Personal statement
Ling Li
With primary dream of making our own advanced chips, I chosen the Electrical Engineering as my
major about three years ago; and then without hesitation I transferred to microelectronics and
nanoelectronics engineering when choosing sub-major for its foundation role in manufacturing
chips one year ago; after more than one year of professional training, I find the area of my
graduate study: …. I know this is just what I want.
这两段我想了很久还是不知道怎么把它们综合起来？
I once fascinated by analog circuits when taking the course, Fundamentals of Analog Electronics. I
would spare more than one hour every day with my already busy schedule, analyzing the circuits
in the textbook which were not required until all the circuits in that book been studied; discussed
those complex analog circuits with the professor every week and one time got the comment,
“Tell me your name. You’re so impressive and brilliant!” I thought that was what I wanted. After
my self-studying the book, Semiconductor Physics and Devices: Basic Principles (Neamen, 3rd), I
got to know semiconductor devices which determine the performance of those circuits,
fundamentally. What’s more, the physics abstract yet concrete, in it were so attractive for me and
my passion inevitably moved on to this field.
Arising from love, I concentrated my heart in studying and did a good job. In the last three years, I
got an average GPA of about 91.4/100 with phys. and circuit courses average 95/100, ranking 7/62.
With full score of 100 in Introduction to Quantum Information Science and nearly full scores in
Quantum Mechanics and Statistical Mechanics, I show my excellent understanding of quantum
mechanics, appreciating the beauty of abstract physics.
Until now, all the projects, the compact model of III-V quantum well FET (QWFET), improving
Kane’s tunneling theory and compact model of Silicon nanowire tunneling FETs (NW t-FET) I have
conducted were strongly related to physics which I think could equip me with solid physics
foundation better for my long term development.
As we know compact model of devices could be a thesis of graduates for its complexity and high
demand of physics and math, but I never hesitated for the difficulties and picked the compact
model of III-V quantum well FET (QWFET) as my first independent academic research, for the
physics of this promising high-speed devices. The two-month short-time study of QWFETs
prepared me with the concert of quantum confinement effects on devices performance which is
also adaptable to nanowire.
And then I transferred to improve Kane’s tunneling theory and the compact model of Silicon
nanowire tunneling FETs (NW t-FET) to continue another undergraduate’s work (who’s now in
MIT for her graduate study). After studying Kane’s tunneling theory, I got an in-depth
understanding of the tunneling which helped me further study the tunneling FETs. Before
constructing the compact model of NW t-FETs, I studied the model of 1-D carbon nanotube FETs
(CNFETs), charge control model, the ballistic transport, quantum capacitance etc., all these
cutting-edge knowledge and also did not ignore other fundamental concepts like band
quantization, Landauer’s equation, even phase coherence influence on ballistic transport. Fully
understanding the physical model of a novel device needs a long time and that’s why until now I
propose a physical based compact model of NW t-FETs and start writing a paper for ASP-DAC to
be held in Sydney, Australia, Jan. 2012.
In studying Kane’s tunneling theory, I derived Kane’s tunneling theory from the basic Schrödinger
equation all the way to the tunneling probability for more than three times and identified many
errs in those classical works. I also found that using crystal momentum representation, the
momentum in the perpendicular tunneling direction and the total energy could be showed
explicitly unchanged in direct tunneling and then we can get the tunneling probability from the
different bands interband matrix element which was the essence of direct tunneling. Knowing
that the accuracy of Kane’s tunneling theory is limited to the linear and low electrical field, we
could just use WKB approximation for its flexibility and simplicity in calculating. Based on the
former work, I continue this project to explore the physical figure of gated tunneling mechanism.
I found that the relation of quantum capacitance (Cq) and gate-insular capacitance (Ci) (ignoring
the electrostatic source/drain capacitance) defined the channel potential and thus determine the
drain current. And the compact model of mine now focuses on the device size Cq comparable to
Ci where is the charge control regime and will continue to study the scaling behavior towards the
quantum capacitance limit regime where1-D nanowire tunneling FET shows its potential by the
power delay product. Meanwhile I proposed an approach to extract the equivalent capacitance in
the source/drain and the quantum capacitance so that we could clarify the influence of these
capacitances on the devices performance, based on its small signal model
Tough work as it is, and I did feel a little depressed without any obvious result at first. Facing
difficulties, I never doubt my capability to deal this problem and keep on going. Soon, the joy of
learning replaced the pity of no fast results and my love and interests in it increase incredibly. And
I treasure this experience a lot because I find I really love semiconductor physics even in this
unfavorable situation. Besides, I have got the chance to learn much cutting-edge knowledge and
ways of academic thinking like rigorousness and in-depth understanding of physics from
discussing with my supervisor Prof. Zhiping Yu. And I even got the chance to work together with
him and jointly delivered an invited talk on modeling of 1D (CNT/NW) tunneling FETs in IEEE
EDSSC (Electron Devices and Solid-State Circuits) just held in Tianjin, China.
With my passion on semiconductor physics, solid physical theory foundation, down-to-earth
altitude, open mind and my related academic research and rich social experience, I confidently
think I am qualified for your prestigious graduate programs.
Key word:
Passion, physics foundation, resilience.
这个东西也许有用，也许没用。
Based on charge control model and Kirchhoff's circuit laws, with the relation of the charge in the
channel and the external equivalent circuit, regarding the channel as an electrode apart from the
source/drain/gate electrodes, I proposed a physical base compact model of silicon nanowire
tunneling FETs.. And now I am working on a paper for ASP-DAC to be held in Sydney, Australia,
Jan. 2012.