ECE 291 -LABORATORY X THE TRANSISTOR COMPARISON OF TWO BASIC TYPES: MOS AND BIPOLAR
OBJECTIVES Familiarity with MOSFET, the most used type of transistor today and its comparison with BJT. Demonstrating extremely high dc impedance of the MOSFET gate. Exploring linear characteristics and switching behavior of the transistors. MOSFET as voltage controlled device and BJT as current controlled device. MOS analog switch.
INTRODUCTION There are two most common transistor types today: the Metal -Oxide - Semiconductor or MOS and the Bipolar Junction Transistor or BJT. The MOS is also designated as MOSFET because it is a field effect transistor (FET). A great majority of both types are made from silicon (Si) and a small fraction (about 2%) from gallium arsenide (GaAs). The BJT dominated the market initially but now most of the transistors, particularly in integrated circuits, are of the MOS type. The BJT, still holds its own, particularly in some analog and high power circuits. While most transistors of any type are made today as elements of integrated circuits (ICs), which can contain millions of circuit elements, single or discrete transistors are still useful in many applications like high frequency or power units. In this laboratory we concentrate on the MOS transistor and compare it with the BJT. The important difference between the gate impedance of MOS and the base impedance of BJT is emphasized.. You will experiment with an N-channel enhancement mode power MOSFET and an npn type BJT.
PRELAB 1. A MOSFET is characterized by a very high input (gate) resistance. Does it mean that no appreciable current ever flows to the gate? Explain. 2. Draw schematics of circuits for experiments with a MOSFET described in sections 1, 2, and 3, below. References: 1. T. C. Hayes and P. Horowitz and The Student Manual for the Art of Electronics Cambridge University Press 1989, pp. 255-260.
LABORATORY Equipment needed from the stockroom: Parts kit, proto-board, analog universal meter, resistance substitution box, leads, scope probe.
1. POLARITY OF THE BIPOLAR TRANSISTOR
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2. TRANSISTOR AS A SWITCH
Replace the BJT with an n-channel enhancement mode MOSFET: the gate terminal replaces the base, the source replaces the emitter and the drain the collector (see Fig. 9.1) Turn the lamp "on" and "off" by connecting resistor Rx either to the positive power supply terminal or to the ground. Now a surprise: disconnect the gate resistor from either ground or power supply. Touch the free end of the resistor (or the transistor gate terminal) with one hand and touch your other hand to either ground or the positive terminal. When the lamp turns "on" remove your hands and wait. After a while "ground" the gate with your fingers. From now on you should remember never leave the MOSFET gate unconnected. What can you say about the input resistance of this circuit? Do you need much current to turn the transistor "on"? Compare with BJT! Measure also the lamp current and the drain voltage. What is the output resistance (resistance between source and drain) of this circuit? Note: The transistor used in these experiments is a power MOSFET capable of carrying a large current and having relatively low channel resistance. A MOSFET used typically in digital circuits would not turn on the bulb; its channel resistance is too high. It could however control an LED (Light Emitting Diode) which requires much less current than a light bulb.
2.3. SWITCHING OPERATION OF A MOSFET.
Note: The input circuit consisting of the resistor and the gate is more complicated than a simple RC circuit because it is affected by the voltage on the drain. As the drain voltage swings it influences the gate voltage (feedback) and the effect looks like changing capacitance of the gate. This explains the somewhat strange shape of the observed waveforms.
3. MOSFET
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4. BJT
5. TRANSISTOR CHARACTERISTICS
6. MOSFET AS AN ANALOG SWITCH
REPORT
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