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Noise analysis of silicon carbide JFETs
Solid-State
Pergamon
NOISE
0038-l
ANALYSIS
101(94)00205-3
OF SILICON
P. FLATRESSE
Vol. 38, No. 5, pp. 971-975, 1995 Copyright 6 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I lOI/ $9.50 + 0.00
Electronics
CARBIDE
JFETs
and T. OUISSE
Laboratoire de Physique des Composants B Semiconducteurs (URA CNRS 840), ENSERG, 23, rue des Martyrs, 38016, Grenoble Cedex, France (Received 4 Ju1.v 1994; in revised form
20 September
1994)
Abstract-The low frequency noise of silicon carbide junction field effect transistors has been systematically studied at room temperature and at T = 77 K. At room temperature, I/f”’ spectra are attributed to surface thermal noise induced by lumped thermal noise generators distributed along the surface, in a way similar to that of GaAs thin-film resistors. At 77 K, generation-recombination noise is observed, and shot noise originates from drain and source contacts.
I. INTRODUCTION
Silicon carbide (SIC) devices are now produced in a large number of companies and laboratories. Since the material quality has greatly improved during recent years, it is now possible to investigate many physical properties of these devices, without being limited by an excessive amount of crystalline or process-related defects. Technology has reached a stage where noise measurements can be useful not only for improving device performance but also for obtaining substantial information on the physical properties of SiC material. Besides, JFETs are known to usually exhibit a good immunity to low frequency (LF) noise, which makes them suitable for analog applications. However, this aspect has received only limited attention to date in the case of SIC JFETs[ 11. Although the various polytypes of Sic exhibit a rather large dielectric constant (- 10). which contrasts with their wide bandgap, the impurity-induced levels remain somewhat deeper than in narrower bandgap semiconductors. In the case of the (6H) polytype, three different sites may induce three different levels for donor-type impurities like nitrogen, which is commonly used for n-type doping (two of them being very close to one another)[2]. In connection with low frequency noise, it was therefore interesting to assess the possible contribution of these levels to room temperature noise. Surprisingly, the main low-frequency noise source does not correspond to generation-recombination (GR) noise, but is shown to be induced by the passivated surface of the device. GR noise is observed only at nitrogen temperature, when the ratio between the free carriers and the electrons bound to the impurities becomes sufficiently small. At T = 300 K, the behaviour of the LF noise power spectral intensity S,d agrees with a model already proposed by other authors for GaAs devices[3]. In our particular case, the dependence of the noise level on the thickness of the buried depletion
layer provides the ability to further confirm the surface origin of the noise source. The model of PouysCgur et a/.[31 is adapted to the Sic JFETs so as to demonstrate that it can account for all the qualitative observed noise features. Improvement of surface passivation should therefore provide the ability to reduce the LF noise intensity of SIC JFETs to an appreciable extent. The paper is divided into three parts. Section 2 summarizes the characteristics of the devices under investigation as well as the experimental procedure. In Section 3 the experimental results obtained at room temperature are given. Described in Section 4 are the low temperature results.
2. DEVICE
CHARACTERISTICS
The (6H) SIC JFETs investigated in this paper were commercially available devices originating from CREE Research Inc. Five different JFETs have been systematically tested, all of them having the same geometry. A schematic of these devices is given in Fig. 1 and further information on similar devices may be found in Refs [2,4]. A buried p + gate controls the depletion layer width of an n-type epitaxial upperlayer. A passivated surface is thus bordering the active part of the channel, consisting in an oxide layer of 20 nm. The channel width, length L and thickness are respectively equal to 1 mm, 5 pm and -0.25 pm. The doping level of the n-layer has been found to be very homogeneous, from capacitance measurements performed at 300 K on the gate junction. Nd - N, was in the 5 x 10’b-10’7cm-3 range for all devices. The LF noise measurements (100 mHz-IO kHz) were performed using an ONO-SOKKI spectrum analyser loaded by a low noise NF 5305 voltage amplifier and an EG&G model 181 current-voltage converter. The number of samples used to calculate the spectra, as well as the cut-off frequency of the
971
P. Flatresse
912
and T. Ouisse
AvG Fig.
I. Schematic
of the SIC devices
investigated
1 T=298K
V,=5OmV
3
2
GATE
in this
paper. high-pass filter used during the systematic measurements were such that the values of power spectral density presented in this paper are reliable only for f > 1 Hz. Nevertheless, measurements with a larger number of samples and without d.c. filtering were performed for a limited number of biasing, and showed that no corner frequency was observable in the low frequency range, at least for f > 0.2 Hz. At room temperature, the devices were biased during 15 mn before the acquisition of the noise spectra, so as to avoid any influence of the transients discussed in Section 3 of this paper.
1
3
VOLTAGE
4
5
(‘/,-VP)
6
(V)
Fig. 3. Apparent variation of the exponent corresponding to low frequency noise spectra with gate voltage; T = 298 and 71 K.
voltage (Fig. 3). Since the white noise level increases with VPand is not much lower than the observed LF noise, it is not clear whether the experimental variation of y has to be attributed to the transition between l/f 3i2noise and the white thermal noise, or to additional LF sources which would appear as VP increases. Moreover, it is worth noticing that the highest gate voltages were close to the built-in potential of the junction (around 2.5 V), so that leakage through the gate might affect the noise characteristics. Since S,d/Z, was varying as l/V,, in the highest frequency range (1 kHz-10 kHz) and always re3. ROOM TEMPERATURE ANALYSIS mained in the order of 4kT/V,, the white noise part of the spectra was identified as thermal noise, and will Typical noise spectra, varying as l/f :, are presented not be given any further attention in this paper. in Fig. 2. They correspond to the ohmic regime of the To explain the existence of an exponent larger than device. As can be seen in Fig. 2, the shape of the 1 in the LF part of the spectra, one could invoke GR different spectra does not exhibit a pure l/f behaviour, but rather a l/f 3,‘2dependence. Such a noise. However, if the observed LF noise was induced by generation-recombination processes in the bulk of dependence is clearly visible in Fig. 2 in the lowest the JFET, the noise power spectral density of the frequency range, for f > 1Hz. However, one can also notice that for frequencies above 50 Hz, the l/f 3’2 drain current S,d should vary as I,. With a fixed drain voltage V,, S,d should also increase with VP. Neither dependence is progressively reduced to an exponent the experimental variation of S,d with Id (Fig. 4), nor value lower than 3/2. In fact, in the 20-200 Hz range, the dependence of S,d on P’, (Fig. 5) agree with the the empirical values found for the exponent y continuously vary from the pinch-off regime, where y is theoretical predictions of GR noise. As can be seen in Figs 4 and 5, S,d increases with Id near the pinch-off nearly equal to 1.5, to 0.5 at the highest values of gate 10-‘8t
-18
(6H) Sic JFET
1 (6H)
SIC
JFET
10-19
f=3Hz 09
10-20 00
10-21
I:;
1o-22
F
f=lOHz
i
I
Fig. 2. Typical
10 Frequency (Hz)
100
noise spectra at 7’ = 298 K and in the ohmic regime.
-21
-
V,=SOmV
T=298K
P
1o-23
J
,,,.
lo-&
10
DRAIN
-5
CURRENT
10
-4
10
-’
ID (A)
Fig. 4. Noise power spectral density of the drain current vs drain current, in the linear regime and at room temperature.
Noise analysis
of silicon carbide
973
JFETs
’ [ sch(bl,2) X
f= 1 OHz
/A WZ LnLu
cc? z
d
i
10 -21, -5
-4
T=298K
-3
GATE
-2
V,=50mV
-1
VOLTAGE
0 V,
1
2
(V)
Fig. 5. Noise power spectral density of the drain current vs gate voltage.
in the linear regime and at room temperature.
regime, and saturates once the effective channel thickness reaches a sufficiently large value. Above this “threshold”, the apparent variation of S,d remains in the range of experimental error. Such a behaviour cannot correspond to a pure bulk noise process. Furthermore, the “white noise” part of the spectra does not correspond to a CR process with a very short time constant, because S,d/l, depends linearly on l/V,, which is characteristic of thermal noise. In the case of GR noise, S,d/I, should vary as
VdPl. For l#‘noise to be explained, one has generally to look for a distribution of time constants, the summation of the associated noise processes leading to l/f spectra. In the case of junction-based devices, such a distribution may arise from the depletion region of the junction[6]. However, in this case, there would have been no reason for observing a saturation of S,d with VP. As a consequence, such a noise mechanism must also be ruled out to explain the experimental data. l/f-‘!’ noise has already been observed in GaAs MESFETs and thin-film resistors[3]. Pouysegur ef aI. have demonstrated that this noise behaviour was mostly dependent on surface passivation[3]. In addition, they have proposed an analytical model consistent with their experiment (from now it will be referred to as the PGC model). Indeed, surface thermal noise is based on the existence of lumped thermal noise generators distributed along the semiconductor-air or semiconductor-dielectric passivation interface. Interface defects are responsible for a surface leakage current, the fluctuations of which induce in turn fluctuations in the depletion layer created at the interface. The PGC model represents the surface as a lumped RC circuit. By summing the contribution of all the thermal noise generators introduced in the surface transmission line, they relate the noise of a thin GaAs film of thickness a to the surface noise by the following relationship[3]: S,,(w)
I;
= 4kTR 2Vp,,S(l
- XwVdqwi,
]?
sh[JmL]
- sin[di%??%]
R and C are the surface leakage resistance and capacitance per unit length, respectively. 1’ is the transmission line constant, equal to $ZZ. X is equal to a/b, where b is the effective thickness of the conductive region (see Fig. I). VP0 is the pinch-off voltage (qN,a*/2t), Id is the drain current and S the film section, equal to W’a. q, n, and pO are the elementary charge, the effective carrier concentration and the bulk mobility, respectively. Two asymptotic regimes may be evidenced from the above formula. There is a frequency-independent region at very low frequencies, and for higher frequencies, expression (I) can be simplified to obtain: S,,(w)
1;
= 4kTR 2Vp,S(]
- X)XV,qwo
(2) A l/f’j2 dependence is thus theoretically derived. The expression of the corner frequency between the two regimes is given in [3]. The only difference which has to be taken into account in our case is that a is now a variable, depending on the gate voltage. Straightforward calculation allows us to adapt the above formula to the Sic JFETs and to express S,, as a function of Id, Vd and fixed quantities: 2
I
(3)
Expression (3) has been obtained by considering that the average thickness tl of the depletion layer induced by the surface charge remains constant with gate voltage. b can also be expressed as Lld/WqpoN,, Vd. In contrast with the simpler structure accounted for by the original PGC model, two asymptotic regimes may now be evidenced. Near the pinch-off voltage VP, the neutral part of the film becomes comparable to 51,and (2) reduces to: (4)
From (4), it can be deduced that S,, should vary as 1: very close to VP. But when the channel thickness becomes much larger than sl. (3) now reduces to: S,d=4kTR
t
w/l,
___ [ ?L?
Vd
1 2
___fi (RCw)“’
(5)
Expression (5) indicates that for large values of Id (i.e. when increasing V,,), the noise intensity saturates. Such a behaviour can easily be explained in a
974
P. Flatresse and T. Ouisse
qualitative manner. The locally fluctuating depletion layer near the surface cannot exceed a given value, of order z(, depending on the amount of defects and/or charges at the surface. Everything otherwise fixed, when the gate voltage is such that the effective channel thickness is much larger than a. the fluctuations of the upper depletion layer only affect a limited and constant portion of the conducting channel, thus leading to a constant noise level. This noise is evidently independent of the current which flows out of the area accessible to the fluctuations. It is only when the depletion layer controlled by the buried gate reduces the total channel thickness to a fraction of space entirely subject to the upper depletion layer fluctuations that S,d varies with the total drain current. The predictions of the PGC model are therefore not only consistent with the shape of the experimental spectra, but also agree with the variation of S,d with VP. Although the experimental S,d does not vary as 1; but rather as I, near the pinch-off, this may simply be due to the fact that what is actually observed is the transition between the constant regime and the regime depicted by expression (4). It is worth noticing that the experimental results were qualitatively similar for all the devices under investigation. The main LF noise source can therefore be attributed to surface-induced fluctuations, and not to CR bulk noise. This conclusion highlights the importance of developing good insulating and passivating layers on Sic. Although the quality of actual thermal and deposited oxides remain lower than that of Si-Si02 structures, it is worth noticing that much work is devoted to thjs particular aspect of Sic technology, and large progress have been made during these last years[7,8]. It should therefore be possible to substantially reduce the noise level of SIC JFETs. since the main noise source is not an intrinsic feature of Sic material, as it would have been in the case of CR noise due to donor impurity levels. Besides, the absence of noticeable Lorentzian spectra indicates that device noise is not controlled by dislocations or deep levels, and thus indicates the high crystalline quality of the SIC material. As regards to CR noise, the situation is different at low temperature. and is addressed in the next part of this paper. As a final comment, it is worth noticing that in the linear regime and for low current levels, large transients (above IO mn) were observed before that a permanent regime was obtained (typical results are given in Fig. 6). Such transients could be due to the very large constant affecting the response of the surface layer, if represented by a transmission line. With a larger drain current, such transients were no longer observable. Suppose that carrier transport at the surface or in the passivated layer is insured by thermally-activated hopping: it will induce large transients before reaching a permanent regime. It will also vanish at low temperature, as experimentally observed and stated in Section 4.
15
(6H) Sic JFET T = 298 K VD=5OmV
12 9
0
200
400 600 Time (s)
800
1000
Fig. 6. Experimental current transients observed in the linear regime and near the pinch-off voltage, at room temperature. 4. NOISE SOURCES AT T = 77 K
Although contact resistance strongly increased and effective carrier concentration was considerably reduced at liquid nitrogen temperature, the devices still operated at T = 77 K (Fig. 7). Since LF CR noise due to donor levels should increase at low temperature, it was therefore interesting to investigate the possible presence of CR noise (such a noise source may become prominent around 100 K in the case of Si JFETs[9]). In contrast with the room temperature measurements, and although l/f i’ spectra are also obtained at low frequency (Fig. 8), S,d does not saturate with gate voltage when the device is biased far from the pinch-off voltage (see Fig. 9). This seems to indicate that the surface noise becomes negligible at low temperature, and agrees with the expected temperature behaviour of hopping transport through surface defects. With a fixed drain voltage and in the linear regime, S,d depends linearly on the drain current, just as for a CR bulk process induced by a donor level, for which the noise intensity is expressed as [5]: s 4
_4qy L2
v
“?
d d (1 + W2T2)
where T is the time constant associated with the donor level, tending to l/u,+,no when no is much lower than
0
2
4 8 6 Drain voltage VD (V)
10
Fig. 7. I,,( V,) characteristics of a Sic JFET at T = 77 K.
Noise analysis
of silicon carbide
97s
primarily limited by thermal noise, but rather by shot noise. Such a limitation is not surprising for SIC at low temperature: since the dynamic resistance of the channel becomes dramatically high, the level of the associated thermal noise becomes lower than that of the contacts.
(6H) SIC JFET T=77K
1O-2’
JFETs
5. CONCLUSION
Frequency (Hz) Fig. 8. Typical
1
1 0 -2’
noise spectra at T = 77 K and in the ohmic regime.
f
(6H)
SIC
T=77K
-8
0
V,=O.5V
10 DRAIN
I
JFET
-’ CGRRENT
10
-6 ID
IO -5 (A)
Fig. 9. Variation of the noise power spectral density of the drain current with drain current, at T = 77 K and for various frequencies.
Nd. v is equal to (N,, - n,,)/(2N, - no), At T = 77 K and in the 10-100 Hz frequency range, the difference between the white noise level and the varying part of the spectra was much smaller than at room temperature, and can account for the apparent variation of y with VB(it is worth noticing that in contrast with the room temperature measurements, y is closer to 2 when the device is biased far from VP). Since the upper limit of the exponent is equal to 2, it is therefore compatible with a GR noise associated with one energy level. The corner frequency f; of the Lorentzian spectrum was too low to be experimentally observable. This is not in contradiction with usual SiC data[2]: at 77 K, n, is probably in the 10’3cm-’ range, and for capture cross-sections of 10m20cm2, the corner frequency would be in the order of I Hz. Furthermore, the influence of the deeper donor level could also prevent the experimental determination off,. As can be seen in Fig. 9, S,d is equal to 2qld in the white noise part of the spectra. Besides, repeating the measurements of Fig. 9 for different values of Vdstill gives the same relationship between S,, and Id, therefore eliminating the possibility of GR noise as the main source of white noise [see expression (6)]. This indicates that the high frequency domain is not
Low frequency noise measurements performed on silicon carbide JFETs provide the ability to assess either the Sic crystalline quality or the specific noise sources which may prevail in such devices. No GR noise process was evidenced at room temperature, therefore indicating the high crystallinity of the epitaxial layer. At T = 300 K. l//‘3’ spectra were obtained and attributed to surface noise induced by the passivation layer, as in thin-film GaAs devices. The noise origin is further confirmed by the dependence of S,, on the gate and drain voltage. Thermal noise is observed for highest frequencies. Improvement of surface treatment should thus allow a substantial progress as regards to the low frequency noise. Although different from previously published data, these results do not contradict the interpretation given in Ref. [I], where the high temperature noise picture was dominated by a superposition of Lorentzian spectra. The origin of such spectra being attributed to multi-phonon capture processes associated with deep levels, their influence may prevail only above room temperature. At T = 77 K, GR noise probably associated to the donor levels and shot noise dominate at low and high frequencies. respectively. of this work was financially supported by GEPRA. The authors wish to acknowledge Drs G. Lenz (Merlin-G&in), J. Lasseur (Schlumberger) and C. Jaussaud (LETI-CEA) for having provided the samples. Many thanks are also due to Professors J. Brini and A. Chovet (LPCS) for hints and discussions. Dr J. Palmour (CREE Research Inc.) is gratefully acknowledged for having supplied technical information and data. Acknowledgemenrs-Part
REFERENCES
I. M. E. Levinshtein, S. L. Rumyantsev and J. W. Palmour, Tech. Phps. Le/r. 19, 513 (1993). 2. G. Pens1 and W. J..Choyke, f%):~ica B 185,264 (1993). 3. M. PouvsPeur. J. Graffeuil and J. L. Cazaux. lEEE Trans. ilecyron Derices ED-34, 2178 (1987). 4. G. Kelner, S. Binari, M. Shur and J. Palmour, Electron. Left. 27, 1038 (1991). 5. A. van der Ziel, Noise in Solid Slate Deuice.7 and Circuits. Wiley, New York (1986). 6. M. Chertouk, A. Chovet and A. Clei, in Noise in Physical Sysrems und l/f Flucluations, AIP Conf. Proc. N 285 (edited by P. H. Handel and A. L. Chung). p. 427. AIP, New York (1993). 7. T. Ouisse, N. Bkourt, C. Jaussaud and F. Ternplier. J. up@. Php. 75, 604 (1994). 8. N. Singh and A. Rys. J. uppl. Phyr. 73, 1279 ( 1993). 9. A. van der Ziel, Proc. IEEE 58, I I78 (1970).
Pergamon
NOISE
0038-l
ANALYSIS
101(94)00205-3
OF SILICON
P. FLATRESSE
Vol. 38, No. 5, pp. 971-975, 1995 Copyright 6 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I lOI/ $9.50 + 0.00
Electronics
CARBIDE
JFETs
and T. OUISSE
Laboratoire de Physique des Composants B Semiconducteurs (URA CNRS 840), ENSERG, 23, rue des Martyrs, 38016, Grenoble Cedex, France (Received 4 Ju1.v 1994; in revised form
20 September
1994)
Abstract-The low frequency noise of silicon carbide junction field effect transistors has been systematically studied at room temperature and at T = 77 K. At room temperature, I/f”’ spectra are attributed to surface thermal noise induced by lumped thermal noise generators distributed along the surface, in a way similar to that of GaAs thin-film resistors. At 77 K, generation-recombination noise is observed, and shot noise originates from drain and source contacts.
I. INTRODUCTION
Silicon carbide (SIC) devices are now produced in a large number of companies and laboratories. Since the material quality has greatly improved during recent years, it is now possible to investigate many physical properties of these devices, without being limited by an excessive amount of crystalline or process-related defects. Technology has reached a stage where noise measurements can be useful not only for improving device performance but also for obtaining substantial information on the physical properties of SiC material. Besides, JFETs are known to usually exhibit a good immunity to low frequency (LF) noise, which makes them suitable for analog applications. However, this aspect has received only limited attention to date in the case of SIC JFETs[ 11. Although the various polytypes of Sic exhibit a rather large dielectric constant (- 10). which contrasts with their wide bandgap, the impurity-induced levels remain somewhat deeper than in narrower bandgap semiconductors. In the case of the (6H) polytype, three different sites may induce three different levels for donor-type impurities like nitrogen, which is commonly used for n-type doping (two of them being very close to one another)[2]. In connection with low frequency noise, it was therefore interesting to assess the possible contribution of these levels to room temperature noise. Surprisingly, the main low-frequency noise source does not correspond to generation-recombination (GR) noise, but is shown to be induced by the passivated surface of the device. GR noise is observed only at nitrogen temperature, when the ratio between the free carriers and the electrons bound to the impurities becomes sufficiently small. At T = 300 K, the behaviour of the LF noise power spectral intensity S,d agrees with a model already proposed by other authors for GaAs devices[3]. In our particular case, the dependence of the noise level on the thickness of the buried depletion
layer provides the ability to further confirm the surface origin of the noise source. The model of PouysCgur et a/.[31 is adapted to the Sic JFETs so as to demonstrate that it can account for all the qualitative observed noise features. Improvement of surface passivation should therefore provide the ability to reduce the LF noise intensity of SIC JFETs to an appreciable extent. The paper is divided into three parts. Section 2 summarizes the characteristics of the devices under investigation as well as the experimental procedure. In Section 3 the experimental results obtained at room temperature are given. Described in Section 4 are the low temperature results.
2. DEVICE
CHARACTERISTICS
The (6H) SIC JFETs investigated in this paper were commercially available devices originating from CREE Research Inc. Five different JFETs have been systematically tested, all of them having the same geometry. A schematic of these devices is given in Fig. 1 and further information on similar devices may be found in Refs [2,4]. A buried p + gate controls the depletion layer width of an n-type epitaxial upperlayer. A passivated surface is thus bordering the active part of the channel, consisting in an oxide layer of 20 nm. The channel width, length L and thickness are respectively equal to 1 mm, 5 pm and -0.25 pm. The doping level of the n-layer has been found to be very homogeneous, from capacitance measurements performed at 300 K on the gate junction. Nd - N, was in the 5 x 10’b-10’7cm-3 range for all devices. The LF noise measurements (100 mHz-IO kHz) were performed using an ONO-SOKKI spectrum analyser loaded by a low noise NF 5305 voltage amplifier and an EG&G model 181 current-voltage converter. The number of samples used to calculate the spectra, as well as the cut-off frequency of the
971
P. Flatresse
912
and T. Ouisse
AvG Fig.
I. Schematic
of the SIC devices
investigated
1 T=298K
V,=5OmV
3
2
GATE
in this
paper. high-pass filter used during the systematic measurements were such that the values of power spectral density presented in this paper are reliable only for f > 1 Hz. Nevertheless, measurements with a larger number of samples and without d.c. filtering were performed for a limited number of biasing, and showed that no corner frequency was observable in the low frequency range, at least for f > 0.2 Hz. At room temperature, the devices were biased during 15 mn before the acquisition of the noise spectra, so as to avoid any influence of the transients discussed in Section 3 of this paper.
1
3
VOLTAGE
4
5
(‘/,-VP)
6
(V)
Fig. 3. Apparent variation of the exponent corresponding to low frequency noise spectra with gate voltage; T = 298 and 71 K.
voltage (Fig. 3). Since the white noise level increases with VPand is not much lower than the observed LF noise, it is not clear whether the experimental variation of y has to be attributed to the transition between l/f 3i2noise and the white thermal noise, or to additional LF sources which would appear as VP increases. Moreover, it is worth noticing that the highest gate voltages were close to the built-in potential of the junction (around 2.5 V), so that leakage through the gate might affect the noise characteristics. Since S,d/Z, was varying as l/V,, in the highest frequency range (1 kHz-10 kHz) and always re3. ROOM TEMPERATURE ANALYSIS mained in the order of 4kT/V,, the white noise part of the spectra was identified as thermal noise, and will Typical noise spectra, varying as l/f :, are presented not be given any further attention in this paper. in Fig. 2. They correspond to the ohmic regime of the To explain the existence of an exponent larger than device. As can be seen in Fig. 2, the shape of the 1 in the LF part of the spectra, one could invoke GR different spectra does not exhibit a pure l/f behaviour, but rather a l/f 3,‘2dependence. Such a noise. However, if the observed LF noise was induced by generation-recombination processes in the bulk of dependence is clearly visible in Fig. 2 in the lowest the JFET, the noise power spectral density of the frequency range, for f > 1Hz. However, one can also notice that for frequencies above 50 Hz, the l/f 3’2 drain current S,d should vary as I,. With a fixed drain voltage V,, S,d should also increase with VP. Neither dependence is progressively reduced to an exponent the experimental variation of S,d with Id (Fig. 4), nor value lower than 3/2. In fact, in the 20-200 Hz range, the dependence of S,d on P’, (Fig. 5) agree with the the empirical values found for the exponent y continuously vary from the pinch-off regime, where y is theoretical predictions of GR noise. As can be seen in Figs 4 and 5, S,d increases with Id near the pinch-off nearly equal to 1.5, to 0.5 at the highest values of gate 10-‘8t
-18
(6H) Sic JFET
1 (6H)
SIC
JFET
10-19
f=3Hz 09
10-20 00
10-21
I:;
1o-22
F
f=lOHz
i
I
Fig. 2. Typical
10 Frequency (Hz)
100
noise spectra at 7’ = 298 K and in the ohmic regime.
-21
-
V,=SOmV
T=298K
P
1o-23
J
,,,.
lo-&
10
DRAIN
-5
CURRENT
10
-4
10
-’
ID (A)
Fig. 4. Noise power spectral density of the drain current vs drain current, in the linear regime and at room temperature.
Noise analysis
of silicon carbide
973
JFETs
’ [ sch(bl,2) X
f= 1 OHz
/A WZ LnLu
cc? z
d
i
10 -21, -5
-4
T=298K
-3
GATE
-2
V,=50mV
-1
VOLTAGE
0 V,
1
2
(V)
Fig. 5. Noise power spectral density of the drain current vs gate voltage.
in the linear regime and at room temperature.
regime, and saturates once the effective channel thickness reaches a sufficiently large value. Above this “threshold”, the apparent variation of S,d remains in the range of experimental error. Such a behaviour cannot correspond to a pure bulk noise process. Furthermore, the “white noise” part of the spectra does not correspond to a CR process with a very short time constant, because S,d/l, depends linearly on l/V,, which is characteristic of thermal noise. In the case of GR noise, S,d/I, should vary as
VdPl. For l#‘noise to be explained, one has generally to look for a distribution of time constants, the summation of the associated noise processes leading to l/f spectra. In the case of junction-based devices, such a distribution may arise from the depletion region of the junction[6]. However, in this case, there would have been no reason for observing a saturation of S,d with VP. As a consequence, such a noise mechanism must also be ruled out to explain the experimental data. l/f-‘!’ noise has already been observed in GaAs MESFETs and thin-film resistors[3]. Pouysegur ef aI. have demonstrated that this noise behaviour was mostly dependent on surface passivation[3]. In addition, they have proposed an analytical model consistent with their experiment (from now it will be referred to as the PGC model). Indeed, surface thermal noise is based on the existence of lumped thermal noise generators distributed along the semiconductor-air or semiconductor-dielectric passivation interface. Interface defects are responsible for a surface leakage current, the fluctuations of which induce in turn fluctuations in the depletion layer created at the interface. The PGC model represents the surface as a lumped RC circuit. By summing the contribution of all the thermal noise generators introduced in the surface transmission line, they relate the noise of a thin GaAs film of thickness a to the surface noise by the following relationship[3]: S,,(w)
I;
= 4kTR 2Vp,,S(l
- XwVdqwi,
]?
sh[JmL]
- sin[di%??%]
R and C are the surface leakage resistance and capacitance per unit length, respectively. 1’ is the transmission line constant, equal to $ZZ. X is equal to a/b, where b is the effective thickness of the conductive region (see Fig. I). VP0 is the pinch-off voltage (qN,a*/2t), Id is the drain current and S the film section, equal to W’a. q, n, and pO are the elementary charge, the effective carrier concentration and the bulk mobility, respectively. Two asymptotic regimes may be evidenced from the above formula. There is a frequency-independent region at very low frequencies, and for higher frequencies, expression (I) can be simplified to obtain: S,,(w)
1;
= 4kTR 2Vp,S(]
- X)XV,qwo
(2) A l/f’j2 dependence is thus theoretically derived. The expression of the corner frequency between the two regimes is given in [3]. The only difference which has to be taken into account in our case is that a is now a variable, depending on the gate voltage. Straightforward calculation allows us to adapt the above formula to the Sic JFETs and to express S,, as a function of Id, Vd and fixed quantities: 2
I
(3)
Expression (3) has been obtained by considering that the average thickness tl of the depletion layer induced by the surface charge remains constant with gate voltage. b can also be expressed as Lld/WqpoN,, Vd. In contrast with the simpler structure accounted for by the original PGC model, two asymptotic regimes may now be evidenced. Near the pinch-off voltage VP, the neutral part of the film becomes comparable to 51,and (2) reduces to: (4)
From (4), it can be deduced that S,, should vary as 1: very close to VP. But when the channel thickness becomes much larger than sl. (3) now reduces to: S,d=4kTR
t
w/l,
___ [ ?L?
Vd
1 2
___fi (RCw)“’
(5)
Expression (5) indicates that for large values of Id (i.e. when increasing V,,), the noise intensity saturates. Such a behaviour can easily be explained in a
974
P. Flatresse and T. Ouisse
qualitative manner. The locally fluctuating depletion layer near the surface cannot exceed a given value, of order z(, depending on the amount of defects and/or charges at the surface. Everything otherwise fixed, when the gate voltage is such that the effective channel thickness is much larger than a. the fluctuations of the upper depletion layer only affect a limited and constant portion of the conducting channel, thus leading to a constant noise level. This noise is evidently independent of the current which flows out of the area accessible to the fluctuations. It is only when the depletion layer controlled by the buried gate reduces the total channel thickness to a fraction of space entirely subject to the upper depletion layer fluctuations that S,d varies with the total drain current. The predictions of the PGC model are therefore not only consistent with the shape of the experimental spectra, but also agree with the variation of S,d with VP. Although the experimental S,d does not vary as 1; but rather as I, near the pinch-off, this may simply be due to the fact that what is actually observed is the transition between the constant regime and the regime depicted by expression (4). It is worth noticing that the experimental results were qualitatively similar for all the devices under investigation. The main LF noise source can therefore be attributed to surface-induced fluctuations, and not to CR bulk noise. This conclusion highlights the importance of developing good insulating and passivating layers on Sic. Although the quality of actual thermal and deposited oxides remain lower than that of Si-Si02 structures, it is worth noticing that much work is devoted to thjs particular aspect of Sic technology, and large progress have been made during these last years[7,8]. It should therefore be possible to substantially reduce the noise level of SIC JFETs. since the main noise source is not an intrinsic feature of Sic material, as it would have been in the case of CR noise due to donor impurity levels. Besides, the absence of noticeable Lorentzian spectra indicates that device noise is not controlled by dislocations or deep levels, and thus indicates the high crystalline quality of the SIC material. As regards to CR noise, the situation is different at low temperature. and is addressed in the next part of this paper. As a final comment, it is worth noticing that in the linear regime and for low current levels, large transients (above IO mn) were observed before that a permanent regime was obtained (typical results are given in Fig. 6). Such transients could be due to the very large constant affecting the response of the surface layer, if represented by a transmission line. With a larger drain current, such transients were no longer observable. Suppose that carrier transport at the surface or in the passivated layer is insured by thermally-activated hopping: it will induce large transients before reaching a permanent regime. It will also vanish at low temperature, as experimentally observed and stated in Section 4.
15
(6H) Sic JFET T = 298 K VD=5OmV
12 9
0
200
400 600 Time (s)
800
1000
Fig. 6. Experimental current transients observed in the linear regime and near the pinch-off voltage, at room temperature. 4. NOISE SOURCES AT T = 77 K
Although contact resistance strongly increased and effective carrier concentration was considerably reduced at liquid nitrogen temperature, the devices still operated at T = 77 K (Fig. 7). Since LF CR noise due to donor levels should increase at low temperature, it was therefore interesting to investigate the possible presence of CR noise (such a noise source may become prominent around 100 K in the case of Si JFETs[9]). In contrast with the room temperature measurements, and although l/f i’ spectra are also obtained at low frequency (Fig. 8), S,d does not saturate with gate voltage when the device is biased far from the pinch-off voltage (see Fig. 9). This seems to indicate that the surface noise becomes negligible at low temperature, and agrees with the expected temperature behaviour of hopping transport through surface defects. With a fixed drain voltage and in the linear regime, S,d depends linearly on the drain current, just as for a CR bulk process induced by a donor level, for which the noise intensity is expressed as [5]: s 4
_4qy L2
v
“?
d d (1 + W2T2)
where T is the time constant associated with the donor level, tending to l/u,+,no when no is much lower than
0
2
4 8 6 Drain voltage VD (V)
10
Fig. 7. I,,( V,) characteristics of a Sic JFET at T = 77 K.
Noise analysis
of silicon carbide
97s
primarily limited by thermal noise, but rather by shot noise. Such a limitation is not surprising for SIC at low temperature: since the dynamic resistance of the channel becomes dramatically high, the level of the associated thermal noise becomes lower than that of the contacts.
(6H) SIC JFET T=77K
1O-2’
JFETs
5. CONCLUSION
Frequency (Hz) Fig. 8. Typical
1
1 0 -2’
noise spectra at T = 77 K and in the ohmic regime.
f
(6H)
SIC
T=77K
-8
0
V,=O.5V
10 DRAIN
I
JFET
-’ CGRRENT
10
-6 ID
IO -5 (A)
Fig. 9. Variation of the noise power spectral density of the drain current with drain current, at T = 77 K and for various frequencies.
Nd. v is equal to (N,, - n,,)/(2N, - no), At T = 77 K and in the 10-100 Hz frequency range, the difference between the white noise level and the varying part of the spectra was much smaller than at room temperature, and can account for the apparent variation of y with VB(it is worth noticing that in contrast with the room temperature measurements, y is closer to 2 when the device is biased far from VP). Since the upper limit of the exponent is equal to 2, it is therefore compatible with a GR noise associated with one energy level. The corner frequency f; of the Lorentzian spectrum was too low to be experimentally observable. This is not in contradiction with usual SiC data[2]: at 77 K, n, is probably in the 10’3cm-’ range, and for capture cross-sections of 10m20cm2, the corner frequency would be in the order of I Hz. Furthermore, the influence of the deeper donor level could also prevent the experimental determination off,. As can be seen in Fig. 9, S,d is equal to 2qld in the white noise part of the spectra. Besides, repeating the measurements of Fig. 9 for different values of Vdstill gives the same relationship between S,, and Id, therefore eliminating the possibility of GR noise as the main source of white noise [see expression (6)]. This indicates that the high frequency domain is not
Low frequency noise measurements performed on silicon carbide JFETs provide the ability to assess either the Sic crystalline quality or the specific noise sources which may prevail in such devices. No GR noise process was evidenced at room temperature, therefore indicating the high crystallinity of the epitaxial layer. At T = 300 K. l//‘3’ spectra were obtained and attributed to surface noise induced by the passivation layer, as in thin-film GaAs devices. The noise origin is further confirmed by the dependence of S,, on the gate and drain voltage. Thermal noise is observed for highest frequencies. Improvement of surface treatment should thus allow a substantial progress as regards to the low frequency noise. Although different from previously published data, these results do not contradict the interpretation given in Ref. [I], where the high temperature noise picture was dominated by a superposition of Lorentzian spectra. The origin of such spectra being attributed to multi-phonon capture processes associated with deep levels, their influence may prevail only above room temperature. At T = 77 K, GR noise probably associated to the donor levels and shot noise dominate at low and high frequencies. respectively. of this work was financially supported by GEPRA. The authors wish to acknowledge Drs G. Lenz (Merlin-G&in), J. Lasseur (Schlumberger) and C. Jaussaud (LETI-CEA) for having provided the samples. Many thanks are also due to Professors J. Brini and A. Chovet (LPCS) for hints and discussions. Dr J. Palmour (CREE Research Inc.) is gratefully acknowledged for having supplied technical information and data. Acknowledgemenrs-Part
REFERENCES
I. M. E. Levinshtein, S. L. Rumyantsev and J. W. Palmour, Tech. Phps. Le/r. 19, 513 (1993). 2. G. Pens1 and W. J..Choyke, f%):~ica B 185,264 (1993). 3. M. PouvsPeur. J. Graffeuil and J. L. Cazaux. lEEE Trans. ilecyron Derices ED-34, 2178 (1987). 4. G. Kelner, S. Binari, M. Shur and J. Palmour, Electron. Left. 27, 1038 (1991). 5. A. van der Ziel, Noise in Solid Slate Deuice.7 and Circuits. Wiley, New York (1986). 6. M. Chertouk, A. Chovet and A. Clei, in Noise in Physical Sysrems und l/f Flucluations, AIP Conf. Proc. N 285 (edited by P. H. Handel and A. L. Chung). p. 427. AIP, New York (1993). 7. T. Ouisse, N. Bkourt, C. Jaussaud and F. Ternplier. J. up@. Php. 75, 604 (1994). 8. N. Singh and A. Rys. J. uppl. Phyr. 73, 1279 ( 1993). 9. A. van der Ziel, Proc. IEEE 58, I I78 (1970).