Prof. Toshishige Yamada, Ph.D. (EE), UC Santa Cruz,



1.            EE135 Electromagnetic fields and waves in Winter 2006

2.            EE135 Electromagnetic fields and waves in Winter 2007

3.            EE293 Fundamentals of semiconductor physics - nanoscale materials and devices in Winter 2014

4.            EE227 Fundamentals of semiconductor physics in Fall 2014


At Santa Clara University (SCU), I have taught ELEN 151 Semiconductor devices, ELEN 152 Semiconductor devices and technology, ELEN 201 Electromagnetism, ELEN 261 Semiconductor physics and devices, ELEN 361 Nanoelectronics, and MECH 121 Thermodynamics.


Toshishige Yamada

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Establish energy band and equivalent circuit methods for nanoscale devices and materials

Perform bottom up theoretical analysis and compare with experimental data

Use thermodynamic ways of thinking in electronic device and material research

(a) Coulomb blockade for ID-VD (staircase) interpreted using an energy band diagram (SPIE 2010, UCSC)


Coulomb staircase is usually described in a highly mathematical way and it is not easy to visualize it. But using the energy band method, it is possible to visualize why the I-V characteristics are staircase-like.


(b) Carbon nanotube gas sensing or detection mechanism (PRB 2004, APL 2006, UCSC UARC at NASA)


This is a model of nanotube gas sensing or detection mechanism. Gas molecules will modify the nanotube-electrode contact properties (contact theory via Schottky barrier modulation) rather than change the nanotube bulk properties (bulk theory via doping).



Carbon nanotube FETs change their channel conductance by orders of magnitude in the gas atmosphere (the figure in the right column). There have been heated discussions for the gas detection mechanism. In doping theory, they assume the nanotube is doped by the gas and thus, there is a large modification of the channel conductance. I have proposed contact theory in "Modeling of Carbon Nanotube Schottky Barrier Modulation under Oxidizing Conditions," Phys. Rev. B 69 (12), 125408 (2004) and "Equivalent circuit model for carbon nanotube Schottky barrier: Influence of neutral polarized gas molecules," Appl. Phys. Lett. 88 (8), 083106 (2006) and considered the gas changes the electrode-nanotube contact properties while keeping the nanotube bulk properties unchanged Recently, scientists at Nanyang have reported an experiment on the gas detection mechanism in Ning Peng, Hong Li, and Qing Zhang, "Nanoscale contacts between carbon nanotubes and metallic pads," ACS Nano 3 (12), 4117-21 (2009) and scientists at Nanyang and MIT Ning Peng, Qing Zhang, Chee Lap Chow, Ooi Kiang Tan, and Nicola Marzari, "Sensing Mechanisms for Carbon Nanotube Based NH3 Gas Detection," Nano Lett. 9 (4), 1626-30 (2009), and ended this controversy. They have concluded "Our results are consistent with Yamada's theoretical prediction that SB (Schottky barrier) modulation is most significant when CNT is operating in the depletion mode."


The same work has been reviewed extensively by scientists at Tales and Ecole Polytechnique in Paolo Bondavalli, Pierre Legagneux, and Didier Pribat, "Carbon nanotubes based transistors as gas sensors: state of the art and critical review," Sensors and Actuators B 140 (1), 304-318 (2009) and scientists at Tales and SKKU, Didier Pribat and Paolo Bandavalli, "Thin-Film Transistors and Circuits Based on Carbon Nanotubes," J. Disp. Tech. 8 (1), 54-60 (2012). "The IBM works have been analyzed and theoretically modeled by Yamada at NASA. Yamada has explained simple Schottky model cannot justify alone the effect of oxygen molecules on the modulation of the Schottky values to obtain a p-type junction in air (Fig. 8)." "Yamada explains that the only way to consistently justify the effect of oxygen is to take into an account a sort of transition between the metal and the SWCNTs. This region is characterized by gold clusters on the electrode surface and charged oxygen molecules." "We can observe that in this case the Schottky barriers for holes in air if CNTs are exposed to air, according to the experimental results obtained by IBM researchers: Yamada's model seems to be satisfying and suitable to describe the effective interaction of gases on metal/SWCNT junctions."


(c) Are nanotubes unconditionally p-type? (APL 2002, NASA)


Usually nanotubes are p-type in air. However, Fan Wu, Taku Tsuneta, Reeta Tarkiainen, David Gunnarsson, Tai-Hong Wang, and Pertti J. Hakonen at Helsinki University of Technology and Chinese Academy of Sciences wrote a paper, "Shot noise of a multiwalled carbon nanotube field effect transistor," Phys. Rev. B 75 (12), 1 (2007) and reported the NT FET behavior at 4.2 K.


They pointed out "Our nanotube sample is clearly n-type doped initially [19]." They continued further "[19] The issue of doping of carbon nanotubes appears to be an intricate one. As argued by T. Yamada "Modeling of Kink-shaped Carbon-nanotube Schottky Diode with Gate Bias Modulation," in Appl. Phys. Lett. 80, 4027 (2002) some of the early experiments can be understood on the basis of negative doping in NTs even though the common belief is to have positive doping either due to adsorbed oxygen or due to a difference in metal-NT work functions." In fact, they have observed high conductance only for Vg > 0. This means that Vg > 0 should correspond to accumulation, and Vg < 0 depletion. Thus, the NT must be n-type in their experiment.


Yamada discussed the current-voltage characteristics of a gated NT metal semiconductor junction in the literature. He showed that the Schottky diode should be placed so that the right is a forward direction from the Id-Vd characteristics. The rectification must occur at the NT metal semiconductor junction. Id-Vg characteristics show that the NT must be n-type.



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[1] Research activities in the Electrical Engineering at UC Santa Cruz


(a) Study of InP nanowire devices in collaboration with Prof. Nobby P. Kobayashi, Dr. Andrew J. Lohn (former UCSC Ph.D. student and presently with Sandia National Labs), and Mr. Hidenori Yamada (presently UCSD Ph.D. student) as shown in figures in the left column


1.       Modeling of staircase-like I-V characteristics behavior in fused InP nanowire devices.

2.       Staircase period ~ 1 V at room temperature.

3.       Staircase clearly visible in dark, but disappearing in light illumination.

4.       Coulomb island formed at a location where a pair of nanowires fuse. The crystalline structure seems continuous there.

The staircase I-V is considered to be due to the partial trap of an electron wave at the fused segment. The Coulomb island there is so tiny that q2/C >> kBT and the presence or absence of a single electron will influence transport significantly. Additionally, Rtot >> RQ so that the number of electrons on the island is quantized. Thus, the Coulomb blockade is considered to occur (the figure in the left column).


Related presentations


1.       T. Yamada, H. Yamada, A. J. Lohn, and N. P. Kobayashi, "Room-temperature Coulomb Staircase in Semiconducting InP Nanowires Modulated with Light Illumination", Nanotechnology 22 (5), 055201 (2011). [pdf]

2.       T. Yamada, H. Yamada, A. J. Lohn, and N. P. Kobayashi, (invited) "Transport in fused InP nanowire device in dark and under illumination: Coulomb staircase scenario," Proc. SPIE 8106, 81060I, San Diego, CA, Aug 22-25, 2011. [pdf]

3.       H. Yamada, T. Yamada, A. J. Lohn, and N. P. Kobayashi, "Reversible suppression of Coulomb staircase in InP nanowires with light illumination," pp. 304-305 of the Proceedings of IEEE Nanotechnology Materials and Devices Conference 2010 (NMDC 2010), Monterey, CA, Oct. 12-15, 2010. [pdf]

4.       H. Yamada, T. Yamada, A. J. Lohn, and N. P. Kobayashi, "Coulomb staircase in fused semiconducting InP nanowires under light illumination," Proc. SPIE 7768, 77680B, San Diego, CA, Aug. 1-5, 2010. [pdf]

5.       H. Yamada, T. Yamada, A. J. Lohn, and N. P. Kobayashi, "Room-temperature Coulomb Staircase in Semiconducting InP Nanowires Modulated with Light Illumination," (P3.3) in the Material Research Society Meeting Spring, San Francisco, CA, Apr. 5-9, 2010.


(b) Study of carbon nanotube FET gas sensing mechanism conducted at UCSC University Affiliated Research Center (UARC) at NASA Ames (figures in the right column)

1.       T. Yamada, "Equivalent Circuit Model for Carbon Nanotube Schottky Barrier: Influence of Neutral Polarized Gas Molecules," Appl. Phys. Lett. 88 (8), 083106 (2006). [pdf]

2.       T. Yamada, "Modeling of Carbon Nanotube Schottky Barrier Modulation under Oxidizing Conditions," Phys. Rev. B 69 (12), 125408 (2004). [pdf]


[2] Research activities at Santa Clara University (present), NASA Ames Research Center, Stanford, Arizona State University, and NEC Microelectronics Labs., JPN


[3] Previous work, present technical interests, and mottos


1.       How: compare experimental data and theoretical analysis, bottom up modeling.

2.       Which structures: nanocarbons such as carbon nanotubes, nanofibers, or graphenes, and metallic/semiconducting nanowires and nanoislands. Also superlattices, quantum dots, thin films, adatoms, and atomic chains.

3.       Which materials: silicon Si, silicon germanium SiGe, strained Si, strained SixGe1-x, gallium arsenide GaAs, indium phosphide InP, indium antimonide InSb, zinc oxide ZnO, indium oxide In2O3, beryllium Be, magnesium Mg, oxygen O2, ammonia NH3, lead Pb, and superconductors.

4.       Which devices: field effect transistor or FET, pn junction, Schottky junction, Schottky barrier, Ohmic contact, solar cell, ultracapacitor, energy generation, energy storage, room temperature Coulomb blockade, low temperature, two terminal device, three terminal device, single electron device, Josephson device, mesoscopic device, nanodevice, interconnect, gas detector (gas sensor), and transmission line.

5.       Which methods: Monte Carlo, molecular dynamics, tight binding, energy band, equivalent circuit, Landauer Buttiker formula, Boltzmann equation, differential equation, and computer simulation.

6.       Which characteristics: device performance at low frequency and high frequency, current voltage characteristics, crystal structure, X ray diffraction, electric field, magnetic field, thermal transport, heat transport, electrothermal transport, nanotube breakdown, nanowire breakdown, and scanning tunneling microscopy modeling.


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