A p-n photodiode is fabricated from a semiconductor with a band gap of $2.5 \mathrm{eV}.$ It can detect a signal of wavelength 

(a) $6000 \stackrel{\circ }{\mathrm{A}}$                                    (b) $4000 \mathrm{nm}$

(c) $6000 \mathrm{nm}$                                  (d) $4000 \stackrel{\circ }{\mathrm{A}}$

The circuit is equivalent to

1. AND gate                              2. NAND gate

3. NOR gate                              4. OR gate 

The electrical circuit used to get smooth output from a rectifier circuit is called 

(1) Oscillator                         

(2) Filter

(3) Amplifier                           

(4) Logic gates

PN-junction diode works as a insulator, if connected

(1) To A.C.                             

(2) In forward bias
(3) In reverse bias                   

(4) None of these

The PN junction diode is used as

(1) An amplifier                       

(2) A rectifier
(3) An oscillator                       

(4) A modulator

Two PN-junctions can be connected in series by three different methods as shown in the figure. If the potential difference in the junctions is the same, then the correct connections will be 

(a) In the circuit (1) and (2)                    (b) In the circuit (2) and (3)
(c) In the circuit (1) and (3)                    (d) Only in the circuit (1)

On increasing the reverse bias to a large value in a PN-junction diode, current

(1) Increases slowly                 

(2) Remains fixed
(3) Suddenly increases             

(4) Decreases slowly

A semiconductor device is connected in a series circuit with a battery and a resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current drops almost to zero. The device may be

(1) A P-type semiconductor               

(2) An N-type semiconductor
(3) A PN-junction                               

(4) An intrinsic semiconductor

In a junction diode, the holes are due to

(1) Protons                               

(2) Neutrons
(3) Extra electrons                     

(4) Missing of electrons

The cause of the potential barrier in a P-N diode is

(1) Depletion of positive charges near the junction

(2) Concentration of positive charges near the junction

(3) Depletion of negative charges near the junction

(4) Concentration of positive and negative charges near the junction