IIIA-5 AlGaAs/GaAs heterostructure bipolar transistor with nonalloyed graded-gap ohmic contacts to the base and emitter

A new type of GaAs bipolar transistor with a base formed by a two-dimensional inversion hole gas was fabricated for the first time. In a conversional GaAs hetero-bipolar transistor, the base thickness is limited to a few hundred angstroms to avoid the unacceptable increase of the base resistance. The present GaAs inversionbase bipolar transistor (GaAs IBT) consists of an n+-GaAs layer as an emitter, an undoped thin AlAs layer, and an n--GaAs layer as a collector, and the transistor has a self-aligned external p+ base region. By applying the emitter-base bias V,, sufficiently, the surface of the n--GaAs at the AlAs interface is inverted, and the two dimensional inversion hole gas is induced. The inversion layer fonns an extremely thin base layer. The valence band discontinuity at AlAs/n--GaAs interface is sufficiently large ( A E , = 0.55 eV) for the holes to be confined at the interface. The electrons, which have a lighter mass tunnel through the thin and low ( A E , = 0.20 eV) AlAs barrier layer, pass across the inversion base layer, and reach the collector after falling down the slope of the collector depletion layer. The fabrication process starts from the crystal growth by MBE of following structure; HB n+-GaAs substrate (Si-dopeci)/n--GaAs (Si-doped 1 X lOI6/cm3, 1.5 pm)/;ndoped AlAs (100 A)/n+-GaAs (Si-doEed, 1 X lOl*/cm3, 5000 A ) . After the crystal growth, a 5000 A tungsten nitride (WN) layer was deposited by sputtering technique on the top n+-GaAs. Then the WN layer and n+-GaAs were selectively etched off down to the AlAs surface and formed the emitter. Using the WN and n+-GaAs emitter electrode as a mask, the magnesium (Mg) ions were implanted to form the selfaligned external base p+-GaAs region. The sample was, then, flash annealed at 800°C for 30 s without a cap under an arsenic pressure. After the annealing process, the ohmic electrodes were formed to the top nC-GaAs, the ion implanted pi layer, and the back side of the HB GaAs substrate for the emitter, base, and collector electrodes, respectively. The transistor characteristics measured at 77 K shows the current gain of CY = 0.77 for the base common circuit, and the current gain of 6 = 3.8 for the emitter common cirucit. The emitter-base voltage of more than V,, = 1.8 V is necessary to obtain the transistor characteristics. This emitter-base voltage of V,, = 1.8 V corresponds to the bias necessary to invert the undoped AlAsIn-GaAs interface and form the two-dimensional hole gas which works as a base. The GaAs inversion base bipolar transistor has an extremely thin base layer with a small base resistance. Therefore, the transistor is a good candidate for an ultrahigh-frequency microwave amplifier.