Diode sensors sensitivity

If a conductor material undergoes a work function change when exposed to a certain chemical species, then clearly one has the foundations of a C-S diode sensor. However, this sensor cannot be made to function if the conductor chemically reacts with the semiconductor. This loss of sensitivity occurs because the new material resulting from the reaction, in general, will not have the same work function sensitivity to the chemical species as the conductor has. The C-I-S configuration solves this problem since a properly chosen I-layer, capable of supporting an electrical current, can be inserted between the conductor and semiconductor to prevent their reaction. The resultant C-I-S structure is able to respond to the effects of the gas species on the conductor. 

One IC-based sensor array approaching commercialization is the Wide Range Ha Sensor [63, 64], which combined CMOS-compatible metal-insulated semiconductor (MIS) diode sensors and resistive sensors (Pd-Ni alloy) with on-chip control electronics. The wide-range claim relies on the MIS sensor for sensitivity to low concentrations of hydrogen gas (0.1%), and the resistive sensor for higher concentrations (1-100%). Non-standard IC processes for deposition of the Pd-based structures were devised at the Sandia foundry. Tests indicated little variability among sensors, e.g. 1-2% error. It was found that sensor exposures... 

Also for CO sensing, the present sensors are available only for the field of security not for environmental use because of the insufficient sensitivity and selectivity to monitor CO in the atmosphere. Examples of CO sensor which have been improved their sensitivity and selectivity are, for example, SnO semiconductor sensors operated under periodic temperature cycle[85-87], a electrochemical sensor using nafion membrane[88], a catalytic combustion sensor composed of catalysts and hydrophobic pol uner[89], a SnOj diode sensor doped with Pd[90] and an optical fiber catalytic sensor with Au/CogO as combustion catalyst[91]. 

Tiinchi A, Woldarski W, Li YX (2004) Hydrogen sensitive Ga O Schottky diode sensor based on SiC. Sens Actuators B 100 94-98... 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

An NIR biosensor coupled with an NIR fluorescent sandwich immunoassay has been developed. 109 The capture antibody was immobilized on the distal end of an optical fiber sensor. The probe was incubated in the corresponding antigen with consecutive incubation in an NIR-labeled sandwich antibody. The resulting NIR-labeled antibody sandwich was excited with the NIR beam of a laser diode, and a fluorescent signal that was directly proportional to the bound antigen was emitted. The sensitivity of the technique increased with increasing amounts of immobilized receptor. There are several factors involved in the preparation of the sandwich type biosensor. A schematic preparation of the sandwich optical fiber is shown in Figure 7.14. 

Kim et al. have compared the hydrogen and methane sensitivity at 400-600°C of Pt and Pd-SiC Schottky diodes fabricated on n-type 6H-SiC. The Pd or Pt (<80 nm) was sputter deposited in 1-mm dots [8]. The sensitivity was measured as the change in current at a constant forward bias of 3V. The Pd-SiC Schottky diodes showed a higher sensitivity, as well as faster speed of response to both hydrogen and methane. The stability of the hydrogen response was tested for 30 days at 500°C and showed excellent results for both types of sensors. 

The hydrogen sensitivity of palladinm-oxide-semiconductor (Pd-MOS) strnctnres was first reported hy Lnndstrom et al. in 1975 [61]. A variety of devices can he nsed as field-effect chemical sensor devices (Fignre 2.6) and these are introdnced in this section. The simplest electronic devices are capacitors and Schottky diodes. SiC chemical gas sensors based on these devices have been under development for several years. Capacitor devices with a platinum catalytic layer were presented in 1992 [62], and Schottky diodes with palladium gates the same year [63]. In 1999 gas sensors based on FET devices were presented [64, 65]. There are also a few publications where p-n junctions have been tested as gas sensor devices [66, 67]. 

SiC capacitor sensors have demonstrated gas-sensitivity to gases such as hydrogen and hydrocarbons [21, 46, 68] up to a maximum temperature of 1,000°C [1, 68]. Devices that can be operated both as MOS capacitors (reverse bias) and as Schottky diodes at temperatures greater than 490°C have also been demonstrated (see Section 2.4.2) [10]. These devices showed sensitivity to combustible gases such as propane, propylene, and CO and were tested at temperatures up to 640°C. 

It was demonstrated that reproducible gas-sensitive silicon Schottky sensors could be produced after terminating the silicon surface with an oxide layer [71, 72]. This interfacial oxide layer permits the device to function as a sensor, but also as a diode, as the charge carriers can tunnel through the insulating layer. The layer made the Schottky diode behave like a tunneling diode, and the ideality factor could be voltage-dependent [73]. 

Weidemann et al. found that wet etching of a GaN surface before Pd deposition also produced an interfacial oxide, which increased the hydrogen sensitivity by approximately a factor of 50 [14], They concluded that comparing device parameters between different GaN Schottky diode gas sensors requires a defined standard treatment of the GaN surface to introduce a controlled interfacial oxide. 

Schottky-barrier diode and metal-oxide-semiconductor (MOS) capacitor gas sensors have established themselves as extremely sensitive, versatile solid state sensors. 

In this review the basis for the chemical sensitivity of these devices will be explored and the various device structures used for these sensors will be discussed. A survey of the performance of the diode-type and capacitor-type structures will be presented and a comparison of characteristics of these two classes of solid state gas sensors will be given. 

The first question to ask when comparing various diode and capacitor sensor structures is how do their sensitivities compare. This question is answered for several hydrogen sensing structures in Table V. 

---