Effective lifetimes in high quality silicon devices

A correct interpretation of pn junction current-voltage and pulsed MOS capacitance-time data allows space-charge region width dependent generation parameters to be separated from bulk controlled recombination parameters. This is very important for the correct extraction of generation lifetime and minority carrier diffusion length, especially for intrinsically gettered devices where the recombination center density varies through the device. Methods to do this are discussed in this paper.     

Boundary conditions for pn heterojunctions

Boundary conditions for the excess minority carrier concentrations at the space-charge region (SCR) edges are developed and used to evaluate the current-voltage characteristics of pn heterojunctions. Carrier transport by drift and diffusion within the SCR, by thermionic emission across the interface, and by minority carrier diffusion within the quasi-neutral bulk are included. When transport across the interface is slower than that across the SCR, the results reduce to the emission-diffusion model of Perlman and Feucht. Alternatively, when carrier transport within the SCR is the limiting process, a result analogous to the diffusion theory of metal-semiconductor diodes results. Finally, if the current is limited by minority carrier diffusion within the quasi-neutral bulk, the quasi-Fermi levels are constant within the SCR and the results are similar to those for pn homojunctions.     

Transition region behavior in abrupt,forward-biased pn-junctions

The transition region for abrupt p-n junctions, symmetric and asymmetric, is analyzed in terms of carrier densities at the neutral bulk-space charge region boundaries and the location of these boundaries with respect to the metallurgical junction. The non-equilibrium semiconductor problem is simplified by defining an effective intrinsic carrier concentration and an effective Fermi energy.The relation between the split in the quasi-Fermi levels and applied junction voltage is obtained. Also, density relationships are derived in terms of externally applied junction voltages. A low, high and very high injection formulation results. The transition region width is determined by solving Poisson's equation for a double region problem approximately. MOS concepts such as inversion and depletion are shown to apply to the forward biased junction.     

On the determination of the minority carrier lifetime from the reverse recovery transient of pnR diodes

The determination of the minority carrier lifetime of pn diodes with abrupt junctions and recombination (R) contacts is discussed. It is confirmed that the base transit time of diodes with sufficiently small base widths (less than approximately three minority carrier diffusion lengths) affects the reverse recovery transient and that large error can be introduced if the minority carrier lifetime of a diode with a small base width is computed using an expression which was derived for diodes with an infinite base width. Approximations to the general expression for the minority carrier lifetime as a function of the storage time and base width are developed, and the errors associated with their use are analyzed. In addition the effect of measurement errors on the determination of lifetime in short base diodes is discussed.A procedure is described which can be used to compute the minority carrier lifetime of pnR diodes which the ratio of the base width to the minority carrier diffusion length is larger than some small number. The applicability of the procedure and the approximations which are developed are a function of the ratio of the diode base width to the minority carrier diffusion length $\text{(}\text{W}\text{L}\text{)}$, and the forward to reverse current ratio $\text{(}\text{I}{}_{\mathrm{f}}\text{I}{}_{\mathrm{r}}\text{)}$ at which the experimental measurements are made. An example is given which shows that less than 1 per cent error is introduced in the computed lifetime by the approximations if the lifetime is computed from measurements made at $\text{I}{}_{\mathrm{f}}\text{I}{}_{\mathrm{r}}\text{= 4}$ and 8 for diodes such that $\text{W}\text{L}\text{> 2 × 10}{}^{\mathrm{?2}}$.     

Analysis of LED degradation; proton-bombarded GaAs

An analysis is given of the degradation of light-emitting, Zn-diffused GaAs diodes after proton bombardment. Use is made of a generally applicable method by which the external bulk quantum efficiency and the injection efficiency of an LED can be determined separately. Owing to the increase of non-radiative recombination being larger in the bulk than in the space-charge region, the injection efficiency at constant current first starts to increase and then decreases as a function of irradiation fluence. Furthermore, it is shown that the apparent bulk quantum efficiency decreases superlinearly with the irradiation fluence. This is consistent with the theory for a linear-graded pn junction and the assumption that the concentration of additional killer centres is directly proportional to the irradiation fluence.     

A simplified theory of the p-i-n diode

A simplified theory of the p-i-n diode is developed for the case of heavy injection in the base, and light injection in the end regions. The current I is taken as the fundamental parameter. The carrier densities n₁ and n₂ at the base boundaries are given directly as functions of the current I by means of simple approximate expressions. The approximation improves with increasing base width and increasing current. It is always within 15% of the correct value for base widths of about 2 diffusion lengths or higher. The accuracy is much better still on the voltage drop across the device, as calculated from the approximate equations for the carrier densities within the base.     

The effect of current suppression in the pn−n+ diode of integrated injection logic

The j-V characteristics of the pn^?n⁺ diode are derived by combining the different current components in the n^? region of the diode for various bias voltages. For infinitly fast recombination, the expression for the current density as a function of bias reduces to that of a conventional one-dimensional theory. For finite recombination, however, the expression deviates significantly from the usual expression. In fact, contrary to what would be expected from the conventional theory, the current increases as the width of the neutral n^? region increases for a constant bias. Finally the results of the analysis of the pn^?n + diode are applied to an analysis of the current gain in the integrated injection logic (I²L) device. A qualitative discussion of the current gain for the lateral pnp transistor and for that of the vertical npn transistor is given.     

Low forward drop JBS rectifiers fabricated using submicrontechnology

This paper demonstrates the impact of using submicron technology (0.5 μm design rules) on JBS Rectifiers to achieve very low forward voltage drops while maintaining good high temperature reverse blocking characteristics. Two dimensional numerical simulations show that decreasing P⁺-junction width and depth improves the on-state voltage drop by improved utilization of the active area for the Schottky region and improved spreading of majority carrier current from the Schottky contact. Experimental results that demonstrate the capability to reduce the forward drop from 0.5 V to 0.25 V, while operating at up to 125°C-175°C with good reverse blocking capability, are presented. The tradeoff curves between forward drop and reverse leakage current show 45× reduction in leakage current for the same forward drop as compared to previous reports on JBS rectifiers. Power dissipation analysis indicates higher operating temperatures, (100°C for Ti-JBS and 175°C for Cr-JBS rectifiers) with reduced heat sink sizes for the JBS Rectifiers when compared to the conventional Schottky Barrier Diode (SBD)     

Mechanisms of device degradation in organic solar cells: Influence of charge injection at the metal/organic contacts

Organic solar cells suffer from device degradation under various undesirable ambient conditions, which limits their efficiencies and applications a lot. The frequently appeared S-shaped current density–voltage (J–V) characteristics just reflect this effect and make its underlying mechanisms even obscurer for experimentalists. By performing device model simulation, we investigate the J–V curves of a organic bulk heterojunction solar cell for different interfacial charge injection conditions. We find the S-shaped kinks appear when both electron and hole injection barriers exceed 0.3 eV, which is attributed to the shortage of dark injection carriers. For the nearly Ohmic contacts, the open circuit voltage is significantly reduced. The quantitative study suggests it arising from the majority carrier accumulation and the induced local field variation at the vicinity of the contacts. S-shaped kink is gradually eliminated with increasing carrier mobilities. Finally, we find the decreasing of fill factor under enhanced light absorption, which results from the drastic bimolecular recombination and thus signifies remarkable internal losses.     

Tunnel injection and power efficiency of InGaN/GaN light-emitting diodes

The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n junction voltage and temperature are presented. It is shown that defectassisted hopping tunneling is the main transport mechanism through the space charge region (SCR) and makes it possible to lower the injection barrier. It is shown that, in the case of high hopping conductivity through the injection barrier, the tunnel-injection current into InGaN band-tail states is limited only by carrier diffusion from neutral regions and is characterized by a close-to-unity ideality factor, which provides the highest quantum and power efficiencies. An increase in the hopping conductivity through the space charge region with increasing frequency, forward bias, or temperature has a decisive effect on the capacitance-voltage characteristics and temperature dependences of the high-frequency capacitance and quantum efficiency. An increase in the density of InGaN/GaN band-tail states and in the hopping conductivity of injection barriers is necessary to provide the high-level tunnel injection and close-to-unity power efficiency of high-power light-emitting diodes.     

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.     

p-i-n diode recovery storage time

Using numerical methods, the high injection equations for carrier concentration in the i-base of a p-i-n diode are solved to determine storage time during diode recovery with equal forward and reverse currents. Graphs are obtained which relate the storage time to the i-base width, the high injection lifetime of carriers in the i-base, the current density, and the h parameters which characterise recombination in the p and n emitters. The most important results are that the storage time increases on decreasing the base width from a large value and becomes smaller as the current increases and that recombination in the p emitter decreases storage time more than recombination in the n emitter. The numerical method used to obtain these results is described.     

The minority-carrier injection in the completely depleted MSM structure

The effect of the semiconductor parameters on the injection of carriers in a metal-semiconductor-metal structure was analyzed. It was assumed that the semiconductor is completely depleted when no external voltage is applied. We found that in that case minority carrier injection dominates over majority carrier injection. Consequently, numerical analysis shows that the current density in a metal-n-type semiconductor-metal structure increases with increasing electron barrier height and/or with decreasing donor concentration. The hole injection current dominates over the electron injection current. These relationships are opposite to those occurring both in a single MS junction and in a partly-depleted MSM junction where the minority carrier injection ratio is known to be negligibly small.     

Organic doped/undoped interface based diode structure: Distinct mechanisms underlying forward and reverse bias

Organic semiconductor diodes fabricated using doped/undoped (high-low) homojunction has the potential of providing controlled and high quality current density-voltage (J-V) characteristics based on majority carrier transport. We study mechanisms of transport underlying such characteristics both in forward and reverse bias regimes of typical doped/undoped homojunction organic diodes fabricated using 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA). We study the J-V characteristics over a wide temperature range (200–300 K), and by varying the intrinsic layer thickness between 10 and 100 nm. The forward bias current, before entering into the space charge limited regime, is exponential over several orders of magnitude with the slope being temperature independent for all intrinsic layer thickness down to 10 nm. The reverse bias characteristics, on the other hand, are highly sensitive to the thickness of the intrinsic layer. While the forward bias is controlled by tunneling at the homojunction interface, the reverse bias is controlled by the interface of cathode (aluminum in this case) and the intrinsic layer. We show that the reverse current is due to Fowler-Nordheim tunneling across a barrier height, which is temperature independent but is sensitive to the layer thickness of the intrinsic layer. The origin of the thickness dependence of barrier height (0.45–0.72 eV for 10–20 nm) is attributed to the change in background carrier concentration in the intrinsic layer due to diffusion of carriers from the highly doped side. The results clearly show that the diffusion length of the majority carriers is approximately 1 nm and is comparable to the nearest neighbor jump distance.     

The exact calculation of the electric field and potential at the metallurgical boundary of abrupt n1−n2 homojunctions

Exact analytical expressions for the potential and electric field at the metallurgical boundary of abrupt n₁−n₂ homojunctions in function of the donor concentrations are given. The graph of the potential drop over the higher doped n₂ region vs the impurity concentration ratio results in a universal curve when the majority carrier densities in both regions are much larger than the intrinsic carrier density. It is also demonstrated that the potential drop over the higher doped region can be approximated by the value of kT/q as soon as the impurity concentration ratio is larger than 10².     

Emission-line broadening of current tunable InAsSbP/InAsSb/InAsSbP heterostructure lasers

The dependence of emission-line broadening on the drive current was studied at 50–80 K for tunable InAsSbP/InAsSb/InAsSbP double heterostructure lasers operating in the 3.3–3.4 μm spectral region. For a small increase of the injection current I over the threshold current I _th, the line width depends on the I-I _th difference hyperbolically, in accordance with the Schawlow-Townes and Henry theories that assume a homogeneous distribution of the nonequilibrium carrier concentration across the resonator width. With the current raised to (3–4)I _th, line narrowing ceases and the line starts to broaden with increasing current. The observed line broadening is explained by the effect of the nonequilibrium carrier concentration gradient between the middle of the resonator and its edges. In tunable lasers, this gradient increases with current, the lasing wavelength simultaneously decreasing. The minimal width of the lasing line is 10–20 MHz. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 12, 2000, pp. 1468–1471. Original Russian Text Copyright ? 2000 by Imenkov, Kolchanova, Kubat, Civish, Yakovlev.     

Microwave oscillations in pnp reach-through BARITT diodes

Studies have been made on the microwave oscillations of reach-through p⁺np⁺ and related structures operated as BARITT diodes (BARrier Injection Transit Time diodes).The mechanisms responsible for the microwave oscillatins are the exponential increase of the local carrier population at the forward-biased pn junction and the transit-time delay of injected carriers transversing the drift region. The small-signal impedance and noise measure of the device are calculated based on (1) the thermionic injection and the space-charge-limited effects and (2) the separation of the drift region into a low-field region and a saturated-velocity region.Microwave CW oscillatins have been obtained from p⁺np⁺ BARITT diodes made from an epitaxial n on p⁺ silicon substrate with epitaxial layer thickness of 8 μm and doping concentration of 5 × 10¹⁴cm^?3. Microwave CW power of the order of a few milliwatts has been obtained at 7 GHz with efficiency greater than 1 per cent. Good agreement has been obtained between the measured and the calculated small-signal impedances.     

Conditions at a p-n junction in the presence of collected current

An abrupt p-n junction, such as occurs at the collector junction of an n-p-n transistor, is considered. The ratio of n- to p-region conductivity is taken to be very high, so that the transition region is restricted almost entirely to the p-region. The electron density distribution n within the transition region is investigated as a function of the applied reverse bias V_c, and of the minority carrier electron current density J which is injected into the transition region from the neutral p-region. It is shown that significant departures occur from the conventional solutions in which the presence of current is neglected. In particular, the electron density n_c at the plane of injection and the transition region thickness w_t, used as collector boundary conditions in the analysis of transistor operation, are shown to be current-dependent.

Two cases are considered. In Case I, applicable to transistors with an epitaxial layer in the base region below the collector, the electron velocity is assumed much less than the limiting drift velocity. For low injection level, where the minority carrier density n is everywhere less than the equilibrium majority carrier density p_p, the transition region is essentially a depletion region and the injected electrons move in an electric field determined uniquely by the applied voltage. It is shown that n_c ∝ J and w_t ∝ V_c^1/2. For high injection level, when n p_p, the transition region is essentially an accumulation region, and conditions of space-charge-limited current flow are established for which n_c ∝ J^2/3 and . The low-level injection results are primarily of interest as analytical extensions of the classical treatment. The high-level injection results are also relevant to the treatment of the dielectric diode.

In Case II, applicable to most alloy and diffused-base transistors, the electron velocity is assumed equal to the limiting drift velocity throughout the transition region. Mobile carrier depletion at low injection again gives way to accumulation at high injection. The functional relationships remain as for Case I at low injection, but become at high injection.

Semi-quantitative and detailed quantitative treatments are developed, and normalized graphs of the minority carrier density as a function of distance within the transition region are given for various junction voltages and injected currents.     

台面尺寸对垂直腔面发射激光器电学特性的影响

采用求泊松方程、电流密度方程、载流子扩散方程以及有源层结压降方程自洽解的方法,计算了不同台面大小的台面结构垂直腔面发射激光器内部的电势分布和有源层中的注入电流密度、载流子浓度及结压降分布.结果表明,台面尺寸对垂直腔面发射激光器内部的电势分布和有源层中的注入电流密度、载流子浓度及结压降分布有重要影响.在一定的外加电压下,随着台面尺寸减小,有源层中心处的注入电流密度、载流子浓度和结压降急剧减小,垂直腔面发射激光器性能恶化.     

Microscopic theory of gain, absorption, and refractive index insemiconductor laser materials-influence of conduction-bandnonparabolicity and Coulomb-induced intersubband coupling

The influence of the conduction-band nonparabolicity and Coulomb coupling between different electronic subbands and different hole subbands on gain, absorption, and refractive index in semiconductor heterostructures is investigated. We implement these features into a fully microscopic approach. At low carrier densities, the nonparabolicity leads to a steeper increase of the absorption for increasing transition energy. In this regime, the Coulomb subband coupling allows for a shift of oscillator strength to energetically lower transitions. In the gain regime, the conduction-band nonparabolicity is shown to reduce the gain width for a given carrier density and to strongly modify the corresponding refractive index. The Coulomb coupling is especially important to determine the correct energetic position and density dependence of the gain maximum. In addition, it leads to a steeper transition from the gain to the absorptive region