Quasi-neutral Base Region Research Articles

Analysis of recombination mechanisms in heterojunction silicon solar cells with rapid thermally annealed thin film emitters

A new family of silicon (Si) wafer heterojunction solar cells fabricated by solid phase crystallization of PECVD amorphous silicon emitters by rapid thermal annealing (RTA) has been analyzed in order to understand the dominant recombination mechanisms. Solar cells fabricated with a broad RTA temperature range of 600–1000 °C were characterized through quantum efficiency, illuminated I–V, and capacitance–voltage measurements. Using the experimental data and theoretical considerations, the influence of carrier recombination in the quasi-neutral and space charge zones as well as at the heterojunction interface were studied. It is established that the carrier recombination in the quasi-neutral base region in the p-type Si substrate predominantly limits the device open circuit voltage. The analysis also showed that the interface recombination velocities at the heterojunction were less than 100 cm s−1. It is also qualitatively established that a post-fabrication forming gas anneal reduces the defect density at the hetero-interface.

Radiative recombination in GaN/InGaN heterojunction bipolar transistors

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The IB-dependent EL intensities for these three recombination paths are discussed. The results also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.

Effects of optical absorption on the quasi-Fermi level splitting at the emitter-base junction in npn heterojunction bipolar phototransistors

This paper analyzes the effects of optical absorption in the quasi-neutral base region of npn heterojunction bipolar phototransistors (HPT's) on the extent of electron quasi-Fermi level splitting at the emitter-base heterojunction interface. The principle of current balance is used to match the thermionic-field-emission from the emitter with the base electron diffusion current, taking into account the nonuniform optical generation in the base. The increase in minority carrier concentration produced by optical generation in the base leads to a decrease in the quasi-Fermi level splitting at the heterojunction interface which reduces the net electron injection into the base, the emitter injection efficiency and current gain to an extent that depends on the incident optical power and the base-emitter bias.

A high optical-gain /spl beta/-SiC bulk-barrier phototransistor for high-temperature applications

A high optical-gain /spl beta/-SiC phototransistor (PT) with a bulk-barrier structure has been fabricated on a silicon substrate. It demonstrated high optical gains of 145 at 25/spl deg/C and 106 at 250/spl deg/C, under a 10-V bias and 10-μW incident optical power with a wavelength of 500 nm. The high optical gains at elevated temperatures are attributed to not only the excellent high-temperature properties of SiC materials, but also the bulk-barrier structure, in which the formed potential barrier, the short base region and an effect of thinning the quasi-neutral base region to zero thickness lead to a greatly enhanced current gain. The developed /spl beta/-SiC bulk-barrier PT possesses a potential for high-temperature high-gain optical-sensing applications.

Analysis and modeling of small-signal bipolar transistor operation at arbitrary injection levels

In this paper a study of small-signal operation of bipolar transistors is presented. Firstly, we derive the governing equations and the associated boundary conditions which describe the small-signal minority carrier transport in the quasi-neutral base region at arbitrary injection levels. Analytical solutions of the transport equations are then discussed. A consequence of the linearity of the transport equations is that the minority carrier currents at the device terminals can be expressed in terms of infinite polynomials of the complex variable. It is then shown that all the hitherto proposed non-quasi-static (NQS) models can be simply obtained by different approximations of the general current expressions. As a consequence, the differences between the various models are clarified, and models previously developed for the low-injection regime are extended to high-injection conditions. The dependence of fundamental model parameters such as the transit time, the partitioning factor, etc., on the injection level is analyzed in detail, and a simple analytical formulation is proposed. Limitations of previous approaches are outlined. Finally, selected NQS models are compared.

A thermionic-field-diffusion model for Npn bipolar heterojunction phototransistors

This article describes the development of a thermionic-field-diffusion model for use in the analysis and design of abrupt, single heterojunction bipolar phototransistors (HPTs) for use in HBT-based optical receivers. In particular, included in this approach are the effects of a dc base bias on the optical gain and device performance. Taking into account the optical generation, the excess electron concentration at the emitter end of the quasi-neutral base is initially determined using a matching of the thermionic field emission across the emitter-base heterojunction with the diffusion current at the emitter end of the base. At the collector end of the base region, a finite electron concentration is used based on the collector current density. The results are used to determine the electron profile in the quasi-neutral base region and the diffusion components to the emitter and collector currents. To determine the device’s optical gain, the photocurrent and the small signal current gain are calculated. The dominant component in the photocurrent is found to be due to optical absorption in the base-collector space charge region. For backside illumination, the collector component becomes larger. The base width and electron diffusion length control the current gain and thereby the optical gain. The model is used to quantify the effects of the device’s structure on the optical gain.

An improved analytical description of the effect of velocity saturation on the collector current of a bipolar transistor

The analytical description of the effect of velocity saturation on the collector current of a bipolar transistor is reconsidered. An improved expression is presented which is consistent with numerical drift-diffusion calculations. It is derived by incorporating velocity saturation into the base Gummel number, as originally proposed by Gummel. Unlike the conventional approach this method makes no use of the boundary condition J n = qv s n (for an npn device) at the edge of quasi-neutral base region. New results are compared with results from the conventional approach.

Injection-induced bandgap narrowing and its effects on the low-temperature operation of silicon bipolar transistors

The authors present evidence that injection-induced bandgap narrowing plays an important role in determining the low-temperature properties of silicon bipolar transistors. This phenomenon occurs when large concentrations of minority-carrier charge are injected into the quasi-neutral base region of the device under high-current conditions. A significant enhancement in the low-temperature transistor current gain in the high-current regime is produced that is unaccounted for in conventional device theory. Comparison of theoretical calculations as well as phenomenological modeling results to measured data supports the authors' claims. The analysis suggests that an understanding of injection-induced bandgap narrowing is required for accurate modeling of bipolar transistors, particularly when they are used in low-temperature applications. >

Lifetime determination in p/n junction diodes and solar cells from open-circuit-voltage decay including junction capacitance effects

A new method for determining the base-region minority-carrier lifetime for p/ n junction diodes or solar cells is presented, which applies provided low-level injection prevails and provided the thickness of the base greatly exceeds that of the emitter. The method uses the traditional open-circuit-voltage-decay measurement. As is customary, the interpretation of the measured response relies on a quasi-static model for the quasi-neutral base region. But the model also includes the capacitive effect of the free electrons and holes stored in the junction space-charge region for the forward-voltage condition intrinsic to open-circuit-voltage decay. Application of the method to solar cells yields values of the minority-carrier lifetime that agree well with those obtained using other techniques. The method requires a numerical procedure, the details of which are described. This method has a potential advantage for monitoring the influence on lifetime of various processing steps in device fabrication.

Transport and related properties of (Ga, Al)As/GaAs double heterostructure bipolar junction transistors

This paper presents a study of the transport and related properties of GaAs double heterostructure bipolar junction transistors with GaAlAs emitters and collectors utilizing both graded and abrupt junctions. By studying the temperature dependence of current-voltage characteristics, the recombination mechanisms in the transition regions associated with the base-emitter, the base-collector junction, and the quasi-neutral base region have been quantitatively separated. The junction corresponding to growth of GaAs on GaAlAs is found to have higher nonradiative recombination. For abrupt junctions, it also shows a lower barrier for electron injection than does the GaAlAs/GaAs junction. At mid-1018cm-3base doping, the junctions show ideal emission/ diffusion transport at high currents, recombination-dominated transport in a medium current range, and transport with a characteristic energy greater than twice the thermal energy at very low currents. Due to excess recombination current in the devices, the offset voltage is shown to be sensitive to the ratio of emitter and collector junction areas. At low temperatures, transport across the injecting interface of abrupt devices is dominated by tunneling and in the quasi-neutral base by diffusion. The devices exhibit gain at temperatures as low as 4.2 K. The current gain below 77 K appears to be limited by radiative recombination lifetime, and at higher temperatures by nonradiative recombination and injection efficiency. Transistors with current gains of 300 to 500 have been achieved at small emitter dimensions of 1.6 µm × 4.0 µm and at current densities exceeding 105A . cm-2. At current densities of 100 A . cm-2, some of the transistors exhibited gains approaching 100. These devices had an offset voltage of less than 100 mV. Self-aligned devices have also been fabricated and show current gains of 20 to 50 at similar dimensions. In the inverted configuration, gains of up to 50 were observed for non-self-aligned transistors. The dependence of device Characteristics upon the structure indicates that surface effects are minimal. The negative resistance exhibited by these devices at high currents is shown to be due to temperature effects resuiting from the 10-µs or lower thermal time constants.

Current gain of narrow-base transistors

It is shown that the total number of impurities per cm 2 in the quasi-neutral base region of narrow-base transistors and their current-gain β strongly depend upon the emitter-base voltage V BE . The computed results demonstrate that the β of these transistors falls considerably with V BE increase, even in the absence of high-injection effects. The analysis also shows that the collector current of narrow-base transistors increases slower than exp ( V BE / V T ) giving rise to a non-ideality factor greater than one. It is concluded that narrow-base transistors with higher base resistivities are more susceptible to these effects.

Diffusion length and lifetime determination in p-n junction solar cells and diodes by forward-biased capacitance measurements

A new method is described and illustrated for determining the value and the temperature dependence of the minority-carrier diffusion length and lifetime in the base region of p-n junction solar cells and diodes. The method applies to devices after the p-n junction is formed, and thus includes the influence of the junction fabrication on the diffusion length and lifetime. The method requires only forward-biased capacitance measurements at the device terminals. It combines the dependencies of the low-frequency and high-frequency capacitance on forward bias in such a way as to yield the component of the capacitance associated with the minority carriers in the quasi-neutral base region. From this quasi-neutral-base capacitance the minority-carrier diffusion length and lifetime are then deduced. The accuracy of this method is estimated to be ±5 percent based on the accuracy of the capacitance bridges and of the measured temperature. To illustrate the method and its accuracy, it is applied to a set of silicon p-n junction diodes having base doping concentrations ranging from about 1014to 1018cm-3, and the values of the diffusion lengths determined by the capacitance method are compared with those obtained using the current response to X-ray excitation. Excellent agreement is seen. In contrast, the open-circuit-voltage-decay method yields values of the diffusion length that differ appreciably from those determined by the capacitance and X-ray methods. The reasons for the relative inaccuracy of the open-circuit-voltage-decay method applied to silicon devices are discussed. The practical limitations of the capacitance method are indicated, and an extension of the method is discussed that makes it applicable to devices having highly doped base regions and surface (emitter) layers. In such devices, the capacitance of the quasi-neutral emitter region can contribute appreciably to the total forward-biased capacitance, and the method can yield information about the material properties of the degenerately doped surface layer. The material properties determined are the phenomenological emitter lifetime and a measure of the energy bandgap narrowing in the emitter.