Sergey N. Vainshtein

Ultrahigh field multiple Gunn domains as the physical reason for superfast (picosecond range) switching of a bipolar GaAs transistor

The superfast (picosecond range) high-current switching observed recently in a GaAs junction bipolar transistor is explained by practically homogeneous carrier generation in the volume of the switching channels by a moving train of avalanching Gunn domains of large amplitude. The very fast (∼200ps) reduction in the collector voltage is determined by shrinkage of each domain, provided the negative electron mobility in ultrahigh electric fields is taken into account and current filamentation takes place. The results of one-dimensional simulations show good quantitative agreement with experimental voltage and current wave forms when the simulated switching area is equal to the summed areas of the filaments observed in the experiment.The superfast (picosecond range) high-current switching observed recently in a GaAs junction bipolar transistor is explained by practically homogeneous carrier generation in the volume of the switching channels by a moving train of avalanching Gunn domains of large amplitude. The very fast (∼200ps) reduction in the collector voltage is determined by shrinkage of each domain, provided the negative electron mobility in ultrahigh electric fields is taken into account and current filamentation takes place. The results of one-dimensional simulations show good quantitative agreement with experimental voltage and current wave forms when the simulated switching area is equal to the summed areas of the filaments observed in the experiment.

Avalanche transistor operation at extreme currents: physical reasons for low residual voltages

Abstract Low residual voltages (70–95 V) were observed in our experiments with Si n + –p–n 0 –n + avalanche transistors at current pulses of a few nanoseconds with an amplitude of ∼100 A. The voltages are much lower than that predicted by a simple theory of avalanche transistor switching. A physical explanation is suggested and a numerical model is produced which explains the low residual voltages by a strong rebuilding of the electric field domain in the n 0 collector. This reconstruction takes place when the current density significantly exceeds a critical value, which is associated with a drift of equilibrium carriers in the collector at a saturated velocity. The final electric field distribution across the collector region was shown to be greatly dependent on both total current density and the ratio of the injection current component to the total current. A voltage drop of less than 50 V was calculated at high total currents (∼10 5 A/cm 2 ) provided that the ratio of the electron injection current to the total current exceeded 0.7. The maximum possible value of this ratio is determined by the fundamental properties of the semiconductor material and plays an essential role in the phenomenon. By contrast, we did not succeed in obtaining any appreciable reduction in the residual voltage for p + –n–p 0 –p + transistors either experimentally or numerically. The physical reasons for this behaviour were found to be mainly determined by the difference in the electron and hole mobilities.

Properties of the transient of avalanche transistor switching at extreme current densities

Avalanche transistor switching at extreme currents is studied under conditions in which the charge of the excess carriers drastically rebuilds the collector field domain, causing fast switching and a low residual voltage across the switched-on device. The dynamic numerical model includes carrier diffusion and considers different dependencies of the velocities and ionization rates for the electrons and holes in the electric field. These dependences determine the principal difference in the switching process between n/sup +/-p-n/sub 0/-n/sup +/ and p/sup +/-n-p/sub 0/-p/sup +/ structures. Reasonably good agreement is found between the simulated and measured temporal dependences of the collector current and voltage drop across the device for a particular type of avalanche transistor. Certain differences in the switching delay can partly be attributed to limitations in the one-dimensional (1-D) approach. It is now certain that collector domain reconstruction defines the transient in a n/sup +/-p-n/sub 0/-n/sup +/ transistor at high currents, but is not very pronounced in a p/sup +/-n-p/sub 0/-p/sup +/ transistor. Some nontrivial features of the device operation are found, depending on the semiconductor structure. In particular, it is shown that the thickness of the low-doped collector region affects mainly the switching delay, and does not significantly effect the current rise time.

Multistreamer regime of GaAs thyristor switching

GaAs bipolar thyristors have been used to obtain current pulses of over 100 A with rise times less than 600 ps and load resistance of approximately 0.4 /spl Omega/. The maximum voltage has been shown to exceeded 500 V in some cases. To interpret the experimental results a multichannel switch regime is proposed. Analysis of the experimental data suggests the possibility of a further increase in the maximum amplitude of the current pulse. >

Negative differential mobility in GaAs at ultrahigh fields: Comparison between an experiment and simulations

Direct measurement of the electron velocity vn at an extreme electric field E is problematic due to impact ionization. The dependence vn(E) obtained by a Monte Carlo method can be verified, however, by comparing simulated and experimental data on superfast switching in a GaAs bipolar transistor structure, in which the switching transient is very sensitive to this dependence at high electric fields (up to 0.6MV∕cm). Such a comparison allows the conclusion to be made that the change from negative to positive differential mobility predicted earlier at E∼0.3MV∕cm should not happen until the electric field exceeds 0.6MV∕cm.

High power gain-switched laser diode using a superfast GaAs avalanche transistor for pumping

Multiwatt single picosecond optical pulses were generated by gain-switched laser diodes using for pumping a superfast GaAs switch, which produces 1–10A current pulses with a duration comparable to the lasing delay. Good quantitative agreement was found between the measured and simulated optical responses and time-resolved spectra when lasing occurred before the trailing edge of the current pulse, while the measured single optical pulse generated near the trailing edge drastically exceeded that in the simulations. This difference is attributed to the effect of additional population of the quantum well by carriers accumulating earlier in the optical confinement region.

Nondestructive current localization upon high-current nanosecond switching of an avalanche transistor

Very good quantitative agreement was found between the experimental and simulated switching transients of a Si avalanche transistor at extreme currents. Two-dimensional (2-D) simulations were performed using the device simulator ATLAS (Silvaco Inc.). Marked current localization was found, which was of a nondestructive character with nanosecond current pulses due to a very significant reduction in the residual voltage across the transistor at high current densities and specific location of the region of intensive heat generation. The device operates reliably at a sufficiently low repetition rate (of a few kilohertz) despite the very high local temperature (/spl sim/750/spl deg/ K) found near the n/sup +/ collector at the end of the switching transient.

Laser diode structure for the generation of high-power picosecond optical pulses

A laser diode structure is designed and tested that permits the generation of high-power (∼100 W) picosecond-range optical pulses. Direct current pumping is used with a current pulse duration of a few nanoseconds and a current amplitude of ∼105 A/cm2. The main distinguishing feature of the structure is separation of the electron injector (p–n junction) from the active region by a potential barrier. The optical gain in the active region is controlled by the transverse electric field, which is determined by the magnitude of the current at each instant. The design allows emission wavelength control by means of band gap engineering.

Analyses of the picosecond range transient in a high-power switch based on a bipolar GaAs transistor structure

The superfast (/spl sim/200 ps) switching observed lately in a GaAs bipolar-junction transistor (BJT) structure is analyzed. Contrary to all known bipolar semiconductor switches, a superfast transient occurs in this GaAs BJT due to practically homogeneous and simultaneous high-rate carrier generation across the entire thickness of the blocking region. This generation is provided by a comb of powerfully avalanching Gunn domains moving across the blocking n/sub 0/ layer and covering whole layer thickness at each instant. Generation of the multiple avalanching Gunn domains is accompanied by current filamentation, and at first glance the total area of the structure should not affect the switching process. It is shown however, that the charge accumulated in the barrier capacitance of the collector junction can cause a further drastic reduction in the switching time and in the residual voltage across the switch. Simulations performed with the barrier capacitance taken into account show excellent agreement with the experimental data for a transistor prototype, while a further significant reduction in the switching time to several dozens of picoseconds, and in the residual voltage to a dozen volts, is predicted for a device area that is an order of magnitude larger.

Significant Effect of Emitter Area on the Efficiency, Stability and Reliability of Picosecond Switching in a GaAs Bipolar Transistor Structure

A drastic reduction in the residual voltage (from ~ 100 V to a few volts) and a significant (factor of ~ 2) increase in the dU/dt switching rate is demonstrated experimentally in the superfast ( ~ 200 ps) avalanche switching of a GaAs bipolar junction transistor with increased emitter area. This result is not a trivial one as only a small number of conductive channels of a few micrometers in diameter participate in the transient independently of the emitter size, while the remaining (passive) part of the structure supplies the switching channels with the currents circulating inside the chip, which makes the impact ionization in the filaments even more powerful. Excellent agreement was found between the experiment and a ¿two-transistor¿ model specially developed here, with one transistor simulating the switching channels and the other the nonswitched part of the structure. Much higher switching stability and reproducibility and much lower power dissipation were observed in the structure with increased emitter area.