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.     

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.     

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.     

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.     

A discrete element small-signal equivalent circuit for forward-biased p+n diodes containing deep levels

A discrete element small-signal equivalent circuit model for p-n diodes containing deep defect levels is developed, by extending an existing model for undamaged devices. With the aid of a simple analytical expression which accurately describes the forward bias d.c. current, the enhanced small-signal conductance due to carrier recombination in the depletion region is included in the model. The influence of trapped charge on the space charge capacitance is incorporated using a simplified version of the analysis of Beguwala and Crowell. The predictions of the model are verified by experimental data from silicon p⁺n diodes, in which deep levels have been induced by electron irradiation. It is shown that the deep level activation energies may be estimated from the forward bias capacitance-voltage characteristics, yielding values which agree well with those obtained by established techniques.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Degradation mechanism for silicon p+-n junctions under forward bias

Leakage current degradation has been observed during forward bias stressing of silicon integrated p⁺-n junctions. Detailed characterization results of the anomalous leakage behavior are discussed in this paper. From these results an electric field-enhanced impurity diffusion mechanism has been proposed to explain both the strong temperature and forward bias dependencies on leakage current time-to-saturation. An activation energy has been determined for this mechanism (0.48±0.04 eV) and is in good agreement with that previously determined for diffusion of interstitial copper in p-type silicon. Subsequent Secondary Ion Mass Spectrometer elemental analysis has confirmed the presence of copper near the surface of the epitaxial layer containing the p⁺-n device.     

Mobility fluctuation1/f noise in silicon p+-n-p transistors

A study of the current dependence of base 1/f noise and of collector 1/f noise in p⁺-n-p transistors shows that the former is most likely of the mobility fluctuation 1/f noise type and that the latter is most probably not of that type. The current dependence of 1/f noise in transistors is a powerful tool in the interpretation of the noise.     

A difficulty in the theory of p+-n junction diode noise

It is shown that the usual theory of p⁺-n junction diode noise leads to the following difficulty. The spectrum of the open-circuit noise voltage of the diode does not go to zero fast enough at sufficiently high frequencies, so that the spectrum cannot be integrated from 0 to ∞. The difficulty is removed when the effect of the capacitance C_j of the junction space charge region is taken into account. This is a particular example of a more general result.     

Analysis of the soft reverse characteristics of n+p drain diodes

Soft leakage current characteristics of n⁺p drain diodes of a p-well C-MOS process have been measured and analyzed. A field dependent component of the leakage current could be determined after subtraction of the diffusion current. The values of the field-dependent generation current found were compared with calculated values owing to thermal generation according to the Poole-Frenkel effect and phonon-assisted tunneling. The measured values for different diode structures were found to agree well with the theoretical prediction of phonon-assisted tunneling through a barrier lowered by the presence of an electric field.     

Carrier compensation in O+ implanted n-type GaAs

Results of electrical measurements on 1 MeV O⁺ implanted n-type GaAs are reported. After annealing the implanted material it is found that the free carrier compenasation rate (k), defined by Favennec[1] as the number of carriers removed per oxygen atom, can be dependent upon both the starting material and the implanted dose.