is achieved. Therefore, when conduction electrons are in the process of accelerating under a high electric field, the excess kinetic energy of conduction electrons can be controlled and lost just before the transition from P valley to L valley. By suppressing the transition to the L valley and repeating acceleration and energy loss due to the electric field, it is possible to enable conduction electrons to travel at high speed in the F valley over long and long distances.

FIG. 1 is a diagram illustrating one method of making it possible to dissipate the above-mentioned excess kinetic energy. 11 indicates the energy for conduction electrons, and 12 indicates the conduction electrons and their movement direction. Conduction electron 12 is in place, Xl, X.

It has been shown that when passing through..., it loses kinetic energy of ΔE.

One method of creating a difference in the energy for conduction electrons, that is, the energy at the bottom of the conduction band, in a semiconductor is to bond semiconductors with different electron affinities. Figure 2 is from book q8J
1f, In1. [GaXAs
, Pl, shows the engineering inertia at the lower end of the conduction band and the upper end of the valence band as the composition (x, y) changes. The horizontal axis 21 represents the group #:
y, and based on the lattice matching condition, the composition X is automatically determined by giving y. The vertical axis 22 indicates the energy for electrons, and the lines 2A and 2A indicate the energy at the lower end of the conduction band and the upper end of the valence band, respectively. What Figure 2 shows is that Y
= 1, that is, I n o, 53 Ga o, u As y decreases from the As mixed crystal, the energy at the bottom of the conduction band increases,
In other words, the electron affinity decreases, and 1nP for y=Q and "1nP for Y=1"
For 0.53Ga and 47As, there is about 0 at the lower end of the conduction band.
．． This shows that there is an energy difference of 25 eV. Therefore, 1n1□G a x A s y with different compositions of y
By sequentially stacking P ty layers, a desired energy distribution structure as shown in FIG. 2 can be obtained.

(Embodiment) FIG. 3 is a diagram showing an embodiment of the semiconductor device of the present invention. 301 is a low-resistance n-type InP substrate crystal, and 302 is an n-type 1nPf epitaxially grown on the substrate crystal with a thickness of about 0.2 μm and a donor concentration of about 5×1016−3.
Layers 303, 304 and 305 are lattice matched to the husband al''1 crystal, have a thickness of approximately o, iμm, and have a donor concentration of 5X.
1.0"m3 of In, GaxAsyP, and layers' with compositions y of approximately 0.18, 0.23, and 0.5, respectively.
By choosing 5, the distance between 13 adjacent layers ζ is approximately 0.06
It is possible to form an energy difference of eV. 306 has a thickness of about 0.05μm and an acceptor concentration of 5×1018c.
p-type layer of rn-3 no, 53Ga o, 4y A,
S-Otori 307 has a thickness of about 05μm and a donor concentration of about 5Ki (31
7 sure l n t, C(] a x A S y P
1 (0.5 in y and 0.22 in r x) n-type layer, and 308 is a low resistance n-type I no , s s G ao , 47A 5 to form a good ohmic contact.
It is a layer. 309, 310: B and 311 Husband 43
Molecular beam epitaxy (
It can be continuously grown on an InP substrate by MBE) or vapor phase epitaxy (PE) technology.

As is clear from FIG. 3, this embodiment
Area 5 is collector, 306 is base and 307
.about.308 form an npn heterojunction bipolar transistor having emitters. The emitter junction in this structure ζ is]: n + x G a x A S
y P 11 (05 in Y″)/I no, si G
So-called wide-gapem consisting of ao, 4□As
itter, and due to its low emitter junction capacitance, it exhibits excellent high frequency characteristics similar to conventional heterojunction bipolar transistors. Furthermore, the structure of this embodiment has a characteristic potential distribution structure in the collector junction depletion layer that can realize the high drift speed described in detail above, as described in detail above, and the collector depletion of electrons from the base to the J collector is The remarkable shortening of the layer running time makes it possible to significantly improve the high frequency characteristics compared to the conventional structure.

Although the above embodiment describes an npn transistor, the same applies to a pnp transistor.

Further, although the above embodiment uses InGaAsP as the basic material, it goes without saying that the characteristic structure of the present invention can also be realized by using other materials such as AlGaAs and changing the composition. do not have.

(Effects of the Invention) According to the present invention, the collector depletion layer transit time τ. can be significantly reduced, and an extremely high-speed bipolar transistor can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a mechanism S in which electrons traveling in a spatially varying electric potential lose kinetic energy. 11 indicates the energy for the electron, and 12 indicates the electron and its direction of movement. FIG. 2(a) is a diagram showing changes in the lower end of the conduction band and the upper end of the valence band when the composition y is changed in In, cGaxAsyPl, mixed crystal under conditions of lattice matching with InP. 21 represents the composition y, 22 represents the energy for electrons, and 23 and ε represent the energy at the upper end of the valence band and the lower end of the conduction band, respectively. FIG. 2(b) shows 1 n I X with different composition y.
A diagram showing a step-like potential distribution formed by stacking GaXAs, P, and □. FIG. 3 is a sectional view showing an embodiment of the bipolar transistor of the present invention. 301 is a low resistance n-type InP substrate, 302 is an n-type InP
layer, low resistance n-type InGaA, Q layer and 309,'3
10 and 311 are 308, 306 and 301 respectively
It is an electrode that makes heomic contact.

Claims (1)

[Claims] In the case of a bipolar transistor in which the collector region adjacent to the base region has a multilayer structure of two or more layers in the vicinity of the junction, and in the case of an npn type, there is an energy difference at the lower end of the conduction band between the adjacent layers, and the collector region The lower end of the conduction band of the layer has a higher energy for conduction electrons than the lower end of the conduction band of the layer on the base side, and in the case of a pnp type, there is an energy difference between the upper ends of the valence bands of adjacent layers, A bipolar transistor characterized in that the upper end of the valence band of the layer on the collector side has higher energy for holes than the upper end of the valence band on the base side.