Current gain in polysilicon emitter transistors

The common-emitter current gain β in a shallow polysilicon emitter transistor is derived by solving the minority-carrier transport equation in the silicon-polysilicon structure. Coupled With the majority-carrier transport, an equation for the current gain is obtained which depends on the physical properties of the polysilicon as well as the silicon emitter and base doping profiles. The calculations are based on the carrier trapping model proposed in the literature and ignore any minority-carrier recombinations. The model predicts the current gain of a polysilicon emitter transistor increases at low current and high temperature and approaches that of a conventional metal contact transistor at high current and low temperature. This is due to the potential barrier across the grain boundaries, while impeding the majority-carrier flow, inject minority carriers as a bias is developed across the grain boundaries. It also predicts a decrease in cutoff frequency fTdue to minority carriers stored in the polysilicon.     

A lateral metal—insulator—p-Si tunnel transistor

We report the fabrication of a lateral MIS tunnel transistor whose emitter and collector are Al/SiO2/p-Si tunnel junctions. All processing is carried out at room temperature except for the growth of the passivating field oxide. The small signal common emitter current gain is 20. Two coupled gain mechanisms exist for such a lateral MIS tunnel transistor. The first mechanism relies on a high minority-carrier injection ratio of the emitter junction. Second, the minority carriers injected into the reverse-biased collector junction may produce additional gain through multiplication of majority-carrier current. Lateral MIS tunnel transistors on n-Si make use of the second mechanism. Our device takes advantage of the high minority-carrier injection ratio achievable with Al/SiO2/p-Si tunnel junctions.     

A silicon phototransistor with a MIS tunnel junction emitter

We have fabricated a silicon phototransistor using low-temperature processing. The emitter of the bipolar transistor is an Al-SiO2-p-Si tunnel junction. The quantum efficiency gain is 200 at λ = 0.6328 µm. The common-emitter current gain of the analogous three-terminal metal-insulator-semiconductor (MIS) transistor also is 200. This demonstrates the high minority-carrier injection efficiency of the MIS emitter.     

重掺杂发射区中禁带宽度和少子复合寿命的确定方法

禁带宽度和少子复合寿命是硅晶体管发射区中重要的物理参数。本文利用p-n结反向扩散电流的温度特性和借助于线性外推法,提出了一种确定绝对零度时禁带宽度的新方法。由于发射区重掺杂,本文考虑了载流子的费米-狄拉克统计分布。提出了确定发射区中少子复合寿命的方法。该方法简便实用。     

Current gain of high-low junction emitter bipolar transistor structure with uniformly doped profiles

The analytic expressions for the maximum current gain and the transit times across the neutral emitter and base region in the case of uniformly doped high-low junction emitter bipolar transistor structure are calculated in detail, which include the effects of energy-gap narrowing in highly doped semiconductor, doping-dependent minority-carrier diffusivities, contact-surface and bulk recombinations. It is shown that the high-low junction emitter with the doping in the low-concentration emitter higher than that of the base region will give higher maximum current gain and better frequency response than those of the LEC structure originally proposed by Yagi et al. Numerical results for the optimal design of the high-low junction emitter are given and discussed.     

Thermal effects on the characteristics of AlGaAs/GaAsheterojunction bipolar transistors using two-dimensional numericalsimulation

Two-dimensional numerical simulations incorporating the spatial distributions of the energy band and temperature were used to study AlGaAs/GaAs heterojunction bipolar transistor characteristics. It was found that the negative differential resistance and the reduction of the base-emitter voltage for a constant base current in the active region are due to thermal effects. The differential current gain and cutoff frequency decrease when the transistor is operated at high power levels. The temperature distribution of the transistor operated in the active region shows a maximum at the collector region right beneath the emitter mesa. When the transistor is operated in the saturation region, the emitter contact region may be at a slightly lower temperature than the heat sink temperature. This thermoelectric cooling effect results from the utilization of the thermodynamically compatible current and energy flow formulations in which the energy band discontinuities are part of the thermoelectric power     

选择离子注入集电极技术研究

为提高超高速双极晶体管的电流增益 ,降低大电流下基区扩展效应对器件的影响 ,将选择离子注入集电区技术 (SIC)应用于双层多晶硅发射极晶体管中。扩展电阻的测试结果显示出注入的 P离子基本上集中在集电区的位置 ,对发射区和基区未造成显著影响。电学特性测量结果表明 ,经过离子注入的多晶硅发射极晶体管的电流增益和最大电流增益对应的集电极电流明显高于未经离子注入的晶体管。因此 ,在双层多晶硅晶体管中采用 SIC技术 ,有效地降低了基区的扩展效应 ,提高了器件的电学特性。     

Transit time and charge storage measurements in heavily doped emitters

We report results of a first direct measurement of the minority-carrier transit time in a transparent heavily doped emitter layer. The value was obtained by a high-frequency conductance method recently developed and used for low-doped Si. The transit time coupled with the steady-state current enables the determination of the quasi-static charge stored in the emitter and the quasi-static emitter capacitance. Using a transport model, we estimated, from the measured transit time, the value for the minority-carrier diffusion coefficient and mobility. The measurements were done using a heavily doped emitter of the Si p+-n-p bipolar transistor. The new result indicates that the position-averaged minority-carrier diffusion coefficients may be much smaller than the corresponding majority-carrier values for emitters having a concentration ranging from about 3 × 10¹⁹cm^-3to 10²⁰cm^-3.     

On the modeling of polysilicon emitter bipolar transistors

In this paper, an attempt is made to derive a general analytical formulation for the current gain and emitter transit time of a polysilicon emitter bipolar transistor (BJT), which includes all previous models as particular cases. Firstly, it is shown that the minority-carrier injection and storage in the polysilicon region can be simply described by effective values of the minority-carrier diffusion length and mobility. These quantities are precisely defined, and depend on the microscopic transport properties of polysilicon grains and grain boundaries. Secondly, a general expression for the effective recombination velocity relative to the poly/mono interface is derived, which includes, and in some cases extends, all previous approaches. This results in a simple and general formulation which avoids some unnecessary simplification present in nearly all previous treatments, and allows easy comparison of the different models for the poly/mono interface and a clear assessment of the relevance of each physical mechanism. Finally, minority-carrier injection and storage in the single-crystal region is addressed. The effect of oxide breakup on both current gain and emitter transit time is also considered, and different models are compared     

Metal migration into polysilicon emitter after very high currentstress

TEM analyses show metal migration into the polysilicon emitter of a bipolar transistor after high current stress. At the edges of the polysilicon emitter where the current density was expected to be the highest, a metal filament was seen penetrating into the edge of the polysilicon emitter after stressing at a current density of 16.3 mA/μm² for 1.68×10⁵ s at 90°C. The metal penetration into polysilicon offers a possible cause for an electrical measurement reported by D.D. Tang et al. (1990), in which a slight lowering of the emitter contact resistance occurs after the same stress     

双极型晶体管发射区物理参量的测定

本文提出了一种测定双极型晶体管重掺杂发射区中禁带宽度和少子复合寿命的方法.利用电流增益的温度特性,可计算出禁带宽度;通过计算发射区中少子反向饱和电流,可计算出少子复合寿命.考虑到发射区中重掺杂效应,本文采用了费米-狄拉克统计分布.此测定方法简便而实用.     

Coherent transistor

The high-frequency operation of an abrupt-heterojunction transistor with ballistic transport in the base is considered. The coherent regime arises at temperatures low enough compared to the injection energy that the injected minority carriers form a nearly collimated and monoenergetic beam. The coherent transistor can have both current gain and power gain at frequencies far above the conventional cutoff. The extended frequency of an intrinsic transistor is limited by the dispersion in the minority-carrier times of flight across the base, rather than the average time of flight itself. The unilateral gain U calculated for an exemplary heterostructure, including the parasitics, demonstrates an active behavior of the coherent transistor in extended frequency ranges     

A fundamental limitation for bipolar transistor scaling

A microscopic model of minority-carrier diffusion in a heavily doped emitter is proposed. Monte Carlo simulation demonstrates that statistical fluctuation in the base current is one of the fundamental limitations in high-speed applications of scaled bipolar transistors. For the transistor presently investigated, with 5.0-μm² emitter area, 0.1-μm junction depth, 8.5-ps measurement time, and 0.75-V emitter/base bias, the base current deviation is 43%. This sets up the maximum operating frequency for the transistor. More lightly doped emitters (such as for heterojunction bipolar transistors) will relax this limitation, but at a cost of increased contact resistance, especially when poly-emitters are utilized. Increasing the emitter/base bias will also make the base current rate more deterministic, but the other limitations such as power dissipation and contact resistance will become more obvious     

Reduction of the current gain of the n-p-n transistor component ofa thyristor due to the doping concentration of the p-base

The reduction of the current amplification factor of a wide-base transistor, with growing doping concentration in the base region, is investigated. A method for the determination of the minority-carrier lifetime τ_n in the base region and the emitter Gummel number G_e is developed. The method is based on transistor structures differing only in the base width. It was found that the lifetime τ_n decreases according to the power law τ_n~N^-0.45_A. This result is analyzed for different recombination processes. Good agreement is obtained if shallow impurities acting as recombination centers are assumed. The injection-limited current gain β_γ decreases significantly with an increase in the total number of the doping concentration of the base, reaches a broad maximum, and then falls slowly. The maximum value of G_e is found to be 1.1×10¹⁴ cm^-4-s in good agreement with theoretical results. Finally, the contribution of the injection efficiency γ and the transport factor α_T to the current gain α are determined. It is found that α is limited mainly by the injection efficiency γ     

The temperature dependence of ideal gain in double diffused silicon transistors

Theoretical treatments predict higher injection efficiency for double diffused silicon transistors than the experimentally observed values. This paper shows that the discrepancy can be partly explained by the difference in the effective energy gaps in the emitter and base regions. Coulomb interaction of the free carriers results in lower energy gap in the heavily doped emitter than in the rest of the transistor. The difference in the energy gaps is experimentally determined from the activation energy difference of the emitter-current and the ideal component of the base Current. It is concluded that too much doping in the emitter lowers the transistor gain, increases the temperature dependence of the gain, and results in a higher excess noise.     

An analytical model for the low- emitter-impurity-concentration transistor

The epitaxial transistor model developed by Grung and Warner is adapted for predicting the characteristics of the low-emitter-impurity-concentration (LEC) transistor. The model predicts the following results for the low region (the lightly-doped emitter region): through a given cross section, the minority-carrier drift current is typically larger than the minority-carrier diffusion current. In other words, the model predicts that the conventional low-level minority-carrier diffusion equation is invalid for the low region, especially for typical bias levels. As a result, the effect of the electric field on minority carriers cannot be neglected in the low region of the LEC transistor and (by extension) in the corresponding low regions of such devices as the epitaxial diode and the integrated-injection-logic transistor.     

Numerical analysis and interpretation of the small-signalminority-carrier transport in bipolar devices

A simple and efficient one-dimensional numerical technique is presented that determines the small-signal minority-carrier transport in the quasineutral regions of bipolar devices, such as diodes and transistors, under sinusoidal excitation. The technique is applied to study small-signal properties of p-n junction diodes and bipolar transistors. Examples treated include the frequency dependence of transistor current gain, the diffusion capacitance of a quasineutral base or emitter, and base-layer carrier propagation delay     

Theoretical current gain of a cylindrical mesa transistor

For the cylindrical mesa transistor, computation of base-region transport efficiency is considered a boundary-value problem; solution of this problem yields mathematical equations applicable to the design and development of practical semiconductor devices. This analytical method is applied to the problem of minority-carrier transport across a solid-cylinder type structure which approximates the transistor base region. Minority-carrier losses—representing a fundamental limitation upon transistor-current gain—are introduced through an assumption of bulk and surface-recombination mechanisms.

Applications of this analysis are illustrated by establishing the common-emitter current gain for typical junction transistors. Assuming, in such computations, physical parameters approximating germanium and silicon diffused devices, the silicon transistor is shown to be less sensitive to surface recombination mechanisms. Further, the existence of an optimum emitter radius is demonstrated for a semiconductor structure similar to the cylindrical hook collector.     

Multiple-state resonant-tunneling bipolar transistor operating atroom temperature and its application as a frequency multiplier

A resonant-tunneling bipolar transistor with two peaks in the direct as well as in the transfer characteristics is presented. The multiple peaks are obtained by sequentially quenching resonant tunneling through the ground states of a series of double-barrier quantum wells, placed in the emitter of a Ga_0.47In_0.53As-Al_0.48In_0.52 As bipolar transistor, thus obtaining nearly equal peak currents and peak-to-valley ratios. The transistor exhibits current gain of about 70 at room temperature and 200 at 77 K. Peak-to-valley current ratios at room temperature and at 77 K are as high as 4:1 and 20:1, respectively. Frequency multiplication by factors of three and five has been demonstrated using the multiple-peak transfer characteristics of the transistor     

用氧化镉(CdO)为发射区的InGaAsP/InP双异质结晶体管

本文提出和研制了一个新型的InGaAsP/InP双极型晶体管.在单片集成电路中它能与1.55μmInGaAsP/InP双异质结激光器共容而组成一个晶体管-激光器器件.该晶体管的主要特点是采用氧化镉(CdO)薄层作为器件发射区,由InP组成收集区而形成NpN双异质结晶体管.测量结果表明晶体管能双向工作,测得的正向共发射极电流增益为40(V_(CE)=5V,Ic=1mA),反向增益为8(V_(CE)=1.5V,Ic=100μA).文中还给出了h_(fe)—I_c特性和晶体管CdO-InGaAsP发射结的伏安特性.