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 (IC, VBC 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:
- P-N-P transistor.
- N-P-N transistor
- Draw I-V 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 N-P-N transistor in the common base circuit, measure and plot IC as a function of VBC for at least four different values of IE (up to ~ 40 mA). Vary VBC (from small negative voltage up to about 10 V). Plot the results for different values of IE on the same graph. Note: In Agilent U2722A source measure unit set a constant value of IE on one channel and step-up VBC on another channel. Download the data and repeat for a different value of IE. You cannot step-up the source setting on both channels.
- Using the same transistor in the common emitter circuit, measure and plot IC as a function of VCE (up to about 10 V) at three different (small!) values of IB. Do not exceed 0.5 W of power dissipated by the transistor!
- Repeat (1) for a P-N-P transistor with appropriate supply polarity.
- Repeat (2) for P-N-P transistor.
- Finally, a fundamental science experiment: measure the electron charge qe. Use the fact that IC follows quite well the Ebers-Moll equation: IC = IS [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 cut-off mode?
- 2N3904 and 2N3906 are so called complementary pair of (N-P-N and P-N-P) 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 = IC/IE as a function of IC for a value of VBC 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 IC for a value of VCE close to the VBC used to calculating α. Check if the relation between α and β (for a given values of IC is what you expect.
- Can you see the effect of the base width modulation in IC(VCE) plots of (2) and (4)? If so, try to derive the Early voltage VA. 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 VBE as given by the Ebers-Moll equation IC = IS [e(VBE /VT) - 1]. Using the data of (5) derive the electron charge qe. 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 qe. If there is a substantial difference, can it be explained by the assumed temperature?