We measured the equivalent noise charge of a-Si:H pin diodes (5 {approximately} 45{mu}m i-layer) with a pulse shaping time of 2.5 {mu}sec under reverse biases up to 30 V/{mu}m and analyzed it as a four component noise source. The frequency spectra of 1/f noise on the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 {mu}m was measured to be the dominant component up to {approximately}100kHz for both saturation and linear regions. 15 refs., 7 figs

Cho, Gyuseong

Drewery, J. S.

Fujieda, I.

Jing, T.

Kaplan, S. N.

Perez-Mendez, V.

Qureshi, S.

Street, R. A. (Xerox Palo Alto Research Center, CA (USA))

Wildermuth, D. (Lawrence Berkeley Lab., CA (USA))

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DISCLAIMER LBL—28836 This report was prepared as an account of work sponsored by an agency of the United States D E 9 0 fll 1 *J 1 Q Government. Neither the United States Government nor any agency thereof, nor any of their VJ.XZXH employees, makes any warranty, express or implied, or assumes any legal liability or responsi­bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer­ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, rccom* mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or rtflect those of the United States Government or any agency thereof. MEASUREMENTS OF 1/f NOISE IN A-Si:H PIN DIODES AND THIN-FILM-TRANSISTORS. Gyuseong Cho, J.S. Drewery, I. Fujieda, T. Jing S.N. Kaplan, V. Perez-Mendez, S. Qureshi, D. Wildermuth, Physics Division Lawrence Berkeley Laboratory University of California, Berkeley, CA 94720 and R.A. Street Xerox PARC, Palo Alto, CA 94304 May 1990 n „ T h i s * ° r k u " a s supported by the Director. Office of Energy Research f f n i J - H ' e h r E»"ft a " d N u C , M r P h y s i c s ' D i v i s i ° " ° ' High^Energy Physics and Office of Health and Environmental Research, Division^ Physics Z Technology! Research of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 MASTER MEASUREMENTS OF 1/f NOISE IN A-Si:H PIN DIODES AND THIN-FIIM-TRANSISTORS. GiUSEONG CHO*, J.S. DREWERY*, I. FUJIEDA*, T. JING*, S.N. KAPLAN* V. PEREZ-MENDEZ*, S. QURESHI*, D. WILDERMUTH*, and R.A. STREET** * Lawrence Berkeley Laboratory, Berkeley, CA 94720 ** Xerox PARC, Palo Alto, CA 94304 ABSTRACT We measured the equivalent noise charge of a-Si:H pin diodes (5 - 45 |im i-layer) with a pulse shaping time of 2.5 Jlsec under reverse biases up to 30 V/|Un and analyzed it as a four component noise source. The frequency spectra of 1/f noise in the soft-breakdown region and of the Nyquist noise from contact resistance of diodes were measured. Using the conversion equations for a CR-RC shaper, we identified the contact resistance noise and the 1/f noise as the main noise sources in the low bias and high bias regions respectively. The 1/f noise of a-Si:H TFTs with channel length of 15 Hm was measured to be the dominant component up to -100kHz for both saturation and linear regions. I INTRODUCTION In previous papers[1-5], we and other groups have covered various applications of reversed biased pin a-Si:H diodes to charged particles, x-ray and Y~ray detection. For charged particle detection in particular, we showed that pin diodes with thin p and n blocking layers and thick (5 - 45 Hm) i regions could serve as efficient position sensitive detectors. Since the collected signal for minimum ionizing particles is -80 electron-hole pairs/|im of the i-layer [1], it is obviously important to design the diodes and associated TFT amplifiers with a minimum amount of electronic noise generation. For thin a-Si diodes, 1/f type noise measurements and theory have been published.[6-8] In this paper we discuss measurements of various noise sources originating from thick reverse biased pin diodes and from the thin-film-transistors (TFTs) that we propose to use for readout of pixel or strip detector configurations. II NOISE AND REVERSE CURRENT IN A-Si:H PIN DIODES II(a) Experimental Sample a-Si:H pin diodes were fabricated by the standard PECVD method at Xerox PARC (Palo Alto, CA) and at Glasstech Solar Inc. (Wheatridge, Co). They consist of five deposited layers or glass substrates: the bottom layer is a metallic electrode formed by Cr or tin oxide; the next three layers are n'-type (-50 nm), intrinsic (5 - 45 flm), and p*-type (-50 nm) a-Si:H layers made by PECVD. The last layer is a top electrode made by evaporation of Cr, Pd, or Al, and it is etched to form circular contacts of radius between 1.5-3 mm. Sample diodes have equivalent capacitances 10 - 250 pF determined by the area of the top electrode and the i-layer thickness. Since the diodes are reverse biased when used as radiation detectors, the dark current and noise characteristics are investigated under this condition. Diode noise was measured by the two configurations shown in Fig. 1. The first method Fig. l-(a) measures the equivalent input noise charge in the time domain. The measurement time is set by a pulse shaping amplifier, usually a CR-RC circuit, which performs a differentiation and integration for shaping the signal pulse. The second method Fig. l-(b) is a 1 set-up to determine the frequency spectrum of the output voltage noise <v2o> following a charge sensitive amplifier. Noise data were recorded on a digital oscilloscope and their Fourier transforms were calculated in a PC. Aliasing due to the digital sampling is removed by a Butterworth filter for each measurement frequency range set by the scope. The output voltage noise is related to the input equivalent current noise by the equation, A2, X <i- r> K' X <iin> (cOC,.,)2 "" (2Kf) 2 "" ( 1 ) where CO is 2;uf, Cin is the dynamic input capacitance of the preamplifier, Av, is the voltage gain of the wide band amplifier and K is the charge-to-voltage conversion gain of the measurement system (~3xl012 V/Coul). HV (a) Time domain noise measurement Main Pulse Pulse Amplifier Shaping Height Amplifier Analyzer (b) Frequency domain noise measurement K ^ @?) Fast Fourier Transform Detector Charge wide- Butter- Digital I Sensitive bandwidth worth Oscillo-V Amplifier Amplifier Prefilter scope PC Fig. 1. Schematic of noise ireasurerrent set-up II(b) Results and Discussion When a-Si:H pin diodes are used as radiation detectors, full depletion is required in order to ensure full charge collection. Usually the operating bias level is set below the point where the pre-breakdown starts shown by a rapid increase in noise. Most of the diodes we have tested show similar I-V and noise characteristics; therefore the discussion here will be confined to measurements on a 26 |im thick pin diode having 3 mm top electrode radius and an equivalent capacitance of 125 pF. Fig. 2 shews the reverse current density (o) and the noise charge (•) measured with a 2.5 Usee shaping time. The measured noise is flat up to a bias of 15 V/pm and then starts to increase rapidly. The increasing noise has been attributed to some reversible soft-breakdown [9] at the high electric field m-p-i junction area. Our present measurements show that the noise frequency spectra in this bias region are 1/f and are proportional to the reverse current density. This result is similar to an earlier report on 1/f noise from a-Si:H Schottky diodes.[7] The measured noise data (•) fits very well to the calculated total noise (solid line) from 0 to 30 V/|lm bias by the equation ®Lz = NlyS + Nls + N L O I + Nf / f (2) where the terms in the right hand side are separately measured or calcula-9 ted from their spectra by equations (3), (5) and (7).[10] (a) system noise N 2sy 3 : It originates mainly from the charge sensitive preamplifier (Smtek A225) and is measured to be 1300 electrons rms by connecting an 125 pF capacitor in place of the diode, in parallel with a load resistor 10 Mil, whose contribution to the total noise is negligible. (b) contact resistance noise N'^: The diode has a finite contact resistance R; of ~500 £2 in series with its equivalent capacitance as illustrated in Fig. 1, which contributes 1500 electrons rms to the system noise. We confirmed that R, is originated from the evaporated Pd top contact. Simulation by a 500 ii resistor in series with an 125 pF capacitor gives the same noise charge as the unbiased sample diode and also agrees well with the estimated noise value by the following equation for a simple CR-RC shaping amplifier,[10] N** = (fP x [ ^ § ^ c ° Is!2 d f = (f) 2 x 4kTR* x — (3) Jo where e is 2.718, q is the electronic charge, X is the shaping time (2.5 jisec), <v 2 H s> is the noise voltage source due to the resistor Rs, and |G| is the transfer function of CR-RC shaper. [10] Iff 5 . 1 • i 1 • tn IQ-10 | L_i J_! I ! [_ 0 5 10 15 20 25 Bias (V/pm) id 1 IO3 io* lo5 io* Frequency (Hz) 3 O Reverse Current • Measured Noise Total Ncise 1/f Noise Shot Noise • • - - Rs Noise System Noise Fig. 2. Reverse current and Noise measurement of a reverse biased 26 pm thick pin diode. Measured noise data (•) is fitted well by the sum (solid line) of four noise components; (a) system noise, (b) contact resistance noise, (c) shot noise of diode, and (d) 1/f noise of diode. Fig. 3. Noise spectra of a reverse biased 26 |Jm thick pin diode. 1/f3 dependency in output noise voltage spectra is equivalent to 1/f when converted into a parallel current noise source in the diode by the equation (1). Nyquist noise from the contact resistance equivalent to 500 £2 is shown as a white spectrum by an arrow in the figure. IO5 I O 4 ^ 103 400 |GP . ("^ (4, (i + (cot)2)2 Fig. 3 shows the excess Nyquist noise component at high frequency due to this series resistance B^. (c) Shot noise N^hot1 Shot noise of the diode in the time domain is calculated from the measured reverse current I r by the equation. IS* x lilted ! i d f = (B» x 2ql r XV ) 0 Af ©2 W (5) (d) 1/f noise N2i/£: The measured output noise voltage spectra <v 2 0> under various bias conditions are plotted in Fig. 3 in which 1/f3 is found to be dominant at low frequency. Equation (1) show that 1/f3 dependence of output noise voltage is equivalent to a 1/f spectrum of input current noise <i2i„>. The measured magnitude of the 1/f3 noise power is proportional to the square of the reverse current so the equivalent input current noise of the 1/f component can be empirically represented by <iin> = K d x -E- Af (6) where Kj is a constant (~3. 6xl0~a) and the measured Ka value decreases from 8xl0~7 to 8xl0~8 almost linearly as the inverse of the square of the i-layer thickness from 10 pm to 45 (im. K^ is equivalent to A/N in Hooge's empirical relation for 1/f noise where A is Hooge's constant [11] t-nd N is the total number of charge carriers in the device. The 1/f noise in the time domain is calculated from r<i!zi> K d f . (e)2 J0 Af w? W (ff X I ^ ^ i 'ii. df = (S-F X K dI? X 6 X2 (7) where the lower frequency limit of the integration was assumed to be about 100 Hz which is equivalent to the trapping level at -0.6 eV below conduction band. The above equation shows that the 1/f noise contribution can be reduced by decreasing the reverse current, or by reducing the surface field, or by using a smaller shaping time T. Since the shot noise is linearly proportional to the current and 1/f noise is proportional to the square of the reverse current, the shot noise is dominant at the lower bias and 1/f noise is dominant at higher bias. At higher bias the diodes start to breakdown irreversibly and are permanently damaged, t-iost of diodes with i-layer thickness range of 5 - 45 Jim show similar characteristics to the one discussed here. However some poorer quality diodes have lower onset bias for sharp breakdown behavior. Ill 1/f NOISE IN A-Si:i: THIN-FIIM-TRANSISTORS III(a) Experimental Sample a-Si:H TFTs were fabricated at Xerox PARC. They are of the staggered inverted type which has a structure shown in Fig. 4. The gate insulator is SijN5 of thickness 0.3 pm and the channel length is 15 pm. The channel width ranges from 16 pm to 256 pm. I-V characteristics and noise spectra were measured in a shielded probe station. A HP 4145 semiconductor parameter analyzer was used to measure the dc I-V curves and transfer characteristics. A HP 3561A spectrum analyzer with a low noise current anplifier was used to measure the spectra 4 of the noise current at each bias condition. Then the equivalent input voltage noise <v2iJ1> was calculated by the following relation. < v2 n> = < i k (8) the where <i2a> is the nveasured drain current fluctuation and g m transconductance at the bias point. IIKb) Results and discussion The dc drain current and its fluctuation change linearly with the channel width of the sample TFTs so the discussion here is given for a TFT with channel width of 128 Jim. Fig. 5 shows I-V curves for the sample TFT. The measured field effect nobility in the saturation region has a range of 0.4 ~ 0.6 crrrVVsec, which corresponds to a range of band mobility of 13 ~ 18 cmVVsec as calculated from the Xerox TFT model [12,13] used in the PSPICE program. 1/f type noise is found to be dominant in the bias and frequency range of interest. The equivalent input voltage noise in the saturation Metal Nitride a-Si:H P773 Glass ! V=25V 20V 15V ^ ~ i i i i_— i i sv 10 IS 20 V,(V) Fig. 4. Schematic diagram of a-Si:H thin-f i im-transis tors made a t Xerox Fig. 5. I-V curves of a-Si:H TFT with L = 15 pm and W = 128 pm nj—TTT t Vg=20V • Vg = 15V \ « Vg = 10V^ • Vg = 5V 101 I02 IO3 IB* 10s lO* Frequency (Hz) Fig. 6. Gate bias dependence of noise spectra of a-Si:H TFT of L = 15 um and W = 128 Jim when V d = 20 V N Iff' > "* • Iff* o l , f f ' ° s I Iff" I Iff12 I lff° 101 ID2 V? 10* 10s 10* Frequency (Hz) Fig. 7. Drain bias dependence of noise spectra of a-Si:H TFT of L = 15 um and w = 128 um when V g = 15 V 1 """"1 """"1 ' -""I ' """» •• ^ itasmajl j • Vd= 5V "--".TK ! , Vd-15V " ' - - j -oHv i • vd'=2ov : •- -. « "V r • «;-»„>.JN. 5. _ . : = ' • • • " - . ' » i , n.'i ' i ' 5 region as shown in Fig. 6 is generally expressed[14] by the equation <v?„> = K t X -d- Af (9) where I d is dc drain-source current. Measured a and R increase slightly from 0.97 to 1.1 and from lxlO-6 to 3xl0~6 respectively as the gate bias increases from 5 V to 20 V. Measured P changes from 0.41 to 0.85 when the frequency changes from 1 Hz to -100 kHz. The range of P agrees with the empirical Si-JFET model.[14] The Nyquist (thermal) noise due to the finite channel resistance of the TFT was calculated by the following equation in the satiiration region, < v » — > = i ^ f g : A f ( 1 0 ) Fig. 7 shows a steady decrease of the equivalent input voltage noise as drain bias increases from 5 V to 20 V when V g = 15 V. Similar effects have been observed in crystal-Si MOSFET.[15] IV CONCLUSION In reverse biased pin diodes, series contact resistance noise is dominant at lew bias and 1/f type noise is dominant in the soft-breakdown region. In order to minimize noise, reduction of contact resistance as well as reverse current is important. For a-Si:H TFTs, 1/f type noise is found to be dominant over the whole operating frequency range. Whether the 1/f type noise is an interface effect or a bulk effect in both a-Si:H pin diode and TFTs is still unclear and requires further investigation. ACKNOWLEDGEMENTS We would like to thank to S. Nelson at Xerox PARC for making sample TFTs and J. Xi and R. E. Hollingsworth at GSI for making sample a-Si:H pin diodes. Also we specially thank to M. Hack at Xerox PARC for valuable discussion on TFTs. This work was supported by the Director, Office of High Energy and Nuclear Physics, and Office of Health and Environmental Research of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. REFERENCES [I] V. Perez-Mendez, et al., Nucl. Instr. and Meth., A2£Q_, 195 (1987) [2] V. Perez-Mendez, et al., Nucl. Instr. and Meth., A273, 127 (1988) [3] V. Perez-Mendez, et al., MRS. Syrap. Proc. 149. 621 (1989) [4] B. Equer and A. Karar, Nucl. Instr. and Meth., A271. 574 (1988) [5] T. Pochet, et al., MRS. Symp. Pioc. 149. 661 (1989) [6] F.Z. Bathaei and J.C. Anderson, Phil. Mag., B S , 87 (1987) [7] F.Z. Bathaei, et al., Phil. Mag., B £7, 259 (1988) [8] A.D'Amico, et al., J. Nbn-Cryst. Sol., 77 & 78. 499 (1985) [9] R.A. Gibson, et al., Appl. Phys., ZL, 307 (1980) [10] F.S. Goulding and D.A. Landis, TFFR Trans. Nuc. Sci., 22, 1125 (1982) [II] F.N. Hboge, Rep. Prog. Phys.. 44, 479 (1981) [12] M. Hac;.-, et al., IEEE Trans. Elect. Dev., M, 2764 (1989) [13] M. Shur, et al., J. Appl. Phys., £&, 3371 (1989) [14] P.R. Gray and R.G. Meyer, Analysis and Design of Analog ICs John Wiley and Sons, (2nd Ed.), pp.647. New York (1984) [15] N. Kutoh, et al.. Solid State Electronics, 3 X 1675 (1988) 6 

Measurements of 1/f noise in A-Si:H pin diodes and thin-film-transistors

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Measurements of 1/f noise in A-Si:H pin diodes and thin-film-transistors

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