Early Voltage of ZMR-SOI Polysilicon Emitter Transistors
at Low Temperatures.

Onno W. Purbo
Inter University Center on Microelectronics
Institute of Technology Bandung
Bandung 40132, INDONESIA
Tel: 62-22-250-1895  FAX: 62-22-438-338
Email: yc1dav@itbgtw.itb.ac.id

	Early voltage of ZMR-SOI Polysilicon Emitter Transistor (PET) is studied as a function of temperature. The unified model of the Early voltage is used to analyze the Early voltage. The carrier freeze-out effect is found to be the major cause of degradation in the Early voltage at low temperatures. Poole-Frenkel temperature effects in EB space charge layer will introduce additional degradation of Early voltage close to liquid N2 temperature. High defect density in ZMR-SOI material is found to be the source of deviations in the measured characteristics of Early voltage between bulk-Si and ZMR-SOI PETs.

1.	Introduction.
	In this work, we studied the Early voltage as function of temperature VA(T) of our Zone-Melting-Recrystallized (ZMR) Silicon-on-Insulator (SOI) Polysilicon Emitter Transistor (PET) [1]. We found that the behaviour of the VA(T) in ZMR-SOI PETs is different from that of bulk-Si PETs (including the one reported in [2]). The major difference between ZMR-SOI and bulk-Si PET is in the quality of starting material. ZMR-SOI is known to have a higher defect density than that of bulk-Si [3].To account for the more defect density in ZMR-SOI PETs, the defect density in the Silicon film on top of the buried oxide is taken into account as in [4][5][6]. We found that taking the defect density into the calculation may account for the discrepancies found in the VA(T) of ZMR-SOI PETs.
	As far as the model is concerned, carrier recombination would be an important issue especially at low temperature conditions [7]. In the EB space charge layer, both SHR carrier recombination [8] as well as Poole-Frenkel effect [9] may exist simultaneously in high performance bipolar transistors at low temperatures. Specially in SiGe HBT [10], carrier recombination in the neutral base region may become quite an important factor in influencing the Early voltage. In a very-narrow base high performance bipolar transistor, the Early voltage may be reduced due to the limitation of the saturation velocity vd in the collector-base (CB) depletion region [11][12].
	The carrier multiplication in the CB junction leads to the well known reverse base current (RBC) [13][14] phenomenon. Lu [13] has reported the importance of carrier multiplication at CB junction of high performance bipolar transistors at low temperature conditions. Thus, it is important to consider the carrier multiplication in influencing VA(T) of bipolar transistors especially for low temperature applications. To the best of our knowledge, no studies have been reported in taking into account the carrier multiplication effects in VA. These models/effects have been developed and reported in [15].

2.	Defects in the CB depletion layer of ZMR-SOI PETs.
	ZMR-SOI material is obtained by recrystallization of a polysilicon layer on top of an oxide layer [3]. It gives rise to certain grain boundaries and defects associated with recrystallized polysilicon in ZMR-SOI material. In this section, the defects in ZMR-SOI material will be taken into account as uniformly distributed donor type traps with density (K - cm-3 V-1) in the band [4][5][6] in the CB depletion layer. The poisson equation in the p-side of CB junction is [5]
	(1)
in the n-side [5],
	(2)
The doping in the base side of the CB junction is approximated as a graded junction with gradient a. The potential V(x) and electric field distribution V(x)/x can be obtained by solving Eqn. (1)-(2) using the value of the voltages at the edges of the depletion layer as the boundary conditions. Using the fact that the electric field is zero at the depletion layer edges and assuming a maximum at the junction, the depletion layer width dnc and dpc in the n-type and p-type of the CB junction, respectively, can be obtained by solving numerically V(x) and V(x)/x. On the p-side of CB junction:
	(3)
(4)
with:

While on the n-side of CB junction, we have
	(5)
(6)
with:

where Vp is the fraction of voltage drop in the C-B depletion region on the p-side. Newton-Raphson iteration [16] is used to solve simultaneously Eqn. (3)-(6) to obtain dnc and dpc as function applied Vcb voltage and defect density. Other effects in VA(T) which might be important for high performance bipolar have been described and modelled in [15].

3.	Experimental Results and Discussion.
3.1.	Experimental set-up.
	We use the experimental measured data of our 7X7 μm2 emitter single poly-emitter double base-contacts bipolar transistors [1] to demonstrate the applicability of the method. Both ZMR-SOI and bulk-Si PETs were prepared using identical and simultaneous processing steps subsequent to the formation of buried collector and epitaxial layers [1]. An HP4145A Semiconductor Parameter Analyzer tied with a MMR Technologies Low Temperature Micro Probe (LTMP) is used to perform the dc measurements at various temperatures in the range of 85K-400K.

3.2.	Low temperature characteristics of ZMR-SOI PETs.

FIGURE 1. VA(T) of bulk-Si (solid) and ZMR-SOI PETs (empty).
	Figure [F1] shows the measured VA(T) of both bulk‑Si (solid symbols) and ZMR‑SOI PETs (empty symbols). The measured VA(T) deviates about 3‑4 Volt (15‑20% of its maximum value) in the range of 85K to 400K. A different temperature variation of the Early Voltages is observed between PET's made on ZMR and bulk‑Si. In bulk-Si PETs, VA(T) is gradually decreasing at lower temperatures. However, in case of ZMR-SOI PETs, VA(T) is gradually decreasing at higher temperatures (>200K) and gradually increasing at lower temperatures. The lower VA of ZMR-SOI PETs with respect to bulk-Si PETs may be due to a significantly different boron profile just near the CB junction although the base Gummel number is still nearly identical for both [1]. It is interesting to note that the shape of the measured VA(T) of our bulk-Si PETs is fairly similar to that presented by Dillard and Jaeger [2] which is however based on high-voltage bipolar transistor process. However, in our case (0.2 μm base PET), the temperature effects of VA(T) are more severe as compared to the transistors reported by Dillard and Jaeger [2]. Thus, it appears that one cannot assume a constant VA(T) for narrow-base bipolar transistors.

FIGURE 2. Measured n(T) of bulk-Si (solid) and ZMR-SOI PETs (empty).
	Figure [F2] shows the measured ideality factor of the base current at low Vbe as function of temperature for both bulk-Si (solid symbols) and ZMR-SOI (empty symbols). It is clearly seen from Fig. [F2] that at low temperatures (<100K) the ideality factor is larger than two (2). Thus, additional effects, such as Poole-Frenkel effects [9][7], are likely to be dominant in our PETs. It is interesting to note that the ideality factor of ZMR-SOI PETs is higher than that of bulk-Si PETs which indicates a more dominant Poole-Frenkel effects occurring in ZMR-SOI PETs. Thus, it is required to incorporate the carrier recombination in the EB depletion region as well as Poole-Frenkel effect in the calculation of VA(T).

3.3.	Calculated Early voltage as function of temperature.

FIGURE 3. VA(T) in ideal case (solid), recombination in neautral base (1), EB depletion (2) and Poole-Frenkel (3).
	Let us now take a closer look at various effects at low temperatures. Figure [F3] shows a typical calculated VA(T) taking into account carrier recombination. During the calculation of Fig. [F3], each recombination process is examined separately. The dashed lines '1' depicts the calculated VA(T) with a significant carrier recombination only in the neutral base region. Curve '2' shows the calculated VA(T) with carrier recombination only in EB depletion region. While, curve '3' is the calculated VA(T) with the existence of Poole-Frenkel effect in the EB depletion region (with no recombination in the neutral base). The ideal calculated VA(T) with no carrier recombination is shown in solid line and used as the reference. Using the default value in the model of carrier life-time [18][19], the calculated VA(T) with carrier recombination in neutral base (curve '1') and in EB depletion layer (curve '2') will follow closely the ideal  VA(T) (solid line) and quite difficult to see significant differences. In order to illustrate the effect of a strong reduction of carrier lifetimes, a four-order decrease in lifetime (with respect to the model [18][19], with typical carrier lifetime of 10-6 sec for doping of 1018 cm-3) was used to obtain curves '1' and '2' in Fig. [F3]. As expected a large decrease in carrier recombination shows a significant degradation of VA [10]. However, the effect of each carrier recombination, namely, in neutral base, in EB region and Poole-Frenkel effect, as functions of temperature is different. The effect of increased neutral base recombination is more significant at high temperatures. While the effect of increase in carrier recombination in EB region is more effective in lowering VA(T) throughout the temperature ranges. Since Poole-Frenkel effect is only significant at lower temperatures [7], as expected the degradation of VA(T) due to Poole-Frenkel effect is dominant only at low temperatures close to liquid N2 temperature. The effect of Poole-Frenkel is quite apparent in our devices especially in bulk-Si PETs (solid symbols in Fig. [F1]) of which VA(T) is reduced quiet dramatically at low temperatures close to liquid N2 temperature. As already observed (see Fig. [F2]), the measured ideality factor of the base current at low Vbe has values more than 2 at low temperatures. Thus, it is clear that Poole-Frenkel effect is a dominant factor in degrading VA(T) of our bulk-Si PETs at low temperatures close to liquid N2 temperatures. Deducing from the ideality factor of the bulk-Si PETs mostly around 1.5 (see Fig. [F2]) and the measured VA(T) in Fig. [F1], it does not suggest a significant carrier recombination in the neutral base region as well as EB depletion region in the bulk-Si PETs.
	We found that the effect of using a finite carrier concentration nc in the calculation of VA(T) is insignificant for transistor with Nepi doping larger than >5x1015 cm-3 as well as for low base current operations (in our case, we use 0.1 mA/cm2). However, as predicted by Lee and Fox [12], the effect of nc is slightly more apparent at high biases.

FIGURE 4. VA(T) with assumed defect of (1)1015,(2)5x1014 and (3)1014 cm-3V-1.
	Figure [F4] shows the calculated VA(T) (by solving Eqn. (3)-(6)) for different assumed defect densities in the range from 1015-1014 cm-3 V-1 in the CB depletion region. The lower defect densities give lower kink in the VA(T) at low temperatures. The highest curve is the Early voltage calculated with the highest defect density. It is shown clearly that when the defect density increases the Early voltage is increasing at lower temperature as well as to reasonably high temperatures and, thus, follow an approximate U-shape function. The increase in Early voltage at low temperatures is mainly because the net defect density increases due to Fermi-level movement at low temperatures [4] and, thus, increase the net negative charge in the CB depletion layer on the base side. Numerically solving Eqn. (3)-(6), for high defect density conditions the depletion layer width dpc in the p-type CB junction will be decreased as compared to low defect density case. This in turn will increase the Early voltage for high defect density conditions (especially at low temperatures). At moderately high temperatures, it follows basically the normal physical phenomenon of Early voltage as function of temperature. The VA(T) of our ZMR-SOI PETs seem to indicate a defect density in the range of 1013-1012 cm-3 V-1.

4.	Summary.
	We found that the major source of deviation in Early voltage as functions of temperature is the carrier freeze-out effect. At low temperature conditions close to liquid N2, the Poole-Frenkel effects in the EB depletion region is a dominant factor in degrading the Early voltage. Carrier freeze-out and Poole-Frenkel effects are found to be the major components that contribute to the characteristics of the Early voltage of our bulk-Si PETs. Degradation due to carrier recombination in the base and EB depletion region may become important only if the material has very short (about >4 order of magnitude lower) carrier life-time which fortunately is not the case in our bulk-Si PETs.
	Incorporating the defect density in the CB depletion region, it is found that the calculated Early voltage follows closely the characteristics (qualitatively) of the measured ZMR-SOI PETs Early voltage. At temperatures closer to the liquid N2 temperatures, the effective defect density increases which in turn reduces the CB depletion width on the base side and, thus, increases the Early voltage.

5.	Acknowledgements.
	The author wishes to thank Prof. C.R.Selvakumar at University of Waterloo, Canada for his encouragement and suggestions during the course of the work. The author wishes to thank the Government of Indonesia and the Canadian International Development Agency (CIDA) for granting the overseas scholarship.

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