Vol.32,No.12Journal of Semiconductors December2011 Light controlled prebreakdown characteristics of a mi-insulating GaAs photoconductive switch
Ma Xiangrong(马湘蓉)1; ,Shi Wei(施卫)2,Ji Weili(纪卫莉)2,and Xue Hong(薛红)2
1Institute of Physics and Electronic Engineering,Xinjiang Normal University,Urumqi830054,China
2Department of Applied Physics,Xi’an University of Technology,Xi’an710048,China
Abstract:A4mm gap mi-insulating(SI)GaAs photoconductive switch(PCSS)was triggered by a pul lar
with a wavelength of1064nm and a pul energy of0.5mJ.In the experiment,when the bias field was4kV,
the switch did not induce lf-maintained discharge but worked in nonlinear(lock-on)mode.The phenomenon
is analyzed as follows:an exciton effect contributes to photoconduction in the generation and dissociation of ex-
魔法师的学徒2citons.Collision ionization,avalanche multiplication and the exciton effect can supply carrier concentration and
energy when an outside light source was removed.Under the combined influence of the factors,the SI-GaAs
PCSS develops into lf-maintained discharge rather than just in the light-controlled prebreakdown status.The
characteristics of the filament affect the degree of damage to the switch.
Key words:light controlled prebreakdown;photo activated charge domain;lf-maintained discharge;exciton effect
DOI:10.1088/1674-4926/32/12/124006PACC:7220H
1.Introduction
A photoconductive miconductor switch has many ad-vantages,such as high power,fast switching speed,small para-sitic capacitance,high repetition frequency and abnce of jit-ter.Therefore,it show
s broad prospective applications in the fields of superspeed electronics,high power electrical pul generation,fine-synchronization control and terahertz technol-ogyŒ1 4 .In1982Œ5 ,the linear photoconduction working mode was investigated in mi-insulating miconductors.Willam-son discovered the high gain mode(which is also called lock-on or nonlinear mode)in a GaAs chip.A number of sub-quent experimental studies indicated that the nonlinear mode of SI-GaAs photoconductive switch(PCSS)can be divided into three phas:triggering(transition process),sustaining(lock-on or steady-state),and recovery.Its typical characteristics are as follows:the electric field threshold and the optical-energy threshold are required.A time delay lies between the trigger optical impul and the electrical pul.The current ritime is not limited by the intensity of the optical pul,conduction capacities are not limited by the photon flux and conduction periods are not limited by the duration of the optical pul. The obrved switch closure time is much shorter than the time for carriers moving at the saturated drift velocity to cross the switch.A stable lock-on electric field exists at the maintaining stage and a filament occurs along with lightŒ6;7 .The behav-iors also indicate that new carriers must be generated during the breakdown in SI-GaAs PCSS.
鞭挞的意思Photo-activated charge domain(PACD)theoriesŒ8 10 can be ud to explain some characteristics of
the nonlinear mode. The high-field domain in the miconductor must comply with nL>1012cm 2condition,where n is the electron concentra-tion and L is the length of the device.The material in a switch chip is made of negative differential conductance(NDC)for photoconductive switches and the carrier concentration is too low to meet the requirement of domain formation.However, under the condition of a high trigger light,the concentration of the photo-generated carriers can be up to1019cm 3and its injection can enable the domain to form.The existence of the PACD is proved by a simulated resultŒ11 .
Many aspects of a high gain photoconductive switch are remarkably similar to streamer formation and electrical break-down in gas.Gas streamers bridge the interelectrode gap in a time shorter than the drift time.Once the conduction chan-nel is ionized,the current is effectively circuit limited.In ad-dition,the filament channels obrved in high gain photocon-ductionŒ6 remble arcs in gasŒ12 15 .Due to this great similar-ity,the applicability of gaous ionization mechanisms,specif-ically a streamer mechanism,to high gain photoconduction in bulk miconductor devices has been investigatedŒ16 .A spe-cial breakdown mode of a SI-GaAs photoconductive switch oc-curs in the nonlinear operation,some breakdown phenomena, such as collision ionization,avalanche multiplication,radia-tion recombination and filaments,which normally cau lf-maintained discharge
in a switch,will appear.However,the breakdown does not develop into lf-maintained discharge, but in the maintaining stage of lock-on.The maintaining volt-age is independent of the bias voltage,but is related to exter-nal environment and the properties of switch chip material. We define this unique breakdown phenomenon as light con-trolled prebreakdownŒ17 ,and more detail has been published in Refs.[18,19],yet tho explanations were not perfect.Light controlled prebreakdown characteristics of SI-GaAs PCSS are
*Project supported by the National Natural Science Foundation of China(Nos.50837005,10876026)and the State Key Laboratory of Electrical Insulation for Power Equipment(No.EIPE09203).
Corresponding author.Email:
Received20April2011,revid manuscript received1August2011c 2011Chine Institute of Electronics
Fig.1.Schematic diagram of the lateral PCSS.
Fig.2.Diagram of the electrical circuit for testing PCSS.
discusd in this paper in order to further rearch the work-ing principle in nonlinear mode,and have the significance in improving performance and extend the life of a switch.
2.Experimental tup and results
In our experiment,the photoconductive material ud for the PCSS was SI-GaAs with a dopant density of 1.5 107cm 3.The resistivity in total darkness was larger than 5 107 cm and the mobility was larger than 5500cm 2/(V s).The overall size of the GaAs wafer was 8mm wide 10mm long 0.6
mm thick.Using the electron beam evaporation tech-nique,a AuGeNi alloy electrode forms ohmic contact by us-ing a standard mixture of Ni/Au–Ge/Au for the metallization at 450ıC and the thickness is 700nm.The electrode dimen-sion is 8 3mm 2and the gap between the two electrodes was 4mm.The geometry of the PCSS is shown in Fig.1.The GaAs chip was placed on the copper clad board with a transmission line and the transmission line was connected with two coaxial connectors.The experimental tup is shown in Fig.2,where a high-voltage power supply charged capacitor (0.01 F)and a 10M resistor were ud to limit the current.The capacitor provided a bias voltage to the PCSS.The Nd:YAG nanocond lar emitted 5ns lar puls with wavelength of 1064nm,en-ergy of 0.5mJ.An energy meter KSDP2210-CAS-1was ud to measure the lar energy.In addition,a Lecory-8600A oscil-loscope (6G)was ud to detect the voltage on the 0.1 sam-pling resistors after being attenuated by a 60dB coaxial atten-uator.When the bias voltage was 4.0kV ,corresponding to the bias electric field of 6.5kV/cm,a nonlinear current waveform of the PCSS was obrved.The nonlinear waveforms super-pod 50times are shown in Fig.3.The switch samples trig-gered continuous 50times and 1500times appeared at 4kV bias voltage,as shown in Figs.4and 5,and damage trace of filament in switch samples was clearly shown in Fig.5.The phenomena show
that the material of switch chip had not been
大学英语四级多少分过
Fig.
3.Output electrical waveform of switch triggered 50
times.客户经理竞聘演讲稿
Fig.4.Switch sample triggered 50times.
Fig.5.Damage trace of filament in switch sample triggered 1500times.
damaged by many triggering,and switch can still be ud not until triggering reached to an extent.
3.Discussion
The process of non-lf-maintained discharge transform-ing into lf-maintained discharge occurs in gas discharge.Lebo and Meek think that lf-maintained discharge can be induced only if there is enough space photoionization,c-ondary electrons generate without cathode action and the breakdown voltage does not depend on the cathode material.When an external electric field is correspondingly weakened,a lot of electrons enter into anode after the initial electron avalanche went through whole electrode gap,also positive
考验的意思
Fig.6.Current–voltage characteristic in gas prebreakdown zone.
ions are gradually neutralized and lo their charge.Then the electron avalanches vanish and gas discharge can not change into lf-maintained discharge.When electron avalanches go through the whole electrode gap and the external electric field reaches the breakdown voltage,the concentration of space charge rapidly increas and a streamer starts to form ahead of an electron avalanche.Many photons constantly radiate from the tip of streamer to its surroundings,and condary electron avalanches occur through space photoionization.Sec-ondary electrons are constantly attracted to the streamer tip,thus streamer channel is extended and pushed forward to the cathode rapidly.The whole electrode gap is connected by con-ducting positive and negative particles.
When the streamer ex-pands to the cathode,current jump occurs,a new current pul will be developed and then breakdown forms.Formation and development of the streamer is taken as the lf-maintained dis-charge stage.Current–voltage characteristic of gas prebreak-down in whole gap is shown in Fig.6.Current slowly ris in T 0with the applied initial voltage,then reaches saturation;electrons obtain enough energy with increasing voltage in T 1;the speed of the current ri becomes very fast due to space photoionization in the T 2.The T 0,T 1and T 2period belong to non lf-maintained discharge.
The sufficient concentrations of the photo-generated car-rier and the bias electric field are necessary conditions to form the PACD.When the bias field surpass its threshold,PACD will generate and grow,and the initial electron concentration in domain is determined by the lar energy.Photoconductive switch chip material can generates 103–105electron–hole pair after absorbing a photon in nonlinear mode.The electric field between PACD and photo-generated hole is opposite to the bias electric field,which screens the bias electric field,so electric field of the domain tip is enhanced and the electron concentra-tion in the domain tip is higher than in other regions.If the bias electric field exceeds the turn-on electric field of NDC,the photo-genetated electron space accumulation can be formed during the drift process and its concentration gradient ris ex-ponential
生日快乐 日语ly,with the results of the exhaustion of rear electrons.Figure 7is the distribution diagram of electrons,holes and built-in electric fields at the formation stage of the PACD.With the growth of the PACD,the electric field in domain increas and then exceeds the electric field with a negative resistance effect.X-axis forward direction is the direction of the bias elec-tric field.When
the PACD is steady-state with its growth com-
Fig.7.Distribution diagram of
electrons,holes and built-in electric fields Œ11 .
Fig.8.Process of exciton formation and radiative recombination by photoactivated Œ20 .
ing to a halt,the PACD begins to drift toward the anode with a stable electron concentration and finally becomes absorbed by the anode to form the output current Œ11 .
Gas streamer models make the assumption that the opti-cal generation of carriers occurs ahead of the impact ionization zone.Since the streamer model hinges on the availability of appropriate photons,it is necessary to explore the conditions under which such photon activity can occur in PACD.GaAs materials having a direct band gap structure are much more likely to support domain io
nization events since optical emis-sions from band-to-band recombination and exciation are criti-cal to PACD process.Some of the recombination radiation pro-duced in domain immediately behind the tip is absorbed ahead of the domain to produce ed carriers that avalanche in the high field,some photons can be absorbed into the head of do-main where the high electric field multiplies the photocharge by impact ionization.
For GaAs material,recombination lifetime of band–band is D 10 7s,but that of exciton is D 10 9s.The time of electron and hole forming exciton is far less than the life-time of band–band transition radiation recombination,so lu-minescence caud by exciton recombination is the main fea-
ture of intrinsic radiation recombination.The physical process can be shown visually in Fig.8Œ20 .When lar trigger GaAs chip,firstly carrier relaxes to conduction band border by emit-ting a photon and phonon to form free exciton.However elec-trons of valence band in GaAs absorb a certain photon energy and relea from valence band,the excited valence band elec-trons still do not have enough energy to transit to conduction band and become free electron,but they will form free excitons under the action of a hole Coulomb field.Excitons can move freely in miconductors and participate in radiation recombi-nation to emit photon finally.Moreover,phonon will partici-pate in free exciton radiation recombination Œ21 23 and energy can be lost in the process.Exciton shows electrically neutral,thus current cannot form,but it can transmit energy by moving and energy releas from recombination.
The formation required optical energy of excition is:
h D E g E n C …k 2
2.m 2e C m 2p
/;(1)
where E g is band gap of GaAs and E n is excition energy level.
The excition energy level Œ24 is defined by
E n D q 4
8"2h 2n
2;
(2)
where "is dielectric constant,q is the electric quantity of elec-tron,n is the number of energy level, is the reduced mass of
electron and hole ( D m p m e =.m p C m e //,and m e ,m
p are electron and hole effective mass respectively.
In Eq.(2),there are infinity energy levels in exciton,ground state level of exciton is E 1D 4.56meV for n D 1;energy level is nearly continuous for n >2;it is E 1D 0for n D 1,which is the same as the energy level of con-duction band bottom,it shows that electron–hole completely move from bondage each ot
her,electrons enter into conduction band,but holes still stay put.So exciton effect can contribute to photoconduction.The lowest single photon energy of absorp-tion for exciton forming is 1.4254eV ,namely long wavelength edge of absorption is 869.8nm.
An exciton is electron–hole pair due to their mutual Coulomb interaction and the interaction is relatively weak,so exciton level is clo to bottom of conduction band.It shows that exciton absorption is most likely to happen when optical wavelength and intrinsic absorption wavelength in GaAs are extremely similar.Becau light wavelength of direct band gap recombination in GaAs is 875˙10nm,exciton easily disso-ciates when it absorbs photons by electron–hole recombina-tion generated in outside domain.Exciton overcome Coulomb binding force and the process of electron transition to conduc-tion band finished.The interactions between PACD and exciton are shown in Fig.9.Electron of high concentration and energy ahead of the domain drift to anode with 5 108cm/s Œ25 ,Elec-tric field of domain tip is more and more strongly with domain approaching to anode,electron density of domain tip is more highly and collision ionization of electron is more intenly,exciton of domain tip is shielded.When strength of avalanche multiplication is reached,equivalent to an incident photon ex-cite many times electron–hole to participate in conduction,this is high gain photoconduction.Also exciton can absorb heat,
Fig.9.Schematic diagram interactions between PACD and exciton.
which domain emits in the drifting process,and heat dissoci-ation will occur.Many electrons by exciton dissociation con-verge domain head from different directions around domain at the same time,the electric field in domain tip is strengthened.Exciton effect contributes to photoconduction with the aid of generation and dissociate.The interaction between radiation recombination of high concentration carrier in PACD,exciton effect and avalanche multiplication supply exactly enough car-rier due to extinction of outside trigger light source induce de-crea of carrier concentration.
The equilibration time between the L and valleys are in near 1ps.GaAs is a direct gap material,but it becomes pudo-indirect under the influence of a high electric field.Hence in-hibiting band-to-band recombination and decreasing radiative emissions,so the optical generation rate in the avalanche zone will be decread.Near the domain tip fields will be far larger;hence the majority of electrons in the impact ionization region at the tip of the domain will lie in the L valley.This extends the carriers lifetime in proportion to the fraction of electrons transferred to the L valley.Radiation recombination reaches the optimum efficiency (this is saturation point).Also,recom-bination coefficient of carrier will change with temperature,the higher the temperature was and the lower the recombina-tion coefficient was.The stability of exciton effect depends on temperature,field,carrier concentration etc,e
xciton effect will quenched at a certain temperature;Coulomb of between elec-tron and hole will be shielded and exciton effect has little or no u.When PACD drifting to anode reaches to a critical level,owing to inhibition of radiation recombination and shielding effect of exciton effect in certain conditions,the numbers and lifetime of carrier have reached a saturation point,correspond-ing to SI-GaAs PCSS in lock-on state.Free electron and hole are generated by the interactions between exciton and PACD,transmission and dissociation of exciton;they provide an es-ntial condition for photoconduction of exciton effect.
The typical pul waveform of the SI-GaAs PCSS in lock-on mode is shown in Fig.3.Compared with the curves in Fig.6,this indicates that it would experience a pul again when gas discharge enters into lf-maintained discharge.However,the electric field outside domain is inferior to the minimum threshold while the PCSS began to nucleate and grow.For im-pact ionization and exciton effect,it will produce fresh elec-
tron–hole pair outside domain zone.The fresh electrons pre-cily supply the insufficiency of carriers when pul removed. External source has been taken away,initial electron and fresh electron enter into anode together and disappear.Becau of radiation recombination,the exciton effect and becau the ex-ternal circuit also constantly consumes and dissipates energy, the electric field in the
switch is not enough to provide lock-on electric field.A new domain can not produce and grow becau the existing conditions of domain had been destructed.How-ever,lifetime of photon-generated carriers is a certain,con-centration of carriers constantly decrea until gradually disap-peared after a long period of lock-on,due to unbalanced carri-ers vanish,the light controlled prebreakdown process destroy, then PCSS turns off and enters into the high impedance state. After the PACD is rapidly absorbed by anode,the hole still stays into electrode gap and slowly drifts to cathode.Field is re-duced becau lots of hole accumulates nearby anode.Streamer cannot form,which does not make lf-maintained discharge development.Owing to the non-uniform electric field in GaAs switch,when non lf-maintained discharge transforms into lf-maintained,local region of high field possibly occurs lf-maintained discharge,however field of broad external domain is so low,that the condition of lf-maintained discharge can not be satisfied.
Initial electron in switch chip generated by lar is ed source on the condition of a certain bias voltage and triggered light energy.Carrier concentration which is provided with light injection and negative resistance effect make the field in do-main increa to the required electrical field of avalanche col-lision ionization rapidly.Although many fresh electrons are produced by avalanche multiplication,the electrons are trig-gered by lar ultimately during the process of domain for-mati
on.Initial electron and fresh electron enter anode and vanish associated with domain,so the process of avalanche collision ionization can not make discharge process transfer to lf-maintain stage.For above,PCSS does not have the induc-ing condition of lf-maintained discharge,but in the non lf-maintained discharge.It is discovered in Fig.4that filaments similar to breakdown have not damage the material in switch chip by many triggering and thus a filament having breakdown can not develop into lf-maintained discharge.The material defect in chip will have certain effect on characteristics of fil-ament.Microdefect absorption will play a dominant role after many repeat lar triggering.Defect absorption will form lo-cal high temperature.When local high temperature reaches a certain degree,it will lead to change of material structure and property that will further enlarge defect.But above-mentioned influences do not create macroscopic damage,they only cau development of inner microcosmic defect in material.The mi-crocosmic defects have low damage degree in initial time and have a little effect on material structure and property.If each defect develops and accumulates,it will increa absorption of following lar pul and a greater microcosmic defect will oc-cur.Owing to the fact that characteristics of microdefect will riously influence damage degree of filament,once lar trig-ger reaches to a certain limit,it will result in the esntial and irreparable damages in the chip when accumulated microdefect reached at a definite scale.Damaged trace of filament can be clearly obrved in Fig.5by repeatedly lar triggering,work-ing performance of the switch significantl
y decread.GaAs PCSS shows non-balance thermal conduction behavior of ther-mal relaxation,namely transient thermal effect of carrier tem-perature more than lattice temperature control the damage char-acteristic of filament and affect the damage degree of GaAs PCSS in the light controlled prebreakdown processŒ26 .
4.Conclusion
Many aspects of SI-GaAs PCSS are remarkably similar to gas discharge.The key distinction is that gas discharge occurs non lf-maintained transforming into lf-maintained discharge process.Some breakdown phenomenon in PACD is similar to lf-maintained action of gas discharge,such as,col-lision ionization,avalanche multiplication,radiation recombi-nation,filament,which normally caus lf-maintained dis-charge in switch.However,the breakdown does not develop into lf-maintained discharge,but is in the maintaining stage of lock-on.Excitons continually produce and dissociate by ra-diation recombination on interaction between exciton effect and domain,collision ionization and avalanche multiplication all generate fresh carrier.The required carrier concentration is supplied and maintaining voltage is controlled in the lock-on stage.Radiation recombination,exciton effect and exter-nal circuit continually consume and dissipate energy;electric field is insufficient to provide lock-on electric field.Carrier concentration continually decreas until to gradually disap-pear after long ti
me lock-on.SI-GaAs PCSS is only in the non lf-maintained discharge and has non-balance transient ther-mal effect characteristic in the light controlled prebreakdown process.
References
[1]Shi Wei,Dai Huiying,Sun Xiaowei.Photon-activated charge
domain in high-gain photoconductive switches.Chin Opt Lett, 2003,1(9):553
[2]Shi W,Tian L.Mechanism analysis of periodicity and weaking
surge of GaAs photoconductive miconductor switches.Appl Phys Lett,2006,89(20):202103
[3]Shi Wei,Jia Wanli,Ji Weili.Ultra-wideband electromagnetic ra-
diation from GaAs photoconductive switches.Chine Journal of Semiconductors,2005,26(1):11专业八级考试时间
[4]Shi Wei,Jia Wanli,Hou Lei,et al.Terahertz radiation from large
aperture bulk mi-insulating GaAs photoconductive dipole an-tenna.Chin Phys Lett,2004,21:1842
[5]Williamson S,Albrecht G F,Mourou M.Lar triggered Cr:
jail
GaAs HV sparkgap with high nsitivity.Rev Sci Inst,1982, 53(6):867
[6]Loubriel G M,Zutavern F J,O’Malley M W,et al.High gain
学习新娘盘头GaAs switches for impul sources:measurement of the speed of current filaments.International Power Modulator Symposium, Costa Mesa,CA,USA,June1994
[7]Loubriel G M,Zutavern F J,Hjalmarson H P,et al.Measurement
of the velocity of current filaments in optically triggered high gain GaAs switches.Appl Phys Lett,1994,64(6):3323
[8]Shi Wei,Zhang Xiangbing,Li Qi,et al.High gain lateral mi-
insulating GaAs photoconductive switch triggered by1064nm lar puls.Chin Phys Lett,2002,19(3):351
[9]Shi Wei.Optically activated charge domain model for high-gain
chromium
GaAs photoconductive switches.Chine Journal of Semicon-ductors,2001,22(12):1481
[10]Shi Wei,Tian Liqiang,Liu Zheng,et al.30kV and3kA mi-
