High Collector Current Density Research Articles

GaN-Based Quantum-Effect Electron Devices Using Quantum Interference of Hot Electron Waves

We theoretically discuss GaN-based quantum-effect electron devices using quantum interference of hot electron waves in the conduction band of the large bandgap collector barrier. In this type of device, the quantum interference of electrons plays a main role in the operation principle and originates the interesting device functions. The device consists of AlN/GaN/AlN double-barrier resonant tunneling (RT) emitter and AlN collector barrier. Double barrier RT emitter provides electrons with narrow energy distribution into the conduction band of the collector barrier, and the modulation of the transmission probability of the hot electron waves occurs due to the quantum interference effect. Multiple negative differential resistance (NDR) with relatively high collector current density is expected from the theoretical results, suggesting new possibilities for a variety of novel device applications.

The role of hydrogen in current-induced degradation of carbon-doped [formula omitted] heterojunction bipolar transistors

At current densities ≥ 5 × 10 4 A cm −2 carbon-doped base GaAs AlGaAs HBTs can display decreases in d.c. current gain which are correlated with the amount of hydrogen incorporated in the base layer during growth by Metalorganic Molecular Beam Epitaxy (MOMBE). During device operation, minority carrier injection-induced debonding of hydrogen from neutral CH complexes leads to an increase in effective base doping level and therefore to a decrease in current gain. In structures grown under conditions where large concentrations of hydrogen are incorporated in the base region, post-growth in situ or ex situ annealing eliminates this effect by breaking up the CH complexes. Properly designed HBTs are stable for >90 h even for very high collector current densities (10 5 A cm −2).

Effect of base dopant species on heterojunction bipolar transistor reliability

The electrical activation of carbon in GaAs grown by metal-organic molecular beam epitaxy has been shown to be excellent in as-grown material. At very high doping levels, however, the activation can be reduced substantially by annealing, even at rather moderate temperatures. This reduction in hole concentration combined with the reduction in gain observed with increasing dopant level in the base suggests a practical limit of less than 10 20 cm −3 for base doping in GaAs/AlGaAs heterojunction bipolar transistors (HBTs). Using moderate base doping, p ≈ 7 × 10 19 cm −3 , carbon-doped devices do not exhibit short-term degradation at current densities less than or equal to 2.5 × 10 4 A cm −2 , even at operating temperatures up to 200 °C, unlike Be-doped HBTs which typically degrade much more rapidly even at twice lower base doping levels. Operation of carbon-doped devices at higher current densities for longer periods of time, however, can result in current-induced decreases in d.c. gain which are correlated with the amount of hydrogen incorporated in the base layer during growth. During device operation, minority carrier injection induced debonding of hydrogen from neutral C-H complexes leads to an increase in effective base doping level and therefore to a decrease in gain. Post-growth in situ or ex situ annealing eliminates this effect by breaking up the C-H complexes. Properly designed HBTs are stable during operation at 125 °C even for very high collector current densities (10 5 A cm −2).

The high-speed performance of [formula omitted] Schottky collector HBTs

We investigated the high-speed performance of Si 1−x Ge x/ CoSi 2 Schottky collector heterojunction bipolar transistors (SCHBTs). The fact that a Si/Si 1−x Ge x/ CoSi 2 SCHBT can suppress the base push out and eliminate the collector charging time, which is a delay component that makes a sizable contribution in high-speed heterojunction bipolar transistors (HBTs), makes it possible to obtain very high and current-stable speeds with this structure. Analysis of the different delay components contributing to the emitter-to-collector delay time shows that for Schottky collector transistors with a base thickness of 300 Å or less, the structure should be optimized to minimize the emitter delay time even at high collector current densities in order to further improve the speed. An advantage is the lower power consumption when this SCHBT is compared with conventional HBTs.

Static and dynamic behavior of a Si/Si0.8Ge0.2/Si heterojunction bipolar transistor using Monte Carlo simulation

A theoretical study of a Si/Si1−xGex/Si heterojunction bipolar transistor using Monte Carlo simulations is reported. The geometry and composition of the emitter-base junction are optimized using one-dimensional simulations with a view to improving electron transport in the base. It is proposed to introduce a thin Si-P spacer layer, between the Si-N emitter and the SiGe-P base, which allows launching hot electrons into the base despite the lack of natural conduction-band discontinuity between Si and strain SiGe. The high-frequency behavior of the complete transistor is then studied using 2D modeling. A method of microwave analysis using small signal Monte Carlo simulations that consists of expanding the terminal currents in Fourier series is presented. A cutoff frequency fT of 68 GHz has been extracted. Finally, the occurrence of a parasitic electron barrier at the collector-base junction is responsible for the fT fall-off at high collector current density. This parasitic barrier is lowered through the influence of the collector potential.

Small-sized collector-up Ge/GaAs heterojunction bipolar transistors with high gain, low base resistance, and high f/sub max/

Small-sized collector-up Ge/GaAs HBT's are successfully fabricated and their operation at a high collector current density and at a high frequency is realized for the first time. The current gain of these devices reaches a peak value as large as 200 at a current density 6/spl times/10/sup 4/ Acm/sup /spl minus/2/, and no degradation in the current gain is observed as the collector width is decreased down to 2 /spl mu/m. The capability of lower voltage operation is also shown. Intrinsic and extrinsic base resistances are as low as 180 /spl Omega///spl square/ and 90 /spl Omega///spl square/, respectively. The calibrated values of f/sub T/ and f/sub max/ are 25 GHz and 60 GHz, respectively. The larger value of f/sub max/ compared with f/sub T/ might be attributed to low base resistance and low base-collector capacitance as expected from the collector-up structure. >

The Role of Hydrogen in Current-Induced Degradation of GaAs/AlGaAs Heterojunction Bipolar Transistors

ABSTRACTCarbon-doped base GaAs/AlGaAs HBTs display current-induced decreases in dc gain which are correlated with the amount of hydrogen incorporated in the base layer during growth by Metalorganic Molecular Beam Epitaxy (MOMBE). During device operation, minority carrier injection induced debonding of hydrogen from neutral C-H complexes leads to an increase in effective base doping level and therefore to a decrease in gain. Post-growth in-situ or ex-situ annealing eliminates this effect by breaking up the C-H complexes. Properly designed HBTs are stable even for very high collector current densities (105 A · cm−2)

Switching characteristics of [formula omitted] Schottky collector HBT

The rise and fall times for both silicon-based Schottky Collector Heterojunction Bipolar Transistor (SCHBT) and Conventional silicon-based Heterojunction Bipolar Transistor (CHBT) were calculated. The results show that SCHBTs have allowed 20% shorter rise and fall times when compared with CHBTs. The effect of the base resistance in charging and discharging the emitter capacitance is included. We use an approximation for the effect of the base resistance as an additional RC time constant. Analysis of the collector transition layer shows that the main advantage of SCHBT over CHBT is that the cut-off frequency degradation is drastically decreased at high collector current density.

High-speed InP/InGaAs double-heterostructure bipolar transistors with suppressed collector current blocking

InP/InGaAs double-heterostructure bipolar transistors (DHBTs), incorporating a new collector structure featuring ‘‘pn pair doping’’ in the heterointerface vicinity, have been fabricated using a low-pressure metalorganic chemical vapor deposition (MOCVD) method. These transistors provide high collector current densities over 1×105 A/cm2, indicating the successful suppression of current blocking. S-parameter measurements determine the high current gain cutoff frequencies of 130 GHz. These values favorably compare with those of conventional InGaAs-collector HBTs fabricated for comparison, suggesting that the InP collectors have excellent electron transport properties.

Transit time of fast bipolar transistors at high collector-current densities

This paper presents a physical description (not an empirical expression) of the forward transit time τ f in fast bipolar transistors at high collector-current densities by using only physical parameters. The transition between the regimes of the weak and strong increase in τ f is defined and physically described with the aid of the critical collector-current density j CC redefined in this paper. Furthermore, the slopes of τ f vs j C for both regimes are expressed analytically as functions only of physical parameters such as technological data (e.g. widths of the base w B and collector w C , collector doping concentration N C , etc.) as well as operating conditions (base-collector voltage V BC and temperature T). A highly-accurate analytical formula derived strictly from physical principles is given for a 1-D description of the forward-transit time τ f for a large range of collector-current density. The accuracy of the physical model is checked and verified with results obtained from device simulations and measurements on fast self-aligned Si n- p- n bipolar transistors for various technological data and temperatures.

Emitter injection and collector current ideality in abrupt heterojunction AlInAs/GaInAs HBTs

We investigated the electron injection process for high-speed N- p- n AlInAs/GaInAs HBTs by measuring collector and base currents as a function of base-emitter voltage with collector-base voltage equal to zero (Gummel plots) at temperatures from 77 to 300 K. We compared the measured collector current with calculations based on electron injection from emitter to base by tunneling through the conduction band spike and thermionic emission over it, using a modified version of the thermionic-field emission theory developed by Crowell and Rideout. Good agreement was obtained between the experimental collector current ideality factor and tunneling-thermionic emission theory for all temperatures and currents. This is an improvement over drift-diffusion and thermionic emission models, which have been used for HBTs but which do not correctly describe the experimentally observed temperature and current dependence of the ideality of the collector current. The tunneling-thermionic emission model explains the increase in collector current ideality factor that occurs as the transistor is biased at high collector current density ( J C ≧ 10 5 A cm −2 ), which is the regime of operation in which f T is maximized and a low ideality factor is most important. The model also explains the experimentally observed variation of h FE with ln I C. Thus the tunneling-thermionic emission model is a useful aid in the design of the epitaxial structure for high-frequency HBTs.

Experimental determination of the internal base sheet resistance of bipolar transistors under forward-bias conditions

A comparatively simple method is described which allows to measure the sheet resistance r si of the internal base even for a strong injecting emitter (strong “forward-bias condition”). For this, a well-known bipolar transistor tetrode with two separated base contacts is used. In order to extend the admissible operating range to sufficiently high collector current densities, corrections of the directly measured (voltage and current modulated) sheet resistance are required, which eliminate the error caused by emitter current crowding and emitter edge effects. For this, analytical correction terms are derived by solving a simplified differential equation for the internal base region. Moreover, the interrelation between r si and the total hole charge of the internal transistor (which can be determined experimentally) can be used to extend the admissible current density range further. The proposed method was verified by numerical device simulations and, in addition, applied to experimental r si determination. It proved to be well suited up to very high collector current densities, where r si is considerably reduced compared to its zero-bias or even weak-injection value.

The new two-dimensional electron gas base HBT (2DEG-HBT): two-dimensional numerical simulation

The bipolar/FET characteristics of the 2DEG-HBT are analyzed extensively by a two-dimensional numerical simulator based on a drift-diffusion model. For bipolar operations at high collector current densities, it is confirmed that the cutoff frequency f/sub T/ is determined mainly by the collector transit time of holes and by the charging time of the extrinsic base-collector capacitance C/sub bc//sup EXT/. The charging times of the emitter and base regions and the base transit time are shown to be negligible. A high cutoff frequency F/sub T/ (88 GHz) and current gain h/sub FE/

InGaAs/InAlAs/InP collector-up microwave heterojunction bipolar transistors

Collector-up InGaAs/InAlAs/InP heterojunction bipolar transistors (HBTs) were successfully fabricated, and their DC and microwave characteristics measured. High collector current density operation (J/sub c/>30 kA/cm/sup 2/) and high base-emitter junction saturation current density (J/sub 0/>10/sup -7/ A/cm/sup 2/) were achieved. A cutoff frequency of f/sub t/=24 GHz and a maximum frequency of oscillation f/sub max/=20 GHz at a collector current density of J/sub 0/=23 kA/cm/sup 2/ were achieved on a nominal 5- mu m*10- mu m device.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Avalanche multiplication in a compact bipolar transistor model for circuit simulation

A simple weak avalanche model valid in a wide range of voltages and currents, is presented. The proposed model is derived by using the base-collector depletion capacitance for predicting the avalanche current. The model needs only one additional transistor parameter; the extraction method and temperature dependence of this parameter are discussed. The decrease in avalanche current for high collector current densities may originate from internal device heating, a voltage drop in the epilayer, or mobile carriers in the depleted part. From experimental results it is concluded that, below a critical hot-carrier current, the decrease in avalanche current due to mobile carriers is negligible.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of silicon bipolar and GaAlAs/GaAs heterojunction bipolar technologies using a propagation delay expression

A performance comparison is presented of silicon bipolar and GaAlAs/GaAs heterojunction technologies for high-speed ECL (emitter-coupled logic) circuits. The propagation delays for state-of-the-art technologies are calculated using a quasianalytical equation which expresses the propagation delay in terms of all the time constants of the circuit. An idealized self-aligned transistor layout is used to eliminate geometry differences from the comparison, and hence to allow the roles of the heterojunction and of the fundamental material, and the device properties to be identified. The gate delays of GaAlAs/GaAs and silicon circuits are predicted. It is shown that the lower gate delays of GaAlAs/GaAs circuits arise primarily from the use of a heterojunction emitter, and also because the peak f/sub T/ of the GaAlAs/GaAs transistor occurs at high collector current density. The limiting time constants are identified for both technologies, and the best approaches for further optimization discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.

A simple regional analysis of transit times in bipolar transistors

In the quasi-static approach the cut-off frequency or unity-gain bandwidth f T of a one-dimensional bipolar transistor can readily be calculated from the doping profile by means of a numerical device simulation program. The physical interpretation of such results in terms of the usual regional model is greatly facilitated by a new partition of the small-signal charge distribution. Only two region boundaries remain to be determined. The criterion employed for this purpose is unambiguous. The total transit time from emitter contact to collector contact (inversely proportional to f T ) is subdivided into five physically meaningful components. The method can readily be implemented in device simulation programs, making these easier to use as a design tool. It provides more detailed insight into the influence of the emitter doping gradient on f T . Examples illustrate the three effects that can cause the f T fall-off at high collector current densities: high injection in the base, base widening and quasi-saturation. In all cases the emitter transit time turns out to dominate the total transit time at very high current densities, albeit beyond the limit of practical interest.

Monte Carlo simulation of AlGaAs/GaAs heterojunction bipolar transistors

A first demonstration of one-dimensional Monte Carlo simulations of AlGaAs/GaAs heterojunction bipolar transistors is reported. The electron motion is solved by a particle model, while the hole motion is solved by a conventional hydrodynamic model. It is shown that the compositional grading of Al x Ga 1-x As in the base region is effective to cause the ballistic acceleration of electrons in the base region, resulting in a high collector current density of above 1 mA/µm2. The current-gain cutoff frequency fT reaches 150 GHz if the size of a transistor is properly designed. Also shown is the relation between the device performances and the electron dynamics investigated.

High Current and High Current Density Pulse Tests of Brushes and Collectors for Homopolar Energy Stores

The incentive exists to use high collector current densities in pulsed duty homopolar generators, the current densities of interest being in the range of 15--45 MA/m2. Tests have been conducted in which current pulses of up to 0.25 MA with an equivalent pulsewidth of 0.3 s have been passed between pancake brush assemblies and fiat plate collectors with collector current densities of up to 78 MA/m2and current densities in the brush fingers of 570 MA/m2. The currents carried per brush have been a comparatively low 800 A. These tests have been successful and are providing the data necessary for the design of the all-iron-rotating homopolar pulse generators.