M Rosling

The ambipolar diffusion coefficient in silicon: Dependence on excess‐carrier concentration and temperature

Recent articles have been published claiming that the ambipolar diffusion coefficient of semiconductors, Da, is basically independent of the excess‐carrier concentration, thus contradicting the conventional result of, for example, Fletcher. However, only a few experimentally verified papers on this subject are published. In the present work, a detailed experimental analysis of the ambipolar diffusion coefficient is presented regarding both temperature and injection‐level dependence. The ambipolar diffusion coefficient was measured in the low‐doped n base of a p‐i‐n type diode at different excess‐carrier concentrations and temperatures using an open‐circuit carrier decay (OCCD) method based on the free‐carrier absorption (FCA) technique. This investigation was performed in the carrier‐concentration range of 1015–2×1017 cm−3 and in the temperature range of 300–420 K, respectively. The ambipolar diffusion coefficient is experimentally found to behave in reasonable agreement with Fletcher’s theory, thus decre...

A study of turn-off limitations and failure mechanisms of GTO thyristors by means of 2-D time-resolved optical measurements

The failure mechanisms of Gate-Turn-Off (GTO) thyristors are investigated. Measurements based on a time-resolved free-carrier absorption (FCA) technique are used to support the presented models. The measurements serve to map the local carrier densities two-dimensionally, at any time of the switching cycle. Inductively loaded GTOs under snubberless operation are studied close to the safe-operating area (SOA) limit. Several important features of the destructive process are established. First, the existence of a quasi space-charge region, QSC, in the n base and charge focusing at the anode side of the device is noticed during the fall-time period. Secondly, a piling up of holes in the p base is followed by current filamentation during the tail period. The sequential behaviour of different phenomena and their possible causal explanations are also established. The expansion of the QSC towards the anode emitter causes the enhancement and focusing of the charge distribution in the vicinity of this junction. Two possible failure mechanisms, viz. local dynamic punch-through in the n base and local dynamic avalanche injection in the blocking junction are discussed in the light of the experimental results. It is also suggested that both the above mentioned failure mechanisms can be part of the chain of events leading to device failure. Regardless of which mechanism dominates, a high peak of excess holes appears in the p base in the beginning of the tail period. This charge debiases the cathode junction locally, i.e. compensates the negative bias of the cathode junction, and current filaments connecting the cathode and anode sides of the device are formed. The excessive power dissipation in the dominant filament causes failure and permanent damage.

A time-resolved optical system for spatial characterization of the carrier distribution in a gate turn-off thyristor (GTO)

The measurement of two-dimensional (2-D) excess-carrier distribution in a gate GTO by a time-resolved infrared-absorption technique is discussed. The optical scanning system employs a wide memory digital oscilloscope for data acquisition and a computer system for control, data processing, and display. Maps of the carrier distribution in the active GTO device are produced under steady-state conditions as well as during turn-on and turn-off operation. The maps are presented as three-dimensional (3-D) views produced from 2-D measurements. >

A novel technique for the simultaneous measurement of ambipolar carrier lifetime and diffusion coefficient in silicon

Abstract Using the classical semiconductor continuity equation, a new method for the simultaneous extraction of the ambipolar carrier lifetime and the ambipolar diffusion coefficient for high injection levels is developed. The method uses carrier decay measurements based on the free-carrier absorption (FCA) technique. The measurements are performed in the base of p-i-n type diodes, where the middle region is a lightly n-doped base region. By varying the forward bias, the ambipolar lifetime and diffusion coefficients are studied for different excess-carrier concentrations ranging approx. 2–4 decades above the base doping. Both the lifetime and diffusion coefficients agree well with theoretical models. The theoretical ambipolar diffusion coefficients agree well with theoretical models. The theoretical ambipolar diffusion coefficient is calculated using the values of Dn and Dp, which, in turn, are determined from mobility models and the Einstein relation. The mobility models used include carrier-carrier scattering effects which are important in the explanation of high-injection dependence.

An improved silicon p-channel MESFET with a BF2 implanted thin channel and ErSi2 gate

Abstract To develop a process for making complementary silicon MESFETs it is of utmost importance to achieve high performance MESFETs. Especially the p -channel MESFETs need improvement of driving capability and switching speed. In this article, we will report results from a p -channel MESFET having an ErSi 2 gate, fabricated using bulk silicon technology. This device exhibits improved values of transconductances as compared to earlier fabricated devices. This is mainly due to the fabrication of a controlled thin channel, manufactured by the BF 2 implantation technique. Also, the subthreshold characteristics of this improved device is presented and is found to be comparable to MOS p -channel devices.

Measurements of failure phenomena in inductively loaded multi-cathode GTO thyristors

The free-carrier absorption (FCA) technique is used for mapping of the carrier content in a dual-cathode gate turn-off thyristor (GTO) at different stages of the turn-off cycle. The FCA technique outputs 3-D maps based on 2-D measurements of local carrier concentration. The measurements are time resolved, thus making a transient response converted into a corresponding sequence of carrier maps. The dual-cathode device is shown to be sufficient for determining the transient behavior in a multicathode structure. The destruction mechanism and reasons for turn-off failure are investigated. The GTO device is inductively loaded and asymmetrically gate contacted in order to emulate a realistic mode of operation. The gate-driving conditions are altered, and the importance of the turn-off gain for turn-off failure is established. The case of equal ON-state cathode currents in both segments is particularly highlighted. The influence of snubber circuits is also discussed. >

A study of design influence on anode-shorted GTO thyristor turn-on and turn-off

Anode-shorted GTO thyristor samples were investigated by means of the free-carrier absorption (FCA) technique. Both the turn-on and turn-off processes were investigated as regards the two-dimensional carrier distribution for different stages of the transient cycles. The results are presented as carrier-map sequences, i.e., 3-D pictures of measured 2-D carrier distributions. Samples were formed as unit segments cut out from large-area devices, and associated with different degrees of anode shorting, silicon thickness, and lifetime treatment. During investigation, the samples were inductively anode loaded, and as regards the turn-off process they were operated near the safe-operation limit. The measurements clearly illustrate the way carriers are transported in the sample when firing the device, and the turn-on process is visualized in steps by means of carrier-map sequences. These measurements are supported by computer simulations. The turn-off process is also visualized in carrier-map sequences measured from two perpendicular directions, and the maps show the critical electric-field expansion which always precedes a turn-off failure due to dynamic breakdown mechanisms. Further on, the effect of design-parameter variations, e.g., anode shorting pattern and carrier lifetime reductions, on destructive GTO turn-off phenomena are discussed. >

Influence of silicon-sapphire interface defects on SOS MESFET behavior

Abstract Silicon MESFETs in silicon-on-sapphire SOS technology were studied and a comparison was made between simulated and measured I - V characteristics of normally-off transistors. The comparisons have shown that the effective conducting-channel depth is only 40% of the actual Si film thickness. A novel simulation approach to compensate for the lack of free carriers close to the sapphire interface is described. Results from measured transistors is presented together with simulated behavior, and also a comparison between simulated and experimental complementary MESFET (CMES) inverter behavior is done. The influence of interface defects on subthreshold current is also discussed.

A time-dependent two-dimensional analysis of the turn-off process in a gate turn-off thyristor (GTO)

A time-resolved free-carrier absorption technique is used to describe transient gate turn-off thyristor (GTO) phenomena two-dimensionally. The measurements are spatially resolved in the anode-to-cathode direction, as well as in the direction determined by the cathode length. Effects associated with charge removal during the GTO turn-off cycle with respect to external circuit conditions are discussed. The behavior of resistively and inductively loaded GTOs is explained on the basis of experimental results. Modeled results published by others are used for comparison. >

Determination of the mobility profile in silicon-on-sapphire material using the “fat” FET principle

Abstract Using a field effect transistor (FET) structure with a Schottky gate, the mobility profile in a silicon layer is calculated from capacitance and conductance measurements. The measurements require ordinary laboratory instruments only. A short theory is presented, followed by experimental values for silicon on sapphire (SOS) material. The results show a sharp decrease in mobility at the defect rich layer close to the sapphire and close to the silicide gate junction. A maximum mobility of 490 cm 2 /Vs for electrons and 190 cm 2 /Vs for holes was obtained for a position roughly in the middle of the silicon layer.