Space Charge Limited Effect Research Articles

A Modified Steady-State Method for Space Charge-Limited Effect of SGEMP

Excitation of the system-generated electromagnetic pulse (SGEMP) effect by high-dose X-rays in a cavity may induce the high space charge-limited (SCL) effect. This nonlinear effect limits the photocurrent from the electron emission surface, thereby limiting the increase in the SGEMP electromagnetic field. A modified steady-state approximation method is derived, which considers the influence of electron velocity and the relativistic effect on the number and motion of electrons, respectively. It can be applied to simulate the motion of electrons with a discrete spectrum. Compared with the original method, this method obtains more accurate results for an increase in energy. The method can be applied to the simulation of a cavity SGEMP with the boundary layer treatment. The results show that the increase in the electron yield may induce a nonlinear change in the peak value of the electric field; however, the frequency spectra are less affected. The influence of the SCL effect on lower-energy electrons is higher than that on higher-energy electrons. With an increase in lower-energy electrons in the energy spectrum, the excited electric field is affected more significantly by changes in the energy spectrum, owing to the SCL effect.

Calculation of Characteristic Time of Space Charge Limited Effect of SGEMP

When an X-ray beam illuminates a material’s surface, the backscattered photo-electrons are emitted from the surface and stimulate the system-generated electromagnetic pulse (SGEMP). With the increase in X-ray flux, the space charge-limited (SCL) effect occurs and a relatively thin layer develops near the emitting surface. The characteristic time is defined as the occurring time of the SCL effect. In this article, an accurate analytical equation is derived by applying the momentum conservation equation for the mono-energetic electrons and verified by the particle-in-cell (PIC) simulation. But for electrons with velocity distribution, the analytic equation using the average velocity approximation, instead of the exponential distribution, is not accurate, as the interaction between the low-energy and high-energy electrons is ignored. PIC simulation results show that the characteristic time of the SCL effect is longer by the PIC simulation than the average energy approximation when the average energy of the electrons is relatively low and vice versa.

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density—voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15 V < V < 0.6 V, the transport property is dominated by diffusion or recombination in the junction space charge region, while at higher voltages (V > 0.6 V), the space charge limited effect becomes the main transport mechanism. The current—voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm2, respectively.

Influence of the a-SiGe:H thickness on the conduction mechanisms of n-amorphous-SiGe:H∕p-crystalline-Si heterojunction diodes

We have fabricated, electrically characterized and simulated n-type hydrogenated amorphous silicon germanium alloys on p-type crystalline-silicon heterojunction diodes with three different a-SiGe:H layer thicknesses: 37, 86, and 200nm. The capacitance–voltage results confirm the existence of abrupt heterojunctions. The conduction and valence-band discontinuities of the heterojunctions and the electron affinity of the n-type a-SiGe:H films were obtained. The conduction mechanisms were determined by analyzing the temperature dependence of the current–voltage characteristics. The results show that at low forward bias (V&amp;lt;0.45V) the diodes with thinner amorphous layers (37 and 86nm) are dominated by recombination in the a-SiGe:H depletion region, whereas the thicker diode (200nm) is dominated by multistep tunneling through depletion region. In addition, at high forward bias (V&amp;gt;0.45V) the space-charge limited effect becomes the main transport mechanism in all the measured devices. The increase in the amorphous layer thickness also causes an increase in the leakage reverse current. Numerical simulations support the proposed transport mechanisms.

Electrical characterization of n-type a-SiGe:H/p-type crystalline-silicon heterojunctions

Heterojunctions with an n-type hydrogenated amorphous silicon germanium (a-SiGe:H) on p-type crystalline-silicon heterojunctions were fabricated and electrically characterized. The electrical characterization was made by current density–voltage (J–V) and capacitance–voltage (C–V) measurements. The C–V results confirm the existence of an abrupt heterojunction. The temperature dependence of the J–V curves shows that in the forward conduction at low bias voltage (V < 0.45 V) the current density is determined by the recombination on the n-type a-SiGe:H depletion region, while at higher voltages (V > 0.5 V), the space charge limited effect becomes the main transport mechanism. The conduction and valence band discontinuities of the heterojunction and the electron affinity of the n-type a-SiGe:H film were calculated using Anderson's model. Under reverse bias conditions the J–V curves suggest that the current density is limited by hopping through the localized states into the gap. One-dimensional (1D) simulations support the proposed transport mechanisms.

TSDC and the role of space charge in polyacrylonitrile/methylacrylate copolymer films

AbstractSpace charge effects on direct current (DC) conductivity and the thermally stimulated depolarization current (TSDC) in the semicrystalline polyacrylonitrile–methylacrylate [P(AN–MA)] films have been investigated. The conduction was found to be controlled by space charge limited effect. On the other hand, the TSDC studies point to a superposition of dipole orientation and space charge effects. With the aid of the thermal sampling technique, the complex spectrum could be resolved into its elementary processes. Curve fitting has been carried out and peak parameters were determined. Information related to the motion of dipolar groups could be obtained. The compensation effects is observed to be valid and the value of the compensation temperature (Tc = 343 K) greatly differs from the glass transition temperature (Tg = 383 K). From the DC and TSDC studies considerations, the origin of the three conduction regions appearing in the 1n σ vs. 1/T could be explored. © 1994 John Wiley &amp; Sons, Inc.

Thermionic injection and space-charge-limited current in reach-through p+np+ structures

The current transport mechanisms of reach-through p+np+ and its related structures have been studied. It has been established that when the applied voltage is slightly greater than the reach-through voltage, at which the n layer is completely depleted, the current increases exponentially with voltage by thermionic injection or diffusion over the potential barrier. The current-voltage relationship is given by J ≅ A*T2 exp[−q(VFB − V)2/4kTVFB], where A* is the effective Richardson constant, T the temperature, V the applied voltage, and VFB the flat-band voltage defined as qNDL2/2εs, where ND and L are the ionized impurity density and the length of the n layer, respectively. When the injected carrier density rises to a value comparable to the impurity density, the space-charge-limited (SCL) effect causes the current to vary less rapidly with the applied voltage. The SCL effect is derived based on an accurate expression of the velocity-field relation, i.e., vs/(1 + Es/E), where vs is the scattering-limited velocity and Es is the critical field given by the ratio of vs to the low-field mobility. In the high-current limit, we obtain the linear current-voltage expression J ≃ qNDvs(V/VFB). Experimental structures are made from epitaxial n on p+ silicon substrate with an epitaxial layer thickness of 8.5 μm and doping concentration of 5 × 1014 cm−3. The second p+ layer of about 1 μm is formed by diffusion. Good agreement has been obtained between the experimental results and theoretical predictions.