20. Read the text “Junction Transistors” only once. How much can you remember? Answer these questions without additional reading.
1. Can you define the P-N-P transistor?
2. What can you say about the characteristics of the various types of junction transistors?
3. What does the emitter-base constitute?
4. Can you give the description of current flow in a transistor?
5. What are the functions of the emitter and the collector?
If you failed try to answer these questions again after doing the exercises given below the text.
Transistors are an extension of the semiconductor diode. The P-N-P transistor is a junction transistor formed by sandwiching a very thin strip of N-type silicon between two ‘wide’ strips of P-type silicon.
In P-N-P transistor (Fig.1) three leads are brought out from individual metallic plates which contact with the respective semiconductor crystals.
The geometry of junction transistors differs depending on the method of construction of the junction. Moreover, the characteristics of the various types of junction transistors depend on the method of fabrication.
Fig.1. The P-N-P transistor
Refer to Fig.1. The P wafer on the left is designated the ‘emitter’, the N wafer in the middle – the ‘base’ and the P wafer on the right – the ‘collector’. The base is about 1 mm thick. For purposes of biasing the transistor may be considered as two diodes.
Fig.2. The operation of a P-N-P transistor
The emitter-base constitutes one diode, the collector-base the other diode. When used as an amplifier, the transistor is biased as shown in Fig.2. The emitter-base is biased in the forward or low-resistance direction; the collector-base is biased in the reverse or high-resistance direction by collector battery.
Holes are the majority current carriers in the emitter-base diode, and they emanate from the the P-type emitter. The presence of the P-type collector and its connection to the negative terminal of collector battery radically alter the path of hole-current flow. Only a small percentage of holes emitted by the emitter combine with the electrons in the base. The other holes (about 95 percent) pass through the very thin base crystal and are attracted to the negative battery terminal on the collector.
The emitter-collector circuit is extremely completed through the two batteries connected in series-aiding. From this description of current flow in a transistor it is clear that the emitter-base current is very small, and that the emitter-collector current is high. It may be seen also that changes in emitter-base bias will result in changes in emitter current. Thus, an increase in forward bias will result in an increase in emitter current and hence in collector current. Base current will increase or decrease very little when emitter current increases or decreases. It is evident therefore, that collector current may be controlled readily by changes in emitter-base bias. The designations ‘emitter’ and ‘collector’ may now be readily associated with their functions.
A junction transistor may also be made with an N-P-N configuration.
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