Modified drift field model for high-low transition in solar cells

A theoretical discussion is presented for the understanding of the back surface pp⁺ transition used in solar cells. It is shown that quasi-neutrality of space charge is a good approximation if the back surface diffusion is fairly deep. The error involved in the drift field model developed by assuming a quasi-neutrality of the space charge is compared with that inherent in the abrupt high-low junction model. The analysis shows that the back surface boundary, when measured from the heavily doped p⁺ side, effectively exists at a distance much larger than the impurity diffusion depth and the recombination current in the base is always less than its value estimated from the abrupt junction model. The voltage in the pp⁺ transition is due to the change in electric field by the excess carriers injected by light and drops across those regions of the cell where the injected carrier density is appreciable.     

Generation-recombination and diffusion currents in HgMnTe n +-p junctions

Special features of the distributions of the space charge, the electric-field strength, and potential in the p-n junction in the narrow-gap HgMnTe semiconductor were considered using the Poisson equation. It is shown that, as the band gap narrows, the effect of free charge carriers induces the coordinate dependence of electric-field strength to deviate from the linear dependence and that of the potential to deviate from the quadratic dependence. As a result of this, and also because of an appreciable increase in the diffusion potential in the n ⁺-p junction with the degenerate n ⁺ region, the mechanisms of the charge transport become peculiar: the voltage dependence of the recombination current deviates from those following from the conventionally used analytical expressions, whereas for higher bias voltages, the diffusion current of holes from the lightly doped p region into the n ⁺ region is prevalent.     

On the forward current-voltage characteristics of p+-n-n+ (n+-p-p+) epitaxial diodes

The forward-biased current-voltage characteristics of p⁺-n-n⁺ and n⁺-p-p⁺ epitaxial diodes are derived theoretically. Effects of the energy-gap shrinkage, the high-low junction built-in voltage, the high-level injection, and the minority-carrier life time on the forward-biased current-voltage characteristics are included. Good agreements between the theoretically derived results and the experimental data of Dutton et al. are obtained. The developed theory predicts that the leakage of the high-low junction is dominated by the recombination of minority carriers in the highly doped substrate, not by the recombination of minority carriers in the high-low space charge region, which is opposite to the previous prediction of Dutton et al.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

Negative photocurrent in GaAs-(AlGa)As heterojunctions

A negative photoresponse in p lightly doped GaAs-n heavily doped (AlGa)As heterojunctions is observed. The a.c. spectral response is presented as a function of d.c. bias and illumination. A model for negative photocurrents is proposed as due to electron diffusion into the p side. The photoresponse sign reversal is caused by the existence of the notch-spike discontinuity and of a notch population. Estimations of the band diagram are made from photoresponse and electrical measurements.     

Recombination properties of a diffused pn junction determined by spectral response measurements

The theoretical spectral responsivity of a diffused pn junction is computed in the case of a silicon n⁺p junction which employes a rather deep (4, 7 μ) and lightly doped N⁺ front region.Comparing experimental results with theoretical predictions the diffusion length L and surface recombination velocity S₀ can be determined. Several cases are examined: the influence of an oxide layer on the front and of gettering processes on L and S₀ are presented and the overall sensitivity of the method is discussed.     

Some aspects of LEC transistor behaviour

Some properties of bipolar transistors with low emitter concentrations are investigated both theoretically and experimentally. It turns out that the base current of LEC transistors at medium and high injection levels is the same as in double diffused transistors and can be explained by Auger recombination in the emitter n⁺ region. The cut-off frequency f_T is rather low, due to extra charge storage in the lightly doped emitter region. Small n⁺ emitter areas, surrounded by a p-ring may introduce anomalies such as kinks in the (_cb, V_be) characteristics and negative resistances.     

The open-circuit voltage of back-surface-field (BSF) p-n junction solar cells in concentrated sunlight

Simple analytical expressions for the open-circuit voltage of the n⁺?p?p⁺ and p⁺?n?n⁺ BSF solar cells, which are valid for both the low- and high-levels of optical illumination, are derived. Based on the principle of superposition the open-circuit voltage of both the n⁺?p?p⁺ and p⁺?n?n⁺ solar cells are expressed in terms of the short-circuit current and the known saturated dark current. Effects of the high-low junction doping, the energy-gap shrinkage, and the dimensions of the BSF solar cells on the open-circuit voltage are included. The numerical results of the derived expressions are found to be in good agreement with the exact numerical analysis of Fossum et al. The optimal design considerations based on the known characteristics of the open-circuit voltage are also discussed.     

Photoelectric properties of p +-n junctions based on 4H-SiC ion-implanted with aluminum

The photoelectric properties of p ⁺-n junctions that were based on 4H-SiC ion-implanted with aluminum and were formed in lightly doped n-type epitaxial layers grown by chemical vapor deposition were studied. It is shown that such photodetectors combine in full measure the advantages of photostructures formed on the basis of Schottky barriers and epitaxial p-n junctions. The results of the theoretical calculation of spectral characteristics of ion-implanted photodetectors are in good agreement with experimental data. The structures feature an efficiency of collection of nonequilibrium charge carriers close to 100% in the spectral range of the photon energies of 3.5–4.25 eV.     

Experimental determination of the avalanche region of one-sided abrupt barriers

In this paper is presented an experimental method for the determination of the width of the avalanche region of one-sided abrupt barriers at breakdown. The ionization rates of both electrons and holes are determined using the same experiments. The method is based on multiplication measurements corresponding to a primary current coming from the highly doped side of the junction. This primary current is obtained by varying the wavelength of a light spot applied to the highly doped side. This method is used to control the avalanche behaviour of P⁺N and N⁺P Si abrupt junctions. The avalanche region and ionization rates obtained are in good agreement with values already published.     

Titanium in silicon as a deep level impurity

Titanium inserted in silicon by diffusion or during Czochralski ingot growth is electrically active to a concentration level of about 4 × 10¹⁴ cm^?3. Hall measurements after diffusion show conversion of lightly doped p type Si to n type due to a Ti donor level at E_C - 0.22 eV. In DLTS measurements of n⁺p structures this level shows as an electron (minority carrier) trap at E_C - 0.26 eV with an electron capture cross section of about 3 × 10^?15 cm² at 300°K. The DLTS curves also reveal a hole trap in the p type material. The e_p (300/T)² activation plot gives the level as E_V + 0.29 eV. The hole capture cross section is about 1.7 × 10^?17 cm² at 300°K and decreases with decreasing temperature and the corrected trap level becomes E_V = 0.26 eV. Ti in lightly doped (360 ohm-cm) n type material does not result in conversion to p type so this level is inferred also to be a donor.A Ti electrically active concentration of about 1.35 × 10¹³ cm^?3 in p type (N_A = 3.35 × 10¹⁵cm^?3) Si results in a minority carrier (electron) lifetime of 50 nsec at 300°K.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

Numerical analysis on abnormal thyristor forward voltage increase due to heavy doping in gated p-base layer

An abnormal forward voltage increase was observed for a p-base gated double diffused n⁺pn^?p⁺ high power thyristor with high impurity concentration at the n⁺-p emitter-base junction. Accurate numerical analysis shows that heavy doping effects are the most responsible mechanism for the abnormality and that depletion layer formation at the center junction accompanies it.It will be shown that appropriate control of the impurity concentration at the emitter-base junction is necessary to avoid this abnormality by realizing the common base transistor current gain of greater than 0.73 for n⁺n^?-portion.     

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.     

Shot noise in back biased p-n silicon diodes

An earlier calculation of the noise due to generation of carriers in the space charge region in a p-n silicon diode by Lauritzen and by Scott and Strutt is corrected for the fact that the field distribution in the space charge region of a p-n junction is linear instead of uniform. If the noise is expressed as S_I(f) = 2eIΓ², we find $\text{Γ}{}^2\text{=}\text{11}\text{15}$ in a p⁺n or n⁺p junction, instead of $\text{Γ}{}^2\text{=}\text{10}\text{15}$ found previously.     

Dependence of carrier mobility on an electric field in gallium selenide crystals

The dependence of the mobility of charge carriers on voltage has been studied in undoped GaSe single crystals and crystals doped with gadolinium; the latter crystals have exhibited various values of dark resistivity (??_d.r ?? 10⁴?10⁸ ?? cm at 77 K) and of the doping level (N = 10^?5, 10^?4, 10^?3, 10^?2, and 10^?1 at %). It is established that the dependence of the charge-carrier mobility on the electric field applied to the sample E ?? 10² V/cm is observed in undoped high-resistivity GaSe crystals (??_d.r ?? 10⁴ ?? cm) and in lightly doped GaSe crystals (N ?? 10^?2 at %) in the region of T ?? 150 K. It is found that this dependence is not related to heating of the charge carriers by an electric field; rather, it is caused by elimination of drift barriers as a result of injection.     

High-voltage planar junction with a field-limiting ring

A twodimensional Poisson equation is solved as part of a program to improve breakdown characteristics of a planar p-n junction by using a field limiting ring. The influences of n^? concentration and n^? layer width of p⁺-n^?-n⁺ diode are investigated. Higher n^? concentration and smaller n^? width make optimum distance between anode and field limiting ring smaller. Breakdown voltages predicted by optimising method reported agree well with the experimental results.     

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.     

Negative capacitance (impedance of the inductive type) of silicon p +-n junctions irradiated with fast electrons

Silicon p ⁺-n junction diodes irradiated with 3.5-MeV electrons (with the dose of 4 × 10¹⁶ cm^?2) are studied. The diodes’ inductance (L) was measured at a frequency f = 1 MHz with the amplitude of alternating current equal to 0.25 mA. Simultaneously with measurements of L at alternating current, a direct current was passed through the forward-biased diode, which brought about the injection of minority charge carriers into the base. In order to identify both of the mechanisms that give rise to the inductive-type impedance in irradiated diodes with the p ⁺-n junction and the main radiation defects that are directly involved in the formation of this impedance, irradiated samples were annealed isochronously in the temperature range T _a = 225–375°C with sub-sequent study of the main characteristics of the defects by deep-level transient spectroscopy. It is shown that the inductive-type impedance in irradiated diodes is caused by the processes of capture and retention of charge carriers injected into the base at the trapping centers for a time ～1/2f, i.e., for a half-period of oscillations. It is also shown that the trapping centers are the vacancy-oxygen complexes introduced by irradiation with electrons.     

Differential capacitance of a <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">p</Emphasis> junction

The differential capacitance of a p ⁺-p junction formed by charge redistribution near the junction, has been investigated taking into account the electric field in the quasi-neutral p region. The dependence of the capacitance and current of the p ⁺-p junction on its voltage is obtained. It is shown that a change in the sign of the p ⁺-p-junction capacitance with an increase in the injection level is caused by a decrease in the bipolar drift mobility in the p-type region. It is also demonstrated that a change in the sign of the p ⁺-p-junction capacitance with an increase in the reverse voltage determines the charge reduction near the junction, as the increase in the negative charge of acceptor ions predominates over the increase in the positive charge of holes.