v i characteristics of pn junction diode experiment

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VI Characteristics of a P-N Junction Diode

Semiconductors is a kind of material that has resistivity and conductivity in between metals and insulators. On the basis of purity semiconductors are of two types: Intrinsic Semiconductors is a kind of pure semiconductor without any significant dopant (impurities) species present. An intrinsic semiconductor is also called an undoped semiconductor and Extrinsic Semiconductors is a kind of semiconductor they are doped with an impurity, it is known as an extrinsic semiconductor.

p-n Junction Diode

Junction generally means the area or point that bounds two different parts, similarly in diodes junction is a boundary of two semiconductor materials i.e. the p-type and the n-type, semiconductor.

The p-side in the p-n junction has a positive side of the semiconductor and it has an excess of holes whereas the n-side has an excess of electrons, therefore, it is the negative side. The p-n junction in semiconductors is developed by the method of doping. adding impurities in a semiconductor is known as doping.

Formation of p-n Junction

Let us understand for an example, consider a thin p-type silicon semiconductor sheet. If we add a small amount of pentavalent impurity(having valency five) to this, a part of the p-type Si will get converted to n-type silicon. This sheet will now contain both regions i.e. p and n-type region and a junction is created between two regions. 

• There are two types of processes that follow after the formation of a p-n junction – diffusion and drift. As we know, diffusion is the process that follows the flow of particles from higher concentration to lower concentration, due to difference in the concentration of electrons and holes at the two sides of a junction, the electrons from the n-side diffuse to the p-side and the holes from the p-side diffuse to the n-side. this leads to raise in diffusion current.
• Also, there is an ionized donor is left behind on the n-side, which is an immobile charge this develop when an electron diffuses from the n-side to the p-side. As the result of this process, a layer of positive charge is developed on the n-side of the junction. 
• Similarly, An ionized acceptor is left behind in the p-side when a hole goes from the p-side to the n-side, resulting in the formation of a layer of negative charges in the p-side of the junction. This region of negative (-) and positive charge (+) on either side of the junction is termed the depletion region. 
• An electric field direction from a positive charge towards the negative charge is developed, Due to this positive charge region on either side of the junction, Due to this electric field, the flow of electrons and holes takes place. This is termed the drift motion. generally, the direction of the drift current is opposite to that of the diffusion current.

Forward Bias

The forward bias of p-n  junction

In biasing semiconductor is connected to an external source. when the p-type semiconductor is connected to the positive terminal of the source or battery and negative terminal to the n-type, then this type of junction is said to be forward-biased. In forward bias the direction of built-in electric field near the junction and applied electric field are opposite in direction. this means that the resultant electric field has a magnitude lesser than the built-in electric field. due to this there is less resistivity and therefore depletion region is thinner. In silicon, at the voltage of 0.6 V, the resistance of the depletion region becomes completely negligible.

Reverse Bias

The reverse bias of p-n  junction

In the reverse biasing, the n-type is connected to the positive terminal and the p-type is connected to the negative terminal of the battery . In this case, the applied electric field and the built-in electric field are in the same direction and the resultant of electric field has higher magnitude than the built-in electric field creating a more resistive, therefore depletion region is thicker. if the applied voltage becomes larger, then the depletion region becomes more resistive and thicker.

p-n Junction Formula The potential difference created by the electric field in the p-n junction is given by: E o = V T ln [N d N a / n i 2 ]  where E o  junction voltage at no bias, V T is the thermal voltage at room temperature i.e. 26mv, N d and N a are the concentrations of impurity and n i is intrinsic concentration.

V-I Characteristics of p-n Junction Diode

V-I characteristics of p-n junction diode

• In forward bias condition p-type is connected to positive terminal of battery and the n-type to the negative terminal of the battery, there is a reduction in the potential barrier, in this condition. For germanium diodes, when the voltage is 0.3 V, and for silicone diodes, when the voltage is 0.7 V the potential barriers decrease and there is a flow of current.
• When the diode is in forward bias , as the voltage applied to the diode is overcoming the potential barrier, the current increases slowly and the curve obtained is non-linear. Once the potential barrier is crossed by the diode, the diode behaves normally and the curve rises sharply as further external voltage increases and the curve obtained is linear.
• When the PN junction diode is under reverse bias , this results in an increase in the potential barrier and resistance also increases. Minority carriers are present in the junction which creates reverse saturation current flows in the beginning.
• If the applied voltage increases rapidly, there is increased kinetic energy due to minority charge carriers which affect the majority charges. In this stage the diode breaks down. or the voltage is called breakdown voltage, This may also destroy the diode.

Sample Questions

Question 1: When silicon is doped with indium it leads to which type of semiconductor?

As we know, Valency of Indium is 3 therefore it is Trivalent in nature, when it is doped in Silicon it has majority of holes, that’s why it is of p-type semiconductor.

Question 2: A transistor has a current gain of 30 Ampere. If the collector resistance is 6 kΩ, the input resistance is 1 kΩ, calculate its voltage gain.

Given, R in =1 kΩ  and R out = 6k Ω   ∴ R gain =  R out /R in   =  6/1 = 6   Voltage gain = current gain × Resistance gain                        = 30 × 6 =180

Question 3: Write characteristics of holes.

Following are the characteristics of holes: A hole is equivalent to a positive electric charge. The mobility of a hole as compare to that of an electron is less.

Question 4: Name the kind of biasing which leads the following result:

a) Increase in resistance,

b) Decrease in resistance and 

c) Increase in width of the depletion region.

a) In reverse bias resistance increases. b) In forward bias resistance decrease. c) In reverse bias there is increase in the width of depletion region take place.

Question 5: What is the ratio of electrons and holes in the intrinsic semiconductor?

Number of electrons = n e Number of holes = n h In intrinsic semiconductor, n e = n h n e /n h = 1 

Question 6: Define the term breakdown voltage of p-n junction.

In reverse bias condition, when the applied voltage increases gradually at a certain point there is increase in reverse current noticed, this is junction breakdown, corresponding applied voltage is known as breakdown voltage of p-n junction diode.

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V-i characteristics of pn junction diode.

Volt-ampere (V-I) characteristics of a pn junction or semiconductor diode is the curve between voltage across the junction and the current through the circuit.

Normally the voltage is taken along the x-axis and current along y-axis.

The circuit connection for determining the V-I characteristics of a pn junction is shown in the figure below.

Fig.1: Circuit Connection for V-I characteristics of a pn junction

The characteristics can be explained under three cases , such as :

• Forward bias
• Reverse bias

Case-1 : Zero Bias

In zero bias condition , no external voltage is applied to the pn junction i.e the circuit is open at K.

Hence, the potential barrier (ref : pn junction tutorial for better understanding) at the junction does not permit current flow.

Therefore, the circuit current is zero at V=0 V, as indicated by point O in figure below.

Fig.2: V-I Characteristics of pn Junction

Case-2 : Forward Bias

In forward biased condition , p-type of the pn junction is connected to the positive terminal and n-type is connected to the negative terminal of the external voltage.

This results in reduced potential barrier.

At some forward voltage  i.e 0.7 V for Si and 0.3 V for Ge, the potential barrier is almost eliminated and the current starts flowing in the circuit.

Form this instant, the current increases with the increase in forward voltage. Hence.  a curve OB is obtained with forward bias as shown in figure above.

From the forward characteristics, it can be noted that at first i.e. region OA , the current increases very slowly and the curve is non-linear. It is because in this region the external voltage applied to the pn junction is used in overcoming the potential barrier.

However, once the external voltage exceeds the potential barrier voltage, the  potential barrier is eliminated and the pn junction behaves as an ordinary conductor. Hence , the curve AB  rises very sharply with the increase in external voltage and the curve is almost linear.

Case-3 : Reverse Bias

In reverse bias condition , the p-type of the pn junction is connected to the negative terminal and n-type is connected to the positive terminal of the external voltage.

This results in increased potential barrier at the junction.

Hence, the junction resistance becomes very high and as a result practically no current flows through the circuit.

However, a very small current of the order of μA , flows through the circuit in practice. This is knows as reverse saturation current(I S ) and it is due to the minority carriers in the junction.

As we already know, there are few free electrons in p-type material and few holes in n-type material. These free electrons in p-type and holes in n-type are called minority carriers .

The reverse bias applied to the pn junction acts as forward bias to there minority carriers and hence, small current flows in the reverse direction.

If the applied reverse voltage is increased continuously, the kinetic energy of the minority carriers may become high enough to knock out electrons from the semiconductor atom.

At this stage breakdown of the junction may occur. This is characterized by a sudden increase of reverse current and a sudden fall of the resistance of barrier region. This may destroy the junction permanently.

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VI Characteristics of a Diode            

Aim of the experiment.

At the end of the experiment, the student should be able to

• Explain the structure of a P-N junction diode
• Explain the function of a P-N junction diode
• Explain forward and reverse biased characteristics of a Silicon diode
• Explain forward and reverse biased characteristics of a Germanium diode

• CBSE Class 12
• CBSE Class 12 Physics Practical
• To Draw The I V Characteristic Curve For P N Junction In Forward And Reverse Bias

To Draw The I-V Characteristic Curve of a P-N Junction In Forward Bias and Reverse Bias

In a standard diode, forward bias and reverse bias takes place. Let us know how to draw the I-V characteristic curve of a P-N junction in forward bias and reverse bias.

To draw the I-V characteristic curve of a P-N junction in forward bias and reverse bias.

Materials Required

• A P-N junction diode
• A 3-volt battery
• A 50-volt battery
• A high resistance rheostat
• One 0-3 volt voltmeter
• One 0-50 volt voltmeter
• One 0-100 mA ammeter
• One 0-100 μA ammeter
• One way key
• Connecting wires
• Piece of sandpaper

Forward bias characteristics

The junction is said to be forward-biased when the p-section of the diode is connected to the positive terminal of the battery and the n-section of the diode is connected to the negative terminal of the battery. With an increase in the voltage, the current also increases. For Si diode, at 0.7 V the current increases suddenly.

Reverse bias characteristics

The junction is said to be reverse-biased when the p-section of the diode is connected to the negative terminal of the battery and the n-section of the diode is connected to the positive terminal of the battery. With an increase in the voltage, there is a small change in the current but the reverse current increases to a higher value with an increase in the voltage.

For forward bias

• The circuit connections should be as shown in the diagram.
• All the connections should be neat, clean and tight.
• For voltmeter (V) and milli-ammeter (mA), the least count and zero error should be noted.
• To get the zero reading from the voltmeter and milli-ammeter, rheostat should be brought near the negative end by inserting the key K.
• To apply the forward bias voltage (V F ) of 0.1V, the contact should be moved towards the positive end. The current remains zero.
• Keeping current zero, increase the forward bias voltage up to 0.3 V for Ge diode.
• To record a small current using milli-ammeter, increase the V F to 0.4 V.
• Increase the V F by 0.2 V and record the corresponding current. When the V F becomes 0.7 V, the current will increase rapidly.
• When V F = 0.72 V, the current increases suddenly and this is known as forward breakdown stage.
• Take out the key if the forward current won’t change as V F increased beyond forward breakdown.
• Record the observations.

For reverse bias

• Note the least count and zero error of voltmeter (V) and micro-ammeter (μA).
• To get zero reading from the voltmeter V and micro-ammeter μA, insert the key K and bring the rheostat near the positive end.
• To apply reverse bias voltage (V R ) of 0.5 V, move the rheostat to the negative end so as to flow the reverse current.
• Increase V R by 0.2 V and record the corresponding current. When V R becomes 20 V, the current will increase rapidly.
• When V R = 25 V, the current increases suddenly and this is known as reverse breakdown stage. Record the current reading and take off the key.

Observations

Range of voltmeter = …….V

Least count of the voltmeter = …….V

Zero error of voltmeter = ……..V

Range of milli-ammeter = …….mA

Least count of milli-ammeter = …….mA

Zero error of milli-ammeter = ……..mA

Table for forward bias voltage and forward current

Range of micro-ammeter = …….μA

Least count of micro-ammeter = …….μA

Zero error of micro-ammeter = ……..μA

Table for reverse bias voltage and reverse current

Plotting of Graphs

Plot a graph between V F and I F taking V F on the x-axis and I F on the y-axis. The graph obtained is known as forward bias characteristic curve.

Plot a graph between V R and I R taking V R on the negative x-axis and negative I R on the y-axis. The graph obtained is known as reverse bias characteristic curve.

Junction resistance for forward bias = …… ohms

Junction resistance for reverse bias = ……… ohms.

Precautions

• The connections should be neat, clean and tight.
• Key should be used when the circuit is being used.
• Beyond breakdown, forward bias voltage should not be applied.
• Beyond breakdown, reverse bias voltage should not be applied.

Sources Of Error

Faulty junction diode might be supplied.

Q1. Define the energy level in an atom.

Ans: Energy level in an atom is defined as the energy value of an electron in the subshell of an atom.

Q2. What are the different types of energy bands?

Ans: Following are the different types of energy bands:

• Conduction band (C)
• Valence band (V)
• Forbidden band (F)

Q3. What are the different types of substances?

Ans: Following are the different types of substances:

• Semiconductors

Q4. What is the SI unit of conductance?

Ans: SI unit of conductance is siemens (S).

Q5. Name the different types of biasing.

Ans: Following are the different types of biasing:

• Forward biasing
• Reverse biasing

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Science Practicals 11 & 12

Search this blog, class 12 physics practical reading to draw the characteristic curve of a p-n junction in forward and reverse bias., apparatus required.

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