TRANSISTORS

TRANSISTORS

• A transistor is a semiconductor device that contains three regions separated by two distinct PN junctions. The two junctions are EB junction and CB junctions. 
• The central region is called base. 
• The two outer regions are called emitter and collector. 
• There are two types of charge carriers, electrons and holes hence transistor is called bipolar transistor. 
• Transistor can be viewed as two PN junction diodes arranged back-to-back with base being common to both the diodes. 
• As soon as the two junctions are formed, majority charge carriers diffuse and form two depletion layers. Depletion layer is narrow at EB junction and wide at CB junction.

There are two types of transistors: 

1.NPN transistor--P-type sandwiched between two N-type. 

2.PNP transistor--N-type sandwiched between two P-type.

FUNCTIONS OF THREE REGIONS IN TRANSISTOR: 

Emitter, Base and Collector are three regions in a transistor. 

Emitter:  

• It emits charge carriers. 
• It is heavily doped and has moderate size. 
• It is located at the one end of transistor. 

Base: 

• The base controls the flow of charge carriers from emitter to the collector. Therefore, it acts as a gate between emitter and collector. 
• It has minimum thickness and is lightly doped. 
• It makes the central region of a transistor. 

Collector: 

•  It collects the charge carriers coming from the base. 
• It has largest size with moderate doping. It is moderately doped. 
•  It is at the other end of a transistor.

Biasing of transistor 

• The Emitter-Base (EB) junction of transistor is always forward biased, and CollectorBase (CB) junction of transistor is always reverse biased. 
• Therefore, it works in an active mode. Hence, it transfers current from low resistance region (EB) to high resistance region (CB). 
• It is seen that almost same current flows through the two junctions. 
• Thus, the device is called as transistor- the shortened form of transfer resistor

CIRCUIT CONFIGURATION 
COMMON BASE CONFIGURATION

TRANSISTOR ACTION

APPLICATIONS OF TRANSISTOR 

• It is used in switching electronic circuits.
• It is used as an amplifier.
•  It is used in integrated circuits.

 

HALL EFFECT

HALL EFFECT 

If a metal or a semiconductor carrying a current I is placed in a transverse magnetic field B, a potential difference VH is produced in the direction normal to both the current and magnetic field directions. This phenomenon is called Hall Effect .

Fig: Schematic of Hall Effect Set Up

Applications / importance of Hall Effect 

(i) to determine the type of semiconductor 

(ii) to determine sign of majority charge carriers concentration 

(iii) to determine concentration of majority charge carriers

(iv) to determine mobility of majority charge carriers 

(v) to determine the drift velocity of majority charge carriers.

1) Hall Voltage 

Let us consider a bar of P-type semiconductor crystal and assume the charge carriers to be holes having a charge +e. Let an electric field Ex be applied to the bar which produced a current I in the x-direction in the crystal.

The Current through the semiconductor wafer is given by

Where p is the hole concentration 

             A is the area of cross section of the end face of semiconductor wafer 

             e electrical charge associated with hole 

            vd is the drift velocity of holes

Let a magnetic field B act in the Z-direction. As holes moves in the bar with a velocity, say v, they experience a Lorentz force FL due to the transverse magnetic field B.As a result they deviate sidewise towards the front face of the bar.

        Because of the deflection due to magnetic field, the holes is case of p-type crystal accumulate on front face and make it positively charged while the rare face becomes negatively charged with respect to the front face. Hence a potential VH called the Hall Voltage appears between the front and rare faces.

        The potential builds up in such a way that the electric field EH due to it discourages the further building up of the charges on the faces. The Process of accumulation of charges on front face lasts till the electric field just balances the Lorentz force FL.  
        Once the balance is established the charge accumulation stops and the crystal attains equilibrium states where holes moves parallel to the faces once again.

In the equilibrium state