Extension of impact-ionization multiplication coefficientmeasurements to high electric fields in advanced Si BJT's

Measurements of the impact-ionization multiplication coefficient M-1 in advanced Si BJTs up to values in excess of 10 (corresponding to a peak electric field at the base-collector junction of about 9×10⁵ V/cm) are presented. The intrinsic limitations affecting M-1 measurements at high electric fields are discussed. In particular, the fundamental role played by the negative base current and the parasitic base resistance in determining instabilities during M-1 measurements is pointed out. An accurate theoretical prediction of the M-1 coefficient on collector-base voltages close to BV_CBO requires that the contribution of holes to impact ionization be properly accounted for     

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     

Measurement of collector and emitter resistances in bipolartransistors

New DC methods to measure the collector resistance R_C and emitter resistance R_E are presented. These methods are based on monitoring the substrate current of the parasitic vertical p-n-p transistor linked with the n-p-n intrinsic transistor. The p-n-p transistor is operated with either the bottom substrate-collector or the top base-collector p-n junction forward-biased. This allows for a separation of the various components of R_C. R_E is obtained from the measured lateral portion of R_C and the collector-emitter saturation voltage. Examples of measurements on advanced self-aligned transistors with polysilicon contacts are shown. The results show a very strong dependence of R_C on the base-emitter and base-collector voltages of the n-p-n transistor. The bias dependence of R_C is due to the conductivity modulation of the epitaxial collector. From the measured emitter resistance R_E a value for the specific contact resistance for the polysilicon emitter contact of ρ_c≅50 Ω-μm² is obtained     

Theory and operation of a GaAs/AlGaAs/InGaAs superlatticephototransistor with controlled avalanche gain

The principle of operation of a bipolar transistor with controlled multiplication of one type of carrier is outlined. The ideal device, with a few periods of a staircase superlattice in the base-collector depletion region, has high current outputs at extremely low bias voltages and high current gains. The principle is experimentally demonstrated in a GaAs/AlGaAs/InGaAs phototransistor where three periods of a periodic pseudomorphic structure, in which electrons should predominantly multiply, are included in the collector depletion region. Independent measurements of the electron and hole avalanche multiplication rates, M_n and M_p, in these structures confirm that M_n/M_p M_n/M_p and α/β are ~2-4, depending on bias voltage. The observed photocurrent characteristics agree reasonably well with Monte Carlo calculations made to simulate the transport of electrons through the collector region. Measured optical gains are as high as 142 in an n-p-n phototransistor with a 2000-Å p-base region     

Breakdown behavior of GaAs/AlGaAs HBTs

Avalanche breakdown behavior at the collector junction of the GaAs/AlGaAs HBT (heterojunction bipolar transistor) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AlGaAs HBT was observed and used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown. The serious current gain degradation of GaAs/AlGaAs HBTs at low current densities was analyzed in connection with the measurement of a large collector-emitter breakdown voltage. The unexpected functional relationship between the collector-emitter breakdown voltage and collector-base breakdown voltage is explained by the absence of a hole-feedback effect for devices not exhibiting transistor action     

Reconciliation of methods for estimating fmaxfor microwave heterojunction transistors

An attempt is made to reconcile the various approaches that have recently been used to estimate the maximum frequency of oscillation f_max in high-performance AlGaAs/GaAs HBTs. f_max is computed numerically from the full expression for Mason's invariant gain using y-parameters derived from the different approaches, i.e., the hybrid-π equivalent circuit, the T-equivalent circuit, and the drift-diffusion equations. It is shown that the results for f_max are essentially the same, irrespective of the source of the y-parameters, provided that the phase delays due to transit of carriers across the base and the collector-base depletion region are properly accounted for. It is also shown, for the particular device studied, that the widely used analytical expression for f_max, involving f _T and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important     

Experimental values for the hole diffusion coefficient andcollector transit velocity in P-n-p AlGaAs/GaAs HBTs

The diffusion coefficient (D_h) and a value for the collector velocity (v_h) of holes in AlGaAs/GaAs P-n-p HBTs (heterojunction bipolar transistors) were obtained from high-frequency measurements on structures with different base and collector widths. Quantities for D_h and v _h of 5.6 cm²/s and 5.5×10⁶ cm/s, respectively, were obtained by plotting the total emitter-collector delay versus inverse emitter current and extrapolating the data to infinite emitter current to obtain the base and collector transit delays. An f_t and f_max as high as 15 and 29 GHz, respectively, were obtained for non-self-aligned (1-μm emitter mesa/base contact separation) devices with a 2.6-μm×10-μm emitter     

High/fmax collector-up AlGaAs/GaAsheterojunction bipolar transistors with a heavily carbon-doped basefabricated using oxygen-ion implantation

AlGaAs/GaAs collector-up heterojunction bipolar transistors (HBTs) with a heavily carbon-doped base layer were fabricated using oxygen-ion implantation and zinc diffusion. The high resistivity of the oxygen-ion-implanted AlGaAs layer in the external emitter region effectively suppressed electron injection from the emitter, allowing collector current densities to reach values above 10⁵ A/cm ². For a transistor with a 2-μm×10-μm collector, f_T was 70 GHz and f_max was as high as 128 GHz. It was demonstrated by on-wafer measurements that the first power performance of collector-up HBTs resulted in a maximum power-added efficiency of as high as 63.4% at 3 GHz     

Submicrometer self-aligned AlGaAs/GaAs heterojunction bipolartransistor process suitable for digital applications

A self-aligned process is developed to obtain submicrometer high-performance AlGaAs/GaAs heterojunction bipolar transistors (HBTs) which can maintain a high current gain for emitter sizes on the order of 1 μm². The major features of the process are incorporation of an AlGaAs surface passivation structure around the entire emitter-base junction periphery to reduce surface recombination and reliable removal of base metal (Ti/W) deposits from the sidewall by electron cyclotron resonance (ECR) plasma deposition of oxide and ECR plasma etching by NF₃. A DC current gain of more than 30 can be obtained for HBTs with an emitter-base junction area of 0.5×2 μm² at submilliampere collector currents. The maximum f_T and f_max obtained from a 0.5×2 μm² emitter HBT are 46 and 42 GHz, respectively at I_C=1.5 and more than 20 GHz even at I_C=0.1 mA     

1/f noise in n-p-n GaAs/AlGaAs heterojunction bipolartransistors:iImpact of intrinsic transistor and parasitic seriesresistances

1/f noise experiments were performed for n-p-n GaAs/AlGaAs HBTs as a function of forward bias at room temperature. The experimental data are discussed with the help of new expressions for the 1/f noise in bipolar transistors where the influence of internal parasitic series resistances has been taken into account. At low forward currents the 1/f noise is determined by spontaneous fluctuations in the base and collector currents. At fixed bias, the collector current noise exceeds the base current noise. At higher forward currents the parasitic series resistances and their 1/f noise become important. Experimental results from the literature are compared with the results     

New low capacitance transverse junction stripe AlGaAs/GaAs laserfor planar laser-MESFET integration

A transverse junction stripe laser structure with a very low parasitic capacitance, compatible with an entirely planar laser-MESFET integration, is presented. The laser, obtained by two successive n (S) and p(Zn) diffusions across undoped AlGaAs/GaAs double heterostructure layers, has a threshold current of I_th =55 mA. A very low parasitic capacitance C_s=0.6 pF is measured, with a corresponding rolloff time constant of t_r=9 ps     

Stability analysis and numerical simulation of multideviceamplifiers

Stability analysis of multidevice amplifiers is made on a generalized circuit comprising two n-ports with S-matrices S (active devices) and S' (passive networks) connected at n interface ports. Open-loop transfer functions defined for a signal-flow graph and its (n-1) subgraphs of incident and reflected waves at the interface ports are expressed in terms of det M_n and its minors, where M_n=S'S-I_n and I_n is the n×n identity matrix. it is shown that the Nyquist plots of the n transfer functions completely characterize the number of right-half complex-frequency-plane zeros of det M_n, and hence the amplifier stability. Insertion of an ideal circulator and isolators at the interface ports enables one to calculate the Nyquist plots and voltage distributions of possible instabilities using commercially available linear circuit simulators. Numerical simulations for two types of parallel-operated GaAs FET amplifiers are performed to verify the usefulness of the analysis in design-phase check of multidevice amplifier stability     

Effect of reverse base current on bipolar and BiCMOS circuits

A detailed study on the effect of reverse base current (RBC) on the switching behavior of bipolar BiCMOS circuits utilizing advanced high-performance bipolar transistors is presented. It is shown that as the collector doping N_c is increased to overcome the Kirk effect (base stretching) during the switching transient, the avalanche-generated reverse base current in the collector-base junction may cause problems for bipolar output devices switching out of saturation. A basic bipolar inverter and various BiCMOS driver circuits were simulated based on measured avalanche multiplication factors from advanced bipolar transistors with various collector doping N _c. In the case of the basic bipolar inverter, the reverse base current may prevent the switching device from being shut off completely during the on-to-off transition and a self-sustained state may result which reduces the output voltage swing. For the common-emitter (CE) BiCMOS driver, a similar self-sustained state may also occur with the added adverse effect of excessive leakage in standby. Design and scaling considerations are discussed     

Near-ideal I-V characteristics of GaInP/GaAsheterojunction bipolar transistors

GaInP/GaAs heterojunction bipolar transistors (HBTs) have been fabricated and these devices exhibit near-ideal I-V characteristics with very small magnitudes of the base-emitter junction space-charge recombination current. Measured current gains in both 6-μm×6-μm and 100-μm×100-μm devices remain constant for five decades of collector current and are greater than unity at ultrasmall current densities on the order of 1×10^-6 A/cm². For the 6-μm×6-μm device, the current gain reaches a high value of 190 at higher current levels. These device characteristics are also compared to published data of an abrupt AlGaAs/GaAs HBT having a base layer with similar doping level and thickness     

Repeated convolutional codes for high-error-rate channels

An error-correction scheme for an M-ary symmetric channel (MSC) characterized by a large error probability p_e is considered. The value of p_e can be near, but smaller than, 1-1/M, for which the channel capacity is zero, such as may occur in a jamming environment. The coding scheme consists of an outer convolutional code and an inner repetition code of length m that is used for each convolutional code symbol. At the receiving end, the m inner code symbols are used to form a soft-decision metric, which is passed to a soft-decision decoder for the convolutional code. The effect of finite quantization and methods to generate binary metrics for M>2 are investigated. Monte Carlo simulation results are presented. For the binary symmetric channel (BSC), it is shown that the overall code rate is larger than 0.6R₀, where R₀ is the cutoff rate of the channel. New union bounds on the bit error probability for systems with a binary convolutional code on 4-ary and 8-ary orthogonal channels are presented. For a BSC and a large m, a method is presented for BER approximation based on the central limit theorem     

Effects of displaced p-n junction of heterojunction bipolartransistors

Two-dimensional simulations that demonstrate the effects of displacements of the p-n junctions from the heterojunctions of symmetrical Al_0.28Ga_0.72/GaAs double-heterojunction bipolar transistors (DHBTs) are reported. When the emitter and/or collector p-n junctions do not coincide with the AlGaAs/GaAs heterojunctions, the electrical characteristics are shown to be drastically altered due to changes in the potential profiles and to changes in recombination rates both in the neutral base and in the space-charge region of the emitter. The effects of a small displacement of the p-n junction from the emitter-base or the base-collector heterojunctions are examined and results for current gain β and cutoff frequency f_T are given that demonstrate enhanced performance for DHBTs with p-n junctions that are not coincident with the heterojunctions     

Super low-noise HEMTs with a T-shaped WSix gate

A T-shaped quarter-micron gate structure composed of WSi_x /Ti/Pt/Au has been developed for low-noise AlGaAs/GaAs HEMTs. The gate resistance R_g was reduced to 0.3 Ω for devices with 200 μm-wide gates despite using WSi_x, and the source resistance R_s reached 0.28 Ω mm by minimising the source-gate distance using a self-alignment technique. This HEMT exhibited the lowest reported noise figure of 0.54 dB with an associated gain of 12.1 dB at 12 GHz     

Self-consistent particle simulation for (AlGa)As/GaAs HBTs underhigh bias conditions

Nonequilibrium electron transport phenomena in the emitter and collector regions under high bias conditions were investigated for standard N-p-n (AlGa)As/GaAs heterojunction bipolar transistors (HBTs) by utilizing a previously developed one-dimensional self-consistent particle simulator. A dramatic increase in the cutoff frequency was observed for a lightly doped collector HBT as the current density increased over 10⁵ A/cm², where the collector transit time was reduced due to the extension of the velocity overshoot region in the collector corresponding to the decrease in electric field near onset of the Kirk effect. A saturation tendency was seen in the collector current versus base-to-emitter bias voltage (V_BE ) characteristic for high V_BE, where V_BE exceeded the base-to-emitter built-in voltage of the conduction band. Simulations indicate that this feature is caused by electron velocity saturation in the neutral n-type (AlGa)As emitter region     

Parasitic energy barriers in SiGe HBTs

Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBTs (heterojunction bipolar transistors) are presented, showing that a parasitic conduction-band barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the f_T degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor I_C(SiGe)/I_C(Si)     

Charge control, DC, and RF performance of a 0.35-μmpseudomorphic AlGaAs/InGaAs modulation-doped field-effecttransistor

The authors describe a study of charge control in conjunction with DC and RF performance of 0.35-μm-gate-length pseudomorphic AlGaAs/InGaAs MODFETs. Using C-V measurements, they estimate that a two-dimensional electron gas (2DEG) with density as high as 1.0×10¹² cm^-2 can be accumulated in the InGaAs channel at 77 K before the gate begins to modulate parasitic charges in the AlGaAs. This improvement in charge control of about 10-30% over a typical AlGaAs/GaAs MODFET may partially be responsible for the superior DC and RF performance of the AlGaAs/InGaAs MODFET. At room temperature, the devices give a maximum DC voltage gain g _m/g_d of 32 and a current gain cutoff frequency f_T of 46 GHz. These results are state of the art for MODFETs of similar gate length