HBTs with a planar-type extended base as a hydrogen-sensitive sensor
C. H. Huang, S. W. Tan, H. Lo, C. Lo and W. S. Lour
A hydrogen sensing transistor fabricated by a heterojunction bipolar
transistor (HBT) with an extended base (EB) formed by a
metal-semiconductor-metal (MSM) hydrogen sensor is reported. The power
consumption in stand-by mode is smaller than 2 µW. Common-emitter
characteristics show that the sensing base (collector) current gains at
25℃ in 0.01%, 0.1%, and 1% H2/N2 are
as high as 75 (512), 134, (977), and 233 (2.89 × 104),
respectively. Low-power consumption and high-sensitive gains are
indicative that our HBT together with planar-type MSM sensor is very
promising for applications to hydrogen sensing transistors using one
voltage source.
Introduction: To develop high-sensitive sensors is demanded since
hydrogen is widely applied as energy carrier. Benefit from first report
on detecting hydrogen with a Si-based metal oxide semiconductor (MOS)
structure [1], a lot of studies on various structures were then
provided. In particular, a metal-semiconductor (MS) diode [2-4] and
a field-effect transistor (FET) [5-7] are two widely accepted
hydrogen sensors. Ohmic and Schottky contacts cannot be formed by using
different high-cost metals on the same layer in fabricating the MS
diode. Besides, a MS diode as a hydrogen sensor is generally forward
biased [3, 4]. Undesirable power consumption has to be concerned in
stand-by situation. Three-terminal FETs having catalytic metal (Pd or
Pt) be their gate metal. However, both a positive and a negative source
are needed for a depletion-mode FET [6, 7]. Power consumption in
stand-by situation is still an issue. Fortunately, a
metal-semiconductor-metal (MSM) diode was reported to have two Schottky
contacts on the same layer [8, 9]. Furthermore, only one source is
required to obtain current-voltage characteristics and a very small
stand-by current is expected. In this letter, a bipolar-type HBT
structure used to fabricate a hydrogen sensing transistor is proposed. A
planar-type MSM diode forming on the low-doping collector layer was
employed as the extended-base hydrogen sensor. Compared to FETs,
bipolar-type transistors also require only one positive source. Thus,
low-power consumption and high-sensitive gains were achieved for the
hydrogen sensing transistor.
Device fabrication and measurement: Fig. 1 shows a schematic
diagram of a proposed hydrogen sensing transistor. An InGaP-GaAs HBT is
used as a current amplify transistor while a MSM diode is employed as a
hydrogen-sensitive sensor. The HBT structure deposited on a
(100)-oriented semi-insulating GaAs substrate was employed. It was
prepared by a metal-organic chemical vapour deposition (MOCVD) system
and consisted of a 0.6 µm n+ -GaAs
(n+ = 5×1018cm-3) sub-collector layer, a 0.8 µm low-dopingn- -GaAs (n- =
8×1016 cm-3) collector layer, a 0.08
µm highly-doping p+ -GaAs
(p+ = 4×1019cm-3) base layer, a 0.03 µm n -InGaP (n =
3×1017 cm-3) emitter layer, and a
0.2 µm n+ -GaAs (n+ =
3×1018 cm-3) sub-emitter layer
together with a 0.1 µmn+ -InxGa1-xAs
(x = 0 to 0.7 in 0.05 µm and x = 0.7 in 0.05 µm) cap
layer. Manufacturing processes of the HBT started with the emitter and
the base mesas. Ohmic contacts for the emitter (E), the collector (C),
and the base (B’) were formed by depositing AuGeNi and AuZn upon the
cap, the sub-collector, and the base layers, respectively. After
removing the cap, the emitter, and the base layers in the
hydrogen-sensor region, two coplanar multiple-finger electrodes were
formed by depositing a 30 nm mixture of Pd and SiO2[8]. The extended base (EB) using the MSM diode was hence achieved.
To measure sensing properties of as-fabricated hydrogen sensing
transistor, 50 ppm to 1% H2/N2 gases
were employed by using a custom-made, flow-through 4-pin testing
chamber. Sensing properties of the EB-hydrogen sensor were obtained by
applying a voltage of VBB’ and introducing
air/N2 and various H2/N2gases into the chamber. Sensing diode currents obtained in
N2 and H2/N2 are denoted
as IDH and IDN ,
respectively. Electrical properties of the HBT were measured by using
the electrodes E, B’, and C. In contrast, sensing properties of the
hydrogen sensing transistor were measured by using the electrodes E, B,
and C. At the present measurement, sensing base and collector currents
(IBN , IBH andICN , ICH ) reflecting
N2 and H2/N2 were
employed as output signals. Furthermore, a sensing base (collector)
current gain of GB =IBH /IBN(GC =ICH /ICN ) was defined to
evaluate our hydrogen sensing transistor.