The PN Junction Diode Viva Questions with Answers
(i) What is a pn junction?
A PN-junction is said to be condition when an N-type material is combined together with a P-type material creating a semiconductor diode.
(ii) Give the symbol for a semiconductor diode.
The 'P' side of a pn junction diode is always positive terminal and is called as anode. Other side which is negative is called as cathode. The symbol is shown in figure.
(iii) What is meant by forward biasing of a pn junction?
If the anode is connected to positive terminal of a battery and cathode to the negative terminal, the set up is called forward bias.
(iv) What is meant by reverse biasing of a pn junction?
When the positive of the battery is connected to the N side and negative of the battery is connected to the P side, then the diode is called to be reverse biased.
(v) What is forward resistance of a diode?
A diode offers an extremely small resistance (not equal to zero) when forward biased and it is called a forward resistance of a diode.
(vi) Distinguish between static and dynamic forward resistances of a diode.
When the diode is forward biased, it creates a small resistance in the circuit. The static resistance is defined as the ratio of DC voltage generated in the diode to the DC current runs through it. Dynamic resistance otherwise called as AC resistance is the reciprocal of the slope of the tangent of the characteristics curve.
ie., Dynamic resistance = change in voltage/resulting change in current = ΔV/ΔI
(vii) What is meant by a semiconductor?
Semiconductors arc substances having resistivity or conductivity lying between conductors and insulators.
(viii) Give some properties of a semiconductor?
i. It has covalent bonding
ii. It is crystalline in nature
iii. It has a negative temperature coefficient of resistance.
iv. Its conductivity increases with addition of impurities.
(ix) What is meant by doping?
Doping is the process of addition of a desirable impurity to a pure semiconductor in order to increase its conductivity.
(x) What is meant by forbidden gap?
The band separating the valence hand and the conduction band is called forbidden gap.
(xi) What is meant by a junction diode?
When a p-type semiconductor is fixed with n-type semiconductor, it forms a junction diode.
(xii) What is depletion region in p-n junction?
Depletion layer is a thin layer formed amid p and n regions of junction diode devoid of free electrons and the holes.
(xiii) Is Ohm's law obeyed in a semiconductor or not?
In a semiconductor, Ohm's law is valid only for low electric fields.
(xiv) What is a rectifier?
Rectifier is a device which converts AC into DC.
(xv) Aim of the PN Junction Diode Experiment
To draw the static current-voltage (I — V) characteristic of a junction diode in forward bias and hence to calculate its ac and dc forward resistances.
(xvi) Apparatus of the PN Junction Diode Experiment
The given junction diode (ex: IN 4007), a 2-volt battery, voltmeter (a low range voltmeter reading upto 1 V with 0.1 V per divisions), milliammeter, key, etc.
(xvii) Procedure of the PN Junction Diode Experiment
Connections are made as shown in the figure. A 2 volt battery is connected to a rheostat Rh. The rheostat is used as a potential divider arrangement. The voltmeter V is connected across the diode. The milliammeter m A is used to measure the diode current. The potential difference across the diode is increased from zero in steps of 0.1 V till the current increases steeply and becomes 25 to 30 mA. In each step the ammeter reading is noted. A mph is plotted with ammeter readings in mA along the Y-axis and the voltmeter readings in volts along X-axis. This gives the forward characteristic of the diode.
(a) To find static (dc) forward resistance (Rf):
Static forward resistance is the ratio of the direct voltage (V) applied at any point P on the straight portion of the characteristic to the direct current (I) corresponding to the point P. The voltage (V) and current (I) corresponding to the point P on the straight portion (i.e., beyond the knee voltage) are noted. The static forward resistance,
Rf= V/I
(b) To find dynamic (ac) forward resistance (rf):
It is defined as the ratio of a small change in the forward bias voltage to the corresponding change in the forward current
rf = (dV/dI)P
The reciprocal of the slope of the graph is determined at the point P. This gives the dynamic resistance of the diode in forward bias at the point P.