Overview: Junction Transistors

A junction transistor is a three-element semiconductor device consisting of two p-n junctions formed by sandwiching a thin layer of doped semiconductor (n-type or p-type) between two thick similar layers of the opposite type (p or n).

I_E ​= I_C ​+ I_B​

• A transistor has two p–n junctions and can be p–n–p or n–p–n type.
• The base is thin and lightly doped, the emitter is heavily doped, and the collector is moderately doped.
• For normal operation, the emitter–base junction is forward-biased, and the collector–base junction is reverse-biased.
• Most charge carriers from the emitter reach the collector; only a small fraction recombine in the base.

The current-amplification is defined as the ratio of change in the collector-current to the change in the base-current for a constant collector to emitter voltage.

Mathematically, β = \[\left(\frac{\Delta I_{C}}{\Delta I_{B}}\right)_{V_{CE}}\]

• In common-emitter mode, the emitter is common; the base is the input, and the collector is the output.
• Base current increases quickly after V_BE​ crosses barrier voltage; input resistance is low (kΩ range).
• The output curve has three regions: cut-off, saturation and active.
• In the active region, I_C is almost independent of V_CE and depends mainly on I_B; this region is used for amplification.

DC Current Gain is defined as the ratio of the collector current to the base current and is denoted by β (DC). Thus,

β(DC) = \[\frac {I_C}{I_B}\]

AC Current Gain is defined as the ratio of the change in the collector-current to the change in the base-current at a constant collector-to-emitter voltage and is denoted by β (AC). Thus,

\[\beta(AC)=\left(\frac{\Delta I_{C}}{\Delta I_{B}}\right)_{V_{CE}}\]

Transconductance is defined as the ratio of the change in the collector-current to the change in the base- to-emitter voltage at constant collector-to-emitter voltage and is denoted by 8m. Thus,

\[g_{m}=\frac{\beta\left(AC\right)}{R_{in}}\]

It is defined as the ratio of the change in the output voltage to the change in the input voltage and is denoted by A_V.

AC Power Gain is defined as the ratio of the change in the output power to the change in the input power.

V_CE ​= V_CC​ − I_C​R_L​