Test: The Transistor As an Amplifier & Transistor Construction - Electrical Engineering (EE) MCQ

The emitter current consists of two parts. It consists of hole current IpE constituted by holes. The other part is that it consists the electron current InE constituted by electrons.

The total emitter current is the sum of I_nE and I_pE. In commercial transistors, the doping of emitter region is made much heavier than base. Hence current by majority charge carriers I_nE is negligible when compared to current by minority charge carriers I_pE.

Clarification: I_E = V/R=400M/20=20mA
I_C=αI_E= 1*20mA=20mA. V_O=I_C*R_L=20m*1k=20V
Amplification, A= V_O/signal voltage=20V/400m=50.

One of the most important application of a transistor is an amplifier. A small change in signal voltage produces an appreciable change in emitter current because the input circuit has low resistance (α=∆I_C/I_E).

The normal working temperature of germanium is approximately 70°C .The normal working temperature of silicon is approximately 150°C. The other advantages of using a silicon material are, it has a smaller I_CBO and its variations are smaller with temperature.

A small change of voltage ∆V_i between emitter and base causes a relatively large emitter current change ∆I_E. We define by the symbol α that fraction of this current change which is collected and passes through R_L.

Emitter current I_E=I_C+I_B=100+0.5=100.5mA.
αdc=I_C/I_E=100/100.5=0.995.

 I_C=βI_B+(1+β)I_CBO
I_C=50*0.25/1000+51/100000=13.01mA.

The cut in voltage of germanium is lower than that of silicon. If both germanium and silicon are in parallel, Ge starts conducting earlier and stops silicon from conducting.

The emitter-base junction is forward biased while collector-base junction is reversed biased. The transistor now operates in active region. Here, it can be used for amplification purpose.

In the process of transistor construction, a crucible is placed in the chamber. This chamber consists of hydrogen or nitrogen. These gases help in the prevention of oxidation. It also contains purified Ge or Si at a temperature few degrees above its melting point.

The grown junction may be formed by suddenly varying the rate of pulling the seed crystal from the melt. This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling.

 In impurity variation method, the impurity content of the semiconductor is altered in its type as well as the quantity. For example, in making NPN germanium grown junction transistor, a small type of N type impurity is added to molten germanium and the crystal growth is started.

This method is based on the fact that proportion in which N and P type impurities crystallise i.e.., enter the grown crystal depends on the rate of pulling. If the pulling rate is small, a P type crystal is grown. If the pulling rate is fast, an N type crystal is grown.

This is similar to soldering and PNP transistor is generally is made by this process. In this method, first of all N type germanium is obtained. The N type wafer and indium dots are placed in a furnace and heated to about 500°C.

Leads for emitter and collector are soldered to the dots making non rectifying contacts. Further, non rectifying base contact is usually made from a welding a strip or loop of gold plated wire to the base plate.

The wafer of crystal has a 3-5m inch thickness and 80m inch square. This is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other.

The wafer is placed in a graphite jig with a dot of prepared indium. One dot of an indium is 3 times larger than the other. Finally the larger dot is used as collector. The smaller dot is used as emitter.

Large area collector junction helps in collecting most of the holes emitted from the emitter ensuring that the collector current almost equals the emitter current. The spacing between two junctions inside germanium wafer is very small and determines the electrical properties.

Grown junction type transistors are manufactured through growing single large crystal which is slowly pulled from the melt in crystal growing furnace. This is generally used for NPN transistors.