Heavy doping effects in p-n-p bipolar transistors

This paper presents the heavy doping effects on the injection current characteristics in p-n-p transistors with a heavily doped but thin base region. The results of the present study indicate that 1) at room temperature the hole current injected into heavily doped base is insensitive to the impurity compensation effect, 2) a linear relationship between the base sheet resistance and the collector-current density is observed when the base doping density is under 1 × 10¹⁹cm^-3. This relationship becomes supralinear as the doping density further increases. As a result, useful current gain exists in thin base transistors even when the base doping is greater than 1 × 10¹⁹cm^-3. From the collector-current-base sheet-resistance relationship and the base doping profile, the effective intrinsic carrier density as a function of the doping density is evaluated and found to increase 8.7 times over that of pure silicon, when the average doping density is 5 × 10¹⁹cm^-3(maximum doping density 1 × 10²⁰cm^-3). 3) The collector current and the current gain of the transistors become less sensitive to the temperature as the base doping density increases. We had observed a current gain up to 30 at 77 K for transistors with the maximum base doping density in the 10¹⁸cm^-3range. The transistors with lower base doping suffer much more degradation in current gain when the temperature is lowered to 77 K.     

InGaAs/In(AlGa)As RHET's with InAs pseudomorphic base

An InGaAs/In(AlGa)As resonant-tunneling hot-electron transistor (RHET) with an INAs pseudomorphic base to increase current gain and to reduce base resistance was designed and fabricated. The conduction band discontinuity between the InAs base and the In(AlGa)As collector barrier was estimated from the thermionic current. The band discontinuity was about 0.38 eV, which agrees well with the calculated band discontinuity taking into consideration the effect of strain. The common-emitter current gain doubled and the base resistance decreased 20% compared to InGaAs-base RHETs with the same doping concentration. A current gain cutoff frequency of 65 GHz and a maximum oscillation frequency of 50 GHz at 77 K were measured     

High-gain pseudomorphic InGaAs base ballistic hot-electron device

A high-gain ballistic hot-electron device is described. The GaAs-AlGaAs heterostructure device, with a 21-mm-thick pseudomorphic In _0.12Ga_0.88As base, had a current gain of 27 at 77 K and 41 at 4.2 K. As characteristically seen in ballistic devices, transfer into the L valley limited the maximum gain. The Γ-L valley separation in the strained In_0.12Ga_0.88As was estimated to be about 380 meV     

Base profile design for high-performance operation of bipolartransistors at liquid-nitrogen temperature

Measurements of thin epitaxial-base polysilicon-emitter n-p-n transistors with increasing base doping show the effects of bandgap narrowing, mobility changes, and carrier freezeout. At room temperature the collector current at low injection is proportional to the integrated base charge, independent of the impurity distribution. At temperatures below 150 K, however, minority injection is dominated by the peak base doping because of the greater effectiveness of bandgap narrowing. When the peak doping in the base approaches 10¹⁹ cm^-3, the bandgap difference between emitter and base is sufficiently small that the current gain no longer monotonically decreases with lower temperature but instead shows a maximum as low as 180 K. The device design window appears limited at the low-current end by increased base-emitter leakage due to tunneling and by resistance control at the high-current end. Using the measured DC characteristics, circuit delay calculations are made to estimate the performance of an emitter-coupled logic ring oscillator at room and liquid-nitrogen temperatures. It is shown that if the base doping can be raised to 10¹⁹ cm^-3 while keeping the base thickness constant, the minimum delay at liquid-nitrogen temperature can approach the delay of optimized devices at room temperature     

低温高注入硅双极晶体管电流增益的定量模拟

本文考虑了基区复合电流，发射结空间电荷区复合电流，基区高注入引起的禁带变窄效应，Ｅａｒｌｙ效应，基区和发射区电导调制效应，有效基区展宽效应以及发射区电流集边效应，定量地模拟了硅双极晶体管电流增益在７７Ｋ和３００Ｋ时与集电极电流的关系，并且与实验结果相吻合，计算还表明在低温７７Ｋ时，电流增益的大注入效应由基区电导调制效应和发射区电流集边效应决定，而在３００Ｋ时则由有效基区展宽效应决定。     

Analytical modelling of current gain and frequency characteristics under high injection levels in Si/SiGe heterojunction bipolar transistors at 77 and 300 K

This paper reports an analytical modelling of current gain and frequency characteristics in Si/SiGe heterojunction bipolar transistors (HBTs) at 77 and 300 K. Important transistor parameters, such as current gain, transconductance, cutoff frequency and maximum oscillation frequency are calculated as a function of Ge concentration in the base under different injection levels. The main physical mechanisms for the current and cutoff frequency rolloff at high injection levels are also analyzed. It shows that the high-level injection effect is more pronounced in the SiGe HBTs as a result of the increasing minority carrier concentration in the base and the Ge concentration and distribution will have a decisive influence of device performance. The results may provide a basis for the design of low temperature operation SiGe HBTs.     

Experimental realization of a new transistor

The authors report on the fabrication and characteristics of a unipolar, three-terminal, resonant-tunneling transistor. The operating principle of this new transistor is based on the fact that the quantum mechanical resonant-tunneling probability of hot electrons between the emitter and the collector is switched almost completely on and off, when either the base or the collector bias is swept. The emitter injects hot electrons to the second lowest subband of a thin (100 Å in this work) GaAs quantum well. Subsequently, the hot electrons will either resonantly tunnel to the collector, or relax to the lowest subband and contribute to the base current. As a result of resonant transmission, at 77 K the current-voltage characteristics of the transistor display negative differential resistance with extremely large (4691) peak-to-valley ratio. Furthermore, when biased near resonance, a maximum DC current gain of ~1.2 and a maximum AC current gain of ~11.9 were observed. The first use of a new `tunneling-in and tunneling-out' scheme in contacting a thin quantum well is also demonstrated     

Operation of SiGe heterojunction bipolar transistors in theliquid-helium temperature regime

We present the first dc measurements of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) operating in the liquid-helium temperature (LHeT=4.2 K) regime. The current gain of the self-aligned, UHV/CVD-grown SiGe HBT increases monotonically from 110 at 300 K to 1045 at 5.84 K, although parasitic base current leakage limits the useful operating current to above about 1.0 μA at 5.84 K. An aggressively designed base profile (peak N_AB≈8×10¹⁸ cm ^-3) is used to suppress base freeze-out at LHeT (R_bi =18.3 kΩ/□ at 4.48 K). We have also identified a non-ideal minority carrier transport mechanism in the collector current at temperatures below 77 K (I_C is not proportional to exp(qV _BE/kT)) which is unaccounted for in conventional device theory. Preliminary calculations suggest that this phenomenon is due to trap-assisted carrier tunneling from the emitter to the collector through the base potential barrier     

An epitaxial emitter-cap SiGe-base bipolar technology optimized forliquid-nitrogen temperature operation

We give the first demonstration that a properly designed silicon bipolar technology can achieve faster unloaded circuit speed at liquid-nitrogen temperature than at room temperature. Transistors were fabricated using a reduced-temperature process employing an in situ arsenic-doped polysilicon emitter contact, a lightly phosphorus-doped epitaxial emitter-cap layer, and a graded SiGe base. At 84 K, transistors have a current gain of 500, with a cutoff frequency of 61 GHz, and a maximum oscillation frequency of 50 GHz. ECL circuits switch at a record 21.9 ps at 84 K, 3.5-ps faster than at room temperature. Circuits which were optimized for low-power operation achieve a minimum power-delay product of 61 fJ (41.3 ps at 1.47 mW), nearly a factor of two smaller than the best achieved to date at 84 K. The unprecedented performance of these transistors suggests that SiGe-base bipolar technology is a promising candidate for cryogenic applications requiring the fastest possible devices together with the processing maturity and integration level achievable with silicon fabrication     

GaAs inversion-base bipolar transistor (GaAs IBT)

A completely new type of GaAs bipolar transistor with a base formed by a two-dimensional hole gas has been fabricated. The transistor has no metallurgical base layer but has an extremely thin inversion hole layer working as a base layer. The current gain β = 5.6 at 77 K and β = 17.1 at 300 K was obtained for the common emitter mode.     

Enhanced modulation bandwidth (20 GHz) ofIn0.4Ga0.6As-GaAs self-organized quantum-dotlasers at cryogenic temperatures: role of carrier relaxation anddifferential gain

Measurements of the threshold current, slope efficiency and optical modulation characteristics of self-assembled InGaAs-GaAs quantum-dot lasers have been made in the temperature range of 20-200 K in order to understand the carrier dynamics in these devices. The dc characteristics of these devices showed a region of almost temperature independent threshold current up to 85 K (T₀=670 K) with a maximum slope efficiency at 150 K. The maximum measured bandwidth increased from 5 GHz at room temperature to 20 GHz at 80 K. This is consistent with the bandwidth being limited by carrier relaxation time through electron-hole scattering     

Frequency response of advanced silicon bipolar transistors at lowtemperature

The high-frequency behavior of advanced bipolar silicon transistors has been measured at temperatures between 83 and 350 K. The cutoff frequency, DC gain, and associated frequency-dependent parameters are reported. Although the transistors are optimized for room-temperature operation, their performance at liquid-nitrogen temperature is not severely degraded. Though decreased, the current gain remains sufficiently high for use in some applications. The cutoff frequency is reduced by about a factor of two. It is suggested that this is primarily due to an increase of the base transit time and that increasing the base doping may improve the low-temperature response. Using the maximum frequency of oscillation to predict circuit switching speed, it appears that small, high-performance transistors suffer a speed degradation at near liquid-nitrogen temperature     

Electrical properties and transport mechanisms of InP/InGaAs HBTsoperated at low temperature

The electrical properties of InP/InGaAs HBTs have been comprehensively investigated between room and near liquid helium temperature. Physical mechanisms for the devices operated in different temperature ranges have been clearly identified. The low temperature measurements indicate that, in the temperature range of 240 K to 300 K, the base current is dominated by electron-hole band-to-band recombination; in the temperature range 77 K to 240 K, trap-related recombination (Shockley-Read-Hall recombination) plays an important role in determining base current; and for temperature lower than 77 K, the collector and base currents are found to be limited by electron tunneling through the barrier formed by the conduction-band discontinuity at the E-B junction. These findings provide us with better physical insight of the device operation at low temperature, which is particularly important for the optimization of InP HBT technology for low temperature applications as well as the development of a quantitative model for circuit design     

InGaAs resonant tunneling transistors using a coupled-quantum-wellbase with strained AlAs tunnel barriers

A bipolar-type resonant tunneling transistor is studied of which the base is identical to a coupled quantum well. On the basis of the InGaAs material system strained AlAs tunnel barriers and a graded InGaAlAs emitter are used. Molecular beam epitaxy growth conditions are studied, showing a specific influence of growth temperature and arsenic pressure. We find clear evidence for resonant tunneling: a saturation of the collector current and a maximum of the transconductance with increasing base-emitter bias in a three-terminal transistor structure. A corresponding effect in a phototransistor structure is found as a maximum of differential current gain with increasing incident light intensity. Room temperature and low temperature (80 K) high-frequency properties are determined and are used to estimate the resonant tunneling time     

Temperature dependent common emitter current gain andcollector-emitter offset voltage study in AlGaN/GaN heterojunctionbipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V_CEOFF at lower temperature is related to an increase of V_BE as the base bulk current is increased, or to the reduction of the ideality factor n_BE     

A GSMBE grown GaInP/GaAs narrow base DHBT exhibiting N-shapenegative differential resistance with variable peak-to-valley currentratio up to 1×107 at room temperature

A Ga_0.51In_0.49P/GaAs DHBT with a heavily doped (1×10¹⁹ cm^-3) narrow base (8 nm) grown by gas source molecular beam epitaxy and fabricated by simple wet chemical etching was demonstrated for the first time. A variable “N” shape negative differential resistance (NDR) controlled by base current was observed in the common-emitter current-voltage characteristics of this device at room temperature. A maximum peak-to-valley current ratio of 1×10⁷ and a maximum current gain of 83 were achieved at room temperature. The largest peak-to-valley current ratio (1×10⁷) achieved is, to our knowledge, the highest reported value to date. The NDR characteristics were explained by the base resistance effect     

MBE-grown Si/SiGe HBTs with high β, fT,and fmax

Si/SiGe heterojunction bipolar transistors (HBTs) were fabricated by growing the complete layer structure with molecular beam epitaxy (MBE). The typical base doping of 2×10¹⁹ cm^-3 largely exceeded the emitter impurity level and led to sheet resistances of about 1 kΩ/□. The devices exhibited a 500-V Early voltage and a maximum room-temperature current gain of 550, rising to 13000 at 77 K. Devices built on buried-layer substrates had an f_max of 40 GHz. The transit frequency reached 42 GHz     

High-temperature DC characteristics of AlxGa0.52-xIn0.48P/GaAs heterojunction bipolar transistors grownby metal organic vapor phase epitaxy

A series of Al_xGa_0.52-xIn_0.48P/GaAs heterojunction bipolar transistors (HBT's) with x=0 to x=0.52 showed ideality factors close to unity for both base current and collector current and small variation in gain with temperature up to at least T=623 K across the whole range of x composition. Hole current injection from the base into the emitter in these devices was shown to be negligible. The current gain, β, which is temperature insensitive was thought to be limited by bulk base recombination for x⩽0.3 and recombination at the graded emitter region for x>0.3. The optimum emitter composition (highest β, and good β stability with collector current and temperature) was found to be x=0.18-0.30. Useful transistor action with very high gain and output resistance is possible up to at least T=623 K, limited only by the thermal performance of the unoptimized ohmic contacts employed in the devices     

Silicon-germanium base heterojunction bipolar transistors bymolecular beam epitaxy

The devices were fabricated using molecular-beam epitaxy (MBE), low-temperature processing, and germanium concentrations of 0, 6%, and 12%. The transistors demonstrate current gain, and show the expected increase in collector current as a result of reduced bandgap due to Ge incorporation in the base. For a 1000-Å base device containing 12% Ge, a six-times increase in collector current was measured at room temperature, while a 1000-times increase was observed to 90 K. The temperature dependence of the collector current of the Si_0.88Ge_0.12 base transistor is consistent with a bandgap shrinkage in the base of 50 meV. For the homojunction transistors, base widths as thin as 800 Å were grown, corresponding to a neutral base width of no more than 400 Å     

The effect of base grading on the gain and high-frequencyperformance of AlGaAs/GaAs heterojunction bipolar transistors

A comprehensive one-dimensional analytical model of the graded-base Al_xGa_1-xAs/GaAs heterojunction bipolar transistor is presented and used to examine the influence of base grading on the current gain and the high-frequency performance of a device with a conventional pyramidal structure. Grading is achieved by varying the Al mole fraction x linearly across the base to a value of zero at the base-collector boundary. Recombination in the space-charge and neutral regions of the device is modeled by considering Schockley-Read-Hall, Auger, and radiative processes. Owing to the different dependencies on base grading of the currents associated with these recombination mechanisms, the base current is minimized, and hence the gain reaches a maximum value, at a moderate level of base grading ( x=0.1 at the base-emitter boundary). The maximum improvement in gain, with respect to the ungraded base case, is about fourfold. It is shown that the reduction in base transit time due to increased base grading leads to a 30% improvement in f_T in the most pronounced case of base grading studied (x=0.3 at the base-emitter boundary). The implications this has for improving f _max via increases in base width and base doping density are briefly examined