Active solid-state devices (e.g. – transistors – solid-state diode – Avalanche diode – Microwave transit time device
Reexamination Certificate
2001-06-12
2004-02-03
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Avalanche diode
Microwave transit time device
C257S006000, C257S197000, C257S603000, C438S091000, C438S403000, C438S412000
Reexamination Certificate
active
06686647
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to Gunn diodes used for oscillation of microwaves and millimeter waves, and is especially related to Gunn diodes which realize improvements in thermal characteristics, yield factor of good products and easy assembly to planar circuits, and manufacturing methods thereof.
Gunn diodes for oscillation of microwaves or millimeter waves are usually comprised of compound semiconductors such as gallium arsenide (GaAs) or indium phosphide (InP). It is the case with such compound semiconductors that the electron mobility is several thousands of cm
2
/V·sec and thus large in a low electric field while the mobility is decreased in case a large electric field is applied since accelerated electrodes transit to a band of large effective mass and thus causes generation of negative differential mobility within the bulk. Consequently, a negative differential conductance is caused in the current-voltage characteristics and leads to thermodynamic instability. Therefore, a domain is generated which transits from the cathode side to the anode side. Repetition of this phenomenon results in vibrating current (oscillation).
An oscillating frequency ft can be determined from transit distance L of the domain and average drift velocity Vd of the electrons with an equation ft=Vd/L in a microwave range. Energy relaxation time consists of time needed for the electron to increase and decrease energy at &Ggr; valley is a main cause for fixing the upper limit of oscillating frequency in a millimeter wave range. It is reported that the relaxation time constant of GaAs is twice as a mode of fundamental frequency of InP and a cut-off frequency of GaAs and InP is 100 GHz and 200 GHz respectively (M. A. di Forte-poisson et al.: Proc. IPRM'89, p.551 (1989)). Since the upper limit of the oscillating frequency of GaAs Gunn diode ranges 60 GHz through 70 GHz in the practical application, higher frequency bands such as 77 GHz band that is used for car mount radar is appropriate to InP Gunn diode.
In case of Gunn diodes for millimeter waves, this distance of transit needs to be extremely short (1 to 2 &mgr;m). In addition, the product of an impurity concentration and a distance of transit for the domain (active layer) needs to be set to be a specified value (e.g. 1×10
12
/cm
2
) to obtain sufficient oscillating efficiency, while the impurity concentration of the active layer becomes rather high in high frequency zones like those of millimeter waves since the oscillating frequency is unambiguously determined by the thickness of the active layer. The current concentration during operation is determined by the product of the impurity concentration of the active layer and a saturation electron speed, and in zones of the millimeter waves, the temperature of the active layer is increased owing to the increase in current concentration, whereby the oscillating efficiency is decreased.
In order to solve such problems, measures had been taken with conventional Gunn diodes for millimeter waves such as employing a mesa-type structure to use elements including the active layer of extremely small sizes, having diameters of approximately several tens of &mgr;m, and assembling the diodes within pill-type packages comprised with a heat portion made of diamond or similar material of favorable thermal conductivity.
A sectional view of InP Gunn diode element
100
of conventional mesa-type structure is shown in FIG.
8
. On to a semiconductor substrate
101
of heavily doped n-type InP, there are sequentially laminated, through MOCVD method, a first contact layer
102
of heavily doped n-type InP, an active layer
103
of lightly doped n-type InP, and a second contact layer
104
of heavily doped n-type InP, and it is employed a mesa-type structure in order to reduce the transit space for the electrons.
Thereafter, a lower surface of the semiconductor substrate
101
is laminated, a cathode electrode
105
is formed onto the surface of the semiconductor substrate
101
while an anode electrode
106
is formed on the surface of the second contact layer
104
, and by performing element separation, the Gunn diode element is completed.
The Gunn diode element
100
thus obtained is built-in in a pill-type package
110
as shown in FIG.
9
. This pill-type package
110
comprises a heat sink electrode
111
and a cylinder
112
of glass or ceramics that serves as an enclosure for enclosing the Gunn diode element
100
, wherein the cylinder
112
is brazed by hard-soldering to the heat sink electrode
111
. The Gunn diode element
100
is electro-statically attracted by a bonding tool of TiC or the like (not shown) and is adhered to the heat sink electrode
111
.
Further, the Gunn diode element
100
and a metal layer provided at a tip of the cylinder
112
are connected by a gold ribbon
113
through thermo-compression bonding or the like. After connecting the gold ribbon
113
, a lid-like metallic disk
114
is brazed onto the cylinder
112
to complete the building-in to the pill-type package
110
.
Conventional InP Gunn diode elements
100
are formed through chemical wet etching by employing a photoresist as an etching mask to obtain the above described mesa-type structure. However, since etching is progressed not only in the depth direction but also simultaneously in lateral directions in this etching method, it is presented a drawback during manufacture that control of the transit space of the electrons (active layer) is made very difficult, whereby ununiformity in electrical characteristics of Gunn diode element is caused.
Also there is a disadvantage that an alloy electrode such as AuGe, which is used for the anode electrode
106
, reacts with In at relatively low temperature, thereby causing deterioration of anode electrode obtaining ohmic contact.
There is another disadvantage that Gunn diode may be burned out since current is concentrated to surface of mesa structure due to the instability of the surface of mesa structure in the active layer
103
of InP.
There is still another disadvantage that the bonding tool intercepted one's field of view, at the time of building-in the Gunn diode element in a pill-type package
110
, during adhesion of the Gunn diode element
100
to the heat sink electrode
111
so that the heat sink electrode
111
could not be directly viewed at. Consequently, the efficiency of building-in operation was quite poor.
Further, utilization of a gold ribbon for assembling the pill-type package
110
incorporated with the Gunn diode element
100
to the microstrip line arranged on the plate substrate resulted in generation of parasitic inductance, whereby ununiformity in electrical characteristics was caused during the assembly.
It is an object of the present invention to provide Gunn diodes and manufacturing methods thereof which solve the above described problems.
SUMMARY OF THE INVENTION
For this purpose, according to the first aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP and a second semiconductor layer of heavily doped n-type InGaAs are formed on a semiconductor substrate of heavily doped n-type InP in said order, further comprising a first electrode of smaller area and a second electrode of larger area both formed on the second semiconductor layer to apply voltages to the active layer, and a high resistance region formed at least deeply to the lower surface of the second semiconductor layer by ion implantation and separating a region covered with the first electrode from the other in the second semiconductor layer, so as to let regions under the first electrode in the second semiconductor layer and the active layer work as a Gunn diode is provided.
According to the second aspect of the present invention, a Gunn diode, in which a first semiconductor layer of heavily doped n-type InP, an active layer of lightly doped n-type InP, a second semiconductor layer of heavily doped n-type InP and a third semiconductor layer of he
Kimura Chikao
Nakagawa Atsushi
Armstrong, Kratz, Quintos Hanson & Brooks, LLP.
Nelms David
New Japan Radio Co. Ltd.
Nguyen Dao H.
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