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.