Download mecve 101 semiconductor devices

M. TECH DEGREE EXAMINATION
Model Question Paper
Branch: Electronics &Communication Engg.
Specialization: VLSI & Embedded Systems
First Semester
MECVE 101 SEMICONDUCTOR DEVICES - PHYSICS AND MODELING
(Regular-2013 Admissions)
Time: 3 hrs.
Maximum marks: 100
Answer all questions
Each question carries 25 mark.s( Part a. carries 5 marks, Part b. carries 8 marks and Part c. carries 12 marks)
I.
a.
b.
c.
Define flatband voltage qualitatively and quantitatively with appropriate diagrams
Discuss the energy band of a MOS Capacitor with a p type substrate in the three regions of
operation with appropriate diagrams
Discuss the C- V characteristics of a MOS Capacitor in all bias regions with appropriate definitions,
derivations and diagrams
Consider an MOS capacitor fabricated on P-type Si substrate with a doping of 5 × 1016cm–3 with
oxide thickness of 10nm and N+ poly-gate. i) Find Cox, Vfb, and Vt. ii) Find the accumulation charge
(C/cm2) at Vg = Vfb –1 V. iii) Find the depletion and inversion charge at Vg = 2 V. iv) Plot the total
substrate charge as a function of Vg for Vg from –2 to 2 V.
OR
II
a.
b.
c.
III
a.
b.
c.
Define threshold voltage qualitatively and quantitatively with appropriate diagrams
Discuss the energy band of a MOS Capacitor with a n type substrate in the three regions of
operation with appropriate diagrams
Discuss the charge distribution in a MOS Capacitor in all regions of operation with appropriate
definitions, derivations and diagrams
Consider an ideal MOS capacitor fabricated on a P-type silicon with a doping of Na = 5 × 1016cm–3
with an oxide thickness of 2 nm and an N+ poly-gate. i) What is the flat-band voltage, Vfb, of this
capacitor? ii) Calculate the maximum depletion region width, Wdmax. iii) Find the threshold
voltage, Vt, of this device. iv) If the gate is changed to P+ poly, what would the threshold voltage be
now?
Differentiate the boundaries between weak, moderate, and strong inversion approximately in terms
of voltages.
In AMI’s 0.5µm CMOS process, µn Cox’ = 116µA/V2 and µp Cox’ = 38µA/V2. Estimate the
boundaries between weak, moderate, and strong inversion (in terms of drain currents) for nMOS
transistors with W/L = 1 and W/L = 100. Assume κ = 0.7.
In the long channel, strong inversion model, the precise current in the linear regime is given
by
, where β is a constant: 0.01 < β < 0.1. Derive an
expression for saturatrion drain voltage VDSsat and saturation drain current, IDSsat (12)
OR
IV
a.
b.
Differentiate the boundaries between weak, moderate, and strong inversion approximately in terms
of voltages
In a 0.5µm CMOS process, µn Cox’ = 116µA/V2 and µp Cox’ = 38µA/V2. Estimate the boundaries
between weak, moderate, and strong inversion (in terms of drain currents) for pMOS transistors
with W/L = 1 and W/L = 100. Assume κ = 0.7.
c.
V
a.
b.
The Enz-Krummenacher-Vittoz (EKV) model of the MOS transistor provides a simple approximate
closed-form expression for the channel current of a MOS transistor in terms of the terminal
voltages. Show analytically that it is valid in all regions of normal MOS transistor operation
Discuss the two key large signal design parameters and their implications
The saturation velocity for electrons in Si is vsat = 8106 cm/sec. Consider an n-channel MOSFET
with Toxe = 3 nm, VT = 0.3V and WT = 50 nm, with gate bias VGS = 1.0V, meff » 260cm2 / V × s . Find
VDsat for i) L = 1 m. Is this a long-channel device or a short-channel device? ii)L = 0.1 m. Is this
a long-channel device or a short-channel device? How would VDsat change if iii)Toxe is decreased?
iv)VGS –VT is decreased?
c.
Propose a non quasi static model of the MOS transistor in transient operation. Derive models of
extrinsic parasitic.
OR
VI
a.
b.
c.
VII
a.
b.
c.
A PMOS transistor with a channel length modulation factor of λ=0.05V-1 is sized so that it has a
drain current of ID=15μA when VSD = VSG-|Vtp|. i) Accounting for channel length modulation what
is the drain current if the drain voltage VD drops by 2.5V?
Consider an n-channel MOSFET with channel doping concentration NA = 41017/cm3, effective
oxide thickness Toxe = 3 nm, source/drain junction depth rj = 0.05 m. i)What is VT for L = 1 m
when VDS = 0.1 V? ii) What is VT for L = 0.05 m when VDS = 0.1 V? iii) What do you infer from
the above?
Discuss the bias dependence of the non quasi static model of the MOS transistor with reference to
Cgs, Cgd, and Cgb
Categorise MOSFET Simulation Models
Compare the small signal capacitances at weak inversion, strong inversion (linear) and strong
inversion (saturation)
Find an equivalent circuit for a MOSSFET which interrelates the incremental changes in iD,vGS,vDS,
etc. and accounts for transconductance, output conductance, backgate transconductance and
MOSFET capacitances
OR
VIII
a.
b.
c.
Write a brief summary of flicker noise models in MOSFET
Differentiate Bulk and Source referred small signal models of MOSFET
Compare Surface Potential, Charge and Threshold voltage based MOSFET models.