Semiconductor Electronics/Bipolar Junction Transistor

In the year 1904 J.A. Fleming introduced the vacuum tube diode. Shortly thereafter in 1906 Lee De Foster added the third element the control grid to the diode, thus making the first vacuum tube amplifier the triode. By 1930, pushed by electronics industry the vacuum tube had four elements in it and was called tetrode, the five element version was called pentode. Vacuum tubes became so popular for electronics that their production that was about 1 million in 1920 rose to 100 millions by 1937.

It was on December 23 1947, the demonstration by Walter H. Brattain and John Bardeen in Bell Telephone Labs introduced the Bipolar Junction Transistor. Its advantage over Vacuum tubes was imminent, it had no heater element, required less power to operate, it was small and light weight, it required no warming up.

Unlike vacuum tube it lasted longer, could have a rugged construction. Hence Transistors gained dominance over vacuum tubes.

Transistor Construction

A transistor is constructed by placing a oppositely doped semiconductor material between two similarly doped semiconductors. Or placing n-type material between two p-type material which forms the pnp-transistor or by placing a p-type material between two n-type semiconductor which forms npn-transistor.

The above diagram shows the schematic construction of a PNP transistor. As you can see an N-type silicon (green layer) is sandwiched between two P type materials (red layer). The left part is indicated by P+ which means its highly doped P-type material. This highly doped portion is called Emitter, that is the piece of semiconductor that supplies majority carriers for the transistor to function. At the extreme right is moderately doped P type material which is called as the Collector. This portion collects the majority charge carriers that is been emitted by Emitter and that manage to cross the collector. The middle region is denoted by n- because it's doped with N-type impurities. The minus'-' sign indicates it's doped very very less compared to the emitter and collector. The middle region is called the Base, and it's this region that serves as a gate, regulating flow of charge from Emitter to collector.

The doping of base is just one tenth of that of collector. In a real transistor, the width of base is very thin. The total width of the transistor will be 150 times that of the width of the base.

In a similar way by sandwiching a lightly doped P region between highly and moderately doped N region we get a NPN transistor as shown below.

Transistor Operation

To see how transistor operates we will look at how an PNP transistor works. NPN transistor works the same way as PNP, but with voltage and currents reversed.

Below is the diagram of PNP transistor. One can see that its emitter is held more positive than base and base more positive than collector. When emitter-base is forward biased and collector based is reversed bias, the transistor is said to be in active region. It's in the active region that transistor acts as a amplifier and so on. So let's study about it.

See the image below for active biasing of PNP transistor.

Now let's say we remove the voltage between base and collector hence the circuit looks as follows.

Let's analyze it.

Common Base Configuration

As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal. The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals as shown. The base terminal is grounded or can be connected to some fixed reference voltage point.

The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of “1” (unity) or less, in other words the common base configuration “attenuates” the input signal.

It has following properties

• low input impedance
• high output impedance
• high voltage gain
• unity (or less) current gain

Transistor Amplifying Action

The transistor is widely used as an amplifying device, and can provide voltage gain, current gain, and power gain. The basic amplifying action may be understood by considering the circuit of Fig 6.1. This circuit is known as the common base circuit due to the fact that the base terminal is common to both input and output circuits. The input voltage is applied between the emitter and the base, while the output voltage is developed across the load resistor RL . A small change in the input voltage Ii Vi produces a change in emitter current AlE given by where re is the input impedance of the circuit. The change in emitter current produces a change in output voltage across the load resistor given by The voltage gain Av, which is the ratio of the voltage across the load to the input voltage is given by Since RL can be made much larger than re, a high voltage gain is possible.

Common Emitter Configuration

As well as being used as a switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor, NPN Transistors can also be used to produce a circuit which will also amplify any small AC signal applied to its Base terminal. If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a Common Emitter Amplifier is produced. One such Common Emitter Amplifier configuration is called a Class A Amplifier. A Class A Amplifier operation is one where the transistors Base terminal is biased in such a way that the transistor is always operating halfway between its cut-off and saturation points, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of the AC input signal superimposed upon the DC Biasing voltage. Without this "Bias Voltage" only the positive half of the input waveform would be amplified. This type of amplifier has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages. With reference to the common emitter configuration shown below, a family of curves known commonly as the Output Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values of base current, (Ib) are applied to the transistor for transistors with the same β value. A DC "Load Line" can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value of Vce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point

Transistor testing

Using multimeter we connect the red and black probe to the lead of transistor first we find the base by moving up the probe to any lead of the transistor for which probe remain on the lead of the transistor and the other is on any two lead of the transistor that the base. Collector (C) we denote that red probe is for finding the collector we notice that when we set off the probe to the lead of the trasistor the multimeter will deflect half of the scale and by changing position we see that multimeter deflect more than half of the scale

Transistor Casing and Terminal Identification

After the transistor has been manufactured using one of the techniques, it leads of, typically, gold, aluminum, or nickel are then attached and the entire structure is encapsulated in a container. Those with the heavy duty construction are high-power devices, while those with the small can (top hat) or plastic body are low- to medium-power devices. Note the very small size of the actual semiconductor device. There are gold bond wires, a copper frame, and an epoxy encapsulation. Four (quad) individual pnp silicon transistors can be housed in the 14-pin