Transistor Physics

• Composed of N and P-type Semiconductors

• N-type Semiconductor has an excess of
electrons
– Doped with impurity with more valence electrons
than silicon

• P-type Semiconductor has a deficit of
electrons (Holes)
– Doped with impurity with less valence electrons
than silicon

P-N Junction (Basic diode):

- Bringing P and N Semiconductors in contact
- Creation of a Depletion Zone

P-Type N-Type

• P-N Junction
• Reverse Biased => No Current
• Applying –ve Voltage to Anode increases
Barrier Voltage & Inhibits Current Flow

• P-N Junction
• Forward Biased => Current Flows
• Applying +ve Voltage > Barrier Voltage to
Anode allows current flow


• Basic Transistor


Water pipe analogy






Properties of the BJT

Common emitter configuration

2 basic laws:

Ie=Ib+Ic
Ic=β.Ib (β=10 to 100)


Operating Point

• Amplifier mode
• Switching mode