8.29 Sspd_chapter 6_part 9_appendix device physics  (Page 2/8)

In CE BJT, Equation I is rearranged as shown below:

At low voltages, Equation II reduces to :

This further simplifies to:

Or

Here I _CBO = Collector Current in CB BJT with Emitter Open that is Emitter current is held at zero magnitude. This current is the reverse leakage current across the reverse biased CB Junction and is of the order of nA.

Here I _CEO = Collector Current in CE BJT with Base Open that is Base current is held constant. This current is not the reverse leakage current across the reverse biased CB Junction. It is the Collector Current with BJT in Forward Active Mode and it is of the order of µA.

Equation IV indicates that at a given ‘V _CE *’ α _F M becomes UNITY and both the terms in the Equation become infinity. Physically this means that as V _CE increases, by Base Width Modulation also known as Early Effect, forward current transfer ratio improves. Simultaneously M is also gradually increasing as seen in Equation III. Hence Collector to Emitter Breakover takes place across Collector to Emitter terminals and this voltage is designated as BV _CEO .

At BV _CEO ,

Rearranging the terms we get:

Therefore if Collector-Base Breakdown occurs at 128 V then Collector-Emitter Breakover with constant base current drive occurs at 60V as shown in Figure III. The range of voltage over which BJT can be safely operated is drastically reduced. The same CE BJT if used with constant voltage drive, as it is in Current Mirror configuration, will experience breakover at 112V.

The physics of this Breakover Phenomena becomes clear from Figure IV and Figure V.

Figure IV shows the electron and hole component of I _E , I _C and I _B at low voltages when reverse leakage current at CB Junction is:

Figure V shows the electron and hole component of I _E , I _C and I _B at high voltages when reverse leakage current at CB Junction is:

Figure IV and Figure VI must be studied in correspondence. Figure VI shows the carrier concentration profile of NPN transistor in Forward Active Mode.

In Forward Active Mode, EB junction is forward biased and BC junction is reverse biased. Built-in barrier potential at EB is reduced (φ _BO - V _EB ) and built-in barrier potential at BC junction is increased (φ _BO + V _CB ).

Reduced barrier potential helps in electron(majority carrier) injection into base. In base electron becomes minority carriers hence increased barrier potential at CB junction is down-hill for the minority carriers. Therefore electrons injected into Base transit through the narrow Base region with negligible recombination and are collected by Collector. Since base is narrow practically all the carriers in Base are collected by the Collector.

While electrons are travelling longitudinally across the Vertical NPN BJT they experience recombination in Emitter as well as in Base.

Recombination in Emitter causes Emitter Injection Efficiency (γ) to be less than 100%.

Recombination in Base causes Base Transport Factor (β*) to be less than 100%.

These two recombinations cause the forward current transfer ratio to be less than UNITY. To provide these two recombinations Base Current is necessitated.