# Electronics and Communication Engineering - Exam Questions Papers

6.
In the circuit shown, in switch S is open for a long time and is closed at t = 0. The current i(t) for t ≥ 0+ is i(t) = 0.5 - 0.125e-1000t A
i(t) = 1.5 - 0.125e-1000t A
i(t) = 0.5 - 0.5e-1000t A
i(t) = 0.375e-1000t A
Explanation:

i(f) = 0.5, i(i) = 0.75

i(t) = Vr + (ii - ij)e-1/p = 0.5 - 0.125e-1000t .

7.
In a semiconductor material. The hole concentration is found to be 2 x 2.5 x 1015 cm-3. If mobility of carriers is 0.13 m2/ v-s. Then find the current density if electric field intensity is 3.62 x 10-19
7.6237 x 10-4 A/cm2
7.6237 x 10-5 A/cm2
7.6237 x 10-3 A/cm2
none of these
Explanation:

Current density J = σE

Where σ = conductivity

Given -> μ = 0.13 m2/v-s = 0.13 x 104 cm2/V sec

P = 2.25 x 1015/cm3

We have, ni = 1.5 x 1010

Also n.p. = n = /p  = (1.6 x 10-19 x 0.13 x 104 x 2.25 x 1015) x = (0.468) (4.5 x 1015)

σ = 2.106 x 1015 μ/cm

J = σE

Current density = 2.106 x 1015 x 3.620 x 10-19

= 7.6237 x 10-4 A/m2.

8.
Consider the system with and where p and q are arbitrary real numbers. Which of the following statements about the controllability of the system is true?
The system is completely state controllable for any non zero values p and q
Only p = 0 and q = 0 result in controllability
The system is uncontrollable for all values of p and q
We cannot conclude about controllability from the given data
Explanation:

Use the condition of controllability.

9.
A MOSFET has a threshold voltage of 1 V and oxide thickness of 500 x 10-8r = 3.9; ε0 = 8.85 x 10-14 F/cm, q = 1.6 x 10-19 c]. The region under the gate is ion implanted for threshold voltage tailoring. The base and type of impant required to shift threshold voltage to - 1 V are __________ .
8.6 x 1011/cm2, p-type
8.6 x 1011/cm2, n-type
0.86 x 109/cm2, p-type
1.02 x 1012/cm2, n-type
Explanation:

VT(new) = VT(odd) +   = 6.903 x 10-8  fB = - 8.6 x 1011

The threshold voltage is always negative for p-channel and hence implant is of p-type.

10.
What is the propagation constant for air filled wave guide with dimensions a = 1.59" and b = 0.795" at 4.95 GHz?
0.6698
0.7698
0.503
0.6598 Here,  .