Research On Operation Mechanism Of GaAs Photoconductive Semiconductor Switches Triggered By Low Optical Excitation Energy

The nonlinear operation of gallium arsenide photoconductive semiconductor switches(Ga As PCSS),producing as many as 103~105 electron-hole(e-h)pairs per absorbed photon because of the avalanche,allows to replace the desktop laser system by laser diode(LD)sources,i.e.,micro-joules excitation energy,which leads to the promise of extremely compact switching systems with low cost for high current switching.In this thesis,the effects of the bias field,the energy storage capacitor,the position of the trigger spot,and switch temperature on the output pulse of the GaAs PCSS are investigated,respectively,by using a single pulsed laser diode for optical excitation.Meanwhile,a Monte-Carlo simulation program considering the photo-activated charge domain is developed to study the operation mechanisms of nonlinear mode in GaAs PCSS,such as the filament current,the lock-in effect,the multiplication effect,and the presence of photoelectric thresholds.Four aspects are summarized as follows:(1)The effects of the bias electric field and energy storage capacitor on the PCSS triggered by low optical excitation energy are studied.At optical excitation energy of 1.6 ?J,the carriers multiplication effects are compared in which the photoconductive evolves from the linear mode into the nonlinear mode as the bias field increases gradually to 24kv/cm.The energy storage capacitor plays an important role on the switched pulse width,i.e.,the smaller the capacitor,the narrower the pulse width.Specially,the nanosecond switchings are observed as an external capacitance of 10 pF is used.This phenomenon is attributed to the repaid depletion of electronic charges in the capacitor.(2)The electric field threshold of the nonlinear mode at low optical excitation energy can be affected by the spot pattern and location of optical pulses as well as the temperature of the PCSS.When the optical pulse pattern is parallel to the electrodes,the electric field threshold increases linearly(15 k V/cm-22 k V/cm)with the increase of the distance between the spot center and the cathode(0.2 mm-2.8 mm).As the optical pulse pattern is perpendicular to the electrodes,the electric threshold firstly drops from a high value to a low value(27 k V/cm-16 kV/cm),then increases again(27 k V/cm),forming a fitting curve similar to a parabola.The influences of the Franz-Keldysh effect and thermal effect are observed as the temperature is changed from-12°C to 24°C.Correspondingly,the electric field threshold decreases linearly as optical pulse pattern is perpendicular to the electrodes.In contrast,when the spot is perpendicular to the electrodes,the electric field threshold shows the trend of a curve.(3)A Monte-Carlo simulation program based the theory of photo-activated charge domain(PACD)is developed to study the operation mechanisms of nonlinear mode in GaAs PCSS.Considering the two cores of PACD theory,the impact ionization and recombination radiation process are simulated.Based on the impcact ionization,the formation and transport of PACD are attributed mainly to the recombination radiation.The internal machnism including nagetive differential mobility,the carriers' satellites valley occupancy and electric-velocity charateristics are investigated,respectively.(4)Based on the simulation of PACD's formation and transport,the non-linear operation of Ga As PCSS at ?J optical excitation is studied.Results show that,there are two critical conditons for this particular characteristic of PACD:(a)electric field for negative differential conduction,i.e.,above 4.0 kV/cm.(b)the density of photo-activated carriers about 1.0×1013cm-3 is necessary that could make the internal electric field is comparable to the bias electric field.It is verified that there are five processes exsit in the non-linear operation,i.e.,high electron density zone,formation of PCAD,formation of current filaments,transport filaments and absorption filaments.The simulated transport velocity of filaments is about 2.0×108cm/s,appearing like the tree branches,which is also in good agreement with the infrared camera photos in literatures.The ultrafast rise time is resulted from the transient absorption of the head of filaments by electrodes.The primary cause of the formation of the lock-on effect and the high multiplication characteristics is explained.It demonstrates the influences of the carrier concentration and bias field on the transport characteristics and reveals the operation mechanism of GaAs PCSS.