Transistor

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.

Unlike the resistor, capacitor, inductor and diode, the transistor  is an active electronic component. This means that , by connection  to  battery ,its is able to act as an amplifier .it is as a current amplifier that the transistor is well known

The transistor is a semi-conductor device made up of two PN junctions and it has three terminals, known as the emitter, Base and collector. Transistors may be of PNP or NPN. Basically, there are two types of transistors. They are:

(a)              The bipolar junction transistor

(b)             The field effect transistor

BIPOLAR JUNCTION TRANSISTOR (BJT)

The BJT is often referred to as simply the transistor. It consist of semi-conductor crystal. These two junctions give rise to three regions called the Emitter, Base and collector.

 fig. 2.5 (a): Structural symbol of NPN transistors

 Fig. 2.5(b): Circuit symbol of NPN transistor

  Fig. 2.5(c): Circuit symbol of PNP transistors

Figure 2.5 (a) above shows an NPN transistor with its symbol. It consists of a layer of P–type material sandwiched between two layers of N–type materials.

Operation: The operation for both type i.e PNP and  NPN are the same. The only difference is in the DC bias condition.

Consider the NPN transistor shown in figure 2.5 (a) below, the base  - emitter (EB) junction is forward biased by voltage Vs. Electrons from the emitter (I_E) will therefore flow across the junction from emitter to base. It is the normal forward current for a forward bias PN junction. As soon as the electrons cross over into the base; they are attracted by positive potential of the collector by making the base very thin.

For a PNP transistor, the polarities of the DC supplies must be reversed as shown in fig. 2.6 (b) below. In this case, the transistor current is a movement of hole from emitter to collector.

 

TRANSISTOR CONFIGURATIONS

There are three possible ways of connecting a transistor when used in a circuit. They are:

The Common Emitter (CE): Configuration where the input signal is feed between the Base and Emitter as shown in Fig 2.6 (a) below. This is the most widely used configuration due to its flexibility and high gain.

 

        Fig. 2.9 (a): Common Emitter (CE) Configuration

Fig. 2.7 (b): Common Collector (CC) Configuration

Fig. 2.7 (c): Common Base (CB) Configuration

The common collector (cc): Where the collector is common, this configuration is also known as the emitter follower since the output is taken at the emitter as shown in figure 2.6 (b).

The common Base (CB) configuration; where the base is common to both input and output as shown in figure 2.6 (c).

TRANSISTOR CHARACTERISTICS

For the transistor to start conducting, its Base Emitter voltage must be up to the range of 0.6v to 0.8. Below this voltage the transistor will cut off and does not actually conduct. The only current that flows is a leakage current which is due to minority carriers.

TRANSISTOR OPERATING REGION   

The bipolar junction transistor has three operating regions namely:

          i.        Cut–off region

          ii.       Saturation region

          iii.      Acute region

The cut off Region: This condition results from reverse bias for both Base – Emitter (EB) and Base–Collector (CB) junctions. Under this condition, both junctions act like open circuit, it is as if the transistor terminals are uncoupled from each other. In cut off condition V_EE-Vcc.

Saturation Region: A transistor is said to be saturated when the current through it is so high that it cannot increase any further i.e. when I_E and I_c are at their maximum value. As I_E increases, V_E also increases.

At saturation, when the transistor is at a maximum, V_E and V_c are approximately equal with Vc_E almost at zero i.e Vc_E = 0.

Active Region: this is corresponding to forward bias for Base Emitter (BE) junction and reverse bias for base – collector junction.

How to test a transistor with a multimeter

The diode test using an analogue multimeter can be extended to give a simple and straightforward confidence check for bipolar transistors. Again the test using a multimeter only provides a confidence check that the device has not blown, but it is still very useful.

The test relies on the fact that a transistor can be considered to comprise of two back to back diodes, and by performing the diode test between the base and collector and the base and emitter of the transistor using an analogue multimeter, the basic integrity of the transistor can be ascertained.

Transistor equivalent circuit with diodes for multimeter test.

It should be noted that a transistor cannot be functionally replicated using two separate diodes because the operation of the transistor depends upon the base which is the junction of the two diodes, being one physical layer, and also very thin.

Step by step instructions:

The instructions are given primarily for an NPN transistor as these are the most common types in use. The variations are shown for PNP varieties - these are indicated in brackets (.. .. ..):

1.     Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.

2.     Connect the base terminal of the transistor to the terminal marked positive (usually coloured red) on the multimeter

3.     Connect the terminal marked negative or common (usually coloured black) to the collector and measure the resistance. It should read open circuit (there should be a deflection for a PNP transistor).

4.     With the terminal marked positive still connected to the base, repeat the measurement with the positive terminal connected to the emitter. The reading should again read open circuit (the multimeter should deflect for a PNP transistor).

5.     Now reverse the connection to the base of the transistor, this time connecting the negative or common (black) terminal of the analogue test meter to the base of the transistor.

6.     Connect the terminal marked positive, first to the collector and measure the resistance. Then take it to the emitter. In both cases the meter should deflect (indicate open circuit for a PNP transistor).

7.     It is next necessary to connect the meter negative or common to the collector and meter positive to the emitter. Check that the meter reads open circuit. (The meter should read open circuit for both NPN and PNP types.

8.     Now reverse the connections so that the meter negative or common is connected to the emitter and meter positive to the collector. Check again that the meter reads open circuit.

9.     If the transistor passes all the tests then it is basically functional and all the junctions are intact.

Notes:

• The final checks from collector to emitter ensure that the base has not been "blown through". It is sometimes possible that there is still a diode present between collector and base and the emitter and the base, but the collector and emitter are shorted together.

As with the germanium diode, the reverse readings for germanium transistors will not be as good as for silicon transistors. A small level of current is allowable as this results from the presence of minority carriers in the germanium.