8.29 Sspd_chapter 6_part 9_appendix device physics  (Page 5/8)

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.

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.

If we take into account of BGN, we must use Eq.XX .

Using Eq.XX we get : β _F = 160.

Section III.3. Polysilicon emitter BJT (PEBJT)

The carrier concentration profile in CE BJT in forward active mode is shown in Figure VI. In 60s EB junction was like a wide base diode but with dimension scaling and improvement in technology, EB junction became shallow and hence narrow base diode. In a narrow base diode the emitter surface contact is decisive in determining the resultant emitter injection efficiency. As shown in Figure IX, metal contact directly to pure single crystal emitter portion causes a poorer injection efficiency as compared to that where metal contact and n+ pure Silicon has heavily doped poly-Silicon Emitter sandwiched between the two layers.

Figure IX. 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. IX a. 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.

Table III. 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].

In Polysilicon emitter BJT (PEBJT), a polycrystalline Si Film lies between Metallic Emitter Contact and Silicon Proper. By ion-implantation, the Polycrystalline Silicon Film is doped with Donor Arsenic Ions. By subsequent annealing, shallow diffusion of Donor Arsenic atoms from N ⁺ Poly-Silicon Source takes place leading to a shallow layer of N ⁺ in Si proper contacted by a similarly doped N ⁺ Poly-Silicon thin film. This structure is known as Polysilicon emitter BJT.

1. This is suitable for shallow EB Junction;
2. Compatible with self-aligned processing. This minimizes parasitic Resistance and Capacitances leading to faster response time.
3. Short Circuit Current Gain (β _F ) in PET is considerably improved as compared to that of shallow all crystalline emitter of equivalent thickness.