GaN Schottky diode sensor

Schottky diodes are the simplest sensor type to fabricate and can be operated in two different modes. In this section we cover their use for gas sensing. 

Wurtzite GaN is a polar material. Therefore, along the c-axis, there are N-face (N-polar) or Ga-face (Ga-polar) orientations on the GaN surface. 

In N-polar GaN Schottky diodes, because of the higher surface reactivity with hydrogen inducing higher polarity, the Schottky barrier heights are reduced much more than those of Ga-polar Schottky diodes. Therefore, the N-polar GaN Schottky diodes have much higher sensitivity than Ga-polar GaN. 

20 (a) Cross-sectional diagram of a Schottky diode made of Ga-polar or N-polar (b) plan-view photograph of device. 

21 (a) l-V characteristics from the three types of diodes (Schottky diodes made of N-polar GaN, Ga-polar GaN and Ga-polar HEMT) before and after exposure to 4% H2 in Nj (b) the absolute and percentage change in current as a function of voltage as a result of hydrogen exposure. 

---

Remote sensing system for hydrogen using GaN Schottky diodes . Sensors and Actuators B-Chemical, 105(2), 329-333. 

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 diode sensors based on other wide bandgap materials have also been investigated, as previously mentioned. GaN Schottky diodes processed on either the Ga or N face have been examined by Schalwig et al. [11,21]. A Pt/GaN Schottky diode with a barrier height of 1-eV has been shown to reversibly transform into an ohmic contact through exposure to [94]. Kokobun et al. have also investigated Pt-GaN Schottky diodes as hydrogen sensors up to 600°C [15]. 

Song, X, Lu, W, Flynn, X and Brandes, G., AlGaN/GaN Schottky diode hydrogen sensor performance at high temperatures with different catalytic metals . Solid State Electron., 2005,49,1330-4. 

Ali M, Cimeilla V, Lebedev V, Romemus H, TQak V, Merfeld D, Sandvik P, Ambacher O (2006) Pt/GaN Schottky diodes for hydrogen gas sensors. Sensor Actual B-Chem 113 797-804... 

AIGaN/GaN HEMT Schottky diode-based hydrogen sensor... 

Matsuo, K., Negoro, N., Kotani, J., Hashizume, T. and Hasegawa, H. (2005) Pt Schottky diode gas sensors formed on GaN and AlGaN/GaN heterostructure Applied Surface Science, 244(1-4), 273-276. 

---