H. Pfleiderer

A very-high-slew-rate CMOS operational amplifier

The amplifier uses a circuit to inject an extra bias current into a conventional source-coupled CMOS differential input stage in the presence of large differential input signals. This measure substantially increases the slew rate of an operational amplifier for a given quiescent current. The performance of the amplifier is compared to a conventional operational amplifier when used in a sample-and-hold circuit. The maximum operating clock frequency of the sample-and-hold increases from 290 kHz to 1 MHz with a hold capacitor of 1 nF. The amplifier has been fabricated in a 5- mu m CMOS process and dissipates a static power of 7.5 mW. >

Photo-induced degradation of a-Si: H diodes with an nin structure

Abstract Diodes with an nin structure consisting of hydrogenated amorphous silicon ( a -Si:H) exhibit space-charge-limited currents. We subjected some diodes to both illumination and voltage stresses. Only the illumination treatment led to a degradation of the I(U) -characteristics. The degradation is reversible by annealing. We deduce from these findings that metastable traps are created by recombining photocarriers rather than by single-carrier trapping. The reversible diode degradation is consistent with the Staebler-Wronski effect.

Influence of bias and photo stress on a‐Si:H diodes with nin‐ and pip‐structures

We observed the current‐voltage characteristics of a‐Si:H diodes having nin‐ and pip‐structures after annealing, bias stressing and photo stressing. Both diode types show photo degradation, the pip diodes also bias degradation. The degradation is reversible in all cases by annealing. Furthermore, photo and bias degradations proceed independently of each other. We conclude that recombination and hole trapping induce different metastable defects.

Ambipolar field-effect transistor

The results of measurements performed on an amorphous-silicon thin-film transistor structure are presented and interpreted. The device characteristics show a continuous alternation between n-channel and p-channel operation, an “ambipolar” effect that is made possible by the provision of ohmic source and drain contacts.

Electron and hole μτ products of slightly doped a-Ge:H films

We measured the spectral photoconductance of slightly boron doped amorphous germanium (a-Ge:H) films through the wavelength interval from 450 to 950 nm, and determined the products ( μτ ) n of electrons and ( μτ ) p of holes by means of an own two-carrier photoconductance model. The doping was achieved by adding diborane (B 2 H 6 ) to the germane (GeH 4 ) gas in the deposition chamber, in concentrations up to 0.3%. In nominally undoped films the product ( μτ ) n exceeds the product ( μτ ) p by two orders of magnitude. A 150 ppm admixture of diborane yields “compensation”, i.e. adjusts the equation ( μτ ) n = ( μτ ) p = 5*10 −8 cm 2 /V. A pin solar cell with compensated i-layer is more red efficient than a cell with undoped i-layer.

Spectral photocurrent-voltage characteristics of crystalline silicon and amorphous silicon solar cells

Abstract Crystalline silicon (c-Si) and amorphous silicon (a-Si) solar cells represent complementary cell types. The generation of photocarriers takes place in the semiconductor bulk (c-Si cell) or is confined to the barrier layer (a-Si cell). Carrier collection hence relies on diffusion (c-Si cell) or drift (a-Si cell). We explain the different behavior of the two cell types by measuring photocurrent-voltage characteristics. The photocurrent of the a-Si barrier cell changes its sign at a certain transition voltage U T . We find that U T depends slightly on the light wavelength λ but remains invariant with respect to alterations in the light intensity. It is possible to interpret both features in terms of a suitable uniform-field picture for a-Si p-i-n cells. The field through the i layer reverses its sign at the flat-band voltage U F . A vanishing field reveals the small contribution of diffusion to the photocarrier transport. The function U T (λ) stems therefore from diffusion.

Surface recombination in a-Si solar cells

The reversal of the electric field in the i-layer of a pin cell under forward voltage stimulates surface recombination. Reversed fields and surface recombination together yield secondary photocurrents and limit the AM1 open-circuit voltage.

Trace of an interface layer between buffer and i-layer in the spectral response of a-SiC:H/a-Si:H solar cells

The comparison of numerical modelling and measurements of the internal collection efficiency q of a solar cell in the annealed and degraded state (state A and B) delivers information about the buffer/i-layer interface defect layer and the position dependent degradation. The plot of the measured internal collection efficiency shows two characteristic features: 1. The variation of the q-curves of blue light with increasing voltage at the crosspoint of the q-curves is like an “S”-shape in state A and B. 2. The q-values at zero voltage show a large spread in state B. The “S”-shape occurs in the voltage range, in which the positive charge of the defect layer is decreasing. The decreasing positive charge reduces the effect of the increasing applied voltage. The large spread of the q-values at zero voltage is the direct consequence of an enhanced buffer/i-interface layer degradation. The comparison of the modelled and the measured q-curves in state B leads to the following conclusion: Degradation occurs by a strong damage of the buffer/i-layer interface region and a weak damage of the i-layer.

Spectral photocurrent-voltage characteristics of a-Si pv and pπ diodes☆

Abstract Standard a-Si pin-solar cells were modified by incorporating small amounts of boron or phosphorous into the “i-layer”, thus producing various psn-diodes. The photocurrent Ip of these diodes decreases with increasing voltage U and vanishes at the transition voltage UT, separating primary and secondary photocurrents. The s-layer fine doping amplifies the dispersion UT(λ) and leads to very strong secondary photocurrents. After a degradation procedure under AM1-light the photocurrent behavior is characterized by a decrease in primary photocurrent and transition voltage but an increase in secondary photocurrent.

BiCMOS for High Performance Analog and Digital Circuits

As a result of its high packing density, low power dissipation and ease of design, CMOS has in recent years emerged as the standard technology for VLSI circuits. Compared to CMOS, bipolar technology offers better analog features, e.g. offset voltage as well as unique speed advantages. ECL processors, for example, operate at clock rates over 100MHz whereas CMOS processors operate typically at 30MHz. Thus bipolar and CMOS devices form an ideal combination. The only drawback is the higher cost of their fabrication on the same chip. BiCMOS processes are more costly than CMOS processes due to additional masks, processing steps and reduced yield. However, this cost factor is less severe today than in earlier years because of the convergence of bipolar and CMOS technologies. This report describes BiCMOS techniques for high-speed/ high-density circuits for digital as well as analog functions. First the features of MOS and bipolar devices are compared. The different aspects of designing a BiCMOS process are described next. Finally the advantages of the BiCMOS technology for systems applications are demonstrated.