Valentin Yuferev

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.

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.

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.

Origin of photoresponse in heterophase ferroelectric Pt/Pb(ZrTi)O3/Ir capacitors

A steady-state short-circuit photocurrent is measured in polycrystalline Pb(ZrTi)O3(PZT) films with Pb excess. It is found that although the photocurrent is directed opposite to ferroelectric polarization, it is not the depolarization current of the ferroelectric. To explain this, the authors assume that Pt/PZT/Ir capacitor consists of PZT grains and semiconductor PbO phase segregated on PZT grain boundaries during the PZT formation. In such medium, the current across the film flows through channels formed by PbO phase, where an electric field is generated by uncompensated polarization charge of PZT grains. Using this concept, the authors described the experimental dependence of photocurrent on polarization.

Thin-film capacitor M/Pb(ZrTi)O3/M as a polarization-sensitive photocell

A steady-state short-circuit photocurrent of preliminarily polarized submicron capacitors with a polycrystalline Pb(ZrTi)O3 (PZT) film is investigated under irradiation by light with a wavelength λ > 0.4 μm. The structures with different M/PZT interfaces that differ in the leakage current by more than an order of magnitude are found to demonstrate virtually the same value of the photocurrent, which is always directed opposite to the ferroelectric polarization of the PZT film. Although the magnitude of photocurrent is determined by the degree of polarization of the film, the observed photocurrent is not a depolarization current of the ferroelectric film. Therefore, the M/PZT/M capacitor behaves like a polarization-sensitive photocell. Within the proposed theory of a heterophase medium, the dependence of the photocurrent on the magnitude of the preliminary polarization is calculated and proves to be in reasonable agreement with the experimental results.

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.

Spreading resistance microscopy of polycrystalline and single-crystal ferroelectric films

Thin films based on lead zirconate titanate with stoichiometric composition near the morphotropic boundary have been studied using atomic-force microscopy methods. The dependence of the local conductivity on the local polarization direction has been observed for all samples, independently of substrate type, deposition method, and film thickness. It has been shown that the current response to the applied voltage exhibits a long current relaxation, about several tens of seconds, which is two to three orders of magnitude greater than the current relaxation time in an external circuit, associated with the ferroelectric domain switching. The conductivity features have been explained by recharging of traps localized at ferroelectric grain boundaries near electrodes and involved in polarization charge screening.

Investigation of the polarization dependence of the transient current in polycrystalline and epitaxial Pb(Zr,Ti)O3 thin films

The polarization dependence of the current in epitaxial and polycrystalline (with conductive grain boundaries) Pb(Zr,Ti)O3 (PZT) films is studied using direct-current (dc) measurements and scanning spreading current microscopy. Both methods show identical results in micro- and nanoscale ranges. The current response from the film to the applied bias contains a long relaxation component that depends on the bias rise rate and polarization direction, exhibiting current peaks near the coercive force value. The polarization dependences of the current for polycrystalline and epitaxial films are found to be fundamentally different. The current of the polycrystalline film is much higher when the bias is directed against the polarization, whereas the current of the epitaxial film is higher if the bias and polarization directions coincide. All films exhibit current hysteresis of non-ferroelectric (clockwise) direction with decreasing bias. It is also shown that the polarization dependences of the transient current in both polycrystalline and epitaxial films are similar to the polarization dependence of the photovoltaic current in these films.

Picosecond range switching of a GaAs avalanche transistor due to bulk carrier generation by avalanching Gunn domains

Superfast high current switching of a GaAs-based JBT in the avalanche mode has been achieved experimentally for the first time. A very fast reduction in the voltage across the transistor was observed (~ 200-300 ps) and the amplitude of the current pulses ranged from 2 to 130 A depending on the load resistance. It was observed experimentally that the switching occurs in a number of synchronized current channels with a characteristic diameter of <~10 microns. A 1D simulation code was developed and the switching transient for a single channel was simulated, with the external circuit incorporated into the simulations. Photon-assisted carrier transport and negative differential electron mobility were taken into account in the theoretical model. The former does not play an appreciable role in the 1D switching transient, although the latter determines superfast switching at extreme current densities (> 1 MA/cm2). Superfast switching occurs due to the appearance of a number of Gunn domains at any instant (up to ~ 20 domains across a collector region ~30 microns in thickness). These domains of huge amplitude (up to ~700 kV/cm) are moving towards the cathode and give rise to extremely high ionization rates across the volume of the channel in the n0 collector region. The simulations provide a fairly reliable interpretation of the experimentally observed switching time, which is shorter than that in Si avalanche transistors by a factor of ~15. The new device is fairly attractive, e.g. for feeding pulsed laser diodes when the current rise time should be shorter than the lasing delay.