Collector Space-charge Region Research Articles

Discussions and extension of van Vliet’s noise model for high speed bipolar transistors

The general van Vliet noise model for transistors was derived for base minority carriers only under adiabatic boundary conditions. This paper extends the model by including the emitter minority carrier noise and the base–collector space charge region (CB SCR) effects based on the mathematical framework developed by van Vliet. Both the finite surface recombination velocity at polysilicon emitter and the finite carrier exit velocity at CB SCR are considered. It is proved that the van Vliet model can be directly used to include the emitter minority carrier noise, and the model holds when the two finite velocity boundary conditions are imposed. A new set of equations are derived to include the effect of CB SCR delay on noise.

Unified model for collector charge in heterojunction bipolar transistors

The base-collector capacitance and the collector transit-time in GaAs-based heterojunction bipolar transistors depend not only on base-collector voltage, but also on collector current. This has to be taken into account in a large-signal model. However, since collector transit-time and capacitance are both caused by the charge stored in the collector space-charge region, it is not possible to model them independently of each other. This paper investigates the interrelation between collector capacitance and transit-time due to transcapacitance effects, and presents an analytical unified description for both quantities, that is derived from measurement-extracted small-signal equivalent circuits. The model is verified by comparison of simulation and measurement data.

Electric field effects associated with the backside Ge profile in SiGe HBTs

A comprehensive investigation of electric field effects associated with the backside Ge profile in SiGe heterojunction bipolar transistors (HBTs) is conducted using calibrated simulations. We show for the first time that the backside Ge retrograde can alter the local electric field distribution in the base–collector space-charge region near the SiGe to Si heterojunction, thereby affecting the impact ionization and the apparent neutral base recombination (NBR) ( I B( V CB)/ I B( V CB=0)) of SiGe HBTs. The changes in the electric field induced by the Ge-induced band offsets contributes to a decrease of the observed impact ionization between comparably doped SiGe HBTs and Si bipolar junction transistors (BJTs), as well as an improved V CB dependence of I B (apparent decrease in NBR). Experimental data on SiGe HBTs with various Ge profiles and a Si BJT control are used to support our claims.

Dead space effect in space-charge region of collector of AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.

Avalanche multiplication and ionization coefficient in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors

The hole-initiated impact ionization multiplication factor Mp−1 and the ionization coefficient αp in AlGaAs/InGaAs p-n-p heterojunction bipolar transistors (HBTs) are presented. A large discrepancy is observed at low electric field when the measured data from the p-n-p HBTs are compared with those given from avalanche photodiode. The results show that the conventional impact ionization models, based on local electric field, substantially overestimate the hole impact ionization multiplication factor Mp−1. We believe that the hole ionization coefficient in p-n-p HBTs where significant dead space effects occur in the collector space charge region.

Modelling of collector signal delay effects in bipolar transistors

A detailed analysis of distributed delay effects related to carrier transport in the collector space-charge region is presented. The analysis is based on the extension of a recent formulation of minority-carrier transport in the base region which applies to nonuniform doping profiles and arbitrary injection levels. It is shown that these effects can be easily included in compact models by modifying the expression of the model parameters. Closed form analytical expressions for the forward transit time, base charge-partitioning factor and excess phase-shift time constant are derived, and the relevance of delay effects in the collector space-charge region is investigated in detail.

Gummel–Poon model for Npn heterojunction bipolar phototransistors

A Gummel–Poon model for abrupt, single heterojunction Npn bipolar phototransistors is described including the effects of the dc base bias on the current and optical gains. Initially, the excess electron concentration at the emitter end of the quasineutral base is determined by matching the thermionic field emission across the emitter–base heterojunction with the diffusion current at the emitter end of the base and including the effects of optical generation. The result is used in determining the electron profile in the base from which the base charge and the electron component to the emitter and collector currents are calculated following the Gummel–Poon model. The photocurrent’s components due to optical absorption in the quasineutral base, the base–collector space charge region, and the collector region are determined taking into account the nonuniform optical generation assuming topside illumination. A comprehensive description of the recombination current components is incorporated including the effects of optical absorption on recombination. The model is then used to calculate the dc and small signal current gain and the device’s optical gain, and to examine the effects of dc biasing and the optical power level. The simulation results are compared with the available experimental results and reasonable agreement is found.

Low-Frequency Fluctuation of the Transport Factor in Junction Transistors*

Noise analysis is carried out in a junction transistor. It is shown that the transport factor of the transistor fluctuates in proportion to the 3/2 power of the current at low frequency. This is derived from the base width modulation caused by mobility fluctuation in the collector space-charge region.

Multicascade impact ionization in Si metal-insulator-semiconductor tunnel emitter auger transistor (Si mis teat)

Abstract This article presents the analysis of the static characteristics of Si metal-insulator-semiconductor tunnel emitter Auger transistor. This study shows that there are some sharp rises of the differential current gain as a function of base-to-emitter voltage β d = f ( U BE ). These sharp rises appear for voltage intervals corresponding to the increase of the energy of tunneling electrons by the value of about the gap E g of Si. Violent retards of the base current I B increase correspond to these sharp rises; that indicates the appearance of an internal hole source. On this basis it was concluded that the multicascade Auger ionization had been possible in the collector space charge region and it had been produced by hot electrons tunneling from the emitter. Due to the considerable current gain ( β d = 700–900 at j >; 10 3 A cm −2 ), satisfactory technological reproducibility and probable good high-frequency characteristics, Si MIS TEAT seems to be a promising device for microelectronics.

The effects of carrier-velocity saturation on high-current BJT output resistance

Modern bipolar junction transistors (BJTs) tend to operate with saturated carrier velocity in the collector space-charge region. A physical analysis of the effects of velocity saturation on the output resistance of BJTs, especially for high current levels is presented. Physical analyses show that when the collector current density approaches a critical value, the Early voltage can increase significantly due to carrier-velocity saturation as the resulting base push-out partially offsets base-width modulation. This effect is also demonstrated in simulations with PISCES and MMSPICE.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Collector signal delay time and collector transit time of HBTs including velocity overshoot

The collector signal delay time τ CT and collector transit time τ′ CT in the heterojunction bipolar transistor (HBT) are studied and discussed using the delay time concept. Based on the assumption that the free carrier travels with the saturation velocity ν s in the base-collector space-charge region, τ′ CT is conventionally modeled as 0.5 τ CT, which equals W C/2 ν s, where W C is the thickness of the collector depletion region. For an HBT in which velocity overshoot can occur in the collector layer, the constant velocity profile used conventionally is questionable. In addition to the constant velocity profile and the step-like velocity profile proposed more recently, the present study considers also a more realistic velocity profile consisting of a linear-dependent velocity in the velocity overshoot region and an average velocity in the rest of the collector space-charge region. This piecewise-linear velocity profile resembles closely that predicted by Monte-Carlo simulations reported in the literature. It is shown that τ′ CT can be notably smaller than 0.5 τ CT, particularly when velocity overshoot is significant. Cutoff frequencies calculated from the present approach using the piecewise-linear velocity profile are in better agreement with Monte-Carlo simulation than that calculated from the constant and step-like velocity profiles.

Analysis of cut-off frequency roll-off at high currents in SiGe double-heterojunction bipolar transistors

Roll-off of the current gain cut-off frequency in NpN double-heterojunction bipolar transistors for large collector currents has been analyzed. The analysis includes such effects as the electron barrier formed at the collector base junction due to electron accumulation. Included in this investigation is also lateral electron diffusion before injection into the collector space-charge region at the base-collector heterointerface once the barrier is formed. The available data obtained in SiGe heterojunction bipolar transistors are in good agreement with this model.

Charge-control analysis of collector transit time for (AlGa)As/GaAs HBTs under a high injection condition

Charge-control analysis of collector transit time ( tau /sub C/) for (AlGa)As/GaAs n-p/sup +/-n heterojunction bipolar transistors has been carried out under the restriction of low-frequency approximation to explain the remarkable tau /sub C/ reduction at the onset of the Kirk effect. The fundamental idea of the model is that the increment of the current density must depend not only on the increment of the carrier density but also on the increment of the carrier drift velocity in the collector space-charge region. The obtained model well explains the previous Monte Carlo results, thus indicating the validity of the above idea as well as the possibility that the actual tau /sub C/ is by far smaller than that estimated from the conventional model. It also leads to a concept for the reduction of transit times, i.e. that an increasing rate of the electron overshoot velocity is as essential for the reduction of tau /sub C/ as the magnitude of the overshoot velocity itself. >

Transient Monte Carlo analysis and application to heterojunction bipolar transistor switching

A self-consistent simulation using the Monte Carlo ensemble particle technique for analysis of heterojunction bipolar transistor (HBT) transient behavior, such as switching performance, is presented. The transient Monte Carlo method has been applied to a self-consistent simulation of two HBT designs with improved collector structures for high-speed and high-frequency applications, and the results are compared with the characteristics of conventional HBTs. The simulation results indicate that the two new collector structures, the inverted field collector and the undoped collector, have better switching performance than the conventional HBT. The study of the switching characteristics' dependence on collector-base bias voltage and collector current suggests that the inverted field HBT is the best approach in terms of switching properties. The results are supported and explained by examining electron transport properties such as overshoot velocity and energy valley occupation, as well as band bending in the collector space-charge region at different current levels. >

Hot-electron InGaAs/InP heterostructure bipolar transistors with f/sub T/ of 110 GHz

A hot-electron InGaAs/InP heterostructure bipolar transistor (HBT) is discussed. A unity-current-gain cutoff frequency of 110 GHz and a maximum frequency of oscillation of 58 GHz are realized in transistors with 3.2*3.2- mu m/sup 2/ emitter size. Nonequilibrium electron transport, with an average electron velocity approaching 4*10/sup 7/ cm/s through the thin (650 AA) heavily doped (p=5*10/sup 19/ cm/sup -3/) InGaAs base and 3000-AA-wide collector space-charge region, results in a transit delay of 0.5 ps corresponding to an intrinsic cutoff frequency of 318 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A proposed structure for collector transit-time reduction in AlGaAs/GaAs bipolar transistors

The impact of velocity overshoot in the collector space-charge region on carrier transport is explored. The effects of overshoot on transit time for conventional collector structures are found to be minor. A new collector structure which exhibits extended velocity overshoot is proposed. This structure promises both simple fabrication and significant improvements (≃ 25 percent) in carrier transit time over conventional collector structures as demonstrated by Monte Carlo simulation.

Electron-velocity saturation at a BJT collector junction under low-level conditions

The consequences of electron-velocity saturation at the collector junction of an n-p-n biopolar junction transistor (BJT) are examined in a manner similar to that employed by Middlebrook twenty years ago. Dimensional shrinkage, especially in base thickness, that has occurred over this time interval causes a change in the prediction of this analysis from a negligible effect twenty years ago to a significant effect today, even in the low-level regime. However, a more detailed analysis considering the electron profile near the collector junction yields the somewhat surprising result that the linear portion of the base-region electron profile is quite unaffected by velocity saturation, in spite of the fact that the minimum electron density n c in the collector region can exceed by many orders of magnitude the equilibrium electron density in the base region. The reason for this is a scaling phenomenon wherein the linear-profile extrapolation rotates about a point on the x axis that is invariant with respect to current throughout the low-level range and approximately invariant with respect to base-region doping. Furthermore, this point on the x axis is very close to the depletion-approximation boundary of the collector-junction space-charge layer. Hence, the classical assumption of vanishing electron density at the boundary of the collector space-charge region constitutes an excellent approximation for low-level conditions. Incidental to the detailed analysis are updated empirical expressions for electron velocity saturation, and an application of the general solution for step junctions that was offered recently.

Current transport in narrow-base transistors

The transport of carriers across the base and the collector space-charge region of a silicon, narrow-base, bipolar transistor, is investigated by means of a Monte Carlo simulation of the transport process. The resulting carrier concentration within the base region turns out to be larger than that determined from the usual drift-diffusion equation, and the carrier transit-time is correspondingly increased. At the beginning of the collector space-charge region, where the field is suddenly increasing to a very large value, an anomalously high carrier mean velocity is observed. The latter than decays to the saturation velocity, with a time constant related to the field and the various kinetic relaxation times. This phenomenon is a non-equilibrium effect which cannot be accounted for by the drift-diffusion equation, but is of a limited influence on the collector transit-time.

Gallium arsenide dual Schottky barrier diodes

Gallium arsenide diodes were made which had Schottky-barriers for both contacts. Devices which were too thick for space change reach-through to occur at breakdown showed microwave oscillations, while thin diodes did not oscillate. Additionally, the structures could be distinguished on the basis of the noise accompanying breakdown. The performance was analysed in terms of transistor theory in which there is avalanche multiplication in the collector space charge region. It was concluded that there is a smooth transition between the reachthrough breakdown characteristic of the BARITT and true avalanche breakdown. The nature of the breakdown depends on the base width and the emitter efficiency.