Department of Electrical and Computer Engineering

COEN 451, Midterm Exam Date: 21st Oct. 2013

Time: 1:15 hour, Attempt all 3 Questions

Calculators are allowed

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Question 1 (7 marks)

Design a circuit which controls a bar display consisting of three LEDs as shown in Fig.1a. The operation of these LEDs is based on the level of the input signal:

LED1 is ON when the input voltage reaches 0.5V

LED1 and LED2 are both ON when the input signal reaches1.0 V.

All LEDs are ON when the input signal reaches 1.5V.

The circuit, which controls the display consists of three transistors, each of a distinct threshold voltage as shown in Fig.1 b

M1 is an NMOS transistor with a VTO=0.5V.

M2 is NMOS transistor with a threshold voltage adjusted by an external voltage Ve.

M3 is the same type transistor of M1 with a threshold voltage adjusted by an ion implantation process.

a. Specify the threshold voltage of each of the transistors to achieve the required operation.

b. Determine the value of the external voltage Ve.

c. Determine the type and dose of the dopant so that the threshold voltage of M3 is adjusted from 0.5V to the required value.

The transistors have the following parameters: tox =200Ao, = 0.5 V1/2, s = -0.6V

Fig. 1b

Question 2.(7marks)

Determine the resistance of the following structure shown in Fig. 2.

Each square is 1μ by 1μ

Polysilicon resistance =10Ω /□, Contact Resistance =5Ω/ contact.

Metal resistance =0.05Ω /□, Diffusion Resistance =70Ω /□,

Transistor Vgs=3.3 V, Vth= 0.6, Vds= 1V, λ= 0.01 V-1

Cox = 1.5fF/m2 , μn= 500cm2/V-sec,

NOTE:Accuracy of calculation of the resistance is important for this question. Do not ignore small resistances.

Fig. 2Structure of Question 2

Question 3. (6 Marks)

A transistor is scaled according to Table 1.

Fill in Table 2, showing the advantage and disadvantage of scaling on performance measures given in Table 2. (7 marks)

Length / L / S / L/S
WIDTH / W / S / W/S
THIN OXIDE / tox / S / tox/S
DIFFUSION DOPING / ND / 1/S / ND. S
SUBSTRATE DOPING / NA / 1/S / NA . S
SUPPLY VOLTAGE / VDD / S / VDD/S

Table 1

PARAMETERS / Equations used / VALUES After Scaling / REMARK
Delay, τ / τ = / τ’=
Area, A / A = / A’ =
Cap/unit area, Cox / Cox = / C’ox =
Gate capacitance / Cg = / Cg’ =
Current,I / I = / I’D =
Current density,ID / ID= / ID’
Power , P / P = / P’ =
Power Density,PD / PD / PD’ =

Table 2

Some Useful Equations

Current Equations

PMOS:

- Cut-off,

- Saturation

- Linear

Transistor resistance: Delay and Power:

R =: Linear region

R =: Saturation

Body effect equation:

or more accurately, below

VTo’ = VTo + (q. DI/Cox), DI= Dose of dopant in the channel (atoms/cm2),

Cox= gate oxide capacitane per unit area, Cox = 1.5fF/m2 , μn= 500cm2/V-sec

Values of some useful constants

Boltzman constant k 1.38 * 10 –23 J/K

Electron charge q 1.6 * 10 –19 C

Thermal voltage T 26 mv (at 300 K)

Electrical permittivity (vacuum) o 8.85 * 10 –14 F/cm

Permittivity of Si si 3.5 * 10 –13 F/cm

Permittivity of SiO2 ox 1.05 * 10 –12 F/cm

Magnetic permeability o 12.6 * 10 –7 Wb/Am

Room Temperature T 300 (=27 0C) K

Appendix B: SPICE Parameters

.MODEL CMOSN mos3 type=n

+PHI=0.700000 TOX=9.6000E-09 XJ=0.200000U TPG=1

+VTO=0.6566 DELTA=6.9100E-01 LD=4.7290E-08 KP=1.9647E–04

+UO=546.2 THETA=2.6840E-01 RSH=3.5120E+01 GAMMA=0.5976

+NSUB=1.3920E+17 NFS=5.9090E+11 VMAX=2.0080E+05 ETA=3.7180E-02

+KAPPA=2.8980E-02 CGDO=3.0515E-10 CGSO=3.0515E-10

+CGBO=4.0239E-10 CJ=5.62E-04 MJ=0.559 CJSW=5.00E-11

+MJSW=0.521 PB=0.99

+XW=4.108E-07

+CAPMOD=bsim XQC=0.5 XPART=0.5

*Weff = Wdrawn - Delta_W

*The suggested Delta_W is 4.1080E-07

.MODEL CMOSP mos3 type=p

+PHI=0.700000 TOX=9.6000E-09 XJ=0.200000U TPG=-1

+VTO=-0.9213 DELTA=2.8750E-01 LD=3.5070E-08 KP=4.8740E-5

+UO=135.5 THETA=1.8070E-01 RSH=1.1000E-01 GAMMA=0.4673

+NSUB=8.5120E+16 NFS=6.5000E+11 VMAX=2.5420E+05 ETA=2.4500E-02

+KAPPA=7.9580E+00 CGDO=2.3933E-10 CGSO=2.3922E-10

+CGBO=3.7579E-10 CJ=9.35E-04 MJ=0.468 CJSW=2.89E-10

MJSW=0.505 PB=0.99

+XW=3.622E-07

+CAPMOD=bsim XQC=0.5 XPART=0.5

*Weff = Wdrawn –Delta_W

Page 1 of 5 Midterm Fall 2013