THE UNIVERSITYo[ EDINBuRGH
Special instructions
SCHOOL OF ENGINEERING MICROELECTRONICS 2 ELEEOBO2O
Exam Date: 1911212019 From and To: 14:30-16:00 Exam Diet: Dec 2019 Please read full instructions before commencing writing
Copyright By PowCoder代写 加微信 powcoder
Exam paper information
• This exam paper consists of TWO sections.
• Candidates should attempt THREE questions, chosen as follows:
o SECTION A: ONE question. Attempt the whole section. o SECTION B: Attempt TWO out of the THREE questions.
• Students should assume reasonable values for any data not given in a question nor available on a datasheet, and should make any such assumptions clear on their script.
• Students in any doubt as to the interpretation of the wording of a question, should make their own decision, and should state it clearly on their script.
• Please write your name in the space indicated at the right hand side on the front cover of the answer book. Also enter your examination number in the appropriate space on the front cover.
• Write ONLY your examination number on any extra sheets or worksheets used and firmly attach these to the answer book(s).
• This examination will be marked anonymously.
Special Items
• Formula Sheet (1 page)
Convenor of Board of Examiners: Professor R Examiner: DrZDurrani& ProfessorJ Morrow
Question Al a)
If the electron mobility for a sample of n-type silicon is 012 m2V’s1 at
50 K, what will the mobility be at 100 K? (2)
Sketch a graph, with appropriately labelled axes, showing how the resistivity of an extrinsic semiconductor is expected to behave as a function of temperature in the following regimes:
Briefly describe what is meant by the term ‘carrier dispersion” in the
context of microelectronics. (2)
Give two reasons for why dry plasma etching is often preferred over
wet acid etching during the microfabrication of semiconductor devices. (2)
For a p-n junction solar cell illuminated with sunlight:
A bipolar junction transistor (BJT) operating at 300 K has a base
current of 10 pA. Calculate the transconductance, assuming the
ideality factor qfor the base-emitter junction is 1 and the common
base current gain a = 0.98. (3)
For a p-channel metal oxide semiconductor field effect transistor (MOSFET) intended to operate normally, state which of the following conditions, if any, would be undesirable. Provide additional explanation for full marks.
(I) Light doping; (2) (H) Extremely heavy doping. (2)
(i) Sketch a one-dimensional energy band diagram to illustrate
the process of absorbing a photon. (2)
(H) Briefly describe why a stacked multi-junction solar cell may be
able to convert a greater proportion of the incident solar energy
to electrical energy than a single junction device. (3)
(i) The MOSFET biased so a layer of holes forms in the semiconductor under the gate oxide.
(H) The MOSFET biased so a layer of electrons forms in the semiconductor under the gate oxide.
(Hi) The gate electrode biased positively with respect to the source. (iv) The gate electrode biased negatively with respect to the
ELEEO8O2O Microelectronics 2— December 2019
SECTION B Question BI a)
Use first principles to derive the following expression for the conductivity (a) of a metal:
An initially intrinsic silicon semiconductor sample, with lateral
dimensions of 10 x 10mm, and thickness 0.4 mm, is subjected to a sequence of photolithography and p-type ion implantation steps,
resulting in a linear lateral gradient in acceptor concentration. At room temperature, when a voltage of 1.7 V is applied across the width of the sample (i.e. in the same axis as the concentration gradient), the total measured current through the system is found to be exactly balanced
(i.e. zero net current flow). If the average concentration of acceptors withinthedeviceis2.5x1011m,whatistheconcentrationgradient
of acceptors? (5)
Briefly discuss all the factors that might cause changes to this balance
of currents if the temperature of the system were to be increased
slightly. (2)
Table B1 provides some typical electrical characteristics of two intrinsic semiconductors. Which of these materials is better suited for the manufacture of each of the following applications? Give reasons for your answers.
where n is the electron number density, e is the charge on the
electron, and p is the electron mobility. (5)
(i) Mass-produced integrated circuit manufacture. (2) (U) Semiconductor lasers. (3) (Ui) Very high frequency electronics devices, (3)
Energygap: Electron mobility: Intrinsic carrier concentration: Resistivity: Defect/impurity concentration:
1.12eV 0.15 m2 V” sl 1.4 x 1016 m3 2.3 x i0 0cm
Gallium Arsenide 1.43eV
0.85 m2 V.1 5I 2.1 x 1012 rn’3
1 x 1080cm 10 rn’3
1019m3 j Table B1
ELEEO8O2O Microelectronics 2— December 2019
Question B2 a)
An un-doped sample of a semiconductor is found to have conductivity valuesof1.10x 106Sm-1and1.05x 1OSni’at300Kand400K respectively. Calculate the energy gap (in eV) for this material. (4)
For the above material, at room temperature, calculate how many
times more likely it is to find an electron at the top of the valence band compared to finding it in the bottom of the conduction band. (4)
Sketch how the probability of finding a hole at different energies varies across the valence and conduction bands for a p-type semiconductor
at room temperature. (2)
For an ideal abrupt p-n junction diode under equilibrium conditions (i.e. there is no external applied bias), and where the concentration of dopant atoms is the same in the p-type and n-type sides of the diode:
Laser diodes require the creation of a population inversion to emit laser light (coherent photons). Sketch the band diagram for a p-n
(i) Sketch the energy band diagram for the p-n junction, indicating clearly the position of the Fermi Energy. (1)
(H) Sketch the variation of charges through the junction, indicating clearly the sign of charges. (1) (Hi) Sketch the variation of the electric field through the junction. (1)
(iv) In terms of the features shown on your band diagram, derive
an expression for the barrier potential across the junction. (2)
(v) What is the maximum forward bias voltage that can be applied
to the junction and why? (2)
junction where a population inversion exists and state what condition regarding the doping of the semiconductor must be satisfied to
achieve the population inversion. (3)
ELEEO8O2O Microelectronics 2— December 2019
Question B3
a) Sketch the structure of a simple n-channel enhancement mode metal
oxide semiconductor field effect transistor (MOSFET), labelling all the
main features, and, where appropriate, the doping types. (6)
b) An n-channel enhancement MOS transistor has the following parameters:
Oxide thickness t0 = 3 x 1o m
Channel width W = 1.2 x 1 06 m
Channel length L = 3.5 x io m
Relative permittivity of the gate oxide Er = 4
The drain-source current IDS is measured for different applied bias voltages with the results given in Table B3, where Vas is the gate- source voltage and VDSi5 the drain-source voltage.
VGS (V) 0.7 1.2 1.6 1.6
VDS (V) 0.05 0.05 0.5 1.2
IDS (A) 3.4×106 14.9×106
(i) Calculate the gate capacitance per unit area, Co.
(ii) From the measurements given in Table B3, deduce values for
the threshold voltage VT, and the channel electron mobility J0. (Hi) Deduce the two values of IDS missing from Table B3.
ELEEO8O2O Microelectronics 2— December 2019
END OF PAPER
‘s[ftJ_’J’
CE c=cDexp—
e= 1.6x c=3×108 m
k= I.38x1023JK’ =8.85×10’2
h= 6.63x1034Js 0 °C=273 K
Formula and Data Sheet for Microelectronics 2
IC=aIE+ICBO
AT =const dE
p 0 ÔV dxc
ELEEO8O2O Microelectronics 2— December 2019
a—nep+peph
F(E)=I÷exp[_EP]
1DPcO7[(VGS_VT)VDS_2]
1DS2L(VGS_VT)
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com