Theory - 32 :- Forward and reverse bias of transistor Junction. General values of junction resistances. Quick testing a transistor‐using Multimeter.

 

Biasing of transistors

Biasing a transistor means giving correct polarity and current level of voltages at the terminals of a transistor, such that, it functions as intended. (as an amplifier or as a solid state switch etc.) between two P‐type layers as shown in Fig 1 b.

the following points are important to note;

• The widths of the outer layers, i.e. emitter and collector layers are much greater than that of the base layer.
• The emitter layer is heavily doped compared to both the base and collector layers.
• The base layer is very thin, of the order of Ill 0th the width of the outer layers, and is very lightly doped

Transistor operation

As transistors have three layers, there are two junctions as shown in Fig 1. The base‐emitter junction behaves as one diode junction. The base‐collector junction behaves as the other diode junction. Fig 2a shows a NPN transistor where the base‐emitter junction is forward‐biased. Hence, the diode conducts resulting in large flow of majority carriers(e1ectrons) from N‐type to P‐type material. Fig 2b shows the base‐emitter junction forward biased and the base‐collector junction is reverse‐biased.  

   The answer is, in a NPN transistor, majority carriers are electrons, because, the emitter and collector are N‐ type materials. Free electrons are generated in the N‐type emitter because of the forward‐biased base‐emitter junction. If the collector voltage is not there, then all the generated electrons flow to the base as shown in Fig 2a. When the base‐collector is reverse‐biased, then, a positive voltage appears at the collector. This positive voltage at the collector completely changes the path of the electron current flow. Because of the thin base and the low base to‐  emitter voltage (0.7V for silicon), about 95 percent   of  .. The electrons pass through the thin base and are attracted to the more positive potential collector as shown in Fig 2b. Only a very small percentage of the electrons from the emitter combine with holes in the base.

It can be seen from Fig 3, that the,

• current carriers come from the emitter
• base current is small( 5% of emitter current)
•  and, the collector current is high (95% of emitter current).

Under such conditions, it can be seen that, small changes in the emitter‐base current will result in large change in the collector current. For example, an increase of say one electron in base current will result in an increase of 19 electrons in the collector current. This is because the collector current is 95% of the emitter current whereas the base current is only 5% of emitter current. This means that the value of the collector current can be easily controlled by changes in the bias on the emitter‐base junction. Summarizing, small changes in the base current results in large changes in the collector current as shown in Fig 4. This is nothing but amplification which is the intended function of a transistor. This behaviour of a transistor is known as Transistor action.

Minority current in transistors

In NPN transistor, as shown in Fig 6, if no voltage is applied across the base‐emitter junction, but a reverse bias is applied across the base‐collector junction, the following things happen, ‐ There is no current in the base‐emitter path as no bias voltage exists. ‐ The base‐collector diode is reverse biased; hence, the forward current due to the majority current Carriers  (e1ectrons)   is zero. ‐ A small quantity minority current of the order of a Few Nano amperes to microamperes flows in the base collector. This small reverse current is due to minority current carriers, electrons in the P‐type base material. ‐ The minority current increases if the voltage applied to the base‐collector increases or the junction Temperature   increases. This is because current increases temperature and temperature releases current carriers from the covalent bond structure

Biasing in Transistor

For proper working of a transistor, emitter base junction should be forward biased and CB junction should be reverse biased.

Transistor test using an analogue 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.

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. 

2 Quick TURN‐ON test

Recall that the base lead of the transistor controls   the   flow  of current carriers from emitter to collector. So, if the base is open, then there can be no current flow through (   emitter collector. This means, the resistance between emitter and collector will be high when the base is open as shown in Fig 9a. This can be checked using an ohmmeter with

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.    While testing, a transistor using ohmmeter, it is suggested to use the middle ohmmeter range (Rx100)· because, ohmmeters in low range can produce excessive current and ohmmeters in high range can produce excessive voltage which may be sufficient to damage small signal transistors