MISFET

Details MISFET MISFET

2002-01-30

2003-09-09

Flynn, Nathan J. (Department: 2826)

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

Field effect device

Having insulated electrode

C257S531000, C257S532000, C257S077000, C257S339000

Reexamination Certificate

active

06617653

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a MISFET formed by utilizing a compound semiconductor layer. More particularly, the present invention relates to a MISFET that has a high breakdown voltage and is suitable for use at a large current.
BACKGROUND ART
Silicon carbide (SiC) is a semiconductor that has a wider bandgap than that of silicon (Si). Accordingly, SiC has a higher breakdown voltage and a higher melting point compared to Si. In view of these beneficial properties, SiC is a semiconductor material that is expected to be applied to next-generation power electronic devices, RF devices, high-temperature-operating devices and so on. It is also known that SiC can take various crystal structures including cubic ones such as 3C-SiC and hexagonal ones like 6H-SiC and 4H-SiC.
FIG. 12
is a cross-sectional view schematically showing the structure of a known NMOS (Metal Oxide Semiconductor)-FET (Field Effect Transistor) that uses SiC.
FIG. 12
shows p-type doped channel SiC layer
102
that has been grown epitaxially on the surface of a p-type SiC substrate
101
doped with aluminum (a p-type impurity) at a concentration of 1×10
18
atoms·cm
−3
; n-type source/drain regions
103
a
and
103
b
doped with nitrogen (an n-type impurity) at a concentration of 1×10
18
cm
−3
and formed in the doped channel SiC layer
102
; gate insulating film
104
of SiO
2
formed on the doped channel SiC layer
102
; gate electrode
105
made of an Ni alloy film and formed on the gate insulating film
104
; source/drain electrodes
106
a
and
106
b
made of an Ni alloy film in ohmic contact with the respective source/drain regions
103
a
and
103
b
; and backside electrode
107
made of an Ni alloy film in ohmic contact with the back surface of the SiC substrate
101
.
In this structure, if a constant voltage is applied between the source/drain electrodes
106
a
and
106
b
, and another voltage (a gate voltage) is applied to the gate electrode
105
, a current flowing between the source/drain regions
103
a
and
103
b
is modulated in accordance with the gate voltage so that a switching operation is performed. In particular, a MOSFET formed on a SiC substrate has higher breakdown voltage characteristic than those of a MOSFET formed on a Si substrate, and is highly regarded as a power electronic device that can supply a large current as well as expected to be implemented as an RF device.
PROBLEMS TO BE SOLVED
A power electronic device with a high-speed operation formed on a SiC substrate has been required to further improve in channel mobility and breakdown voltage with respect to its applications. This improvement has been continuously required by all semiconductor industries using compound semiconductor layers such as GaAs, GaN, SiGe, and SiGeC layers as well as a SiC layer for an active region.
In addition, the known MOSFET has problems peculiar to semiconductor devices including compound semiconductor layers. Specifically, a lot of interface states and charges exist at the interface between the gate insulating film
104
and the doped channel SiC layer
102
in the known NMOSFET, thus causing harmful effects on its characteristics as an ideal MOS device. A gate insulating film in a MOSFET formed on a Si substrate is generally made of a SiO
2
film (a thermal oxide film), which is formed by thermally oxidizing the Si substrate. For this thermal oxide film, since dangling bonds of Si atoms exist in the surface of the Si substrate, a certain amount of interface states are inevitably generated. It is known that the density of the interface states is about 10
10
.
On the other hand, it is known that even when the surface of a SiC layer is thermally oxidized to form a SiO
2
film (a thermal oxide film), about 10
12
interface states or fixed charges still exist at the interface between the SiC layer and the SiO
2
film thereon. Hence, the amount of interface states or fixed charges is greater than that in the Si substrate by about two orders of magnitude. It has been considered that this is because carbon, for example, which should have been removed during the thermal oxidation, remains as an impurity in the surface of the SiC layer and because an impurity for carriers (an n-type or a p-type impurity) in the SiC layer to be thermally oxidized is incorporated into the resultant thermal oxide film.
FIG. 13
illustrates an energy band in the gate electrode
105
, gate insulating film
104
, and doped channel SiC layer
102
when carriers flow, i.e., in an inversion state, in a known NMOSFET. As shown in
FIG. 13
, in the known NMOSFET, the threshold voltage thereof, for example, varies according to high-density interface states and positive charges trapped as fixed charges. Simultaneously, carriers (electrons) flowing through the channel are affected by interaction with the charges, resulting in decrease in channel mobility and deterioration in characteristics such as transconductance and high frequency response. Likewise, in a PMOSFET, negative charges are trapped in a gate insulating film, resulting in deterioration in characteristics of the device.
In addition to the device using the SiC substrate, devices using a substrate made of a compound semiconductor such as GaAs or GaN also have the same problems. Presumably, one of the reasons is that a compound semiconductor is composed of two or more elements. At the present time, even if an oxide film formed on the surface of a compound semiconductor substrate is used as a gate insulating film, it is difficult for the device to obtain characteristics suitable for practical use. Not only in the MOSFET but as long as an oxynitride film, a nitride film, or other metal oxide film (such as a tantalum oxide film) is used as a gate insulating film, the same problems might occur due to either positive or negative charge trapping.
DISCLOSURE OF INVENTION
It is therefore an object of the present invention to ensure a high-speed operation and a high breakdown voltage in a semiconductor device with a MISFET structure provided on a compound semiconductor substrate. It is another object of this invention to provide a semiconductor device exhibiting excellent electronic characteristics by using means for preventing harmful effects on characteristics of a transistor resulting from interface states or fixed charges created between a gate insulating film and a channel region.
An inventive first MISFET includes: a compound semiconductor layer formed on a substrate; two heavily doped layers, which are defined and spaced apart from each other in the compound semiconductor layer and contain an impurity of a first conductivity type; an active region, which is sandwiched between the two heavily doped layers and contains an impurity of a second conductivity type; a gate insulating film formed on the active region; and a gate electrode formed on the gate insulating film. The active region is formed by alternately stacking at least one first semiconductor layer functioning as a region where carriers flow and at least one second semiconductor layer containing an impurity for carriers at a high concentration and smaller in film thickness than the first semiconductor layer such that carriers spread out therein under a quantum effect. A region in the active region that is in contact with the gate insulating film is occupied by the first semiconductor layer.
According to this structure, the impurity concentration is low in the first semiconductor layer. Thus, scattering of impurity ions is reduced in the first semiconductor layer, and an especially high channel mobility is achieved. On the other hand, since the impurity concentration is low in the first semiconductor layer, the number of charges of the second conductivity type, trapped in the gate insulating film or near the interface between the gate insulating film and the active region, decreases and flowing of carriers is less prevented by the charges. When carriers spread out under a quantum effect, charges of the first conductivity type are trapped in an impurity contained in the second semico

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