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Experiment :C-8
Drawing of the characteristic curve for three semiconducter diodes.
Submitted to Mr. Golam Dastegir Al-Quaderi and
Dr. Ratan Chandra Gosh
Submitted by Muhammed Mehedi Hassan Group A ;Batch-09 Second
Year, RollSH 236 Student of Physics Department, Uinversity of Dhaka.
Date ofexperiment May 03,2011. Date of submission July 04,2011.
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Experiment : Drawing of the characteristic curve for three semiconducter diodes. Theory : One of the
crucial keys to solid state electronics is the nature of the P-N junc- tion. When p-type and n-type materials
are placed in contact with each other, the junction behaves very differently than either type of material
alone. Specif- ically, current will flow readily in one direction (forward biased) but not in the other (reverse
biased), creating the basic diode. This non-reversing behavior arises from the nature of the charge
transport process in the two types of ma- terials. In the p-type region there are holes from the acceptor
impurities and in the n-type region there are extra electrons.
Figure 8.1 Forward and reverse bias of P-N junction diodes.
When a p-n junction is formed, some of the electrons from the n-region which have reached the
conduction band are free to diffuse across the junction and combine with holes.
Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. A space charge
builds up, creating a depletion region which inhibits any further electron transfer unless it is helped by
putting a forward bias on Equilibrium of junction.
With the application of sufficient reverse voltage, a p-n junction will experi- ence a rapid avalanche
breakdown and conduct current in the reverse direction. Valence electrons which break free under the
influence of the applied electric field can be accelerated enough that they can knock loose other electrons
and the subsequent collisions quickly become an avalanche. When this process is taking place, very
small changes in voltage can cause very large changes in current. The breakdown process depends
upon the applied electric field, so by changing the thickness of the layer to which the voltage is applied,
zener diodes can be formed which break down at voltages from about 4 volts to several hundred volts.
These characteristics can be shown by suitable graph.
Apparatus : A circuit board,three different diodes and some connecting wires.
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Table -8.1:Reading for the forward connection:
Diode -1 Diode -2 Diode -3 Voltage Current Voltage Current Voltage Current V mA V mA V mA 0.00 0.00
0.00 0.00 0.00 0.00 0.10 0.00 0.10 0.01 0.10 0.00 0.15 0.00 0.12 0.02 0.15 0.00 0.20 0.00 0.15 0.05 0.20
0.00 0.21 0.00 0.16 0.06 0.25 0.00 0.27 0.00 0.18 0.08 0.30 0.00 0.30 0.00 0.19 0.11 0.35 0.00 0.35 0.00
0.21 0.12 0.40 0.00 0.40 0.00 0.22 0.14 0.45 0.00 0.42 0.01 0.23 0.15 0.50 0.00 0.46 0.01 0.24 0.17 0.600.00 0.51 0.02 0.25 0.19 0.65 0.00 0.56 0.03 0.26 0.20 0.70 0.00 0.60 0.04 0.27 0.21 0.75 0.00 0.61 0.05
0.28 0.23 0.80 0.01 0.62 0.06 0.29 0.25 0.81 0.02 0.64 0.06 0.30 0.27 0.82 0.03 0.65 0.07 0.31 0.30 0.85
0.06 0.66 0.07 0.32 0.32 0.89 0.08 0.67 0.07 0.33 0.34 0.92 0.12 0.68 0.08 0.40 0.50 0.95 0.21 0.69 0.19
0.45 0.64 0.97 0.25 0.70 0.28 0.50 0.74 0.98 0.28 0.71 0.37 0.60 1.03 1.00 0.34 0.75 0.54 0.70 1.34 1.01
0.42 0.80 0.90 0.80 1.62 1.03 0.49 0.90 3.48 0.90 1.94 1.05 0.67 1.00 8.12 1.00 2.29 1.10 0.82
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Table -8.2:Reading for the reverse connection:
Diode -1 Diode -2 Diode -3 Voltage Current Voltage Current Voltage Current V A V A V A 0.00 0.00
0.00 0.00 0.00 0.00 0.05 0.00 0.03 1.50 0.30 0.00 0.10 0.00 0.05 2.30 0.50 0.00 0.15 0.00 0.07 2.60 1.50
0.00 0.20 0.00 0.09 2.90 2.00 0.00 0.25 0.00 0.10 3.00 2.50 0.00 0.30 0.00 0.15 3.50 3.00 0.00 0.35 0.00
0.18 3.70 3.21 0.00 0.40 0.00 0.20 3.85 3.30 0.00 0.45 0.00 0.25 4.00 3.50 0.10 0.50 0.00 0.30 4.10 4.000.10 0.55 0.00 0.35 4.20 4.07 0.20 0.60 0.00 0.40 4.40 4.13 0.30 1.00 0.00 0.45 4.60 4.20 0.40 1.50 0.00
0.50 4.80 4.25 0.45 2.00 0.00 0.55 5.00 4.30 0.51 3.00 0.00 0.60 5.10 4.40 0.60 4.00 0.00 0.65 5.30 4.50
0.70 5.00 0.00 0.70 5.40 4.60 1.00 6.00 0.00 0.75 5.50 4.70 1.30 7.00 0.00 0.80 5.60 4.80 1.60 8.00 0.00
0.85 5.70 4.90 2.00 9.00 0.00 0.90 5.80 5.00 2.50 10.00 0.00 0.95 6.10 5.30 3.80 - - 1.00 6.50 5.50 6.00 -
- 1.20 6.90 6.00 16.00
Calculation : Diode equation
I = I
o
(eqv/nkT 1) lnI = lnI
o
+ ln(eqv/nkT 1) Therefore lnI = lnI
o
+ qv/nkT.........(i) Assume that: eqv/nkT >> 1 qv/nkT >> 0 here n
Si
= 2 and n
Ge
= 1 Therefore ln(I/I
o
) = qv/nkT Theoritically, q/k= 1.381023JK1
1.61019C
= 11594.2CJ1K
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We know the equation for straight line in x-y plane is:
y=mx+c..........(ii)
Comparing equation (i) and (ii) we get ,
slope =q/nkT..........(iii)
So q/k can be determined for the diodes
For diode-1 slope is 11.81 so,
q/k=11.81 2 306CKJ1 = 7227.72CKJ1
For diode-2 slope is 5.04 so,
q/k=5.04 1 306CKJ1 = 1512CKJ1
Percentage of error for diode-1(Silicon)=|115947227
11594
| 100% = 37.6%
Percentage of error for diode-2(Germenium)=|115941512
11594
| 100%=86.9% Discussion :
We have used three different diode and from the graph the material of the diode can be figure out.
Here the break down voltage for first diode is nearly 0.7V , by theoritical knowledge this is the break down
voltage for Silicon. For the second diode it is nearly 0.3V, which refers to a Germenium diode. And the
third is a Zener diode, whose charcteristic knee voltage is -4.0V.
For these intrinsic semiconductor(silicon and germanium),this charcteristics can be explained by the
band thoery.According that the Fermi level is essentially halfway between the valence and conduction
bands. Although no conduction occurs at 0 K, at higher temperatures a finite number of electrons can
reach the conduction band and provide some current. In doped semiconductors, extra energy levels areadded.The application of band theory to n-type and p-type semiconductors shows that extra levels have
been added by the impurities. In n-type material there are electron energy levels near the top of the band
gap so that they can be easily excited into the conduction band. In p-type material, extra holes in the
band gap allow excitation of valence band electrons, leaving mobile holes in the valence band.
The nature of the p-n junction is that it will conduct current in the for- ward direction but not in the
reverse direction. It is therefore a basic tool for rectification in the building of DC power supplies.
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