IEEE TRA~"SACTlO0:S ON INSTRU:vIEl\TATlON AND MEASUREMENT. VOL. 40. NO.3. JUl\E 1'191

25

Input signal ; 500 jJV 20 gain ~ A Simulation result continuous curves Measured error points x x x

A ; 200 o ::J 10 '"0 A 400 (f) ClJ 750 D::::

o 20 40 60 80 100 120 140 160 180 Phase difference

effcct [7 J. The schemc can also be successfully employed in auto­ A Fiber-Optically Triggered Avalanche matic test equipment involving ac bridges [8[. R. Jacob Baker, Gregory T. Perryman. and Phillip W.

Y. CONCLUSION It has been demonstrated that using feedback technique for phase Abstract-A bipolar transistor operating in the avalanche re­ compensation in a PSD, the amplitudc of thc measurand can bc got gion is optically triggered into secondary breakdown, This transistor has been given the name fiber-optically triggered avalanche transistor directly without the need for a post-processing of the in-pha,e and (FOTAT), The FOTAT acts as an optical power discriminator. That quadrature components. Experimental results presented serve to is, secondar) breakdown occurs when the triggering optical power ex­ establish the easy applicability of the method to obtain a high de­ ceeds tbe triggering tbreshold of tbe FOT AT, This secondary break­ gree of phase compensation coupled with a fine resolution. Quan­ down is seen as a between the collector and emitter. High voltage (> 100 V) nanosecond transition duration pulses arc gen­ titative comparison of the extent of compensation obtained exper­ erated using this negative resistance, imentally with that estimated by PC-simulation indicates good agreement.

I. iNTRODlICTIOi': ACKl\;OWLEDG:vtEl\;'I Avalanche have found many uses in the design of in­ The second author is obliged to Prof. A. ]. Rogers of King's strumentation [1]-[4], but some problems exist when trying to use College. , U.K .. for kindling his interest in this area. The these transistors in electro-optic instrumentation. Time jitter re­ authors are grateful to Prof. Y. G. K. Murti of lIT Madras for quircments for this instrumentation can be much more stringent encouragement. valuable suggestions. and criticism. than in electrical instruments. The jitter developed when triggering these transistors electrically is mainly due to the stray inductance

REFERENCES and capacitance. Also. when trying to generate a trigger frulll an optical signal a photodetector of some sort is usually required. A II J K. L. Smith. "Thc ubiquitous phase sensitive detector.·· Wireless fiber-optically triggered avalanche transistor (FOT A T) can help World. voL 78. pp. 367-.. 50. Aug. 1972. solve both of these problems. By injecting the light directly into 12J 1. G. Lac). "A current ,witching phase sensitive detector and it, ap­ the transistor, not only can the secondary breakdown mechanism plication to a resolved components voltmeter." EleClwlL EI1~ .. voL 39. pp. 1.. 8-151. Mar. 1967. . be triggered. but the jitter is theoretically nonexistent. 13J D. P. Blair and P. H. Syndenham. "Phase ,en,itive detection a, a The optical triggering of an avalanche transistor was first shown me am to recoYer signals buried in noise." I PIII'sics E: Scientific In- by Thomas and Coleman [5]. A lens was used to focus part of a struments. YoL 8, pp. 621-627, 1975. . signal onto a transistor die to start the second brcakdow n pro­ I"J v. Brunstein. "Frequency doubler and flip-flop make adju'table phase cess. This method of triggering is diffieult to incorporate into a shifter." Declrol1ics. \01. .. 9. p. 100. 1976. 15J 1. Austin and 1. R. Forre,\. "A preci,c digital phase shifter using portable piece of instrumentation due to the alignment necessary in PLL." Eiecrwl/. Lell .. vol. 14. pp. 254-255. Apr. 1978. 16J E. W. Tay and V. G. K. Murti, "Unity gain frequcncy independent Manu,cript received June 16. 1990: revised November 17, 1990. Thi, phase shifter.' Decrroll. Lett .. vol. 20. no. 10, pp. 431-432. May work was performed under the au'pice, of the U.S. Department of Energy 19~". by E.G.&G. Energy Measurements Inc. under Contract DE­ 17J A. J. Rogers. "Optical methods for mea,urement of voltage and cur­ AC08-X8NV 10617. rent on power systems." 01'[. Laser. Tech .. pp. 273-283, Dec. 1977. The authors are with E.G.&.G. Energy Mca,uremcnts. Las Vegas. NV 18J H. K. P. Neubert. 1l1Slnllllellf Transducers. London: Oxford Univer­ 89125. sity Press. 1975. pp. 196-197, 2nd ed. IEEE Log Number 9143082.

0018-9456/91 /0600-0649S01 .00 C 1991 IEEE 650 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT. VOL. 40. NO.3. JUNE 1991

·V cc

(, ~ R C

(2 ~

(a)

Fig. 2. FOTAT test circuit.

junction is given by

M= m I - (~)"

where n is in the range of 2-6 for silicon, V is the potential across (b) the junction, and Vb is the of the junction. If RB Fig. 1. (a) Epitaxial transistor used as a FOTAT. (b) Packaged FOT AT. is zero, some simplifying assumptions may be made. The break­

down voltage Vb is approximately BVCBO , the breakdown voltage of the collector-base junction with the emitter open. The emitter directing the laser signal and the requirement that the optical source current is negligible compared to MlcBo and can be assumed to be be close to the transistor. For these reasons, the optical triggering zero. of an avalanche transistor has been used very little. With the ma­ With the aid of Fig. 3 the dynamic operation of the transistor turing of fiber optics within the last decade these problems can be circuit shown in Fig. 2, with RB equal to zero, may be described. eliminated. Fig. I(a) shows the epitaxial transistor used as a FO­ A value of Rc is selected such that the static operating point of the TAT. The top of a transistor unit, packaged in a metal TO-39 type transistor is at point A, a voltage just slightly below BVCBO and at package, is removed. This exposes the transistor die. One end of a currenth. The value of Rc is typically> 100 kO. When the tran­ an optical fiber is cleaved, positioned over the die, and epoxied in sistor is triggered the operating point moves to point B. Point B is place, while the other end is terminated with a fiber optic connec­ determined by BVCBO , RL , and RAo where RA is the "on" resistance tor. The result, shown in Fig. I(b), is a versatile device that can of the transistor. The negative resistance of the transistor while be used in electro-optic equipment, performing functions from that switching is given by the slope of the load line between points A of a nonlinear detector to that of an optically triggered electrical andB, pulse generator. (3) II. DEVICE OPERATION

The static characteristics of a bipolar transistor, as shown in Fig. The transition duration from A to B is essentially independent of 2, while operating in the avalanche mode, are given by (la)-(ld), the load resistance. The upper limit of the load resistance is deter­ mined such that the transistor will not enter the active region when (Ia) switching. The lower limit is set by the amount of power the tran­ sistor can dissipate without being destroyed. From point B the col­ (I b) lector current decays exponentially with a time constant of (RA + (Ie) Rd C until reaching point C. This is the point where the transistor is no longer in second breakdown. The collector to emitter voltage (I d) at this point is not great enough to sustain the avalanche condition and is denoted by VA' The collector current drops off quickly from

The base current, I B , is assumed to be flowing out of the base. The point C to point D. From point D the collector capacitor voltage collector current, while operating in the avalanche region, is called charges back to BVCBO (point A). the leakage current, I L • The only parameter of operation not yet described analytically is As the collector-base junction becomes reverse biased a point is the optical trigger pulse. Some problems exist in fuJly character­ reached where avalanche multiplication, M, becomes greater than izing the effects of this pulse. The emitter and base metalizations one. The empirical expression for avalanche multiplication of a p-n shown in Fig. I present the first problem. The etched area between IEEE TRAl"SACTlOl"S ON I~STRUMENTATlON AND MEASURB1EKT. VOL. ~(). MJ. 1. JUNE IY~I 651

f­ Z W 0: 0: ::::> o 0: o Fig. 4. Output pubc of FOT AT -100 V / div and 20 ns / div. of­ W --' --' 8 ! value of voltage should be set very close to the breakdown voltage of the transistor so as to keep leakage current to a minimum. I The output of this circuit into a 50-n load is shown in Fig. 4. The output amplitude is approximately 500 V with a first transition duration of about 10 ns. The optical trigger pulse is a 1.0-ns FDHM. Gaussian. at 806 nm. Fig. 5. This optical pulse is coupled BV CBO to an attenuator. through a 50/125 graded index fiber. for adjust­ ment of the optical power. The output of the attenuator is con­ V CE (COLLECTOR-EMITTER VOLTAGE) nected through a 50/125 graded index fiber. to a beamsplitter. The Fig. 3. 1- V Characteristics of FOTA T. purpose of the beam splitter is to split the light pulse into two parts; one for monitoring the optical amplitude and the other for trigger­ ing the FOTAT. these contacts is the only area where the light can penetrate the Monitoring the optical amplitude is accomplished by taking 2 o/c and generate carriers. The position of the fiber with of the light pulse. one side of the beamsplitter. and converting it respect to the transistor will also have a part in determining the to an electrical pulse for display on channel one of the 7104 oscil­ actual intensity of light radiated on the junction. If the intensity of loscope. The amount of optical power used for triggering can be light striking the semiconductor can be determined. the continuity calculated using the responsivity of the P670 I detector and the equation can be used to determine the transport and decay of car­ beam splitter ratio. The other output of the beamsplitter is coupled riers. that is. to a 100/140 graded index fiber. This fiber is connected through an X\'z translation stage to the transistor under test. The electrical II + j(x. t) (4) output of the FOTAT is displayed on channel 2 of the 7104. 7 It has been found that positioning the fiber at different places above the transistor changes the amount of power needed to start where j (x. t) is the carrier generation function. D is the carrier the second breakdown process. The main objective in positioning diffusion constant, 11 is the carrier concentration. and 7 is the mean the fiber above the transistor is to select the location where the carrier lifetime. The carrier generation function for single photon optical power required to trigger secondary breakdown is a mini­ absorption is given by [6]. mum. This would allow triggering from a lower power source. The X\'Z stage was used for positioning the fiber at different locations (5) above the transistor. The distance between the fiber and die was approximately 20 /lm. The required optical power to trigger the FOT A T versus fiber ,.here C¥I is the single-photon absorption coefficient, q is electron position is shown in Fig. 6 for three transistors. These positions charge. hl' l is the energy of an incident photon. and 1 is the inten­ sity of light incident on the semiconductor. The optical penetration correspond to labeled positions shown on the transistor die in Fig. distance. which is a function of wavelength and material type. is I. The curves in Fig. 6 represent the most general characteristics also a factor which is difficult to detem1ine under these circum­ of a total of 10 2N3742's used as FOT A T's with additional curves stances. providing no additional information. Qualitatively. position 4 ap­ Because of the difficulty in determining the intensity of light ra­ pears to be the optimum place for positioning the fiber. diated on the junction and the optical penetration distance. the op­ The optical pulse duration did have an effect on whether the FO­ tical trigger will be characterized by the optical power required to TAT did or did not trigger as the incident optical power became start the second breakdown process. This will allow the greatest close to the threshold. This means that the energy in the pulse be­ flexibility when designing instrumentation around these devices. comes the main factor determining whether the FOT AT will fire when using fast « I ns) pulses at optical powers close to thresh­ old. Also the leakage current through the transistor had some effect III. EXPERIMENTAL RESUl.TS on the tiring threshold. Increasing the Icakage current caused a For the experimental tests the circuit shown in Fig. 2. with RB lowering of the threshold. = O. R[ = 50 n. Re = 10 Mn. Rt = 510 n. Cc = 100 pF. was Other transistors. such at the 2N2222. were used as FOT AT's. used. A 2N3742 transistor was used because it had a large die area. The triggering optical power. the output amplitude. and transition c approximately 600 /lm . Other transistors may be uscd as well. duration varied among different transistors. For instance. the provided that the avalanche breakdown occurs in them before the 2N2222 required approximately 15 mW to trigger. The output am­ punch-through effect. The value of Vee was typically 900 V. This plitude was> 100 V. with a first transition duration of < Ins. 652 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 40, NO.3, JUNE 1991

PHALO BEAMSPLITTER TEK 7104 @

CH2

Fig. 5. Test setup used to determine triggering optical power.

12 - [6] C. H. Lee, Ed., Picosecond Optoelectronic Devices. New York: Ac­ ademic, 1984, chap. 5, pp. 127-128.

10-

P o W Recent Developments in the PTB RF Standard E 8- R Attenuation Measuring Equipment I N Transistor 1 Dieter Stumpe M 6 Transistor 2 I L Transistor 3 , L I Abstract-The frequency range of the RF standard attenuation mea­ W A 4 suring equipment at the Physikalisch-Technische Bundesanstalt (PTB), T Germany, has been extended up to 40 GHz in the waveguide bands R T S 220 and R 320. The total uncertainty of the system is estimated to be equal to or smaller than 0.002 dB for a 30-dB attenuation step.

I. INTRODUCTION o ------+---+--4---+---+----'----' 10 The standard equipment for carrying out attenuation measure­ 100u FIBER POSITION ments at the PTB, Braunschweig, works on the principle of the Fig. 6. FOTAT optical input power requirement versus fiber position. power ratio method applying dc substitution [1]. For the lower GHz range this equipment has been described in several papers [2]-[4]. It has the advantage of simplicity over all the other methods where mixers and local oscillators are used (e.g., IF and AF substitution IV. CONCLUDING REMARKS techniques). Attenuation values up to 30 dB can be measured with The purpose of this paper was to present the idea of a FOTA T small uncertainties. so that the device can be used in the design of electro-optic instru­ In recent years the demand for attenuation calibrations at fre­ mentation. This device was shown to provide an effective method quencies above 18 GHz traceable to national standards has in­ of generating fast transition duration electrical pulses from an op­ creased because an increasing number of commercial and military tical signal. communication links and systems are operated at frequencies Future work wiJI be concerned with the device physics, as well between 18 and 40 GHz. The frequency range of the standard as circuit design methods to exploit the operation of these devices. equipment for attenuation measurements at PTB has therefore been Problems such as the dependence of pulse duration on triggering extended up to 40 GHz in two waveguide bands (R 220, R 320). and the dependence on leakage current will also be investigated. This development is described in the paper.

REFERENCES II. THE PTB STANDARD EQUIPMENT FOR PRECISE ATTENUATION MEASUREMENTS [I] W. P. Mitchell, "Avalanche transistors give fast pulses," Electronic Design, vol. 6, pp. 202-209, Mar. 1968. A block diagram of the standard equipment forming a waveguide [2] H. M. Rein and M. Zahn, "Subnanosecond-pulse generator with system working on the principle of the power ratio method using variable pulse width using avalanche transistors," Electron. Lett., vol. dc substitution is shown in Fig. I, and a more detailed diagram of 11, no. I, pp. 21-23,1975. the substitution section in Fig. 2. [3] R. J. Baker and M. D. Pocha, "Nanosecond switching using Power MOSFET'S" Rev. Sci. [nstr., vol. 61, no. 8, pp. 2211-2213,1990. [4] R. J. Baker, "High voltage pulse generation using current mode sec­ Manuscript received September 11, 1990; revised November 22, 1990. ond breakdown in a bipolar junction transistor," Rev. Sci. Instrum., This work was supported in part by the Arbeitsgemeinschaft Industrieller vol. 62, no. 4, pp. 1031-1036, 1991. Forschungsvereinigungen e.V. (AIF), Kiiln, Germany. [5] S. W. Thomas and L. W. Coleman, "Laser-triggered avalanche-tran­ The author is with the Physikalisch-Technische Bundesanstalt (PTB), sistor voltage generator for a picosecond streak camera," Appl. Phys. 3300 Braunschweig, Germany. Lett., vol. 20, no. 2, Jan. 1972. IEEE Log Number 9143083.

0018-9456/91/0600-0652$01.00 © 1991 IEEE