The reliability of InGaP/GaAs N–p–n heterojunction bipolar transistors (HBTs) with different base metal contact systems (Au/Zn/Au, Ti/Au, Ti/Pt/Au and the novel Ti/ZrB2/Au) under current and temperature stress is studied in this paper. We further report results of current stress on three p-GaAs doping impurities namely Zn, Be and C. The effect of O+/H+ and O+/He+ ions, used in the fabrication of planar self-aligned HBTs, is also investigated in the stability of device dc current gain. The instability phenomena typical of each factors and their effects on the HBT characteristics are reported.

NiGeAu and PdGeTiPt ohmic contacts to n-GaAs and TiPd and PdGeTiPt ohmic contacts to p(+)-GaAs are examined by comparing their contact resistances, chemical intermixing as determined by Auger electron microscopy, interface structure as determined by transmission electron microscopy, and surface roughness as determined by surface profiling all measured as a function of annealing time and temperature. The n-PdGeTiPt contact annealed for short times, less than or equal to 15 s, and at low temperatures, less than or equal to 395 degrees C, was superior to the NiGeAu contact because it had a comparable contact resistance, less interface mixing, better lateral homogeneity, and a smoother surface. However, its contact resistance increased substantially with the annealing time and temperature, whereas the NiGeAu contact was relatively unaffected. For all annealing times and temperatures except the one at 550 degrees C, the TiPd contact to p(+) GaAs was superior as it had a lower contact resistance and a comparable amount of interface intermixing, lateral homogeneity, and surface roughness. However, it had a complete chemical breakdown at 550 degrees C, whereas the PdGeTiPt contact resistance remained relatively stable.

A key issue in the design of power microwave HBTs for high efficiency is the gain roll-off as device size is increased. Careful design is required to maintain adequate gain in order that the high power-added efficiency potential of these devices may be realized. This paper reviews the options for finger combination and the mechanisms controlling the gain scaling are quantified by modelling. This insight is then used to optimize the device layout of an improved common-emitter power device structure. Experimental devices have been fabricated and measured. Excellent gain scaling is demonstrated which verifies the design approach.

A wide range of base ohmic contacts have been studied to determine each of their suitability for usage in InGaP/GaAs HBTs for operation at high temperature. A novel base ohmic contact using Ti/ZrB/sub 2//Au has been developed with ZrB/sub 2/ as a barrier layer to prevent Au indiffusion from the contact layer to the base region. Current stress at high temperature on these HBTs show that the devices with this novel contact remain unchanged beyond 20 hours whereas devices with conventional contacts show clear signs of degradation after only a few hours of stress.

The effects of annealing on the distribution of elements in a Au(400 nm)/Ti(75 nm)/Pd(75 nm) Ohmic contact structure on zinc-doped p-GaAs epilayers, have been investigated using secondary ion mass spectrometry and cross-sectional transmission electron microscopy. The structure remained layered upon heat treatment up to 380°C in spite of considerable elemental mixing and the formation of new phases. The metallisation/semiconductor interfacial region was found to be very reactive. At room temperature, interaction between the contact and the GaAs resulted in the formation of a 20 nm thick Pd–Ga–As ternary layer (phase I) adjacent to the substrate. Annealing the structure at temperatures of 200 and 260°C led to further interaction at the contact/GaAs boundary and to the creation of protrusions, composed of a second Pd–Ga–As ternary compound (phase II), extending 90 nm into the semiconductor substrate. Heat treatments at 320 and 380°C resulted in a uniform multi-phase layer without protrusions, of total thickness 170 nm, next to the GaAs substrate.

In this study, various N-p-n heterojunction bipolar transistors (HBT) structures with high C-doped bases, grown by metal-organic chemical vapour deposition (MOCVD), have been fabricated with identical geometry and processing steps. Measured results show that in addition to the intrinsic heterojunction emitter injection efficiency, the base bulk recombination plays an important role in maintaining the current gain at high temperature. Furthermore, it is shown that emitter/base space-charge region recombination has a significant detrimental effect on the variation of current gain with increasing temperature. A theoretical model is presented which predicts well the current gain variation with temperature for both AlGaAs/GaAs and InGaP/GaAs HBTs.

In this paper the effect of C-, Be- and Zn-doped GaAs base on the bias stress reliability of n-p-n AlGaAsGaAs microwave HBTs, fabricated under an identical processing technology and layout design, have been investigated. It was found that, following 24 hours of identical bias stress at room temperature, both Be- and Zn-doped devices exhibited current gain degradation of 68 and 57%, respectively, compared with only 7% for the C-doped device. The increase in the emitter/base turn-on voltage in the degraded devices is attributed to the p-n junction displacement.

The GaAs based heterojunction bipolar transistor (HBT) continues to show improvements in power, efficiency and bandwidth making it very desirable for power amplifier applications. Among these are military phased array radar, secure communications, mobile phones, wireless networks and satellite communications. The foundation for the HBT is firmly rooted in its material base. The early candidate for HBT devices GaAlAs/GaAs has in many instances now been replaced by GaInP/GaAs due to its superior electrical and physical properties. While the inclusion of GaInP does offer numerous benefits for HBT devices it is comparatively immature and is not without its own difficulties. In this paper the technology and problems associated with the practical realisation of power HBT devices based on the material system GaInP/GaAs are discussed.

A comparison between MOCVD grown AlGaAs/GaAs and InGaP/GaAs HBTs showed a relatively constant DC current gain as a function of temperature upto 300/spl deg/C for the latter devices. A novel high temperature ohmic contact using ZrB/sub 2/ diffusion barrier has been developed and found to be suitable for HBTs operating at elevated temperatures. This barrier has been successfully employed in the fabrication of InGaP/GaAs HBTs which operated at 200/spl deg/C for 18 hours without any significant change to device performance.

Reactive ion etching is becoming increasingly widespread in the fabrication of III-V semiconductor devices. Particular applications include optical waveguides, heterojunction bipolar transistors (HBTs) and FET/HEMT type devices. Etch depth control is a critical issue in the fabrication of many devices. Optical waveguides require accurate etch depths for optical mode control whereas in the fabrication of HBTs there is a requirement to access a thin base layer. It is possible to fabricate devices using a “dead reckoning” method-however, this has proved not to be very accurate and a form of end point detection is required. A number of end point detection techniques are available and include optical emission spectroscopy, optical reflectometry and mass spectroscopy. This paper will compare all three methods and demonstrate weaknesses and strong points for each. Particular attention will be given to optical reflectometry and mass spectroscopy in the fabrication of HBTs

Summary form only given. We compare the electrical, physical, and chemical properties of TiPdAu and PdGeTiPt contacts for GaAs HBTs. Contact and specific contact resistances are measured using the TLM method and devices made with the two different types of emitter contacts are compared, the surface structures are examined using optical and scanning electron microscopy, and the chemical structures are investigated with Auger and transmission electron microscopy. In addition, comparisons are made between the TiPdAu and PdGeTiPt contacts to heavily C-doped GaAs. The advantages of being able to substitute PdGeTiPt contacts for TiPdAu are that all of the contacts could be made with a single metallization, and the Pt would be a more effective diffusion barrier to the in-diffusion of Au.

Au/Pd/Ti and Au/Ti/Pd ohmic structures to thin p+-GaAs layers have been investigated for use as contacts to the base region of HJBTs. The Au/Pd/Ti contact system yielded specific contact resistivities at or above 2.8 × 10-5Ωcm2. Heat treatments up to 8 minutes at 380°C caused only limited interaction between the metallization and the semiconductor. The metal penetrated to a maximum depth of ≃2nm. Specific contact resistivity values less than 10-5Ω2 were achieved using the Au/Ti/Pd (400/75/75nm) scheme. The nonalloyed Au/Ti/Pd contact showed the best combination of electrical and structural properties with a contact resistivity value of 9 × 10-6Ωcm2 and Pd penetration of the GaAs epilayer to a depth of ≃30nm.