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Built-in drift fields are employed to enhance the performance of GaAs-based heterojunction bipolar transistors (HBTs) with reduced turn-on voltage. Specifically, we explore in detail the dc and RF device property improvements enabled by using compositionally graded GaInAsN base layers. Experimental results are compared to predictions of the standard drift-diffusion base transport model employing a finite exit velocity. In large area devices, graded base samples with built-in fields of /spl sim/7 kV/cm (i.e. 40 meV over 500 /spl Aring/) typically have a dc current gain 1.8/spl times/ larger than constant base composition samples. In small area devices, the peak cut-off frequency is typically 10%-15% higher than constant composition samples. These results are shown to agree reasonably well with predictions, thereby demonstrating that analytical drift-diffusion based models can be extended to HBTs with GaInAsN base layers.

AlGaAs/GaAs/GaAs and GaInP/GaAs/GaAs n-p-n heterojunction bipolar transistors (HBTs) are now in widespread use in microwave power amplifiers. In this paper, improved HBT structures are presented to address issues currently problematic for these devices: high offset and knee voltages and saturation charge storage. Reduced HBT offset and knee voltages (V/sub CE,os/ and V/sub k/) are important to improve the power amplifier efficiency. Reduced saturation charge storage is desirable to increase gain under conditions when the transistor saturates (such as in over-driven Class AB amplifiers and switching mode amplifiers). It is shown in this paper that HBT structures using a 100-/spl Aring/-thick layer of GaInP between the GaAs base, and collector layers are effective in reducing V/sub CE,os/ to 30 mV and V/sub k/ measured at a collector current density of 2/spl times/10/sup 4/ A/cm/sup 2/ to 0.3 V (while for conventional HBTs V/sub CE,os/=0.2 V and V/sub k/=0.5 V are typical). A five-fold reduction in saturation charge storage is simultaneously obtained.

AlGaAs emitter heterojunction bipolar transistors (HBTs) are demonstrated to have excellent dc and RF properties comparable to InGaP/GaAs HBTs by increasing the Al composition. Al/sub 0.35/Ga/sub 0.65/As/GaAs HBTs exhibit very high dc current gain at all bias levels, exceeding 140 at 25 A/cm/sup 2/ and reaching a maximum of 210 at 26 kA/cm/sup 2/ (L=1.4 /spl mu/m/spl times/3 /spl mu/m, R/sub sb/=330 /spl Omega///spl square/). The temperature dependence of the peak dc current gain is also significantly improved by increasing the AlGaAs mole fraction of the emitter. Device analysis suggests that a larger emitter energy gap contributes to the improved device performance by both lowering space charge recombination and increasing the barrier to reverse hole injection.

InGaP/GaInAsN double heterojunction bipolar transistors (HBTs) with compositionally graded bases are presented which exhibit superior dc and radio frequency performance. Reducing the average base layer energy gap and optimizing the emitter-base (e-b) and base-collector (b-c) heterojunctions leads to a 100-mV reduction in the turn-on voltage compared to a baseline InGaP/GaAs process. Simultaneously grading the base layer energy band-gap results in over a 66% improvement in the dc current gain and up to a 35% increase in the unity gain cutoff frequency. DC current gains as high as 250 and cutoff frequencies of 70 GHz are demonstrated. In addition, the InGaP/GaInAsN DHBT structure significantly reduces the common emitter offset and knee voltages, as well as improves the dc current gain temperature stability relative to standard InGaP/GaAs HBTs.

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.