APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 5
31 JULY 2000
GaNÕWÕW-oxide metal base transistor with very large current gain
and power gain
K. Mochizuki,a) K. Uesugi,b) and P. M. Asbeck
Department of Electrical and Computer Engineering, University of California, San Diego,
9500 Gilman Drive, La Jolla, California 92093-0407
J. Gotohc) and T. Mishima
Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan
K. Hirata and H. Oda
Hitachi ULSI Systems Corp., Kokubunji, Tokyo 185-8601, Japan
共Received 10 April 2000; accepted for publication 7 June 2000兲
We demonstrate a GaN/W/W-oxide metal base transistor 共MBT兲 whose collector is formed by
oxidizing the intrinsic W base. The thickness of the nonoxidized intrinsic base of the fabricated
collector-up MBT on a sapphire substrate was estimated to be 2–3 nm. Although the MBT showed
large leakage, subtraction of the leakage from collector current revealed that the transistor had a
very large small-signal direct current 共dc兲 current gain of 87 dB and a dc power gain of 50 dB. This
indicates that the GaN-based MBT is a possible candidate for microwave and millimeterwave
amplifiers as well as for high-speed integrated circuits used in optical fiber communication system.
© 2000 American Institute of Physics. 关S0003-6951共00兲04231-5兴
Among GaN-based electronic devices, AlGaN/GaN high
electron mobility transistors have already demonstrated
record figures for high power microwave performance.1
AlGaN/GaN heterojunction bipolar transistors 共HBTs兲 are
now being extensively studied2,3 because of such advantages
as normally off characteristics and low phase noise properties expected from a bulk device. However, their base resistance is very high due to the difficulty in achieving a high
hole concentration and mobility.
The metal base transistor 共MBT兲, which uses lowresistivity metal as a base, should greatly improve the direct
current 共dc兲 and radio frequency 共rf兲 performance while
maintaining the advantages of HBTs. So far, MBTs with
high common-emitter current gain ␤ and common-base current gain ␣ have been demonstrated in several material systems: ␤ ⫽100 in Si/Mo/Si,4 ␤ ⫽1.4 in GaAs/W/GaAs,5 ␣
⫽0.95 in Si/CoSi2 /Si, 6 and ␣ ⫽0.8 in GaAs/Nb/InSb.7 However, there have been no reports on GaN-based MBTs. In this
letter, we report an MBT consisting of GaN emitter, W base,
and W-oxide collector in collector-up configuration. Very
high ␤ and dc power gain were attained through ultrathin
base formed after selective oxidation of the W.
Schematic cross section of the fabricated MBT is shown
in Fig. 1. The MBT had a Au/Pt/Ti Schottky collector contact area of 30⫻30 ␮ m2, a W-oxide/W base-collector junction area of 40⫻40 ␮ m2, and a W/GaN base-emitter junction
area of 75⫻145 ␮ m2. First, a 1.0-␮m-thick GaN subemitter
layer (Si:1⫻1018 cm⫺3) and a 0.50-␮m-thick GaN emitter
layer (Si:5⫻1016 cm⫺3) were grown by metalorganic chemi-
cal vapor deposition on a C-plane sapphire substrate. Subsequently, a W film, whose thickness was 4.5⫾0.5 nm, was
deposited on the GaN emitter layer by rf sputtering. This was
followed by deposition of a 0.25-␮m-thick SiO2 film using
plasma-enhanced chemical vapor deposition 共PECVD兲 at
300 °C and by formation of SiO2 opening using photolithography and wet chemical etching. The sample was then
mounted on a Mo holder with In solder and was transferred
to molecular beam epitaxy chamber for selective oxidation
of the W at 500 °C for 2 min under ultrahigh vacuum
(1⫻10⫺8 Pa). After removing the SiO2 using wet chemical
etching, we observed the color of surface using optical microscopy and confirmed that while negligible oxidation occurred in the area where the SiO2 covered, severe oxidation
of the W took place in the collector area. X-ray photoelectron spectroscopy revealed that the surface composition of
the W oxide is WO3. Depth profiles of atomic composition
in WO3 /W/GaN structure determined by Auger electron
spectroscopy is shown in Fig. 2. Although surface roughness
of the GaN decreases depth resolution, the thickness of the
nonoxidized W can be estimated to be half the thickness of
the initial W film, i.e., 2–3 nm.
a兲
Electronic mail: motizuki@crl.hitachi.co.jp; present address: Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan.
b兲
Present address: Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan.
c兲
Present address: Image-Related Device Development Center, Hitachi, Ltd.,
Displays, 3300 Hayano, Mobara, Chiba 297-8622, Japan.
FIG. 1. Schematic illustration of cross section of the fabricated GaN/W/Woxide collector-up metal base transistor on a sapphire substrate.
0003-6951/2000/77(5)/753/3/$17.00
753
© 2000 American Institute of Physics
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754
Appl. Phys. Lett., Vol. 77, No. 5, 31 July 2000
Mochizuki et al.
FIG. 2. Depth profiles of atomic composition in W-oxide/W/GaN structure
determined by Auger electron spectroscopy.
Next, a 1.5-␮m-thick SiO2 film was deposited on the
selectively oxidized W film by PECVD at 300 °C. This was
followed by base mesa photolithography, wet chemical etching of the SiO2, and reactive ion etching 共RIE兲 of the W
using CF4. After removing the photoresist with acetone,
0.50-␮m-thick GaN emitter and 0.15-␮m-thick GaN subemitter were etched by CCl2F2 RIE using the SiO2 as a mask.
The SiO2 mask was then removed by wet chemical etching,
and an Al/Ti emitter electrode was formed on the GaN subemitter surface by a lift-off technique followed by alloying at
650 °C for 40 s. Finally, a Au/Pt/Ti electrode was formed by
a liftoff technique on the W-oxide as a Schottky collector
contact, and on the W base and the Al/Ti emitter electrodes
as probing pads.
Transmission line model measurement showed that the
sheet resistance of the extrinsic base 共nonoxidized W兲 was
0.40 k⍀/sq, while that of the intrinsic base 共partially oxidized W兲 was 7.0 k⍀/sq. This indicates that the mobility in
W is severely degraded when the film thickness becomes less
than 5 nm.
Figure 3共a兲 shows the common-emitter current–voltage
characteristics measured at room temperature using an HP
4155 curvetracer. The large collector leakage with a turn-on
voltage of 0.4 V indicates that the Au/Pt/Ti collector electrode touched the n-GaN emitter through pores in the W
base.5 Careful control of the W deposition condition is expected to decrease the collector leakage. The slope of 53 ⍀
in Fig. 3共a兲 corresponds to the sum of emitter and collector
resistance. Here we define ⌬I C as the difference between the
collector current I C at a certain base current I B and the I C at
I B ⫽0. Figure 3共b兲 shows the collector-to-emitter voltage
(V CE) dependence of the ⌬I C with I B from 20 to 80 nA 共20
nA step兲. When I B ⭌100 nA, ⌬I C did not increase from the
results with I B ⫽80 nA. We have not yet understood what
caused the ⌬I C saturation. The peak ␤ reached 2.2
⫻104 (87 dB) at V CE⫽4.7 V when I B ⫽20– 40 nA. This ␤
almost agrees with the observed ␣ ⫽0.999 95 in commonbase current–voltage characteristics. The dc power gain was
calculated from the curves of I B ⫽20 and 40 nA in Fig. 3共b兲
with the obtained base-emitter voltage, and the result is
shown in Fig. 4 as a function of the mean ⌬I C between
⌬I C (I B ⫽20 nA) and ⌬I C (I B ⫽40 nA). The maximum dc
power gain of 50 dB was achieved at ⌬I C ⫽0.90 mA. The
FIG. 3. 共a兲 Common-emitter current–voltage characteristics with base currents of 80 nA 共open circles兲 and 0 共solid triangles兲. 共b兲 Collector-to-emitter
voltage dependence of ⌬I C , which is the difference between the collector
current I C at a base current I B of 20–80 nA 共20 nA step兲 and the I C at I B
⫽0.
FIG. 4. Small-signal power gain as a function of the mean ⌬I C between
⌬I C (I B ⫽20 nA) and ⌬I C (I B ⫽40 nA).
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Mochizuki et al.
Appl. Phys. Lett., Vol. 77, No. 5, 31 July 2000
detailed mechanisms of current flow leading to the very high
␤ are not yet fully understood; however, the large incremental power gain unequivocally confirms transistor operation.
No alternating current measurements of the device characteristics have been made. It is likely that the device has the
potential for very high cutoff frequency ( f T ), as a result of
the ultrashort dimensions involved. Although the maximum
oscillation frequency ( f max) may be limited due to large
base-collector capacitance in the present implementation,
very high f max can be expected by increasing the thickness of
the W oxide, particularly because of the potentially low resistance of the metal base.
In conclusion, the fabricated collector-up GaN/W/Woxide metal base transistor on a sapphire substrate showed a
very large small-signal dc current gain of 87 dB and a dc
power gain of 50 dB. These results indicate that the GaNbased MBT is a possible candidate for microwave and millimeterwave amplifiers as well as for high-speed integrated
circuits used in optical fiber communication system.
755
The authors would like to thank D. Qiao of Department
of Electrical and Computer Engineering, University of California, San Diego, for his support for making ohmic contacts
to GaN. The author K. M. is grateful to Dr. I. Ohbu of
Central Research Laboratory, Hitachi, Ltd., Japan for his
support for preparing the W-deposited GaN on sapphire
wafers.
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