Part B  Semiconductor Devices
Experiment Series 4 – BJT
BJT Prelab Assignment

Read carefully the description of the experiments described below. Based on
specifications of the transistors you will test (2N3904 and 2N3906), design the
experiments using Agilent U2722A source measure unit. This instrument will be your
source of current and voltage, using its two channels. Draw schematics of circuits with a
BJT transistor and two sources in the common base and the common emitter
configurations. Show source polarity and indicate relevant currents and voltages with
appropriate symbols (I_{C}, V_{BC} etc.). Specify the type of source (voltage or current) and
which source output will be varied to obtain the specified characteristics. Consider also
the ranges of these values. You can simplify your work by providing circuit schematics
form Multisim simulations in point 2, below.
Show schematics for:
 PNP transistor.
 NPN transistor
 Draw IV curves for the collector current in both common base and common emitter configuration for either 1a or 1b. Label the graphs axes with the same symbols for current and voltage as used in the schematics. Indicate on the graphs where the transistor operates in the saturation and the active modes. Use MULTISIM simulations for two transistors to be used in the experiments: 2N3904 and 2N3906. Consider that the instrumentation in the laboratory has the voltage range 0  20 V and current up to 120 mA.
 Define transistor parameters: α and β (current gain) in terms of values measured in the experiments, and the relation between the two parameters.
Experiments with BJT
 Using an NPN transistor in the common base circuit, measure and plot I_{C} as a function of V_{BC} for at least four different values of I_{E} (up to ~ 40 mA). Vary V_{BC} (from small negative voltage up to about 10 V). Plot the results for different values of I_{E} on the same graph. Note: In Agilent U2722A source measure unit set a constant value of I_{E} on one channel and stepup V_{BC} on another channel. Download the data and repeat for a different value of I_{E}. You cannot stepup the source setting on both channels.
 Using the same transistor in the common emitter circuit, measure and plot I_{C} as a function of V_{CE} (up to about 10 V) at three different (small!) values of I_{B}. Do not exceed 0.5 W of power dissipated by the transistor!
 Repeat (1) for a PNP transistor with appropriate supply polarity.
 Repeat (2) for PNP transistor.
 Finally, a fundamental science experiment: measure the electron charge qe. Use the fact that I_{C} follows quite well the EbersMoll equation: I_{C} = I_{S} [e^{(VBE /VT)}  1].Use either type of transistor but make sure it operates in the active mode at a current in a few mA range.
Reference: Jasprit Singh Semiconductor Devices, John Wiley & Sons 2001. pp. 257  283.
Report and Analysis

Present all graphs. Comment on the curves:
 Are they what you expected and in agreement with the simulations?
 Where the transistor operates in the active, saturation of the cutoff mode?
 2N3904 and 2N3906 are so called complementary pair of (NPN and PNP) transistors. Do you see symmetry between the characteristics of the transistors? Plot any of the characteristics of the two transistors on one graph (reverse polarity, if needed).
 From (1) and (3) calculate and plot α = I_{C}/I_{E} as a function of I_{C} for a value of V_{BC} in the active mode. The plot will have as many points as the number of I(V) curves you obtained in (1) and (3). Similarly, from (2) and (4) calculate β and plot it as a function of I_{C} for a value of V_{CE} close to the V_{BC} used to calculating α. Check if the relation between α and β (for a given values of I_{C} is what you expect.
 Can you see the effect of the base width modulation in I_{C}(V_{CE}) plots of (2) and (4)? If so, try to derive the Early voltage V_{A}. Hint: extrapolate linear fit trendlines of the active mode parts of the curves until their intersection with the horizontal axis (see Jasprit Singh Semiconductor Devices, John Wiley & Sons 2001, p. 174  283).
 The collector current is controlled by V_{BE} as given by the EbersMoll equation I_{C} = I_{S} [e^{(VBE /VT)}  1]. Using the data of (5) derive the electron charge q_{e}. Assume the temperature of 300 K, which is reasonable if you did not drive your transistor too hard. Compare your result with the known published value of q_{e}. If there is a substantial difference, can it be explained by the assumed temperature?