Influence of the law of electrical conductivity on thermal switching and breakdown

Influences of the law of electrical conductivity σ on thermal switching and breakdown events were investigated. The conductivities selected weresigma_{1}propto exp(aT), orsigma_{1}propto exp(-B/T), and also voltageVdependentpropto exp (bV), whenTis temperature and a, B, and b are constants. Simple models, thin film, and cylindrical specimens were considered. The influence of σ was examined by comparing steady-state V-I characteristics, the values of maximum voltages Vm, times for temperature runaway when voltages larger than Vmwere applied, and instabilities in the negative differential resistance ranges. Some significant differences were found as σ1rises much faster with temperature than σ2. Thus current filaments were very narrow whensigma_{1}propto exp(aT), but orders of magnitude wider whensigma_{1}propto exp(-B/T). For a given current, much larger temperature rises occur in the σ2than in the σ1case of conductivity. Transition to a branch line V-I characteristic and filament formation can occur in the σ2case at considerably higher temperatures than in the σ1case.     

A New Emitter Switched Thyristor (EST) employing Trench Segmented p-base

A new emitter switched thyristor (EST) employing trench segmented p-base, which successfully improves the forward I-V and switching characteristics with decreasing the device active area, is proposed and verified experimentally with using shallow trench process of novel junction termination extension (JTE) method. The latching current of EST is determined by the p-base resistance of upper npn transistor. Floating n+emitter of conventional EST is enlarged to obtain large base resistance. However, the proposed EST increases the p-base resistance with shorter floating n+ emitter than that of conventional one. Shallow trench in floating emitter region forms the highly resistive p-base region under the bottom of trench. The experimental results show that the shortened floating n+ emitter and lowered latching current of proposed EST decrease experimentally the forward voltage drop by 17.7% and snap-back phenomenon with small active area. The breakdown voltage of series lateral MOSFET of proposed EST is increased from 7 to 14 V due to the trench filled with oxide which results in vertical redistribution of electric field, therefore current saturation capability and forward biased safe operating area (FBSOA) of proposed EST are enhanced. The simulation results show that the switching operation is performed successfully at the blocking voltage of 600 V and Eoff of the proposed one is reduced by 3.7%. The measured inductive load switching characteristics also shows that Eoff of proposed one is improved by 7.2%.     

Static V-I relationships in transistors at high injection levels

At high injection levels the static V-I characteristics of p-n junction devices must be modified to account for nonlinear transport mechanisms and for relationships between the injected carrier concentrations and the applied voltages which differ from the familiar low-injection relationships. These carrier-concentration-voltage relationships differ in part because of ohmic voltage drops and in part because of conductivity-modulation effects. It is difficult to explore experimentally the high-level extensions of p-n junction theory in diodes because ohmic drops usually dominate the VI characteristics at high levels. However, these effects can be studied experimentally in a transistor which operates in the avalanche mode, with zero base current, because the ohmic drops can be suppressed. Experimental results are presented which support an analysis of the high-level V-I characteristics of the emitter junction of a transistor. This analysis does not predict ane^{qV/2kT}dependence of the junction current at high levels, but yields instead ane^{qV/(1 pm m)kT}dependence, where m is a parameter having a value of about 0.3 in germanium, and V is the applied voltage less the ohmic drops. The plus sign applies for a p⁺-n junction while the minus sign holds for an n⁺-p junction. In accordance with this theory, complementary n-p-n and p-n-p transistors exhibit markedly different behavior at high injection levels.     

Numerical analysis of turn-off characteristics for a gate turn-off thyristor with a shorted anode emitter

Turn-off current waveform for a gate turn-off thyristor (GTO) with a shorted anode emitter has been calculated numerically by solving the semiconductor basic equations in an equivalent one-dimensional model device. This model is derived from the analysis of current and carrier distributions obtained by a two-dimensional calculation of the on-state of GTO. A calculated turn-off current waveform agrees well with the experimental waveform. The computational time of one case is about 2 min. It is shown that this one-dimensional analysis method is useful for the calculation of the turn-off time. Using this one-dimensional model during the turn-off process and the two-dimensional model in the on-state, the relation between turn-off time and the forward voltage drop can be obtained in relation to the shorted emitter structure. It is shown that the shorted emitter structure is useful to improve this tradeoff relation.     

One-dimensional analysis of reverse recovery and dv/dt triggering characteristics for a thyristor

Reverse recovery anddv/dttriggering characteristics of a thyristor are analyzed using a one-dimensional numerical model, which consists of the solutions of the full set of semiconductor device equations, including the effect of the shorted emitter. The calculated waveforms of the anode voltage and current are in good agreement with the experimental ones. The reverse recovery characteristic is discussed for the case of an inductive load, and the dependence of capacitive and resistive components of the space-charge region on residual carriers in the n base is discussed on the basis of carrier distributions. Also, the role of the shorted emitter ondv/dttriggering is investigated in connection with the rates of supply and removal of carriers in the p base. It is shown that the shorted emitter improvesdv/dtcapability by causing not only a reduction in the injection efficiency of the n emitter, but also rapid turn-off of the n-p-n transistor section of a thyristor.     

Turnover phenomenon in N ν N Si devices and second breakdown in transistors

Turnover phenomenon in N νl N Si devices was studied in relation to second breakdown in NP ν N Si transistors. The devices were fabricated by a usual diffusion method using P2O5powder as diffusant source. Four groups of the devices, each having a diameter of 1.0 mm, a thickness of 0.25 mm, a resistivity in ν layer ranging from 3 to 210 ohm-cm, and a surface concentration in N layer of 2 × 10²⁰cm^-3, were tested under two kinds of applied voltage. The applied voltage was a half-cycle voltage from ac source and single-pulse voltage with a rising and flat part. The field strength in ν layer was below 3 × 10³v/cm. The voltage vs. current characteristics and the transient figures of voltage and current were observed on a memoriscope, and the temperature of the device was determined using temperature sensitive paints. The current increases linearly with the increasing voltage at the onset of the V-I curve, and saturates with further increases in voltage. Turnover occurs after the large departure from ohmic behavior in V-I characteristics. At the turnover, the temperature of the device was estimated to be near intrinsic; 160°C for the device with ∼ 210 ohm-cm, 330°C for ∼ 3 ohm-cm. The discussions on turnover characteristics are made in terms of the decrease in carrier mobility, the increase in carrier concentration in ν layer, and the current constriction in the device.     

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     

Performance and Stability of Large-Area 4H-SiC 10-kV Junction Barrier Schottky Rectifiers

The forward and reverse bias dc characteristics, the long-term stability under forward and reverse bias, and the reverse recovery performance of 4H-SiC junction barrier Schottky (JBS) diodes that are capable of blocking in excess of 10 kV with forward conduction of up to 10 A at a forward voltage of less than 3.5 V (at 25 $^{circ}hbox{C}$) are described. The diodes show a positive temperature coefficient of resistance and a stable Schottky barrier height of up to 200 $^{circ}hbox{C}$. The diodes show stable operation under continuous forward current injection at 20 $hbox{A/cm}^{2}$ and under continuous reverse bias of 8 kV at 125 $^{circ}hbox{C}$. When switched from a 10-A forward current to a blocking voltage of 3 kV at a current rate-of-fall of 30 $hbox{A}/muhbox{s}$, the reverse recovery time and the reverse recovery charge are nearly constant at 300 ns and 425 nC, respectively, over the entire temperature range of 25 $^{circ}hbox{C}$–175 $^{circ}hbox{C}$.      

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     

Thermal switchback in high ftepitaxial transistors

To investigate the reasons for second breakdown¹- or thermal switchback--and then to extend the safe operating area of power transistors, the behavior of an array of twenty small devices, mounted in parallel on a common heat sink, first without, then with an emitter series resistance, was evaluated. The first configuration suffers from β changes occurring at relatively low currents, an abrupt decrease of the base-to-emitter voltage, and failure to dissipate power. The configuration with emitter resistance shows a very tight collector-current base-current distribution, and normal base-to-emitter voltage characteristic. The same effect was obtained on a single large area device by employing a distributed thin-film resistor in series with the emitter. The uniform current distribution and the better thermal spreading of the heavily inter-digitated epitaxial device compensate for the small increase in saturation voltages due to the emitter resistance. Improved operating current levels and safe operating area are results observed on relatively small chip sizes.     

Floating base thyristor

The floating base thyristor (FBT) is a new thyristor structure in which its p-base region, containing a p⁺ region is not shorted to the n⁺ emitter. Using the DMOS process, an n-channel and a p-channel MOSFET are integrated with the thyristor structure. The device operates in the thyristor mode with a low ON-state voltage drop at even high current densities when a positive bias is applied to, both gates. When a negative bias is applied to the OFF gate, the device operates in the IGBT mode with the saturated current controlled by the positive bias applied to the ON gate     

Second breakdown in simplified transistor structures and diodes

The use of high-frequency silicon power transistors with interdigitated comb emitter structure was successful for proving the essential features of the theory of lateral thermal instability and second breakdown. For a more detailed study of the appearance of hot spots and their relationship to the onset of second breakdown, the use of a simpler structure looked advisable. This paper describes an uncomplicated geometry which offers many possibilities for modifications. Calculations, experimental data, and photographs are presented which show the means by which second breakdown and possible burnout occurs after the formation of a hot spot. In particular, measurements demonstrate the localized thermal runaway in the collector-base diode and the relationship to the presence of the emitter. The role of the emitter and base layer resistances as well as the resistance of the contacting metal layers (series and contact resistance) in reducing the thermal instability is shown. The shorted emitter structure proved to be a successful method to postpone or even avoid the appearance of second breakdown. The tool for the determination of current and voltage distributions within the small silicon structures was a potential microprobing apparatus with a resolution of 2µ.     

Computer-aided design consideration on low-loss p-i-n diodes

Two methods are presented to realize a low forward-voltage p-i-n diode with a thin i region. One is to increase recombination current by appropriately controlling carrier lifetime in the i region. The recombination current for a constant voltage reaches a maximum at a carrier lifetime given by(q/kT) Wmin{i}max{2}/8mu_{i}, where Wiand μiare thickness and effective mobility of the i region, respectively. The other is to increase diffusion current in the p emitter by decreasing carrier lifetime only for the high impurity concentration region. On the basis of the first method, diodes with a 0.83-V forward voltage at 150 A/cm², a 200-V reverse voltage, and a 50-ns reverse recovery time were obtained.     

Current-temperature feedback effects in III-V heterojunctionbipolar transistors

The consequence of the reciprocal relation between the temperature and current distributions in heterojunction bipolar transistors (HBTs) has been determined. The dc current voltage (I-V) characteristics, RF small-signal parameters, and temperature distributions of discrete devices with emitter fingers of varying lengths were analyzed empirically and their thermal profiles calculated numerically. The lateral temperature gradient induced in the finger due to power dissipation under normal operating conditions is shown to directly affect the current distribution in the transistor. The negative temperature dependence of the HBT base-emitter junction turn-on voltage results in positive feedback between current and temperature. This current temperature relationship leads to higher localized current densities in the hottest portion of the device, the center of the emitter. The temperature of the hot section rises with increasing power dissipation, continually drawing more current. Ultimately, the current through HBTs is localized to a comparable area at the finger center, independent of the emitter length     

Quantum-well resonant tunneling bipolar transistor operating at room temperature

The first resonant tunneling bipolar transistor (RBT) is reported. The AlGaAs/GaAs wide-gap emitter device, grown by molecular beam epitaxy (MBE), contains a GaAs quantum well and two AlAs barriers between the emitter and the collector. In the common emitter configuration, when the base current exceeds a threshold value, a large drop in the collector current (corresponding to a quenching of the current gain β) is observed at room temperature, along with a pronounced negative conductance as a function of the collector-emitter voltage. These striking characteristics are caused by the quenching of resonant tunneling through the double barrier as the conduction band edge in the emitter is raised above the bottom of the first quantized subband of the well. Single-frequency oscillations are observed at 300 K. The inherent negative transconductance of these new functional devices is extremely valuable for many logic and signal processing applications.     

Extraction of DC base parasitic resistance of bipolar transistorsbased on impact-ionization-induced base current reversal

A method for the evaluation of the DC base parasitic resistance, R_B, of bipolar transistors is described. The method is based on impact-ionization-induced base current reversal and enables R_B to be evaluated independently from the emitter parasitic resistance in a wide range of emitter current and collector-base voltage, without requiring any special device structure. The method can also extract R_B in the impact-ionization regime, where current crowding due to negative base current induces an increase in R_B at increasing emitter current     

Characteristics of polysilicon contacted shallow junction diodeformed with a stacked-amorphous-silicon film

A high-performance shallow junction diode formed with a stacked-amorphous-silicon (SAS) film is presented. Since the boundaries of stacked silicon layers and the poly/mono silicon interface act as a diffusion barrier for implanted dopants, the junction depth of SAS emitter contacted diode is about 500 Å shallower than that of the as-deposited polysilicon emitter contacted diode. The fabricated SAS emitter contacted diodes exhibited a very low reverse leakage current (⩽1 nA/cm² at -5 V) and a forward ideality factor m ≈1.001 over seven decades on a log scale. The reverse I-V characteristics were found to be nearly independent of the reverse voltage from room temperature to 200°C, and it was also found that the leakage current was due almost completely to the diffusion current. The plots of the diode leakage current versus the perimeter to area ratio showed that the periphery-generation current contributed little to the total leakage. The processing temperature for the SAS emitter contacted p⁺-n diode can be as low as 600°C     

An investigation of nonideal base currents in advanced self-aligned`etched-polysilicon' emitter bipolar transistors

The authors report a physical analysis of nonideal base currents in advanced self-aligned etched-polysilicon emitter bipolar transistors. By studying the dependence of the leakage currents on bias voltage, temperature, device geometry, and process parameters, the authors identify the nature of these currents and isolate the main critical fabrication steps. The abnormal base current at forward bias is found to be a generation-recombination current involving defects along the spacer oxides. These defects also control the reverse base characteristics through a trap-assisted tunneling current mechanism. A simple modification of the spacer structure is shown to result in high-performance emitter/base junction properties     

4 kV 4H-SiC epitaxial emitter bipolar junction transistors

In this paper, we present 4H-SiC bipolar junction transistors (BJTs) with open-base blocking voltage (BV/sub CEO/) of 4000 V, specific on-resistance (R/sub on,sp/) of 56 m/spl Omega/-cm/sup 2/, and common-emitter current gain /spl beta//spl sim/9. These devices are designed with interdigitated base and emitter fingers with multiple emitter stripes. We assess the impact of design (emitter stripe width and contact spacing) on device performance and also examine the effect of emitter contact resistance on the device forward conduction characteristics.     

微波功率SiGe HBT的温度特性

通过研究SiGe异质结双极型晶体管(HBT)的温度特性，发现SiGe HBT的发射结正向电压随温度的变化率小于同质结Si双极型晶体管(BJT). 在提高器件或电路热稳定性时，SiGe HBT可以使用比Si BJT更小的镇流电阻.同时SiGe HBT共发射级输出特性曲线在高电压大电流下具有负阻特性，而负阻效应的存在可以有效地抑制器件的热不稳定性效应，从而在温度特性方面证明了SiGe HBT更适合于微波功率器件.