Band Theory

Band Theory

Band Theory

Doping

• Impurities that donate electrons called – DONORS• Impurities that ACCEPT electrons called – ACCEPTORS

• Semi conductors can be - DOPED – with an impurity which changes the conduction properties.

Lattice

• The semi-conductor forms a LATTICE structure around the impurity.

N-type semiconductors

Forward Bias• Electric field of

 battery overwhelms interface electric field allowing holes and electrons to be pushed acrossinterface.

Such a connection is called a "forward bias", because it allows current to go forward.

Reverse bias

Graph

• pn junction diode "turns on" at about 0.6 V, but that variesaccording to the doping

concentration.

Notice the sharp rise in currentnear the turn-on voltage.  This behavior will be exploited laterin the construction of the transistoramplifier.

Graph

 

This is the "characteristic" curveof a pn junction diode.  It showsthe slow, then abrupt, rise ofcurrent as the voltage is raised.

Under reverse bias, even verylarge voltages will cause onlyvery small currents, essentiallyconstant reverse bias currents.

The reverse bias current dependsmainly on the rate of thermal generation of electron-hole pairs

Reverse current exaggerated; typicalreverse current:  10 microamperes.

                                     

Black band corresponds to the "point" in thediode symbol on the right.

                 

 It points as "point" is spelled, with "p" first, and"n" later.

 

Rectification of an AC Signal

                               

Smoothing

Capacitor charges up during the positive half-cycle, then releases its charge through the resistor during the negative half-cycle, causing current through the resistor that otherwise wouldn't exist.

Bipolar Junction Transistors                                                            

                             

     

                              

Bipolar junction transistors (BJT).

Two types of charge carriers are involved.

The PNP Transistor

                                    

     

The junction on the left side ofthe base is forward-biased;the junction on the right sideof the base is reverse-biased.

Base is very narrow, and not drawn to scale.

Amplification

                                         

     

Emitter-base junction is biased at the turn-on voltage, so even very small changes in voltage there will lead to very large changes in the emitter-base current (see detail).

Electric field at the base-collector junction sweeps up almost all holes which enter the base. Weak input signal is amplified by the transistor (details follow).