Operating conventional electronic devices in the noninstantaneous multiplication(amplification) regimeR. P. Jindal Citation: Journal of Applied Physics 68, 324 (1990); doi: 10.1063/1.347136 View online: http://dx.doi.org/10.1063/1.347136 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/68/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Direct simulation of proton-coupled electron transfer across multiple regimes J. Chem. Phys. 138, 134109 (2013); 10.1063/1.4797462 Tomography in the multiple scattering regime of the scanning transmission electron microscope Appl. Phys. Lett. 82, 3943 (2003); 10.1063/1.1579116 Operating Regime in Low Energy Plasma Focus Devices AIP Conf. Proc. 299, 281 (1994); 10.1063/1.2949165 The Operating Principles of the Smallest Electronic Devices AIP Conf. Proc. 241, 346 (1991); 10.1063/1.41383 A Monte Carlo simulation of a photodetection scheme employing noninstantaneous multiplication J. Appl. Phys. 64, 6845 (1988); 10.1063/1.341974

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Operating conventional electronic devices in the noninstantaneous multiplication (amplification) regime

R. P. Jindal AT&T Bell Laboratories. Murray Hill, New Jersey 07904

(Received 2 January 1990; accepted for publication 20 March 1990)

One requirement for operating a device in the noninstantaneous mUltiplication mode is that its response to an incident photon is the generation of a pulse of constant current. This current is allowed to have a fluctuating duration and can exhibit full shot noise. It is shown that a majority of conventjonal current-te-current and voltage-to-current converting devices should be operable in the noninstantaneous mode. Ideal photoconductive detectors (current-to-current converters) operated in the conventional (temporally deterministic) mode are factor of 2 below the ideallimit on the signal-to-noise ratio set by the shot noise of the input light signal. When operated in the noninstantaneous mode these devices can be made to approach this ideal limit. The issue of resettability, i.e., the guaranteed turn off of each pulse of constant current, is examined with the help of a specific detector structure. Junction field-effect transistors are presented as an example of voltage-to-current converting devices operated in the noninstantaneous mode. The consequent improvement in the noise performance is also quantified.

t INTRODUCTION

We have made considerable progress towards under­standing the noise-free detection limits inherent in conven­tional detection schemes. Further, we have proposed and examined the consequences of departing from the classical wisdom of a temporally deterministic approach! where the device response time Tp is very small compared to the sig­nal duration time tm• We have thus analytically examined2

a general noise problem where 'Tp can be small, comparable to or larger than tm• Later, the idea of having Tp larger than tm (noninstantaneous mode) has been applied to the prob­lem of level and pulse detection3

,4 in the case of light wave communication systems. These analytical results are sup­ported by a subsequent numercials investigation of the pulse detection case. Further, criteria that a physical phe­nomenon must exhibit in order to be a suitable candidate for implementing this concept have also been developed. 6

Also, finite medium avalanching, as one such possible phe­nomenon, has also been proposed.6 In the present work we would like to further examine the issue of device imple­mentation of this concept in more general terms.

It is realized that an essential ingredient for imple­menting noninstantaneous multiplication is that the response to a photon is the generation of a constant cur­rent. This current is allowed to exhibit full shot noise and can have a fluctuating duration Tpo Now, in general, an amplifying device converts an input voltage or current into an amplified output voltage or current. Thus if one can modulate the device input voltage or current with a photon-generated charge, one should investigate the possi­bility of· operating the device in the noninstantaneous mode. With this basic idea in mind, as an example of a voltage-to-current converting device, we shall examine the operation of a JFET as an ideal light pulse detector. As an example of a current-to-current converting device we shall examine the possibility of operating a photoconductive

detector7 (PCD) in the noninstantaneous mode. Note that the proposa16 of using finite medium avalanching for im­plementing random multiplication is yet another example of a current-to-current converting amplifying device.

II. REVIEW OF RANDOM (NONINSTANTANEOUS) PHOTODETECTOR OPERATION

For a light wave system operating at a bit rate B per second, the time available for the transmission of one bit of information is liB seconds. This is referred to as tm. For each bit interval we have photons arriving at random in­stants oftime at the receiver. At the beginning ofa bit time slot we flush the dectector. Then, for each incident photon the detector initiates a constant current exhibiting full shot noise. This current terminates when the carrier generation process initiated by the photon terminates or when the detector is flushed at the end of the bit interval. We thus permit randomness both in the number of carriers gener­ated per photon and also the times at which these carriers are released into the external circuit. Near the end of the time slot we count the number of carriers that have been emitted in a time interval 'T short compared to the bit time slot tm and make a decision as to whether a "one" or a "zero" was transmitted. At this point, the detector is flushed and is poised for receiving the next bit in the fol­lowing time slot.

III. CONVENTIONAL OPERATION OF A PCI)

Although PCDs have been around for along time7

there has been a renewed interest in them lately for light wave applications. This interest has been primarily in the long wavelength ( > 0.8 /LID) regime where several authors have published experimental results using binary and ter­nary semiconductors.8 It is instructive to briefly review the conventional mode of operation of a PCD. As the name suggests, a PCD can be thought of as a light sensitive

324 J. Appl. Phys. 68 (1). 1 July 1990 0021-8979/90/130324-04$03.00 @ 1990 American Institute of PhYSics 324

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resistor. The incidence of a photon results in the generation of one positive and one negative charge. It is important to note at this point that the only way these charges can disappear is by mutua! annihilation. Hence, by definition these charges have a common life time rp' One of these charges is hlghly mobile with a device transit time ty when the device is under bias. The other charge is relatively slow. This slow charge could correspond to an ionized donor in which case the mobile carner is an electron. rp is then the electron lifetime. Alternatively, this slow charge could be a hole, in which case Tp is the hole transit time across the device. (Once the hole has traversed the device the two charges effectively recombine.) Now, so long as the slow charge is present in the device, under the influence of an applied bias, the fast charge keeps circulating around the circuit including the device, generating a current. The number of times it recycles around the device is precisely equal to the photodetector gain M = r pit,.. Since the device bandwidth is determined by Tp the gain bandwidth product of the device is equal to reciprocal of the transmit time t r of the fast charge.

IV. NOISE MECHANISMS IN APeD

A PCD suffers from a number of noise mechanisms9

including generation~recombination (OR) noise, 1/fnoise, and thermal noise. Of all these noise mechanisms, the one that cannot be suppressed conventionally is the GR noise,9 This noise can essentially be viewed as fluctuations in car­rier lifetime rpo This gives rise to fluctuating gain M and hence excess noise, When GR noise is the dominant noise mechanism, for the conventional mode of operation, the SIN ratioJO of a peD can be simplified to

(S/N)rms = TJm2(PoPt/hv)/8BW, (1)

where 11 is the quantum efficiency of the photodetector, m is the modulation index of the light signal, POP! is the av­erage incident optical power, and BW is the bandwidth. Then assuming a 100% quantum efficiency (11 = 1), mod­ulation index m of unity, and a bandwidth equal to halfthe bit rate E, we get

(2)

In the above expression (PoPtlhv) is the average photon arrival rate nand 1/B is the measurement time tm• The peak (SIN) ratio for the transmission of a "I" is given by

(SIN)peak= Ii tm/2. (3)

This value of the SIN ratio is a factor of 2 below the ideal (SIN) ratio ntm associated with the input photon flux. Thus, at least for the case of a two-level GR process9 there is a 3~dB associated penalty.

Another way of analyzing the above situation is by considering the fluctuations in rp as the source of noise. (This approach is more i.nstructive in view of the nonin~ stantaneous mode of operation which we will discuss i.n the next section.) Let the primary input photo current be given by lin- Then the output current IOllt is given by

(5)

325 J. Appl. Phys., Vol. 68, NO.1, 1 July 1990

Then from the variance theorem I (which is applicable to the conventional mode of operation), we get

Sj (/)=(r /ttr)2S], (f) + 2qlio var(r Ittr ). (6) out P In JY

Translating this into a SIN ratio, we find

(SIN)in (SIN)out = 1 + var( rpV:;:; . (7)

Then for the case when var(1'p) = ~ we recover Eq. (3), For situations where Tp is more noisy this penalty could be worse.

v. pee OPERATION IN THE NONINSTANTANEOUS MODE

A peD needs to satisfy the following criteria6 in order to operate in the noninstantaneous mode. It should have a relatively extended response time, should be quickly reset­table, and be able to respond to several photons in parallel by generating a constant current of possibly variable dura­tion which may exhibit full shot noise. The PCD can easily be arranged to have a relatively extended response time 'Tp

which is comparable to or larger than the optical pulse width tm• It can respond simultaneously to several photons by generating a constant current per photon of a variable duration with zero shot noise. The only question that re~ mains is that a resettability. This may not be straightfor~ ward for some peD structures and hence needs to be ad­dressed. With operation in the noninstantaneous mode one should be able to get rid of the OR noise limitation of the peD since the effective gain4

,5 G = MrlTp = Tlty is essen­tially free of any noise associated with fluctuations in 'Tp'

VI. pel) RESETT ABILITY

As pointed out eadieI', conventional PCD structures satisfy all but one requirement for implementing nonin­stantaneous multiplication. This requirement is the reset­tability of the detector in a time short compared to the optical pulse width. In this section we shall examine this issue in some detail and evaluate the available alternatives with the help of a specific device structure. Issues such as spectral sensitivity range, coupling, and quantum efficiency are not central to the following discussion, its sole purpose being to demonstrate the feasibility of improved noise per­formance.

Consider the sectional diagram of a device as shown in Fig. 1. The device consists of alternating n + and p + dif­fusions on a standard nl n + epitaxial substrate. The device is biased with its substrate grounded, and the n + diffusions biased positive with respect to the substrate. The p + dif­fusions are biased at a potential negative with respect to the n + diffusions. The sample temperature is such that the photoactive impurities in the substrate are not ionized. Electromagnetic radiation coupled from the top (or the side) ionizes the impurities liberating an electron and leav­ing behind an ionized impurity (positive fixed charge) for each photon, The liberated electron travels to the n -l- dif~ fusion constituting a current flow. As this electron is col­lected by the n + diffusion another electron is introduced

R. P. Jindal 325

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:):::::::::/:::::::::}:::::t:::::::::::::::::::::;::;::::!::;:::: 11+ :::::::::::::::::!?'::::::::::::::;:;:::::;!::::t::,,::,:,!::::

CONTACT METAL

I

FIG. 1. A conceptual photoconductive detector structure dealing with the issue of fast rest.

from the substrate n + contact (in order to maintain charge neutrality). Thus there is a generation of a constant current in the external circuit so long as the impurity re­mains ionized. A similar process is generated by other in­cident photons during the optical pulse. The sequence of events outlined so far is nothing but the conventional pro­cess of photoconductor operation. At the end of the light pulse we sample the current flowing in the external circuit in a time 7" small compared with the light pulse width tm.

Next we examine the question of quenching this detec­tor. It has been observed that the response generated by visible light may often be reduced by the addition of longer wavelength light. This involves the population of selective states by the longer wavelength radiation. However, we shall not pursue this phenomenon of infrared quenching? any further. The underlying principle of our approach is to increase the electron population to a high level so as to enhance the impurity deionization rate by several orders of magnitude. This excess electron population can in princi­ple be generated by optical, electrical, or thermal excita­tion. We prefer the electrical route. By momentarily for­ward biasing the p + diffusion we introduce a large number of holes into the epitaxial substrate. This results in a cor­responding injection of a large number of electrons from both the top and bottom n + diffusions into the epitaxial region. For a semiconductor where direct band-to-band recombinations are improbable, provided the electron-hole plasma density is not too high, this will result in a quick neutralization of an the ionized impurities (localized midgap states). Then, as we reverse bias the p + diffusion the injected holes will be recollected by it. The injected electrons will be correspondingly collected by the n + dif­fusions in a single transit time. The detector is now ready for use for the next optical pulse.

VII. FET OPERATION AS A RANDOM DETECTOR

Although, in principle, any transconductance exhibit­ing device is suitable for the following discussion, we shall discuss the operation of a JFET in the noninstantaneous mode. Shannon and Lohstroh 11 have shown how a silicon JFET can be used as an optical detector. Consider the

326 J. Appl. Phys .• Vol. 68, No.1. 1 July 1990

v

FIG. 2. Biasing circuit for the JFET.

JFET biased as shown in Fig. 2. The essential idea is to apply a voltage reset pulse Vr to the gate of a JFET through a series capacitor Cs• This results in a momentary forward conduction of the gate diode charging this series capacitor. When this resetting pulse terminates, the capac­itor charge results in a gate reverse bias with the JFET biased at pinch-off. Then for each incident photon the gen­erated charge results in a gate current partially neutralizing the charge responsible for the gate bias. This results in an increase in the channel conductance and hence the channel current. This process of gate current integration continues for the "frame time," which in our nomenclature is the bit time tm, till the next resetting pulse is applied. During this time the device can be read repeatedly in a nondestructive manner at the drain.

Although the issue of noise performance of this device is not dealt with in the previous work!1 we make the fol­lowing observations. As suggested,! I for maximum charge gain one needs to integrate the drain current over the whole frame time. Now, the effective charge generated by a photon is directly proportional to the gap between its ar­rival time tar and the following reset pulse. Thus the gain per photon is given by

(8)

where C is a constant. The charge gain for each photon will therefore fluctuate in sympathy with its arrival time tar resulting in a degradation in the signal-to-noise ratio. Cal­culating var(tar) based on Poisson statistics of the photon arrival process, one can evaluate var(M) and hence arrive at the signal-to-noise ratio at the output of this FET am­plifier as

(S/N)in (S/N)out= 1 13 . + ntm

(9)

The above expression assumes gain fluctuations as the dominant noise source and hence quantifies the degrada­tion in the SIN ratio due to them. Going back to the problem of random detection, to eliminate this gain fluc­tuation noise, one needs to sample the drain current at the end of the optical pulse (frame time) over a time window T small compared with optical pulse width tmo In this man­ner, although with a reduced gain of Or, the device can be made to approach the ideal limit on the signal-to-noise ratio. Note that sampling in a shorter time will increase the effective bandwidth and hence the contribution due to the JFET channel thermal noise. However, by properly scaling the FET one can keep this noise contribution small.

R. P. Jindai 326

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VIII. CONCL.USIONS

We have shown that both current~to-current and voltage-to~current converting conventional devices can be operated in the noninstantaneous multiplication mode. Specific examples discussed are for a photoconductive de~ vice and a JFET both attaining the ideal limit set by shot noise limited input signal-to-noise ratio. The structural de­tails along with related tradeoff's are beyond the scope of the present general treatment and will be discussed else­where.

ACKNOWLEDGMENTS

I am reminded of a discussion with Serge Luryi regard­ing the possibility of the existence of such an effect in the operation of a photon drag detector12 structure. Discus­sions with John Brews and Serge Luryi regarding the re~

327 J, Appl. Phys., Vol. 68. No, 1. 1 July 1990

settability of the pen were extremely helpful. Support from Bill Lynch and Simon Sze is greatly appreciated.

'W. Schockley and J. R, Pierce, IRE 26,321 (1983), 2R. P. Jindal, IEEE Electron Device Lett. EDI..!!, 315 (1987). 3R. P, Jindal, "The Principle of Random Multiplication" 9th Interna­tional Conference on Noise in Physical Systems, Montreal, Canada, edited by C. M. Van Vliet (World Scientific, Singapore, 1987), pp. 241-244.

4R. P. Jindal, J. Appl. Phys. 63, 2824 (1988). 5R. P. Jindal, J. App!. Phys. 64, 6845 (1988). 6R. P. Jindal, IEEE Electron Device Lett. EDL-lO, 49 (1989). 7 A. Rose, Proc. IRE 43, 1850 (1955). 8See for example, D. K. W. Lam, R. I. Macdonald, J. P. Noad, and B. A. Syrett, IEEE Trans. Electron Devices ED-34, 1057 (1987).

9 A. van der Ziel, Noise in Measurements (Wiley, New York, 1976), Chap. 11.

lOS. M. Su, Physics a/Semiconductor Devices (Wiley, New York, 1981). Chap. 13.

II J. M, Shannon and J. Lohstroh, IEEE Trans. Electron Devices 1£0·21, 720 (l974).

125. Luryi, Phys. Rev. Lett. 58, 2263 (t987).

R. P, Jindal 327

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