C. van Berkel

Time and temperature dependence of instability mechanisms in amorphous silicon thin‐film transistors

We have measured the time and temperature dependence of the two prominent instability mechanisms in amorphous silicon thin‐film transistors, namely, the creation of metastable states in the a‐Si:H and the charge trapping in the silicon nitride gate insulator. The state creation process shows a power law time dependence and is thermally activated. The charge trapping process shows a logarithmic time dependence and has a very small temperature dependence. The results for the state creation process are consistent with a model of Si dangling bond formation in the bulk a‐Si:H due to weak SiSi bond breaking stabilized by diffusive hydrogen motion. The logarithmic time dependence and weak temperature dependence for the charge trapping in the nitride suggest that the charge injection from the a‐Si:H to the nitride is the rate limiting step and not subsequent conduction in the nitride.

Bias dependence of instability mechanisms in amorphous silicon thin‐film transistors

We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon‐silicon nitride thin‐film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a‐Si:H) layer, whereas the charge trapping takes place in the a‐SiN:H. The metastable states are suggested to be Si dangling bonds in the a‐Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin‐film transistor means that states can be created simply by populating conduction‐band states i...

Resolution of amorphous silicon thin-film transistor instability mechanisms using ambipolar transistors

Bias stress measurements on amorphous silicon‐silicon nitride ambipolar thin‐film transistors give clear evidence for the co‐existence of two distinct instability mechanisms: the metastable creation of states in the a‐Si:H layer and charge trapping in the a‐SiN:H layer. The creation of metastable states in the a‐Si:H is found to dominate at low positive bias, while charge trapping in the nitride dominates at larger positive bias and negative bias.

Quality factor in a‐Si:H nip and pin diodes

We analyze the forward characteristics of a‐Si:H nip and pin diodes. At low bias, a well‐defined exponential region exists, described by a noninteger quality factor n between 1.2 and 1.7. With increasing temperature, the quality factor decreases. This behavior can be understood with a model based on electron and hole recombination in the i layer, which relates the temperature dependence of the quality factor to the distribution of localized states in the amorphous silicon. The predictions of the model are supported by numerical calculations in which the diode device equations are solved for a given distribution of localized states. The different ideality factors are due to different energy dependencies of the density of deep states in the i layer.

Photo‐field effect in amorphous silicon thin‐film transistors

Amorphous silicon thin‐film transistors show a marked photosensitivity, with the illumination producing an increase in the off‐current and a negative shift of the threshold voltage. Here we present the results of a complete theory of this photo‐field effect, which includes the steady state flux of electrons and holes perpendicular to the source‐drain current path. We show that illumination always produces a change in the band bending and a consequent redistribution of the space charge. The theory gives an excellent agreement with the experimental results using a simple model for the density of states in the amorphous silicon.

Adding synchronous and LSSD modes to asynchronous circuits

A synchronous mode as well as a scan mode of operation are added to a large class of asynchronous circuits, in compliance with LSSD design rules. This enables the application of mainstream tools for design-for-testability and test-pattern generation to asynchronous circuits. The approach is based on a systematic transformation of all single-output sequential gates into synchronous and scannable versions. By exploiting dynamic circuit operation in scan mode, the overhead of this transformation in terms of both circuit cost and circuit delay is kept minimal.

REVERSE CURRENT MECHANISMS IN AMORPHOUS SILICON DIODES

We analyze the dark steady‐state reverse current of a‐Si:H nip diodes at high voltages. The reverse current shows a strong voltage dependence and has a temperature dependence characterized by a voltage‐dependent activation energy. A model, based on the simultaneous field enhanced generation of electrons and holes, is developed to describe this voltage and temperature dependence. In this model the effective mass of electrons and holes is a model parameter. Good fits with experimental results are obtained for an effective mass value of 0.05me. The low effective mass value is tentatively explained as a parameter that accounts for a field dependent narrowing of the band gap due to the presence of localized band tail states.

Instability mechanisms in amorphous silicon thin film transistors and the role of the defect pool

The threshold voltage shift in amorphous silicon thin film transistors at moderate applied bias is detemined by changes in the density of dangling bond states in the a-Si:H. Positive bias-stress creates dangling bond states at a low energy (D e states), in both oxide and nitride transistors. Negative bias-stress creates dangling bond states at higher energy (D h states) in oxide transistors, but mainly reduces the density of D e states in nitride transistors. These results are explained using a defect pool model for the dangling bond states. The difference for oxide and nitride transistors is due to a different zero bias Fermi energy position at the interface. For nitride transistors, charge trapping at higher bias, followed by thermal annealing leads to a new zero bias thermal equilibrium density of states. Transistor characteristics can be optimised in this way.

The photosensitivity of amorphous silicon thin film transistors

Abstract Amorphous silicon thin film transistors show a marked photoresponse. We describe results of a complete theory of the photofield effect, which includes the steady state flux of electrons and holes perpendicular to the source-drain current path. We indicate the nature of recombination centres required to reduce the photosensitivity and still maintain a good field effect.

Deep trapping controlled switching characteristics in amorphous silicon thin-film transistors

We report transient effects in amorphous silicon thin‐film transistors occurring upon switch‐on and switch‐off, which are controlled by trapping and emission from the deep states in the amorphous silicon. We develop a unifying theoretical description which is applicable to both switch‐on and switch‐off. The model is based on carrier thermalization to the deep states and includes the spatial dependence of the thermalization process in the band‐bending region. The model is able to explain the experimentally observed switch‐on and switch‐off behavior. In the case of switch‐on, electrons are progressively trapped into the deep states in the bulk a‐Si:H throughout the entire thickness of the layer. The range of trapping times is large and this leads to a dynamic threshold votage shift and a time dependence of the source‐drain current extending between 1 μs and 1 s. In the case of switch‐off, two processes occur sequentially. First, there is emission of electrons from the bulk a‐Si:H deep states, which leads to...