Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
2002-01-31
2004-09-07
Kang, Donghee (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C257S021000, C257S024000, C257S027000, C257S194000, C257S195000, C257S196000, C438S167000, C438S172000, C438S285000, C438S571000, C438S590000
Reexamination Certificate
active
06787820
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to field effect transistors (FETS) using a hetero-structure of nitride semiconductors represented by the general formula In
x
Ga
y
Al
1-x-y
N (with 0≦x≦1, 0≦y≦1, 0≦x+y≦1).
Semiconductors including gallium nitride, such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and indium aluminum gallium nitride (InAlGaN), have a high dielectric breakdown electric field strength, high thermal conductivity, and high saturated electron drift velocity, for example, so they are preferred as materials for high frequency power devices. More specifically, a so-called two-dimensional electron gas is formed in the heterojunction structure of an AlGaN film serving as the top layer and a GaN film serving as the bottom layer (hereinafter referred to as “AlGaN/GaN hetero-structure”), by accumulating a high concentration of electrons near the heterojunction interface in the GaN film.
This two-dimensional electron gas exhibits high electron mobility because it is spatially separated from the donor impurities added to the AlGaN film. Consequently, the source resistance component can be reduced by using an AlGaN/GaN hetero-structure in field effect transistors.
The distance d from the gate electrode formed on the AlGaN/GaN hetero-structure to the two-dimensional electron gas is usually short at about several dozen nm, so even if the gate length Lg is short at about 100 nm, the ratio Lg/d (aspect ratio) of the gate length Ld to the distance d can be increased to about 5 to 10. Therefore, an excellent characteristic of AlGaN/GaN hetero-structures is that they make it easy to fabricate field effect transistors with little short channel effect and good saturation properties.
Additionally, in high electric field regions of about 1×10
5
V/cm, the two-dimensional electron gas in the AlGaN/GaN hetero-structure has at least twice the electron velocity of currently available AlGaAs/InGaAs hetero-structures, for example, and therefore its application as a high frequency transistor material in high frequency power devices is anticipated.
However, one problem in field effect transistors using AlGaN/GaN hetero-structures or GaN is that there may be instabilities in transistor operations, depending on the approach used to apply the gate voltage or drain voltage. More specifically, it has been reported that the drain current decreases for other than thermal reasons when the drain voltage is increased, and also that the drain current gradually decreases when the strength or frequency of signals applied as the gate voltage is increased.
Drain current is thought to decrease for the following reasons:
(1) Poor crystal quality in the AlGaN film of the AlGaN/GaN hetero-structure leads to deep energy levels in the AlGaN film caused by numerous defects, with the deep energy levels acting as electron trapping centers (electron traps).
(2) Numerous defects in the surfaces of the GaN and the AlGaN films cause deep energy levels that contribute to the trapping and releasing of electrons.
On the other hand, a method for reducing the drop in drain current is to form a GaN film doped with n-type impurities in high concentration as a cap layer on the AlGaN/GaN hetero-structure, that is, on the AlGaN film.
FIG. 4A
is a cross-sectional view of a conventional semiconductor device, or more specifically, a field effect transistor, using an AlGaN/GaN structure with this cap layer.
As shown in
FIG. 4A
, a buffer layer
13
made of a GaN film, and an electron supply layer
14
made of an n-type AlGaN film, are sequentially formed on a substrate
11
made of sapphire or silicon carbide (SiC) via an AlN (aluminum nitride) film
12
. A cap layer
15
made of an n-type GaN film covers the upper surface of the electron supply layer
14
. A gate electrode
16
is formed on the electron supply layer
14
within a recessed portion provided in a predetermined region of the cap layer
15
, and a source electrode
17
and a drain electrode
18
are formed on either side of the gate electrode
16
on the cap layer
15
.
In this conventional semiconductor device, a high-concentration two-dimensional electron gas
19
is formed in the buffer layer
13
near the interface with the electron supply layer
14
so that the semiconductor device can be operated as a FET by controlling the concentration of the two-dimensional electron gas
19
with the voltage applied to the gate electrode
16
. That is, the upper portion of the buffer layer
13
functions as a channel layer.
In this conventional semiconductor device, the cap layer
15
protects the surface of the electron supply layer
14
, so that the formation of deep energy levels caused by defects in the surface of the electron supply layer
14
can be inhibited. As a result, fluctuations in the potential energy of the electrons (hereinafter referred to simply as “potential”) caused by electrons being trapped and released at the surface of the electron supply layer
14
can be suppressed. At this time, adding n-type impurities to the GaN film serving as the cap layer
15
can increase the distance from the surface of the electron supply layer
14
to the two-dimensional electron gas
19
, thereby lessening the effect that fluctuations in potential in the surface of the electron supply layer
14
have on the potential of the channel layer.
Conventional semiconductor devices, however, have drawbacks in that a reduction in drain current cannot be adequately prevented, and the contact resistance of the source electrode
17
and the drain electrode
18
, which are ohmic electrodes, is increased.
SUMMARY OF THE INVENTION
In light of the above, it is an object of the present invention to reliably prevent a reduction in drain current so as to stabilize FET operations and to reduce the contact resistance of the ohmic electrodes in FETs using a hetero-structure semiconductor including GaN.
To achieve this object, the inventors assessed the problems with conventional semiconductor devices, namely, a first problem of inadequate prevention of a reduction in drain current, and a second problem of an increase in contact resistance of the ohmic electrodes.
As mentioned earlier, in order to reduce the effect that traps in the surface of the hetero-structure of a semiconductor including GaN have on operation of the FET, it is effective to increase the distance from the surface of the hetero-structure to the region in which the two-dimensional electron gas is formed, that is, to the channel layer of the FET. In other words, increasing this distance makes it possible to reduce the effect that fluctuations in the surface potential of the surface of the hetero-structure caused by the trapping and releasing of electrons have on the potential of the channel layer. However, in the case of an AlGaN/GaN hetero-structure, that is, when an AlGaN film is used as the electron supply layer, the AlGaN layer itself cannot be made thick for attaining this effect because the AlGaN film and the GaN film have different lattice constants.
Accordingly, in the conventional semiconductor device, the cap layer
15
made of the n-type GaN film is formed on the electron supply layer
14
made of the AlGaN film to achieve the above-described effect.
In assessing the first problem, the inventors found that in the conventional semiconductor device, the difference between the spontaneous polarization of the GaN film serving as the cap layer
15
and the spontaneous and piezoelectric polarization of the AlGaN film serving as the electron supply layer
14
results in a drop in electron concentration in the channel layer of the FET, thereby causing the first problem of inadequate prevention of the reduction in drain current.
Next, in assessing the second problem, the inventors found that in the conventional semiconductor device, a potential hill caused by the above difference in polarization between the GaN film and the AlGaN film is formed at the interface between the cap layer
15
and the electron supply layer
14
, because a
Ikeda Yoshito
Inoue Kaoru
Masato Hiroyuki
Kang Donghee
McDermott Will & Emery LLP
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