Metal-N-type semiconductor ohmic contact with a shallow N+ surface layer

Most papers covering metal-semiconductor ohmic contact theory which have been published up to date consider systems with homogeneous impurity concentration in the semiconductor. However, there are techniques of ohmic contact formation on nondegenerate semiconductor where only a very shallow surface layer is impurity enriched. In this paper a model of such contacts is proposed and a simple approximate analytical expression for the specific resistivity is derived. If the impurity concentration in the surface layer is very high, the contact specific resistivity is essentially proportional to N_B^?1, N_B being the semiconductor substrate impurity concentration. To make a good ohmic contact, it is sufficient that the width of the heavily doped surface layer be equal to the equilibrium contact depletion region width. Any further enlargement of the enriched layer practically does not influence the total sample resistance due to the dominant share of the semiconductor body resistance. Experimental results confirm these conclusions qualitatively.     

Noise in near-ballistic n+nn+ and n+pn+ gallium arsenide submicron diodes

Direct-current (d.c.) characteristics and noise measurements in the range 1 Hz-25 kHz are reported for n⁺nn⁺ and n⁺pn⁺ near-ballistic devices, with n regions (p regions) of 0.4 μm (0.45 μm), fabricated by molecular beam epitaxy at Cornell. The n⁺nn⁺ mesa structures show very low 1/? noise. indicating a Hooge parameter α_H = 6.0 × 10^?. This very low noise is attributed to the near absence of phonon collisions. The thermal (? like) noise above 1 kHz is equal to Nyquist noise at the lowest currents, rising to slightly above Nyquist noise for high currents, indicating the presence of carrier drag effects. The n⁺pn⁺ noise, on the contrary, is quite high. It seems to be associated with the ambipolar effects occurring for low injection of electrons in the p region. The importance of noise measurements for confirming ballistic or near-ballistic behavior is discussed.     

Junction potentials of strongly illuminated n+ - p - p+ solar cells

The low-junction (LHJ) model is applied to an n⁺ - p - p⁺ solar cell having finite dimensions, in order to investigate its performance under intense illumination. Ambipolar transport equations are solved in the three sections of the cell using appropriate boundary conditions. Expressions for junction currents are derived, and the junction potentials under open-circuit conditions are computed by the Newton-Raphson method.The theory presented here includes the effects of high level injection. The generalized current density equations which are derived here for an n⁺ - p - p⁺ device are shown to reduce to the ideal Shockley diode equation with appropriate modifications. The effects of p - p⁺ low-high junction on the open-circuit voltage of the cell are explained. The theoretical results of this paper are consistent with the experimental results of others.     

Low field study of P+NN+ or P+PN+ structures in the forward direction

It is shown, on the basis of P⁺NN⁺ and P⁺PN⁺ models, how injection processes modify the field distribution in two-electrode systems, and thereby the effective resistivity.     

Post-breakdown bulk oscillations in gold-doped silicon p+−i−n+ double-injection diodes

Experimental work on post-breakdown bulk oscillations in n-type gold-doped phosphorus-compensated p⁺?i?n⁺ double-injection diodes is presented. An empirical relationship for the frequency of oscillation in this region is derived for the first time and discussed.     

Specific contact resistivity of TiSi2to P+and n+junctions

Specific contact resistivities of the Al/TiW/TiSi2/Si system are characterized. It is found that without a TiW barrier layer, Al can penetrate through the TiSi2layer and significantly affect the TiSi2/Si interfacial contact resistance. Intrinsic TiSi2contact resistivities to n⁺and p⁺silicon are characterized with a TiW barrier between the silicide and the aluminum. TiSi2contact resistivity to n⁺silicon is found to be about one order of magnitude lower than that of Al to n⁺silicon. However, TiSi2to p⁺silicon contact resistivity is higher than that of Al to p⁺silicon and is very sensitive to the boron implant dose.     

Epitaxial n+ layer GaAs mesa-finger interdigital surfacephotodetectors

Several varieties of interdigital surface photodetectors have been fabricated on semi-insulating GaAs using mesa-etched n⁺ epitaxial layers as the carrier collection electrodes. This structure provides a significant responsivity improvement over conventional metal-semiconductor-metal (MSM) photodiodes by eliminating the surface reflection of the metal fingers and by providing increased photoconductive gain. High-speed testing using 100-ps doubled Nd:YAG pulses gave FWHM (full width at half maximum) responses of less than 500 ps for a 4-μm finger width and spacing and showed minimal degradation from standard MSM detectors of the same surface geometry. By introducing no additional process steps, the mesa-finger detectors are also monolithically compatible with mesa-etched GaAs MESFET technology     

High injection phenomena in p+in+ silicon solar cells

A very simple analytic analysis of p⁺in⁺ solar cell structures is undertaken based in the quasi-neutrality condition (n = p), which validity is verified. The only differential equation to be solved in the model is the classical continuity one. Non linearities, appea ronly in the boundary conditions. This model can be applied to p⁺nn⁺ or n⁺pp⁺ cells under very high injection conditions. High carrier concentrations are required to support the current flow even in short-circuit conditions, so enhancing the recombination which, on the other hand, is reduced by high injection lifetime increase. Under-linearity between the short-circuit current and the photon flux is also deduced at very high irradiance. The short-circuit current under bifacial illumination is higher than the sum of those currents under front and back illumination alternatively, so leading to inherently better bifacial cells. In the V_oc vs J_sc curve values of m in the logarithmic slope mkT/q range from values slightly below one to two. The value m = 1 is not to be expected even if the base recombination is negligible due to Dember potential effects.     

Characteristics of three-terminal metal-tunnel if oxide-n/p+ devices

The device described here comprises a p⁺ substrate containing an epitaxial n-layer, on the surface of which is grown a thin (～50 Å) tunnel oxide. A metal cathode is deposited on the oxide surface, and a metal anode on the back side of the p⁺ substrate. A third terminal, the gate electrode, is connected to the n epilayer to provide for biasing the n-p⁺ junction.The I-V characteristic exhibit two stable states: a high-impedance state and a low-impedance state which are separated by a negative-resistance region. The high-impedance state is stable for applied voltages up to the intrinsic threshold voltage, V_s. When the switching voltage is exceeded, the device switches rapidly to the low-impendance state, which is characterized by a current that increases with little increase in the voltage across the device.The switching voltage may be reduced below V_s by current or voltage biasing of the n-p⁺ junction by means of the gate electrode. Gate efficiencies, the ratio of the change in switching voltage with d.c. gate voltage or current, of 10 V/V and 1.0 V/μA have been observed. Pulsed gate measurements are also presented, and it found that for pulse widths down to 0.1 μs the gate switching characteristics follow the d.c. characteristics. For pulse widths less than 0.1 μs the gate efficiencies are degraded. Suggestions for improving the device characteristics and the turn-on and turn-off time of the device and device reliability are discussed.     

Base current and dominant time constant of an n+n−pn+ transistor operating in the upward mode

An attempt will be made to put together all information on I²L base current available from literature. The aim is to get explicit relations for all base current components at low, medium and high injection. This requires knowledge of excess concentration patterns and stored excess charge as well as of the associated time constants. As a final result the upward current amplification β_u and the upward time constant τ_u are obtained. In the latter no depletion charges have been taken along and a quasi-static condition is assumed. Lateral base current effects have not been taken into account.     

Stabilizing effects for the supercritical GaAs n+nn+-transferred electron amplifier

In this paper effects of importance for the stabilization of supercritical n⁺nn⁺ GaAs transferred electron devices are considered. By small-signal impedance calculations and measurements it is shown that doping- as well as temperature gradients of correct polarity reduce the device negative resistance and enhance stability. It is also found that an increasing doping density reduces the negative resistance. Finally it is demonstrated that relaxation effects have a profound influence on the impedance, and that such effects have to be included in a small-signal analysis in order to give reasonable agreement with measurements.     

Effective recombination velocity at the NN+ interface

The effective recombination velocity S_nn⁺ at the nn⁺ interface in buried layer (nn⁺p) and n epi-n⁺ substrate structures has been studied using a model which takes into account the retarding outdiffusion region, recombination and bandgap narrowing. The variation of S_nn⁺ with diffusion length and bandgap narrowing has been estimated taking into consideration their doping-dependence. An attempt has been made to explain the wide range in the reported values for S_nn⁺ using the results of this study.Results indicate clearly the difference between the S_nn⁺ of the two structures. This difference arises from the collection by the p-substrate which accounts for a significant part of the S_nn⁺ of the buried layer structure over a wide range of values of diffusion length. This collection component of S_nn⁺ is sensitive to bandgap narrowing.On the other hand, the S_nn⁺ of the nn⁺ structure is largely determined by the recombination in the outdiffusion region which is sensitive mainly to the value of diffusion length in that region. The component of S_nn⁺ representing recombination in the n⁺ substrate is sensitive to bandgap narrowing. The present study indicates the dependence of S_nn⁺ on the structure and processing of the devices in which the nn⁺ interface occurs.     

Heat flow resistance measurements of silicon p+-v-n+ diodes under forward and reverse biased conditions

A detailed study of the heat flow resistance measurements in a p⁺-v-n⁺ diode is studied in both forward and reverse biased conditions. Measurements are made by continuously switching the diode from the power dissipation state into the temperature measuring state. Safe operating power limits are identified for the diodes depending upon their mode of operation either as a microwave switch or as an IMPATT oscillator.     

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.     

Quenched-in centers in silicon p+n junctions

Silicon p⁺n junctions heat treated at high temperatures (1200°C) for a long time (2–20 hr) and then quenched to room temperatures or below shows two deep donor levels (E_C?264meV andE_C?542meV) in the n-type side of the junction depletion layer which appear to originate from the same imperfection center. The concentration of these levels ranges from 10¹³ to 10¹⁴ cm^?3. The junction leakage current comes from carrier generation at the deeper level in the depletion region. Phosphorus gettering was found ineffective in reducing the concentration of these quenched-in levels, but they are annealed out by very slow cooling (25°C/hr to 650°C then quench to room temperatures). The thermal emission and capture rates of electrons and holes at these levels are measured as a function of temperature and electric field by the junction high frequency capacitance and d.c. leakage current transient techniques. It is demonstrated that the detailed balance relationship does not hold. The origin of this double donor center is yet to be identified.     

A comparative study of n+ ohmic contact resistance forPdInGe and NiGeAu metallizations on GaAs

NiGeAu and PdInGe metallizations are used for n⁺ ohmic contacts to GaAs. One-dimensional analytic and two-dimensional numerical analyses are presented using experimental data obtained from transmission line and cross bridge Kelvin test structures. Results from the 1-D and 2-D models show that the former analysis overestimates the specific contact resistance by at least an order of magnitude. This discrepancy is shown to be test structure dependent and metallization independent     

Avalanche multiplication noise characteristics in thin GaAs p+-i-n+ diodes

Avalanche noise measurements have been performed on a range of homojunction GaAs p⁺-i-n⁺ and n⁺-i-p ⁺ diodes with “i” region widths, ω from 2.61 to 0.05 μm. The results show that for ω⩽1 μm the dependence of excess noise factor F on multiplication does not follow the well-established continuous noise theory of McIntyre [1966]. Instead, a decreasing noise factor is observed as ω decreases for a constant multiplication. This reduction in F occurs for both electron and hole initiated multiplication in the thinner ω structures even though the ionization coefficient ratio is close to unity. The dead-space, the minimum distance a carrier must travel to gain the ionization threshold energy, becomes increasingly important in these thinner structures and largely accounts for the reduction in noise     

Effect of injection efficiency and dimensions on the performance of a forward biased P+−I alloyed junction

P⁺?I junction has a high carrier injection, when forward biased. The bias voltage is dropped across the junction and the intrinsic region. The injection efficiency γ at the junction has been found to depend upon the voltage drop V_J across the barrier and the thickness of the intrinsic layer expressed as $\text{d}\text{L}$ where d is the thickness of the intrinsic layer and L is a measure of the diffusion length of the carriers in the intrinsic region. As V_J is varied the injection efficiency remains close to unity for low values of V_J but decreases for larger values of V_J, the decrease being more for larger values of $\text{d}\text{L}$. The ratio of the voltage drop across the intrinsic layer V_I to the voltage drop V_J across the junction depends upon the injection efficiency at the junction γ and the ratio $\text{d}\text{L}$. As γ decreases $\text{V}{}_{\mathrm{I}}\text{V}{}_{\mathrm{J}}$ increases, the increase being more for larger values of $\text{d}\text{L}$. The forward J?V characteristic is found to depend upon $\text{d}\text{L}$. As $\text{d}\text{L}$ decreases a lower resistance is offered by the diode. Modifications arising out of carrier-carrier scattering have also been considered. For a given V_J and $\text{d}\text{L}$, carrier-carrier scattering reduces γ. A reduced value of γ increases $\text{V}{}_{\mathrm{I}}\text{V}{}_{\mathrm{J}}$ and makes J?V curve less steep. The analysis has been carried out for $\text{V}{}_{\mathrm{J}}\text{?}\text{4kT}\text{e}$.     

Specific contact resistivity of InGaAs/InP p-isotype heterojunctions

The specific contact resistivity of lattice matched InGaAs/InP p-isotype heterojunction has been measured through the use of an interface transmission line model structure. The measured resistance values are comparable to or greater than those of the metal/semiconductor interface and depend heavily on the doping and the abrupt or graded nature of the interface.<>     

1/f noise in n+-p-n microwave transistors

1/f noise measurements in the base current and the collector current of NEC 57807 n⁺-p-n microwave transistors show that the noise is not of the mobility fluctuation type, since the current dependence of S_IR(f) and S_IC(f) differs from the theoretical predictions. The low collector current 1/f noise makes it doubtful whether the mobility fluctuation concept is applicable in this case.