Physics of Semiconductor Devices Institute of Microelectronics and Optoelectronics Cao Tianlin 21131044 1 1

2）high-level injection 2.83 High-current Gain 1）low-level injection 2）high-level injection High-level injection effects in a bipolar transistor 2

This lead to a decrease in gain at high currents. The slope of the collector characteristic changes from q/KT to q/2KT on entering high-level injection. This lead to a decrease in gain at high currents. 3

2.84 Basewidth Modulation ( Early effect) When the base doping is low or the basewidth very small,the application of a reverse bias to the collector can cause the width of the base to be significantly modulated 4 4

A greater reverse bias across the collector–base junction, for example, increases the collector–base depletion width, decreasing the width of the charge neutral portion of the base. 5

for low collector–base reverse bias; Bottom: narrower NPN Top: NPN base width for low collector–base reverse bias; Bottom: narrower NPN base width for large collector–base reverse bias. 6

Base-narrowing has two consequences that affect the current: 1)There is a lesser chance for recombination within the "smaller" base region . 2) The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases. 7

Tangents to the characteristics at large voltages extrapolate backward to intercept the voltage axis at a voltage called the Early voltage 8

2.9 JUNCTION BREAKDOWN 9

2.91 Punch-through In transistors with very narrow or very lightly doped bases the application of a reverse bias to the collector can cause the depletion region to extend across the whole width of the base and join up with the emitter/base depletion region. 10

2.92 Zener Breakdown Zener break-down is a tunnelling mechanism in which large numbers of electrons penetrate through the energy barrier imposed by the bandgap of the semiconductor. 11

zener diodes

2.93 Avalanche Breakdown Avalanche breakdown is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. 13

The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons. 14

High gain and high breakdown voltage cannot be obtained simulataneously in a BJT

The I-V curve for a diode showing avalanche and Zener breakdown The I-V curve for a diode showing avalanche and Zener breakdown. Note that the voltage increments in the negative range of the x-axis are larger than those in the positive range 16

Zener breakdown can be distinguished from avalanche breakdown by messuring the temperature dependence of the breakdown voltage. If Zener breakdown is the dominant mechanism,the breakdown voltage decrease with increasing temperature,whereas for avalanche breakdown it increases. 17

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