Notes on BJT & FET Transistors.

Comments.

The name transistor comes from the phrase �transferring an electrical signal across a resistor.�

In this course we will discuss two types of transistors:�

The Bipolar Junction Transistor (BJT) is an active device.� In simple terms, it is a current controlled valve.� The base current �(I_B) controls the collectorcurrent (I_C).�

The Field Effect Transistor (FET) is an active device.� In simple terms, it is a voltage controlled valve.� The gate-source voltage �(V_GS) controls the draincurrent (I_D).�

Regions of BJT operation:

Cut-off region:� The transistor is off. There is no conduction between the collector and the emitter.� (I_B = 0 therefore I_C = 0)

Active region:� The transistor is on. The collector current is proportional to and controlled by the base current (I_C = βI_B) and relatively insensitive to V_CE.� In this region the transistor can be an amplifier.

Saturation region:� The transistor is on. The collector current varies very little with a change in the base current in the saturation region.� The V_CE is small, a few tenths of volt.� The collector current is strongly dependent on V_CE unlike in the active region.� It is desirable to operate transistor switches in or near the saturation region when in their on state.

Rules for Bipolar Junction Transistors (BJTs):

• For an npn transistor, the voltage at the collector V_C must be greater than the voltage at the emitter V_E by at least a few tenths of a volt; otherwise, current will not flow through the collector-emitter junction, no matter what the applied voltage at the base.� For pnp transistors, the emitter voltage must be greater than the collector voltage by a similar amount.
• For the npn transistor, there is a voltage drop from the base to the emitter of 0.6 V.� For a pnp transistor, there is also a 0.6 V rise from the base to the emitter.� In terms of operation, this means that the base voltage V_B of an npn transistor must be at least 0.6 V greater that the emitter voltage V_E; otherwise, the transistor will not pass emitter-to-collector current.� For a pnp transistor, V_B must be at least 0.6 V less than V_E; otherwise, it will not pass collector-to-emitter current.

BASIC EQUATIONS FOR THE BJT.�