A new method is presented for the direct extraction of hybrid-T equivalent circuits for heterojunction bipolar transistors. The method differs from previous ones by extracting the equivalent circuit without using test structures or numerical optimization techniques. Instead, all equivalent circuit parameters are calculated analytically from small-signal S-parameters measured under different bias conditions. The analysis includes the distributed nature of the HBT base. The calculated parameters are essentially frequency-independent and they exhibit systematic bias dependence over the typical operating range of the transistor. Thus, the present method ensures unique and physically meaningful parameters for transistor design improvement and large-signal circuit simulation. In addition, the present method is much faster than the numerical optimization method. >

Direct extraction of equivalent circuit parameters for heterojunction bipolar transistors

A self-aligned InGaP/GaAs heterojunction bipolar transistor with a compositionally graded In/sub x/Ga/sub 1-x/As base has been demonstrated with f/sub T/=83 GHz and f/sub max/=197 GHz. To our knowledge, these results are the highest reported for both parameters in InGaP/GaAs HBT's. The graded base, which improves electron transport through the base, results in a DC current gain and a cutoff frequency which are 100% and 20% higher, respectively, than that achieved by an identical device with a nongraded base. The high f/sub max/ results from a heavily doped base, self-aligned base contacts, and a self-aligned collector etch. These results demonstrate the applicability of InGaP/GaAs HBT's in high-speed microwave applications.

High-speed InGaP/GaAs HBTs with a strained In/sub x/Ga/sub 1-x/As base

A new heterojunction bipolar transistor (HBT) small-signal equivalent-circuit parameter-extraction procedure employing multibias S-parameter data is presented. The algorithm combines analytical and empirical parameter evaluation techniques and results in a bias-dependent HBT model. To minimize the risk of nonphysical parameter estimation, elements such as the DC transport factor, /spl alpha//sub o/, and the emitter-base conductance are evaluated from the device DC characteristics, and the frequency dispersion of /spl alpha/ is related to the RC time-constant of the emitter-base junction. Moreover, initial values for the extrinsic device parasitics are obtained from "hot" as well as "cold" S-parameter data. The method results in excellent fit between measured and modeled S-parameter data in the frequency range DC-40 GHz and for a wide range of bias operating points.

DC to high-frequency HBT-model parameter evaluation using impedance block conditioned optimization

During elevated-temperature bias stress, InGaP/GaAs HBT's grown by MOCVD show a medium-term degradation in current gain of about 20%, with an activation energy of 0.64 eV. They also show a corresponding decrease in base resistance and an increase in turn-on voltage. InGaP/GaAs HBTs grown by GSMBE, however, do not show this degradation. SIMS measurements show a five times greater than GSMBE-epi hydrogen concentration of about 10/sup 19/ cm/sup -3/ in the base layer of the MOCVD-grown epi. The degradation can be explained by acceptor depassivation due to hydrogen out-diffusion from the epi during stress.

Reliability investigation of InGaP/GaAs heterojunction bipolar transistors

Direct comparison between the HBT small signal Tee model and the hybrid pi topology is made to 100 GHz. It is shown that a one to one correspondence exists between the two topologies, but that some of the pi model parameters exhibit a frequency dependence with respect to the Tee model parameters. Using this analysis, an enhanced Gummel Poon large signal model has been developed which extends the model accuracy (usually up to mm-wave) by properly including collector current delay, self heating, and avalanche breakdown. A collection of measured versus modeled results are given.

Comparison of hybrid pi and Tee HBT circuit topologies and their relationship to large signal modeling

A new parameter extraction technique for heterojunction bipolar transistors (HBTs) is described. Utilizing a novel low frequency extraction algorithm, the intrinsic elements and the resistive parasitics are obtained. The overall small-signal equivalent circuit of HBTs is then determined based on those extracted element values. This technique advances current equivalent circuit modeling capability of HBTs by minimizing the interactive computer optimization/simulation process and removing the need of special test structures. >

D.L. Miller

Papers

Unique determination of AlGaAs/GaAs HBT's small-signal equivalent circuit parameters