1.4 The journey of i.c.technology from micro (1959) to nano (2009  (Page 2/2)

IV . 1.1. Band Gap Narrowing in Emitter, its deleterious effects and its cure.

Heavy doping of emitter causes Band Gap Narrowing in Emitter which limits the improvement in Injection Efficiency due to heavy doping of Emitter. Emitter doping may be N _D = 10 ²⁰ /cc but in effect we get the Injection Efficiency corresponding to 10 ¹⁸ /cc [Appendix IV]. Hence we say that BGN limits N _Deff = 10 ¹⁸ /cc. This effect(BGN) is mitigated by using heavily implanted PolySi layer used as emitter contact.

Due to heavy doping in Emitter, degeneracy is introduced which leads to Band Gap Narrowing(BGN) by ∆E _g in

As shown in Appendix IV,

For a doping level of N _D = 10 ²⁰ /cc, ∆E _g =0.12eV.

This gives an effective doping level of N _Deff = 10 ¹⁸ /cc.

The classical formula for current gain without BGN is:

If we take the following data: (N _D ) _E = 10 ²⁰ /cc, (N _A ) _B = 10 ¹⁷ /cc, W _E = W _B = 1µm and D _pE =1.25 (cm) ² /sec and D _nB =20 (cm) ² /sec then β _F = 1.6×10 ⁴ ;

But taking into account of BGN, β _F = 160.

The other component responsible for improved Current Gain in Poly_Si Emitter Transistor is due to flatter slope of excess minority carrier in Emitter region [Appendix V ]

Figure 4. Linear Gradient is a strong function of surface condition. Metal contact gives a large gradient and Poly Silicon contact to Emitter gives a much lower linear gradient on the emitter side thereby improving I Dn /I D (Injection Efficiency) by several orders of magnitude.

To achieve higher degree of integration we had to go for smaller feature size as well as shallow devices. Metallic contact gives infinite surface recombination velocity, hence it gives a much steeper gradient as shown in Fig 4a. whereas Poly-Silicon gives a much lower gradient resulting in a very low hole component of the total current thereby giving a much higher injection efficiency.[Appendix V ]

Table 7. Room Temperature current gain as a function of emitter contact for device run BIP-8.

[“Effect of Emitter Contact on Current Gain of Silicon Bipolar Devices”, T.H.Ning&R.D. Isaac, IEEE Int. Electron Devices Meeting, pp.473-476, 1979].

“ The current gain of silicon bipolar transistors with shallow emitters depends critically on the emitter contact technology…………….The conventional contact by metal or metal silicide degrades the current gain, while contact by a thin layer of poly-silicon is effective in improving the current gain” [ibid].

IV.1.2. Base Spreading Resistance r x and the frequency response of BJT:

This is due to the narrow sandwich of high sheet resistance P base. P Base is of 200Ω/▄ to keep N _A|B /N _D|E = 10 ¹⁷ /10 ¹⁹ as low as possible. This results in r _x = 100Ω. This creates serious deterioration in high frequency performance of BJT especially in terms of Unity Power Gain Frequency (f _max ).

It can be shown that (f _max ) = [f _T /(8πr _x C _μ )] ^1/2

Using the dimensions of BJT used in 70s we get [Appendix IV], we get

f _t (Unity gain BW)= 624MHz;

f _max = [f _T /(8πr _x C _μ )] ^1/2 = 18.6GHz;

From this example it is clear that base spreading resistance r _x must be minimized to get best frequency performance but reduction of r _x requires increased base conductivity but increased base conductivity means lower Injection Efficiency because

Injection Efficiency =γ = 1/[1+σ _B W/(σ _E L _p )]

From injection efficiency point of view: σ _B <<σ _E ;

These are contradictory requirements hence to achieve Short Circuit Current Gain of 100 we cannot allow Base Spreading Resistance to go below 100ohm.

By using Poly Silicon , γ can be improved several orders of magnitude. This results in β of the order of 10,000. Here base conductivity can be increased to minimize Base Spreading Resistance. Thus with a reasonable β of 100, considerably lower Base Spreading Resistance can be achieved which plays crucial role in frequency performance and in improving the Figure of Merit of BJT which happens to be

GBP= 1/( r _x C _μ );