A. Neugroschel

Direct-current measurements of oxide and interface traps on oxidized silicon

A direct-current current-voltage (DCIV) measurement technique of interface and oxide traps on oxidized silicon is demonstrated. It uses the gate-controlled parasitic bipolar junction transistor of a metal-oxide-silicon field-effect transistor in a p/n junction isolation well to monitor the change of the oxide and interface trap density. The dc base and collector currents are the monitors, hence, this technique is more sensitive and reliable than the traditional ac methods for determination of fundamental kinetic rates and transistor degradation mechanisms, such as charge pumping. >

Systematic analytical solutions for minority-carrier transport in semiconductors with position-dependent composition, with application to heavily doped silicon

For quasi-neutral regions of semiconductor devices with position-dependent composition, we have derived expressions for the position dependence of the excess minority-carrier density and for relevant recombination currents. To make the development concrete, we study nonuniformly and heavily doped emitter regions of silicon p-n junction devices. The expressions developed differ from those previously advanced in that they are in the form of a multiple integral series, yielding, by truncation, many different orders of approximation. Correspondences exist between some of the different orders of approximation and various solutions previously obtained. All of the mechanisms relating to hole and electron transport in position-dependent heavily doped semiconductors are accounted for in the new expressions. These mechanisms include bandgap narrowing, majority-carrier degeneracy, Auger recombination lifetime, etc. To assess the accuracy of the various orders of approximation, we compare their predictions with a numerical solution. We determine that the simplest approximation, which contains only one term of the integral series, is accurate to within about 5 percent of the numerical solution for thin emitters (∼0.2 µm) provided the surface recombination velocity is less than 105cm/s. It thus applies directly to bipolar transistors with polysilicon contacts, and to surface-passivated solar cells. This new solution, which we call the zeroth-order or quasi-neutral quasi-equilibrium approximation, is simpler than solutions previously put forward. If the emitter junction is deep or the contact is ohmic, the higher-order approximations provide whatever accuracy is needed. We separate the emitter recombination current into charge and a characteristic time, enabling the calculation of the contribution of emitter to capacitance and to delay in the time domain or phase shift in the frequency domain.

Degradation of bipolar transistor current gain by hot holes during reverse emitter-base bias stress

Experimental evidences are given which demonstrate that degradation of the common-emitter forward current gain h/sub FE/ of submicron silicon npn bipolar transistors at low reverse emitter-base junction applied voltage is caused by primary hot holes of the n/sup +//p emitter tunneling current rather than secondary hot electrons generated by the hot holes or thermally-generated hot electrons. Experiments also showed similar kinetic energy dependence of the generation rate of oxide/silicon interface traps by primary hot electrons and primary hot holes. Significant h/sub FE/ degradation was observed at stress voltages less than 2.4 V.

Measurement of collector and emitter resistances in bipolar transistors

New DC methods to measure the collector resistance R/sub C/ and emitter resistance R/sub E/ are presented. These methods are based on monitoring the substrate current of the parasitic vertical p-n-p transistor linked with the n-p-n intrinsic transistor. The p-n-p transistor is operated with either the bottom substrate-collector or the top base-collector p-n junction forward-biased. This allows for a separation of the various components of R/sub C/. R/sub E/ is obtained from the measured lateral portion of R/sub C/ and the collector-emitter saturation voltage. Examples of measurements on advanced self-aligned transistors with polysilicon contacts are shown. The results show a very strong dependence of R/sub C/ on the base-emitter and base-collector voltages of the n-p-n transistor. The bias dependence of R/sub C/ is due to the conductivity modulation of the epitaxial collector. From the measured emitter resistance R/sub E/ a value for the specific contact resistance for the polysilicon emitter contact of rho /sub c/ equivalent to 50 Omega - mu m/sup 2/ is obtained. >

A method for determining energy gap narrowing in highly doped semiconductors

A general experimental method for the determination of the phenomenological energy gap narrowing \DeltaE_{G} in regions of semiconductor devices that have high concentrations of donor or acceptor impurity atoms is presented. The theoretical grounds for the method are discussed in detail, including the strong influence of Fermi-Dirac statistics on minority-carrier recombination in heavily doped regions. The method requires measurements only of the temperature dependence of de current; therefore it is very accurate. The values of \DeltaE_{G} deduced from the method are insensitive to the mechanisms controlling recombination and to the value of minority-carrier mobility in the heavily doped region; they are also independent of the value of n i the intrinsic carrier density, at a specific temperature. The values for the Si:As emitters of transistors and diodes were measured in the majority-carrier concentration range from 4 × 1018cm-3to 2 × 1020cm-3.

A methodology for experimentally based determination of gap shrinkage and effective lifetimes in the emitter and base of p-n junction solar cells and other p-n junction devices

An experimentally based methodology is described that determines the effective gap shrinkage and lifetime in the emitter of a p-n junction solar cell. It provides the first experimental means available for assessing the importance of gap shrinkage relative to that of large recombination rates in the highly doped emitter. As an additional result of the procedures employed, the base lifetime is also determined. The methodology pertains to a solar cell after the junction is formed. Hence each material parameter determined includes the effects of the processing used in junction fabrication. The methodology consists of strategy and procedures for designing experiments and interpreting data consistently with the physical mechanisms governing device behavior. This careful linking to the device physics uncovers the material parameters concealed in the data. To illustrate the procedures, they are applied to an n+-p solar cell having substrate resistivity of about 0.1 Ω cm.

A method for determining the emitter and base lifetimes in p-n junction diodes

A method is described that provides an experimental means for the first time to separate and determine the emitter and base lifetimes in a p-n diode after the junction has been fabricated. In the method, several static and transient measurements are analyzed using physical models of the diode characteristics. To illustrate the method, diffused silicon diodes are fabricated having substrate (base) impurity concentrations ranging from 1014to nearly 1017phosphorous atoms per cubic centimeter. The results show an emitter lifetime that is much smaller than the base lifetime in the diode having the highest base doping concentration. In this diode, the recombination current from the emitter is 65 percent of the recombination current from the base, demonstrating the significance of the emitter in governing the static current-voltage dependence. The importance of emitter recombination to the transient characteristics is also demonstrated. The paper emphasizes the techniques by which the base and emitter lifetimes are distinguished. It also demonstrates the need for carefully basing the quantitative analysis of the measurements on the underlying diode physics. The method described here applies not only to p-n diodes but also to junction solar cells and transistors.

Degradation of silicon bipolar junction transistors at high forward current densities

The physical degradation mechanisms of silicon bipolar function transistors at high forward current densities were delineated quantitatively using three n/p/p and one p/n/p state-of-the-art submicron polysilicon-emitter transistor technologies. The increase of the operating current gain and decrease of emitter series resistance from million-ampere per square centimeter stress current are related to hydrogenation of the electronic traps at the metal-silicide/polycrystalline-Si and polycrystalline-Si/crystalline-Si emitter contact interfaces. A demonstration of the 10-year operation Time-to-Failure extrapolation methodology is also presented.

Minority-carrier transport parameters in n-type silicon

Minority-carrier diffusion length L, lifetime tau , and diffusion coefficient D in n-type Si are measured at 296 K in the doping range from 10/sup 18/ cm/sup -3/ to 7*10/sup 19/ cm/sup -3/. The measurement is based on a lateral collection of carriers generated by a spatially uniform light. The distance between the illumination edge and the collection junction is defined by photolithography. This allows simultaneous and independent determination of all transport parameters in the same material. A self-consistency and accuracy check is provided by the relation L/sup 2/=D tau . Details of experimental procedures are described. Empirical best-fit relations for the three parameters are given. The extraction of lifetime and diffusion coefficient was done in the frequency domain, which allows for straightforward elimination of parasitic effects in the nanosecond and subnanosecond range. >

Heavily doped polysilicon-contact solar cells

We report the first use of a (silicon)/(heavily doped polysilicon)/(metal) structure to replace the conventional high-low junction or back-surface-field (BSF) structure, of silicon solar cells. Compared with BSF and back-ohmic-contact (BOC) control slimples, the polysilicon-back solar cells, show improvements in red spectral response (RSR) and open-circuit voltage. Measurement reveals that a decrease in effective surface recombination velocity S is responsible for this improvement. Decreased S results for n-type (Si:As) polysilicon, consistent with past findings for bipolar transistors, and for p-type (Si:B) polysilicon, reported here for the first time. Though the present polysilicon-back solar cells are far from optimal, the results suggest a new class of designs for high efficiency silicon solar cells. Detailed technical reasons are advanced to support this view.