M. M. Shahidul Hassan

Control of current-mode second breakdown in transistors through use of double-graded collectors

Abstract Bipolor transistors with double-graded collectors offer the prospect of achieving a substantial measure of protection against device failure from current-mode second breakdown, which cannot be achieved from a uniformly doped epitaxial transistor where the scope for improvement is fundamentally limited by a need to support the required blocking voltage. Optimal values of double-graded collector parameters of transistors are numerically computed, and empirical expressions for optimal parameters are found.

Characteristics of an epitaxial Schottky barrier diode for all levels of injection

Abstract The effect of the minority carrier recombination on injection ratio and charge storage time of an epitaxial Schottky barrier (SB) diode has been presented. The dependence of injection ratio and storage time on the minority carrier reflecting properties of the epitaxial-substrate interface is also studied when the drift region of a SB diode is fully invaded by the minority carriers. The study shows that at a relatively low current density, the conductivity modulated region partially occupies the drift region and the charge stored within the injection region cannot be neglected. In the present work, this region is considered for the first time in studying the characteristics of the SB diode. Analytical expressions for the diode forward current, minority carrier injection ratio and storage time parameters are obtained; and the derived expressions are applicable for all levels of injection. In the previous works, to model the complete range of injection level in the SB diode, empirical formulas were introduced to combine the high-level injection model with the traditional low-level current–voltage equations.

Base transit time of a Heterojunction Bipolar Transistor with Gaussian doped base

Base transit time for an npn SiGe HBT is calculated assuming Gaussian doped base and a generalized trapezoidal Ge profile considering field dependent mobility in excess to doping dependent mobility and diffusivity. Band-gap narrowing (BGN) due to heavy doping, due to presence of Ge and due to change in the density of states (DOS) are also considered. For presence of Ge, a different saturation velocity has been used. Ge profile variation has been incorporated by a single parameter. Base transit time of SiGe HBT is calculated and computed for different Ge contents. The calculated base transit time shows similar variations as found in literature for BJT.

New expression for base transit time in a bipolar transistor for all levels of injection

Abstract A new and compact formula for the base transit time in an npn bipolar junction transistor is derived. The collector current density and minority carrier charge within the base are separately expressed as a function of injected electron concentration in the neutral base in order to find an expression for base transit time. The modeling of collector current density, base stored charge and base transit time is essential for the design of high speed bipolar transistors. The derived expressions are applicable for arbitrary injection regions before the onset of the Kirk effect and they are simple and straightforward to give a physical insight into device operation.

Breakdown voltage of high-voltage bipolar transistors

Abstract An empirical expression for open-base bulk breakdown voltage of an epitaxial n+pn−n+ bipolar transistor as a function of collector doping density and common-emitter current gain is useful in the design of the collector region of the transistor. In this paper, analytic expressions for optimal collector doping density and epitaxial layer thickness of high-voltage transistors with a lightly-doped collector are obtained as a function of breakdown voltage subject to a minimum collector resistance. A relation between open-base and open-emitter bulk breakdown voltages is also established in this paper.

Increase of critical current density and voltage for triggering avalanche injection through use of graded collector doping

Abstract In this paper it is shown that both critical current density and voltage of an epitaxial bipolar power transistor with an inductive load, which are taken as a measure of its susceptibility to avalanche injection, can be significantly increased by using double graded collector doping profiles. The first graded portion gives the required current protection level and the steeper second graded profile can be used to increase the voltage protection level. Calculations are carried out for optimum collector parameters to achieve minimum resistance for the required open base breakdown voltage. Results are compared with uniformly doped profile for the same collector resistance and open base voltage. It is shown that a device with graded collector is less prone to failure due to avalanche injection.

Estimate of minimum collector current for inducing current mode second breakdown in reverse biased epitaxial bipolar transistors

Abstract When collector current of a reverse biased epitaxial bipolar transistor exceeds a critical value, the transistor goes into current mode second breakdown and its terminal voltage abruptly decreases to a low value. The current is localized at the centre of the emitter stripe where high current densities will be reached. At high collector current densities, high level injection is encountered in the base region. Here, a simple analytic expression accounting for high injection effects in the base is derived for the critical collector current. The result is applied to a planar transistor structure. The approach presented in this paper is different from those given by previous authors where they treated current crowding phenomena only for low level injection.

Performance of CZTSxSe1−x solar cell with various mole fractions of sulfur for different buffer layers

Among the thin film solar cells, CZTSSe is the most promising one. In this article, the performance of CZTSSe solar cell with different potential buffer layers for different mole fractions of S of CZTSSe has been numerically investigated. It has been found that ZnSe buffer layer provides better performance in terms of efficiency than others within 0.7 mole fraction of S of CZTSSe. Above this value ZnO and ZnS both provide almost identical performance. The obtained results might be helpful in developing CZTSSe thin film solar cells by using proper mole fraction of sulfur in CZTSSe which has the potential to provide higher efficiency.

Parameters optimization of a heterojunction thin film solar cell to improve the conversion efficiency

Thin Film Solar Cell is a technology with great potential because of its low material consumption and relatively high efficiency. In this paper several techniques have been taken into consideration which can improve the efficiency of thin film solar cell like CIGS and ZnTeO. Simulations have been performed to optimize the parametric values that can greatly influence the efficiency of thin film solar cell. Effect of change of cell width, transmittance, recombination lifetime etc. on the conversion efficiency has been studied and optimized in MATLAB. It has been found that thin film solar cells with larger cell width can provide better efficiency only when the minority carrier mobility of the cell material crosses a critical value. If the mobility of the cell material is below this critical value, the efficiency of the cell decreases with increase in cell width.

Analytical modeling of base transit time considering recombination in the non-uniformly doped base

The main objective of this paper is to show that recombination in the base needs to be taken into account in determining base transit time τB. In previous analytical works for τB, recombination in the base was neglected. In this paper both drift and diffusion currents for electron and hole are considered in obtaining minority carrier profile n(x). In the model, both SRH and Auger recombination are considered. The energy-bandgap-narrowing effects due to heavy doping, velocity saturation as well as doping and field dependent mobility are considered. The model shows that recombination has significant effects on the base transit time of a heavily doped base.