1.The V-I characteristic of a silicon diode is shown in the figure. Calculate the resistance of the diode at             

Considering the diode characteristics as a straight line between I = 10mA to I = 20 mA passing through the origin, we can calculate the resistance using Ohm's law.
(a) From the curve, at I = 20 mA, V = 0.8 V, I = 10 mA, V - 0.7V
                              
(b) From the curve at
                                

2.In a Zener regulated power supply a Zener diode with V_z =6.0 V is used for regulation.  The load current is to be 4.0mA and the unregulated input is 10.0V.  What should be the value of series resistor R_s?

The value of R_s should be such that the current through the Zener diode (I_z )is much larger than the load current (I_L ).  This is to have good load regulation.
Let us choose I_z =5I_L , i.e., I_z = 20 mA.  The total current through R_s is, therefore, 24 mA.  The voltage drop across R_s. is 10.0-6.0 = 4.0 V.  This gives R_s = 4.0 V / The nearest value of carbon resistor is 150 Ω is appropriate.  Slight variation in the value of the resistor does not matter, what is important is that the current I_z should be sufficiently larger than I_L.
3. (a)The number of electron-hole pairs in an intrinsic semiconductor is   Calculate the number of electron-hole pairs at 227^o C.  Given that Boltzmann constant is
   (b)  If this semiconductor is doped by a donor impurity such that the number of conduction electron becomes calculate the number of holes at 27^o C.  Also approximately calculate the dopant concentration.

(a)
   

4. In half wave rectification, what is the output frequency if the input frequency is 50 Hz?  What is the output frequency of a full wave rectifier for the same input frequency?

Output frequency in Half Wave Rectifier : 50 Hz.
Output frequency in Full Wave Rectifier : 100 Hz.

5.For a CE transistor amplifier, the audio signal voltage across the collector resistance of 2 k is 2 V.  Current amplification factor of the transistor is 100.  Find the input signal voltage and base current if the resistance is 1 k.

6.You are given two circuits as shown, which consist of NAND gates. Identify the logic operation carried out by the two circuits.

 The truth table for first circuit :

                                 
If we remove the intermediate output Y', the overall result stands for the AND gate.
                                  
Hence, the circuit is an equivalent of AND gate.  The truth table for second circuit :
        
If we remove the intermediate output the overall result stands for the OR gate.
This can be represented as Y = A + B
Hence, the circuit is an equivalent of OR gate.
               

7. A transistor has a current amplification factor (current gain) of 50.  In a CE - amplifier circuit, the collector resistance is chosen as 50 k and the input resistance is 1 k.  Calculate the output voltage if input voltage is 0.01 V.

8.The number of silicon atoms per . This is dipped simultaneously with atoms per m³ of Arsenic and atoms of Indium. Calculate the number of electrons and holes. . Is the material n-type or p-type?

9.In an intrinsic semiconductor the energy gap E_g is 1.2 eV. Its hole mobility is much smaller than electron mobility and independent of temperature.  What is the ratio between conductivity at 600 K and that at 300 K?  Assume that the temperature dependence of intrinsic carrier concentration n_i is given by