R.M. Swanson

Ion-implanted complementary MOS transistors in low-voltage circuits

Simple but reasonably accurate equations are derived which describe MOS transistor operation in the weak inversion region near turn-on. These equations are used to find the transfer characteristics of complementary MOS inverters. The smallest supply voltage at which these circuits will function is approximately 8kT/q. A boron ion implantation is used for adjusting MOST turn-on voltage for low-voltage circuits.

27.5-percent silicon concentrator solar cells

Recent advances in silicon solar cells using the backside point-contact configuration have been extended resulting in 27.5-percent efficiencies at 10 W/cm2(100 suns, 24°C), making these the most efficient solar cells reported to date. The one-sun efficiencies under an AM1.5 spectrum normalized to 100 mW/cm2are 22 percent at 24°C based on the design area of the concentrator cell. The improvements reported here are largely due to the incorporation of optical light trapping to enhance the absorption of weakly absorbed near bandgap light. These results approach the projected efficiencies for a mature technology which are 23-24 percent at one sun and 29 percent in the 100-350-sun (10-35 W/ cm2) range.

Studies of diffused phosphorus emitters: saturation current, surface recombination velocity, and quantum efficiency

The surface recombination velocity s for silicon surfaces passivated with thermal oxide was experimentally determined as a function of surface phosphorus concentration for a variety of oxidation, anneal, and surface conditions. This was accomplished by measuring the emitter saturation current density J/sub 0/ of transparent diffusions for which the J/sub 0/ is strongly dependent on s. At the lowest doping levels, the value of s was confirmed by measurements of s on substrates with uniform phosphorus doping. The impact of these measurements on solar cell design is discussed. >

Modeling and measurement of contact resistances

This paper presents a generalized model of ohmic contacts and a unified approach for the accurate extraction of specific contact resistivity (ρc) for ohmic contacts from measured contact resistance using the cross bridge Kelvin resistor, the contact end resistor, and the tranmsission line tap resistor test structures. A general three-dimensional (3-D) model of the contacts has been developed from the first principles and has been reduced to 2-D, 1-D, and 0-D (one lump) models with the necessary approximations. It is shown that the conventional I-D models overestimate the value of ρcbecause of the parasitic resistance due to 2-D current flow around the periphery of the contact window. Using 2-D simulations, we have accurately modeled the current crowding effects and have extracted accurate values of ρcindependent of contact size and the test structure type. A theory of scaling of contacts has been developed and is applied to commonly used structures. A universal set of curves has been derived for each particular contact resistance test structure and, given the geometry of the structure, these allow accurate determination of ρc, Without the actual use of the 2-D simulator. Experimental and theoretical accuracy of the three test structures has been compared. Accurate values of ρcfor various contact materials to n+and ρ+Si have been determined. The data confirm that in the past researchers have overestimated ρc, and that ρcwill not limit device performance even with submicrometer design rules.

Calculation of surface generation and recombination velocities at the Si‐SiO2 interface

Using deep level transient spectroscopy in the current transient mode, the interface trap density and electron and hole capture cross sections have been measured for thermally oxidized 〈100〉 silicon. We have compared oxides grown with and without HCl in the growth ambient, and also investigated the effect of the postoxidation inert ambient anneal. Values of the depleted surface generation velocity and surface recombination velocities in low‐ and high‐level injection were then calculated from the measured interface trap parameters using Shockley–Read–Hall theory. We report here the results of the calculations and compare them with a few direct experimental measurements.

Measuring and modeling minority carrier transport in heavily doped silicon

Abstract From a fundamental transport formulation it is demonstrated that in heavily doped regions, only two independent parameters control the transport and recombination of minority carriers. The extraction of the minority carrier lifetime, diffusion coefficient, or bandgap narrowing is therefore impossible from DC measurements only. At least one additional AC measurement is necessary. The reported experimental data in the literature are critically revisited. When the published data are interpreted in terms of the original measurements a comprehensive picture appears, with surprising agreement among authors. Modeling of heavily doped regions in devices is shown to be possible, and good predictions of the emitter saturation current are demonstrated.

The physics and modeling of heavily doped emitters

The physics of minority-carrier injection and internal quantum efficiency of heavily doped emitters is studied through a novel computer simulation. It is shown that in the shallow emitters of modern devices, the transport of carriers through the bulk of the emitter, and the surface recombination rate are the dominant mechanisms controlling the minority-carrier profile. Carrier recombination in the bulk of the emitter only produces a small perturbation of this profile. This observation permits us to develop a simple and accurate analytical model for the saturation current and internal quantum efficiency of shallow emitters.

A 720 mV open circuit voltage SiOx:c‐Si:SiOx double heterostructure solar cell

For maximal performance solar cells should resemble semiconductor lasers, i.e., they should be constructed in the form of a double heterostructure. We have found rather good performance in SIPOS‐crystalline silicon‐SIPOS double heterostructure solar cells, where SIPOS≡SiOx. The processing of these solar cells gives insights into the truly outstanding performance of the n+‐SIPOS: p‐Si heterojunction which has a forward saturation current coefficient J0=10−14 A/cm2, or equivalently an ‘‘emitter Gummel number’’ Ge=3.3×1015 s/cm4. This suggests that crystalline silicon solar cells can be much more efficient than had been suspected.

Recombination in highly injected silicon

Recent advances in solar cells designed to operate under high-level injection conditions have produced devices that are approaching some of the limits imposed by the fundamental band-to-band Auger recombination in Silicon. A device has been optimized to study this recombination by using the fabrication technology developed for point-contact solar cells. Using both steady-state and transient measurements, the recombination rates in high-resistivity Si in the injected carrier density range of 1015to 2 × 1017carriers / cm3were investigated. The coefficient of the recombination, which depends on the carrier density cubed, is found to be 1.66 × 10-30cm6/s ± 15 percent. This result is four times higher than the ambipolar Auger coefficient commonly used in the modeling of devices that operate in this injected carrier density range and lowers the expected limit efficiencies for silicon solar cells.

Point-contact silicon solar cells

A new type of silicon photovoltaic cell designed for high-concentration applications is presented. The device is called the point-contact-cell and shows potential for achieving energy conversion efficiencies in the neighborhood of 28 percent at the design operating point of 500× geometric concentration and 60°C cell temperature. This cell has alternating n and p regions that form a polkadot array on the bottom surface. A two-layermetallization on the bottom provides contact. Initial experimental results have yielded a cell with 20-percent efficiency at a concentration of 88.