8.29 Sspd_chapter 6_part 9_appendix device physics  (Page 3/8)

I _B = I _BB + I _Ep = provides the holes for recombination in Base + provides the holes to be injected into Emitter and cause hole diffusion current in Emitter.

Therefore

In Figure VI, charge stored in the Base is shown by the shaded region and is given by the following expression:

In Figure VI, minority carrier densities are depressed near the depletion layer of BC junction. Here EHP are thermally generated since minority carrier density is less than the thermal equilibrium value. The EHP (electron-hole pair) which survive to reach the reverse biased BC junction depletion width are swept across the depletion width as the reverse leakage current. The electrons on Base side and holes on Collector side are swept across the depletion width because the enhanced built-in barrier potential across the BC junction acts as the down-hill for minority carriers on two sides. The electron component of this reverse leakage I _CEO adds to the injected electron current being collected by the collector and hole component of the reverse leakage I _CEO provide the holes in Base for recombination as shown in Figure IV.

As V _CE increases , down-hill potential gradient becomes very steep which accelerates the minority carriers being swept across the BC junction to the extent that IMPACT IONIZATION and IMPACT GENERATION starts as shown in Figure V. Now a copious amount EHP are generated in reverse biased BC junction. Electrons add to I _C increasing it considerably and holes povide recombination centers in Base.

Eventually when α _F M becomes UNITY, holes generated by impact ionization provides all the holes required in Base for recombination with injected electrons as well as holes generated by impact ionization provide the holes to be injected into Emitter. Thus requirement for I _B from Base terminal becomes zero, current gain becomes infinite and I _C shoots up and breakover occurs. At the breakover point S-type negative impedance region is caused as shown in Figure VI. The PHYSICS of BREAKOVER is as follows:

At α _F M = 1 the impact generated holes just suffice the recombination needs of the device. At α _F M>1 the impact generated holes are more than what is required for recombination in the Base and for injection into Emitter. Hence the excess part of impact generated holes starts building the concentration of the carriers and their concentration gradient in the Base. This leads to a sudden shoot up in the Collector Current.

The S-type NIR occurs as shown in Figure III, because before breakover α _F1 is of the order of 0.7 4 because of very small I _C and after the breakover α _F2 is of the order of 0.8 because of large I _C hence

Rearranging the terms we get:

For 2N2119A: n = 2, BV _CBO = 128V, BV _CEO = 66 V, α _F1 = 0.74; α _F2 = 0.8.

Substituting these values we get: V _S = 58V.

Section III.2. Band Gap Narrowing in BJT.

In a classical BJT, Emitter Bulk Width(2µm)>>Diffusion Length of minority carriers in Emitter(fraction of a micron). Hence hole (minority carrier in Emitter) diffusion current is non-dominant and d.c. current gain in a classical BJT is :