Compilation of experiments on ‘Semiconductor Devices’ for engineering students.
Experiment # 1. Characteristics of a Zener Diode:
To draw the characteristics of a zener diode and study of its use as a voltage regulator.
Circuit Diagram:
The basic principle of the circuit diagrams for studying the characteristics of a Zener diode under forward bias and reverse bias arrangement are shown in Fig. 2.10(a) and (b) respectively. In the figures E represents a variable d.c. source shown positive and negative terminals are connected with the Zener diode Z in a manner as shown R is a constant resistance, mA is the milliammeter and V the voltmeter connected across the Zener diode.
The circuit diagram for studying the Zener diode as a voltage regulator is shown in Fig. 2.11. In the circuit a fixed load resistance RL and a variable load resistance R’L are there to use the Zener diode for the purpose.
Working Formula:
If a curve is drawn by plotting voltage across the Zener diode as the abscissa and the corresponding current passing through it as the ordinate then the curve so obtained is called the static characteristic curve of the Zener diode. In the reverse bias region when the voltage increases beyond a certain value the Zener breakdown is said to occur. In this region the voltage across the Zener diode changes by a very small amount with the change of Zener current and is called the Zener breakdown voltage.
ADVERTISEMENTS:
The current voltage relationship for a semiconductor diode is given by-
Where I is the current at a voltage V and I0 is the leakage current. The symbol VT stands for the volt equivalent temperature and is equal to T/11,600 while η is a characteristic constant which is unity for germanium and nearly 2 for silicon at rated current.
Special Remarks:
So the current increases exponentially with a very small slope at low forward voltage. As V increases, the current is also found to increase following the above relation.
Again, when V = 0 we get I = 0.
For reverse bias arrangement since V is negative we get, I – I0 i.e., the current then becomes independent of the applied voltage.
Log I is plotted against V. If one extrapolates this graph to V = 0 the value of I0 can be obtained. Of course, the measured value of I0 is always greater than the theoretically predicted value.
ADVERTISEMENTS:
The dynamic or a.c. resistance (impedance) of a Zener diode is defined as-
Zener diode can be used (i) as a shunt regulator (or load regulator) and (ii) as a line regulator. To study the Zener diode as a load regulator, a variable load resistance is connected across the Zener diode. The supply voltage is kept constant. If a curve is drawn taking load current (lL) as abscissa and corresponding voltage (VL) across the Zener diode as ordinate then the curve such obtained is termed as the load regulation curve.
This means that with change in voltage drop i.e., (VNL – VL) with change in current through the Zener diode is negligible for a perfect voltage regulator. Hence the smaller the value of ρ, the greater is its stabilization capability. Of course, there is a certain regulation voltage spread upto which the Zener diode acts as a good regulator which is the characteristics of the Zener diode.
ADVERTISEMENTS:
For a poor regulation, the voltage drops with flow of current i.e., (VNL – VL) has a certain positive value depending on the current range chosen. Hence as the value of VNL|VL increases, the regulation becomes poorer.
Since in the working region of the Zener diode the current-voltage curve has a certain slope, the value of regulation voltage depends on the choice of the operating point of the Zener diode.
1. Specifications of the Zener diode from the semiconductor manual are to be noted (if possible).
The main specifications for the purpose of the experiment are:
(i) Power rating
(ii) Maximum Zener current (d.c.),
(iii) Nominal test current, and
(iv) Nominal Zener voltage.
Otherwise, the values are to be collected from the instructor.
2. Circuit connections (showing the basic principles) are to be drawn on the laboratory note book and connections are made as shown in Fig. 2.10(a) for studying the forward characteristic, while Fig. 2.10(b) for studying the reverse characteristic. The connections are to be done if the active and passive elements are supplied in discrete forms.
For the forward bias arrangement the negative terminal of the variable d.c. source is connected through a resistance R to the negative terminal of the Zener diode. For the reverse bias arrangement, on the other hand, the polarity of the d.c. source and the meters are reversed, i.e., the positive terminal of the source is connected to the negative terminal of the Zener diode.
A milliammeter (mA) and a voltmeter (V) are connected in the circuit in the positions shown. Value of the smallest division of the different meters are noted.
3. For drawn the forward characteristic, the applied voltage is kept to a very low value so that the forward current can be recorded from the meter supplied.
For each forward voltage the value of the current is to be noted avoiding parallax. This current is recorded as observation I. The forward voltage is changed slightly from the adjusted value and is again set to the previous forward voltage. Value of the corresponding current is cautiously noted under identical condition as before. This current is recorded as observation II. The mean of the two currents from observation I and II is the actual value of current for that forward voltage.
The forward voltage is increased in steps (depending on the calibration of the metre) and for each forward voltage the mean current as before is recorded.
Caution:
Do not exceed the power rating.
4. For studying the reverse characteristics the circuit connections are to be made (if closed box device is not supplied) as shown in Fig. 2.10(b). The reverse voltage is kept to the minimum value of obtain a small amount of current as permitted by the apparatus. The
mean current for that reverse voltage is noted from observations I and II.
The voltage is now increased in regular steps (depending on the calibration of the apparatus and the maximum reverse Zener voltage) and for each step the mean current.
Caution:
Do not exceed the power rating and maximum Zener current of the Zener diode supplied. Study of Zener as a voltage regulator.
5. For studying voltage regulation the circuit connections as shown in Fig. 2.11 are made. The key K is to be closed.
For load regulation the supply voltage (i.e., line current) is kept fixed while the variable load R’L is to be varied for obtaining the regulation. To use the Zener diode as a load regulator the combined load K, and R’L is kept to the maximum value by means of the rheostat. Supply voltage is now slowly increased so that the voltage across the Zener diode attains nearly the maximum value as obtained during the study of reverse characteristic.
If now the load resistance is decreased the change of Zener voltage becomes very small until the current through the Zener diode attains the knee region of the reverse characteristic. This is the rough checking of the range upto which good regulation can be attained. This rough checking is to be made before recording the observations regulation characteristics.
After this rough checking, the Zener voltage is set to the maximum reverse voltage is set to the maximum reverse voltage as permitted by the apparatus (keeping the load resistance maximum). The load voltage and the corresponding load current are noted twice as observation I and II and the mean load current for a particular load voltage is noted.
6. The load resistance is now gradually decreased and in each case the mean load current is noted corresponding to a particular load voltage.
Caution:
If the load resistance is made too low the current through the Zener diode tends to the breakdown region and hence regulation becomes poor with decrease of load resistance.
7. The line voltage or the line current is decreased by a small amount so that the initial maximum Zener voltage is slightly less than that adjusted in the previous operation. During this adjustment the load voltage is kept at the maximum value. Now load is gradually decreased and for each load voltage the corresponding load current is noted.
8. The above operation is repeated for other line voltage or line current.
9. For each line voltage (or line current) the recorded values of voltages are plotted against the corresponding load currents. This gives the load regulation characteristic for that supply voltage.
The load regulation characteristics for a Zener-diode set at different initial supply voltages are shown in Fig. 2.12. Considering a particular regulation characteristic [say, (Vz)II = 4.8 volt], the value of the rated load voltage [i.e., (VL)II] for a particular rated current [say (IL)R] is noted from the regulation characteristic (Fig. 2.12).
Also the value of VN-L can be obtained either from the extrapolation or from the experimental data by opening the key K in the circuit (Fig. 2.11). Hence, the percentage regulation can be calculated using the theoretical relation. This is done by choosing different operating rated currents on the same regulation characteristic.
Also, the same procedure may be done using other load-regulation characteristics. In all cases, the percentage regulation of voltages are calculated for different rated currents.
10. For studying the line regulation, the load is kept fixed but the supply voltage (i.e., line voltage) is changed. Here also for different fixed load resistances VL – IL data are noted for different initial line currents and curves are drawn. The initial line current is to be chosen to such a value so that its change does not lead the Zener voltage to the knee region.
Important Points:
One of the method for studying the reverse characteristic of Zener diode is shown in Fig. 2.10(b). Here, the voltages and currents can be directly read from the meters supplied. Sometimes the circuit shown in Fig. 2.13 may also be used.
Here Vjn can be measured, also, the voltage across R (i.e., VR) can also be measured.
Hence, the current I flowing through the Zener-diode can be calculated by using the relation I = VR/R. Also, the value of Zener- voltage (i.e., VZ) can be calculated by using the relations VZ = Vin – VR. Hence, the reverse characters tic can be drawn, similarly, the forward characteristic can also be drawn by reversing the Zener-terminals.
Works with a Regulated Power Supply:
The power supply unit used for studying the semi-conductor characteristics consists of the combinations of suitable step-down transformers and proper I.C’s. Finally, the step-down a.c. voltages are converted to d.c. voltages with suitable rectifier unit. Hence, the power supply unit should be connected to a.c. mains. It should never be connected to d.c mains because it will damage the supply unit.
After connecting the input terminals of the power supply unit to the a.c mains, the connections are made as shown in Figs. 2.10(a) for forward characteristic, and 2.10(b) for reverse characteristic. Sometimes, some of the connections are made inside the black-box, while some connections are to be done by the experimentor. The basic principle is the same as shown in Fig. 2.10(a) and 2.10(b). Before, switching on the power supply unit, the circuit should be approved by the instructor.
Experimental Results:
(A) Evaluations of one smallest division of different meters
N.B.:
Data are shown only for illustrations.
(B) Zener voltage (VZ) and the corresponding Zener current (lZ) reading for forward biasing
Supply voltage = volt, (15 volt, say)
(C) Zener voltage and the corresponding Zener current reading for reverse biasing.
(Data are given only for illustrations for a 4.7 V Zener diode).
N.B.:
For a high quality Zener diode, the voltage across the Zener diode remains practically the same for a wide variation of Zener-current in the Zener-region.
(D) To draw (VZ – IZ) curve.
Zener voltage (VZ) is plotted along the positive X-axis and the Zener current (Iz) along the positive I-axis for drawing the forward characteristic. For drawing the reverse characteristic, reverse voltage is plotted along the negative X-axis, while the corresponding current along the negative U-axis. The graph is shown in Fig. 2.14.
Calculation of Zener Dynamic Impedance:
For a non-linear region of reverse-characteristic curve, (say at a point P) in Fig. 2.14, a tangent is drawn as shown by mp. Finally a triangle mnp is constructed. Hence ZD = mn/np × 103 ohm. For the linear region say at a operating current i0 given by i0 = i1 + i2/2, a triangle is drawn (say abc) from which ZD is again calculated. In this way ZD is calculated at different operating currents from the reverse characteristic.
Similarly, from the forward characteristic (Fig. 2.14), different triangles efg, xyz are considered at different operating currents. Finally ZD is calculated at different operating currents.
N.B.:
The rate of change of the value of the dynamic impedance is a relative measure of the circuit as a voltage regulator. If with change in current, the value of ∆V is zero (or nearly zero), the device can be treated as a good voltage regulator. For example, consider that a freshly charged battery under normal service condition is taken.
Let the current change through the resistance in the battery circuit but the voltage across the battery remains that same. Hence, through ∆I changes, yet ∆V does not change. Hence, such battery is a good voltage regulator. On this analogy, for a larger value of current, with a Zener diode, in the reverse characteristic, the Zener diode acts as a good voltage regulator.
This is because, ZD → 0 as current increase. Sometimes, the forward characteristic curve is also used as a voltage regulator. But, it is rarely done. A graph of ZD Venus operating current may be drawn using reverse characteristic. The variation of ZD with operating current with change of temperature may also be studied.
(E) Load voltage (VL) and the corresponding load current (IL) reading for the study of the Zener diode as a voltage regulator (see Fig. 2.12).
(F) To draw (VL – IL) graph and hence to find the percentage regulation of voltage (for load regulation.
IL is plotted along the positive X-axis and VL along the positive Y-axis for each set. The nature of the curves is as shown in Fig. 2.12. From the graph for a specific load current (IL)R value of (VL)R is noted and the following table is done.
Note:
Make similar table for line regulation.
i. Before starting the experiment, the forward and reverse biasing condition for the Zener diode supplied should be checked. If the polarities cannot be identified properly then after connecting the Zener diode in the circuit, the voltage is gradually increased and the maximum voltage across the Zener diode is noted (say, V1 volt).
Next the terminals of the Zener diode connections are changed and again the supply voltage is slowly increased. Let now the maximum voltage across the Zener diode be V2 volt. If V2 > V1 then the second arrangement of the Zener diode gives the reversed biasing condition.
ii. Special diodes with sharp (vertical) change in current voltage relationship when operated in reverse-bias condition are called Zener diode. Such diodes which are designed with adequate power dissipation capacities to operate in the breakdown region can be used as a constant voltage device.
iii. The value of ∆Vz/∆Iz is zero in ideal cases for a perfect voltage regulator and it is exhibited by a Zener diode at high reverse current region.
iv. Zener diode is specially manufactured in the breakdown region. Practically all Zener diodes are made of silicon because of the superior thermal qualities i.e., they exhibit high voltage stabilization even if function temperature is high enough.
v. The manufacturers specify the value of the breakdown voltage known as Zener voltage (Vz). This is on the linear portion of the reverse characteristic and corresponds to approximately one quarter of the maximum power dissipation capability of the diode. As for example, for a 5V Zener diode if the maximum power rating is 400 mW, then the maximum safe current is 80 mA. For such diode Vz is taken at 20 mA.
vi. Both reliability and performance of a Zener diode can be improved considerably with the oxide passivation process. Zener diodes may be used even upto 200°C.
vii. Some Zener diode are available with Zener voltages below 2V. Usually diodes are then used in the forward direction.
viii. For a high voltage reference it is preferable to use two or more Zener diodes in series rather than using only one. Because in that case the combination allows higher dissipation, lower dynamic resistance and also lower temperature coefficient.
ix. The Zener voltage regulators are used in general, for applications of low power without going into much complexity.
x. In order to measure the Zener voltage, the voltmeter to be used must be highly sensitive. If a voltmeter has a high in put impedance then practically a very small current is lost in the voltmeter circuit and it can be ignored. Hence the resistance of the voltmeter must be high enough.
Extra Work for Zener Diode:
i. Place a hot filament lamp at a distance of about 3 cm. from the Zener diode and draw the forward and reverse characteristics. Remark for the difference in readings.
ii. Some of the transistors like BO 147, BO 148 and BO 107 possess the breakdown voltage near about 8 volt between their base emitter junctions. Hence they can be used as V-Zener. For such transistors, the base and the collector terminals are shorted by a connecting wire. They behave as a Zener diode when the emitter and base terminals are considered. Perform the same experiment with such converted Zener diode.
Experiment # 2. Static Characteristics of a P-N-P Transistor under C-B Configuration:
To draw the static characteristics of a P-N-P transistor under C-B configuration.
Circuit Diagram:
The circuit arrangement for studying the characteristics under common base configuration is shown in Fig. 2.15. Note that for the common base configuration two milliammeters are connected to the emitter and the collector terminals as shown in the figure.
Working Formula:
A P-N-P transistor is a combination of two P-N junctions. It is formed by sandwiching a thin N-layer between two P layers. Of the two P-N junctions one is forward biased while the other is reverse biased. The n-layer in the middle is the base, the forward biased P-layer on the left is the emitter while the reverse biased P-layer on the right is the collector.
In a common base circuit the collector current is controlled by the variation of emitter current. This gives one important control characteristic of a transistor designated by α, where α is defined as the ratio of the change in collector current (δIc) to the change in emitter current (δIE) when the collector voltage (VCB) is maintained at a constant value.
Note 1:
In a common emitter circuit, the input signal is applied to the base. This gives another important control characteristic designated by β, where β is defined as the ratio of the change in collector current (δIB) to the change in base current (δIB) when the collector voltage (VCE) is kept constant.
Mathematically, the a.c. current gain for common emitter configuration is expressed as,
Experimental Procedure:
1. The range and the value of one smallest division of the meters supplied are noted. The transistor number and other necessary specifications are also noted from the semiconductor manual.
2. Before performing the experiment, the given two milliammeters are compared to see whether they are calibrated with the same scale.
3. Circuit is now made as shown in the common base configuration to get the data of collector current for different collector-to-base voltage (VCB) when the emitter current is kept fixed. For each of value VCB the collector current (IC) is observed twice and the mean is taken.
4. For different fixed values of emitter current (IE) the operations as indicated in procedure no. (3) are repeated.
5. A graph is now drawn showing the variation of collector current with different collector- to-base voltage. Another graph is to be drawn to show the variation of collector current with emitter current for a fixed VCB. From the graph the current gain a is calculated with is the ratio of increase in collector current to a small change in emitter current for a given collector-to-base voltage.
Experimental Results:
The data given are only for illustrations.
To draw IC – IE curve.
A graph is drawn by plotting IE (in mA) along X-axis and lC (in mA) along Y-axis for a fixed value of VCE. The nature of the graph is shown Fig. 2.17.
The value of VCB is adjusted to another value and the above procedure is performed to get another set of values of IC and IE. A separate graph is again drawn for this value of VCB.
Discussion:
1. For a common base configuration, the collector current is found to be almost independent of the collector voltage for a fixed emitter current.
2. From the relation α = β/(1 + β) it is clear that as α approaches towards unity, β becomes increasingly larger. So during experiment a must be measured very accurately to avoid error in β.
3. From the semiconductor manual, the maximum values of different variables (i.e., voltage, current etc.) are to be noted. During experiment, the values of IB, VCE etc., must not exceed the values as specified by the manufacturer.
Experiment # 3. Static Characteristics of a Transistor (under CE mode):
To draw the static characteristics of a transistor (under CE mode) and to calculate hfe, hoe etc.
Circuit Diagram:
The basic principle of the circuit diagram for this purpose is shown in Fig. 2.18 using N-P-N transistor.
The indices are as follows:
Q → Transistor (E: emitter, B: base, C: collector); µA → Microammeter; mA → Milliammeter; Rh1, Rh2 Rheostats; VBE, VCE → Voltmeters.
The emitter of the transistor is forward biased and the collector is reverse-biased with the help of external sources. The sources may be batteries or a regulated power supply unit.
N.B.:
In some circuits, the voltmeter marked by VBE is not supplied. This voltmeter is connected in the circuit only when the drawing of input characteristic curves is necessary.
Working Formula:
The collector current versus collector voltage curves are generally known as the output characteristic curves of a transistor. To draw the characteristic curves of a transistor in the CE- mode, the base current (IB) is kept to a constant value and the collector currents are noted by varying the collector voltages. The operation is repeated for different base currents and then the curves are drawn by polting collector current versus collector voltage.
i. The a.c. current gain for a CE configuration for small signal mode is denoted by β, which is the ratio of small change in collector current to the small change in base current when collector-emitter voltage is maintained at a constant value.
Mathematically, the a.c. current gain for a common emitter configuration is expressed as-
Since, the experimental results are concerned with finite and measurable quantities, hence it is customary to write it as–
Experimental Procedure:
1. The circuit connections are made as shown in Fig. 2.18. Before switching on the circuit, it should be approved by the instructor.
2. The type number and different characteristics of the transistor supplied are noted from the manual (if available).
3. The value of the smallest division of different meters are evaluated.
N.B.:
Zero (or initial) error, if any, for any one of the meters supplied are minimised by adjusting the base-screw of the pointer, (if possible), otherwise, proper correction should be done.
4. For a particular value of the base current (say at 10µA) the collector voltage is increased in regular steps (e.g. VCE = 1, 2, 3 volt etc.) and the corresponding collector current at each value of the collector voltage is recorded. The collector current is noted twice as obs. I and II. The collector voltage is increased gradually until the recordings for the active region of the curve may be taken. If possible, the recording for the saturation region should also be taken for lower values of VCE.
5. The base current is fixed to other values and similar procedure as described in operation (4) is repeated.
6. VCE is kept to a fixed value and the variations of lB and VBE are noted. This is done for different values of VCE.
7. VCE is kept to a fixed value and variations of lB and lC are noted.
Experimental Results:
N.B.:
Do not exceed the maximum power rating during recordings. Data shown in different tables should never be taken as the typical. These are shown only for illustrations.
(A) Identification number of the transistor = …
Crystal type of transistor = …
Name of the manufacturer = …
Different characteristics.
N.B.:
This should be filled up if the semiconductor manual is available. This may also be filled up as directed by the instructor.
N.B.:
This table is important in the sense that during recording the values VB, lB, VC, IC, VE, lE etc. should be kept to such a value so that the transistor is not damaged.
(B) Evaluation of the Smallest Division of Different Meters:
(C) Collector Current and Collector Voltage Reading for Different Constant Value of Base Current:
N.B.:
The maximum collector voltage will be set at a value as directed by the instructor.
[The above operations are to be repeated for other fixed base currents and similar tables as shown in table III are to be done.]
(D) To Draw (IC – VCE) Curve:
For each constant value of lB (in µA) different VCE (in volt) are plotted along the X-axis and the corresponding values of lC (in mA) along the Y-axis using Table III. The nature of the curves obtained is as shown in Fig. 2.19.
(E) To Find the a.c. Current Gain, or a.c. (i.e., hfe) from Graph:
N.B.:
∆lc and ∆IB are to be converted to the same unit before calculation of hfe. Different operating points are considered to calculate the hybrid parameters.
(F) To Find the Output Admittance (h0e) from Graph:
(For this different operating points are chosen as shown in Fig. 2.19.)
(G) Data for Drawing of IB – Ic Curves (at Constant VCE):
N.B.:
This table is to be done if directed by the instructor.
[Repeat the above type of observations for other higher constant values of VCE (say 4V, 6V etc.)]
(H) To Draw (IB – lc) Curve:
A graph may be drawn by plotting IB in µA along X-axis and the corresponding value of IC in mA along Y-axis for fixed VCE (in volt). The nature of the curves are obtained using Table VI.
Taking different operating points the hybrid parameter h0eare determined. The mean value of it is considered. The value of h0e can also be determined from the graph. It should be noted that these parameters are quiescent point dependent.
(I) Data for Drawing of VBE – VB Curve at Constant VCE:
[These recording are to be done if a voltmeter is supplied to measure VBE. This part of the experiment should be done if directed by the instructor.]
N.B.:
This procedure is done for different values of VCE. Do not exceed the maximum power rating.
(J) To Draw (VBE – IB) Curve:
A graph is drawn by plotting IB in µA along Y-axis and corresponding VBE along X-axis for VCE. The nature of the curves obtained using Table VII is shown in Fig. 2.20.
For non-linear region tangents are drawn, while for the linear region different triangles are drawn to calculate the value of hie at different operating points.
i. The current gain for the common emitter configuration is very large in comparison to that for the common base configuration.
ii. The operating points must be recorded since the values of hybrid parameters change with the change of operating points in the active region for the same transistor.
iii. The output characteristics of a transistor are not parallel to each other due to slight variation of α with VCB. But because of early effect, a slight change in a (say 5%), β may vary 34% or so. Output characteristics for larger values of collector current are crowded together for constant base current increments.
iv. For calculating the mean value of the current, observation I and observation II should be taken under identical equilibrium conditions.
Extra Work on P-N-P Transistor:
To be performed if directed by the instructor.
1. From the output characteristics (i.e., lC – VCE curves) calculate the variation of- (i) a.c. current gain (hfe) and (ii) d.d. current gain hFE, when the operating collector current varies from 10 mA to 50mA for the transistor supplied. Comment on the result.
Also show graphically the variation of hFE and hfe with lC and VCE.
2. Place a hot electric bulb (60 W, 220 V) at a distance of about 3 to 4cm from the transistor so as to heat the transistor. Draw the output and the transfer characteristics, and investigate the effect of temperature rise on collector current. Comment on the result. Also plot hFE versus lC at the two temperatures on the same graph paper.
3. Investigate the variation of hre, hoe, hie and hfe with change of the operating collector current. Plot the curves showing the variations of hre, hfe, hice and hoe with operating collector current on the same graph paper. Use the output characteristics and transfer characteristics for this purpose.
Notes:
The value of hFE at first rises to a maximum value with increase of lC. The extent of the maximum region and its value depend on the value of hFE of the transistor supplied. For a transistor with a high d.c. β value, the position of maximum is very sharp, while for the low d.c. β value, the maximum is not prominent.
4. Calculate the value of lCEO (i.e., cut off current) at different collector current using the relation
IC = βIB + ICEO
5. For the transistor supplied, keep the base current at 50 µA. Plot the collector voltage along the X-axis and the base voltage along Y-axis (keeping the base current constant at 50 µA). In this way draw the VBE – VCE curve at a constant IB. Change the value of IB to other values and obtain the characteristics curves. Calculate hre and hie from these characteristic curves. Compare these values with those obtained before.