Graded Heterojunction Bipolar Transistors Research Articles

An analytical model of the inverse base width modulation effect in SiGe graded heterojunction bipolar transistors

The analytical model of the collector current ideality factor degradation at high Vbe in modern SiGe graded base heterojunction bipolar transistors (HBTs) has been developed for the first time. It is subsequently used for the analysis of the inverse base width modulation (IBWM) effects in SiGe HBTs with respect to collector current degradation, current gain premature roll-off and SiGe base transit time. Comparing the IBWM effects calculated for HBTs with various base impurity concentration doping profiles and the Ge grading, we have found that the careful optimization of SiGe base parameters may substantially minimize negative influence of the IBMW effects on the HBT electrical parameters.

Ensemble Monte Carlo analysis of self-heating effects in graded heterojunction bipolar transistors

Self-heating effects in graded AlGaAs/GaAs heterojunction bipolar transistors are analyzed using an ensemble Monte Carlo description of electron transport from emitter to subcollector including space-charge effects through a self-consistent solution of Poisson’s equation. For a given collector current density, the temperature rise in the intrinsic portion of the device is calculated using a simple analytical treatment. The temperature dependence of the scattering rates and various device parameters is included in the Monte Carlo simulations. The extent of velocity overshoot in the collector and its dependence on the device temperature is analyzed. The collector delay time is most affected by self-heating effects leading to a degradation of the unity current gain cut-off frequency of graded heterojunction bipolar transistors. For the biasing conditions considered here, the unity current gain cut-off frequency is found to be as much as 18% below its value calculated while neglecting self-heating effects.

Heterostructure-emitter and pseudomorphic base transistor (hepbt) with a graded Al xGa 1− xAs confinement layer

In this paper, we will demonstrate a new heterostructure-emitter and pseudomorphic base transistor (HEPBT) with a graded Al x Ga 1− x As confinement layer. The device studied takes the advantages of the heterostructure-emitter bipolar transistor, the graded heterojunction bipolar transistor (HBT), and an AlGaAs/GaAs HBT with an InGaAs base layer simultaneously. Owing to the presence of the GaAs emitter and graded Al x Ga 1− x As confinement layer, a low offset voltage of 100 mV is obtained. Also, attributed to the insertion of a InGaAs quantum well, a high emitter efficiency yielding the high common-emitter current gain of 120 is achieved. Consequently, the HEPBT studied shows a promise for circuit applications.

An analytical model for the current transport of graded heterojunction bipolar transistors

An analytical model for the current transport of graded heterojunction bipolar transistors

Analysis of abrupt and linearly graded heterojunction bipolar transistors with or without a setback layer

Analytical equations of electric field, electrostatic potential, and junction capacitance for abrupt and linearly graded heterojunctions with or without a setback layer have been derived. The equations are general and applicable to both abrupt and linearly graded heterojunction bipolar transistors. Collector and base currents and small-signal parameters such as transconductance and cutoff frequency of the abrupt and linearly graded heterojunction bipolar transistors with or without a setback layer are evaluated. Comparisons between the isothermal model, the numerical model, and the present model including the self-heating effect are demonstrated.

Resolving degradation mechanisms in ultra-high performance N-p-n heterojunction bipolar transistors

The conduction band barrier caused by base dopant outdiffusion is investigated for MBE-grown Al/sub x/Ga/sub 1-x/As/GaAs N-p-n linearly graded heterojunction bipolar transistors (HBTs). The change of the B-E heterojunction conduction band barrier can be directly revealed by a novel technique based on the diffusion-thermionic emission current model. This is accomplished by measuring the inverted collector current ratio at two different heterojunction reverse biases. In addition, this ratio is found to correlate with an anomalous base current component measured at low temperature. The heterojunction potential barrier and the anomalous base current component are attributed to beryllium redistribution during MBE growth and forward current stress. This suggests that the diffusion and incorporation of beryllium dictate V/sub BE/ uniformity and long-term reliability of HBTs. >

A physics-based, analytical heterojunction bipolar transistor model, including thermal and high-current effects

A detailed, analytical model for predicting the DC and high-frequency performance of AlGaAs/GaAs graded heterojunction bipolar transistors (HBTs) is presented. The model is developed based on the relevant device physics, such as current-induced base pushout and thermal effects. The current gain, cutoff frequency, and maximum frequency versus the collector current density, which is a function of the applied voltage as well as the corresponding temperature in the HBT, are calculated. The results suggest that the conventional HBT model, which assumes the HBT temperature is the same as that of the ambient, can overestimate the three figures of merit considerably when the collector current density is high. Furthermore, it is shown that the present model correctly explains such experimentally observed HBT high-current behavior as the rapid falloff of the current gain and cutoff frequency. The model predictions compare favorably with the results obtained from a model which solves numerically the Poisson and continuity equations coupled with the lattice heat equation. >

Effect of recombination current on current gain of HBTs

In order to predict more accurately the current gain behaviour of HBTs at low collector current region, surface recombination and interface recombination have to be considered. Recombination current has been obtained by the combination of bulk recombination in the neutral base, interface recombination at the heterointerface, space charge recombination, and surface recombination components. Results from the present model have been compared with experimental results, and the most recent analytical models for abrupt and graded heterojunction bipolar transistors (HBTs). The study shows that this model can predict the device gain more accurately.

A Gummel-Poon model for abrupt and graded heterojunction bipolar transistors (HBTs)

A Gummel-Poon model for abrupt and graded GaAlAs/GaAs/GaAs heterojunction bipolar transistors (HBTs) is developed. The effect of carrier recombination at the emitter-base heterojunction, space charge region (SCR) width modulation effect, and base-widening effect at large collector currents have been considered. Results from this model are compared with numerical results, experimental results, and results from the most recent analytical models. The results show that the common-emitter current gain behavior in the low collector current region can be predicted more accurately by this model, and that interface and surface recombination affect the current gain more dominantly than the other recombination processes. Dependence of cutoff frequency on collector current obtained from the present model agrees well with the experimental results. This model can also predict both current gain and cutoff frequency falloffs at large collector current. This model can be easily implemented in the SPICE program.

Improved current gain and f/sub T/ through doping profile selection in linearly graded heterojunction bipolar transistors

Analytical and experimental results are used to show that extension of a thin p-doped layer of base doping into the graded-gap region, close to the base, of an n-p-n AlGaAs/GaAs heterojunction bipolar transistor and removing n-type dopant from the rest of the linearly graded AlGaAs region improves current gain beta and unity gain cutoff frequency f/sub T/. Current gain is significantly improved by reducing recombination near the metallurgical interface and using the effective electric field from the grading to accelerate electrons as they are injected into the p-base. The doping profile also inhibits the formation of a potential minimum in which electrons can be stored in close proximity to the base. This greatly improves f/sub T/, and does not hamper the current injection or increase the turn-on voltage. Space-charge recombination current is also reduced, due to the carrier density reduction associated with the effective electric field due to the graded gap. >