Early effect or base width modulation –

The early effect is the variation in the width of the base in a bipolar transistor due to a variation in the applied base-to-collector voltage.

For example a greater reverse bias across the collector- base junction increases the collector-base depletion width. If VCE increases VCB increases too.

$V_{BE}\lt V_{CE1}\lt\lt V_{CE2}$

The emitter-base junction is unchanged because the voltage $V_{BE}$ is the same.

Base narrowing has two consequences that affect the current:

There is a lesser chance for recombination within the “smaller” base region.

The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases.

To countervail the Early- effect is a lightly doping of the collector region and a heavy doping of the emitter region.

Avalanche breakdown –

• If bias voltage exceeds the reverse bias breakdown voltage of diode then the avalanche breakdown occurs.

• Breakdown voltage in turn depends on doping concentration of each site of junction.

• If the field at junction exceeds some critical value, impact ionization or Zener tunneling will occur within junction leading to breakdown.

Effects due to high current condition:

a) Lateral voltage drop –

a. First effect is geometry related. BJTs are fabricated as long narrow strips.

b. If the base current is sufficiently high, the base voltage varies laterally across device, reducing current drive capability.

c. Similar effect can occur at emitter and collector contacts. Low resistance contacts must be provided to collector and emitter since collector and emitter currents are typically very high than base current.

b) Kirk effect or base push out effect –

a. When density of majority carriers injected into the base collector depletion region become comparable to doping level in collector Kirk effect arises.

b. These additional carriers compensate the ions in collector side of depletion region, reducing the electric field.

c. When these electric field approaches zero, base encroach to metallurgical collector, increasing the base width and hence reducing the current gain.

d. Increasing collector doping increases the voltage at which Kirk effect becomes important but it also increases the collector capacitance.