Electron device and junction transistor

Details Electron device and junction transistor Electron device and junction transistor

2001-08-08

2003-05-20

Nelms, David (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Heterojunction device

Having graded composition

C257S010000, C257S101000, C257S103000, C257S256000, C438S020000, C438S022000, C438S048000

Reexamination Certificate

active

06566692

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electron device which functions as a high-output power transistor employed, for example, in base stations for mobile radios.
In the past, electron emissive elements had a structure provided by the hot cathode method (or an electron gun method). The electron emissive element is provided with a cathode formed of a material having a high melting point such as tungsten (W) and an anode spaced opposite to the cathode. The cathode is heated to high temperatures to launch hot electrons from the solid into a vacuum. Also available is a so-called NEA emissive element which the inventors suggest to replace those employing the hot cathode method. The NEA electron emissive element employs a semiconductor material or an insulating material having a negative electron affinity (NEA). Described below is the principle of an electron device that functions as an electron emissive element (hereinafter referred to as the NEA electron device).
FIG. 1
is a perspective view illustrating the structure of a prior-art NEA electron device that employs aluminum nitride (AlN) as an example of a NEA material. As shown in
FIG. 1
, the NEA electron device includes an electron supplying layer
101
for supplying electrons and an electron transport layer
102
for transporting the electrons supplied from the electron supplying layer
101
toward the solid surface side. The NEA electron device also includes a surface layer
103
formed of a NEA material and a surface electrode
104
used for the application of a voltage to allow electrons to travel from the electron supplying layer
101
to the surface layer
103
.
In this example, the electron supplying layer
101
is formed of an n-type GaN (n-GaN), and the electron transport layer
102
for allowing electrons to travel smoothly from the electron supplying layer
101
to the surface layer
103
is formed of non-doped Al
x
Ga
1−x
N (where x is a variable increasing in general continuously from 0 to 1) having a graded composition with an Al content ratio x varying continuously. The surface layer
103
is formed of AlN which is an intrinsic NEA material, and the surface electrode is formed of a metal such as platinum (Pt).
Now, described below are the electron affinity that is significant to the basic characteristics of this element and the structure of the electron transport layer that is required for smooth transportation of electrons.
1. Electron Affinity
The “electron affinity” in a semiconductor material is defined as the energy required to launch an electron present on the conduction band edge into a vacuum and unique to the material. Now, described below is the concept of “negative electron affinity (NEA)”.
FIGS.
2
(
a
) and (
b
) are energy band diagrams of semiconductor materials having a negative and positive electron affinity, illustrating the respective energy states. As shown in FIG.
2
(
b
), the electron affinity &khgr;=E
vac
−E
c
>0 in a typical semiconductor, where E
f
is the Fermi level of the semiconductor, E
c
is the energy level of the conduction band edge, E
v
is the energy level of the valence band edge, E
g
is the bandgap, and E
vac
is the vacuum level. That is, the semiconductor has a positive electron affinity. In contrast, for some types of semiconductors, &khgr;=E
vac
−E
c
<0 as shown in FIG.
2
(
a
). That is, semiconductors such as AlN have a negative electron affinity.
Now, consider a semiconductor having a positive electron affinity as shown in FIG.
2
(
b
). In this case, to launch an electron present on the conduction band edge into a vacuum, the presence of the energy barrier of a magnitude of &khgr; requires to give the amount of energy to the electron. For electron emission, it is therefore necessary in general to give an energy to an electron by heating or to allow an electron to tunnel the energy barrier by application of a high electric field.
On the other hand, consider a semiconductor having a negative electron affinity as shown in FIG.
2
(
a
). In this case, absence of energy barrier allows an electron present on the conduction band edge of the surface to be easily emitted into a vacuum. In other words, no additional energy is required to launch the electron present on the semiconductor surface into a vacuum.
2. Electron Transport Layer
It is conceivably effective in efficient electron emission to employ, as the surface layer of an electron device for emitting the electron, a material having a substantially zero or negative electron affinity like the one mentioned above. However, no electron is present in general on the conduction band of a NEA material in an equilibrium state. Therefore, it is necessary to efficiently supply electrons in some way to the surface layer formed of a material that allows electrons to be emitted easily.
As shown in
FIG. 1
, the inventors have suggested a structural example. The structure has an intermediate layer (the electron transport layer
102
) having gradually decreasing values of electron affinity to effectively supply electrons from the electron supplying layer
101
(a positive electron affinity), having a number of electrons therein, to the surface layer
103
in a NEA state (a negative electron affinity).
FIGS.
3
(
a
) and (
b
) are energy band diagrams of the structural example of
FIG. 1
, provided when no voltage is applied between the electron supplying layer
101
and the surface electrode
104
(an equilibrium state) and a forward bias V is applied therebetween. Here, the structure includes the electron supplying layer
101
, the electron transport layer
102
, the surface layer
103
, and the surface electrode
104
. As mentioned above, the electron transport layer
102
is selected from materials that gradually decrease in electron affinity &khgr; toward the surface.
In the equilibrium state shown in FIG.
3
(
a
), there exist a number of electrons in the conduction band of the electron supplying layer
101
. However, the high energy level of the conduction band edge of the surface layer
103
prevents the electrons from reaching the outermost surface on the other hand, when a forward bias is applied to such a structure (a positive voltage to the surface electrode side), the energy band is bent as shown in FIG.
3
(
b
). As a result, the gradients of the concentration and the potential cause electrons present in the electron supplying layer
101
to travel toward the surface layer
103
. In other words, an electron current flows. In addition, the electron transport layer
102
or (Al
x
Ga
1−x
N) and the surface layer
103
or (AlN) are non-doped. Accordingly, the electrons injected from the electron supplying layer
101
to the electron transport layer
102
and the surface layer
103
can travel without being captured by recombination with holes or the like. Furthermore, the electron transport layer
102
is continuously graded in composition and thereby no energy barrier, which prevents electrons from traveling, is formed on the conduction band edge. Thus, this is advantageous in that electrons are efficiently transported to the surface.
As described above, the compositionally graded Al
x
Ga
1−x
N layer is employed as the electron transport layer
102
. This allows electrons to efficiently travel from the n-GaN layer having a positive electron affinity to the surface layer
103
(AlN layer) having a negative electron affinity. Then, since the surface layer is in a NEA state, the electrons injected to the electron transport layer
102
and the surface layer
103
can pass easily through the surface electrode
104
to be emitted outwardly into a vacuum or the like.
However, such a phenomenon was also observed in the NEA electron device employing the structure shown in
FIG. 1
that the application of a predetermined voltage to the surface electrode
104
would not serve to provide the expected amount of electrons.
A diagnosis of the cause of the phenomenon showed that defects such as fine cracks had occurred in the Al
x
Ga
1−x
N layer that cons

Affiliated with

Also associated with

Rating

Electron device and junction transistor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Electron device and junction transistor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electron device and junction transistor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3065375