H. Bleichner

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...

THE AMBIPOLAR AUGER COEFFICIENT : MEASURED TEMPERATURE DEPENDENCE IN ELECTRON IRRADIATED AND HIGHLY INJECTED N-TYPE SILICON

From free-carrier absorption measurements of spatially and temporally resolved excess carrier concentrations in the n-base of electron irradiated, low doped, highly injected p-i-n type diodes, the effective ambipolar lifetime has been extracted with respect to different injection levels and temperatures. By measuring lifetimes up to injection levels of 3×1017 cm−3, the ambipolar Auger coefficient Ca and its temperature dependence has been determined in the range of 300–420 K. In accordance with recent publications, the room temperature value of the Auger coefficient was found to be about three times higher than one of the most commonly used values. Further on, the ambipolar Auger coefficient is empirically estimated to vary with temperature as Ca(T)=1.1×10−30(T/300)1.8, were T is the absolute temperature. Also the ambipolar diffusion coefficient has been investigated and compared with different models. The results are important and useful in simulation of high-injected silicon devices, i.e., solar cells a...

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.

Subthreshold behaviour of silicon MESFETs on SOS and bulk silicon substrates

Abstract The subthreshold characteristics of silicon MESFETs manufactured using both bulk silicon and silicon-on-sapphire (SOS) technology, have been studied. n - and p -type devices have been investigated and their characteristics are presented here. The results show that the subthreshold behaviour for bulk devices in fully comparable with that of MESFETs, while the SOS devices show a somewhat lower value of subthreshold swing. A comparison between calculated and experimental behaviour is presented which yields information about the influence of the geometry and processing parameters of the subthreshold behaviour. It is proposed that the saturation subthreshold current for bulk transistors can be decreased by a proper design. An improvement for SOS devices, however, requires improved substrate material.

An optical system for bilateral recombination-radiation diagnostics of the carrier redistribution in switching power devices

An optical scanning system is developed for the detection of recombination radiation from power devices. The instrument facilitates two-dimensional characterization of the excess-carrier content in a device. A bilateral technique enables measurements along various perpendicular surfaces of the devices under investigation. Hence, measurements of processes not present, or less dominant, in one direction may be observed in another. Examples taken from gate turn-off (GTO)-thyristor investigations are presented. All stages of the switching cycles as well as during the ON-state can be investigated with the instrument. Detailed three-dimensional carrier maps are produced for the visualization of measurement results, and sequences of maps are used to observe time-dependent phenomena involved in the transient operation of a device. Features associated with placement of the optical focus are explained, and possibilities of tomographic mapping of power devices are discussed. >

Flying-spot scanning for the separate mapping of resistivity and minority-carrier lifetime in silicon

Abstract A new method to perform local measurements of the material parameters in bulk-semiconductor material is presented. Charge carriers are generated optically in the material by a sinewave-modulated HeNe laser and cause an electric current through a rectifying contact at the opposite side of the sample. The detected current will carry information of all the electrical parameters in the bulk of the sample such as resistivity and minority-carrier lifetime as well as surface effects like surface-recombination velocity and optical reflectance. The presented method employs sinewave-modulated light, which is absorbed in the surface layer of the sample. The minority carriers will cause a current after their diffusion through the material when reaching the depleted layer of the reverse-biased rectifying contact. This current is also sine-shaped. A measurement of its amplitude and phase shift relative to the source of excitation is performed, as the reverse-bias is varied. These two measured quantities give absolute values of the local resistivity and minority-carrier lifetime by means of the theoretical relationships presented in the paper. When the light spot is scanned across the wafer lateral variations in these parameters are exposed. The method to scan a sample by moving the point of excitation gives the possibility of a complete quantitative mapping of a sample as to the spreading of the bulk parameters, i.e. a resistivity map and a lifetime map. The experiments were performed using typical material for power components, i.e. relatively high-resistive n -type silicon.

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. >