Pwm Paul Blom

Increasing the noise margin in organic circuits using dual gate field-effect transistors

Complex digital circuits reliably work when the noise margin of the logic gates is sufficiently high. For p-type only inverters, the noise margin is typically about 1 V. To increase the noise margin, we fabricated inverters with dual gate transistors. The top gate is advantageously used to independently tune the threshold voltage.

Optimization of barrier thickness for efficient carrier capture in graded-index and separate-confinement multiple quantum well lasers

We present results from numerical calculations on the carrier capture efficiency in separate‐ confinement and graded‐index separate‐confinement multiple quantum well (MQW) lasers. We find that the capture time oscillates as a function of the well width as well as the barrier width between the wells, due to a changing overlap of the barrier wave functions with the bound states in the wells. We show that one order of magnitude improvement in the carrier capture efficiency can be accomplished by properly choosing the dimensions of the layers in the active region of the MQW laser.

Semitransparent organic solar cells with organic wavelength dependent reflectors

Semitransparent organic solar cells employing solution-processable organic wavelength dependent reflectors of chiral nematic (cholesteric) liquid crystals are demonstrated. The cholesteric liquid crystal (CLC) reflects only in a narrow band of the solar spectrum and remains transparent for the remaining wavelengths. The reflective band is matched to the absorption spectrum of the organic solar cell such that only unabsorbed photons that can contribute to the photocurrent are reflected to pass through the active layer a second time. In this way, the efficiency of semitransparent organic solar cells can be enhanced without significant transparency losses. An efficiency increase of 6% was observed when a CLC reflector with a reflection band of 540–620 nm was used, whereas the transparency of the organic solar cells is only suppressed in the 80 nm narrow bandwidth.

Injection-limited electron current in a methanofullerene

The dark current of bulk-heterojunction photodiodes consisting of a blend of a methanofullerene (PCBM) as n-type electron acceptor and a dialkoxy-(p-phenylene vinylene) (OC1C10−PPV) as a p-type electron donor sandwiched between electrodes with different work functions has been investigated. With ohmic contacts for hole and electron injection, the dark current appears completely dominated by the electron current in the PCBM, as a result of a much higher electron mobility. This electron current is bulk space-charge limited. With Au as a high work function metal, the electron current becomes injection limited. The injection-limited electron current from the Au electrode into PCBM is explained within a thermally assisted hopping model. In spite of the presence of an injection barrier of about 0.76 eV, the injection-limited electron current from a Au electrode into PCBM still exceeds the bulk-limited hole current in OC1C10−PPV.

Ordered Semiconducting Self-Assembled Monolayers on Polymeric Surfaces Utilized in Organic Integrated Circuits

We report on a two-dimensional highly ordered self-assembled monolayer (SAM) directly grown on a bare polymer surface. Semiconducting SAMs are utilized in field-effect transistors and combined into integrated circuits as 4-bit code generators. The driving force to form highly ordered SAMs is packing of the liquid crystalline molecules caused by the interactions between the linear alkane moieties and the pi-pi stacking of the conjugated thiophene units. The fully functional circuits demonstrate long-range order over large areas, which can be regarded as the start of flexible monolayer electronics.

Charge transport in dual-gate organic field-effect transistors

The charge carrier distribution in dual-gate field-effect transistors is investigated as a function of semiconductor thickness. A good agreement with 2-dimensional numerically calculated transfer curves is obtained. For semiconductor thicknesses larger than the accumulation width, two spatially separated channels are formed. The cross-over from accumulation into depletion of the two channels in combination with a carrier density dependent mobility causes a shoulder in the transfer characteristics. A semiconducting monolayer has only a single channel. The charge carrier density, and consequently the mobility, are virtually constant and change monotonically with applied gate biases, leading to transfer curves without a shoulder.

Monolayer dual gate transistors with a single charge transport layer

A dual gate transistor was fabricated using a self-assembled monolayer as the semiconductor. We show the possibility of processing a dielectric on top of the self-assembled monolayer without deteriorating the device performance. The two gates of the transistor accumulate charges in the monomolecular transport layer and artifacts caused by the semiconductor thickness are negated. We investigate the electrical transport in a dual gate self-assembled monolayer field-effect transistor and present a detailed analysis of the importance of the contact geometry in monolayer field-effect transistors.

Analysis of the shallow and deep center occupancies in Si‐doped AlxGa1−xAs using a multilevel donor model

The concentration of occupied deep centers in Si‐doped AlxGa1−xAs for x≥0.2 has been calculated from a three‐level donor model, in which the shallow levels are treated as excited states of the deep (DX) ground state. The deep level is assumed to be tied to the L valley, and the shallow levels to the Γ and X valleys. The behavior of the free‐electron density and the thermal activation energy as function of composition is in good agreement with experimental results reported in the literature. In this model of dependent donor levels the deep‐level occupancy can be directly calculated without needing deep‐level transient spectroscopy measurements. A two‐level donor model is used to calculate the pressure dependence of the deep level from a hydrostatic pressure experiment on a GaAs/Al0.3Ga0.7As heterostructure reported in the literature. We assume a shallow level tied to the Γ valley and an arbitrary deep level which is not coupled to any of the conduction bands. The calculation of the position of the deep lev...