Vertical scaling in heterojunction bipolar transistors with nonequilibrium base transport

For the iirst time, we show a departure from the conventional dependence of fi on base thickness xs in abrupt junction n-p-n heterojunction bipolar transistors (HBTs). This is to be contrasted with the familiar fl a l/xi found in homojunction bipolar transistors where current gain is limited by diffusive base transport. Our data, combined with high frequency and collector breakdown measurements, coniirm the fact that, in the regime where extreme nonequilibrium electron transport in the base dominates, fi scales as l/x,. In addition, there exists another regime where current gain scales approximately as l/xi, but base transport cannot be described using the commonly accepted notion of diffusive electron transport. In a classical n-p-n homojunction (and graded-juno tion) bipolar transistor, electrons introduced from the emitter diffuse across the base. If the current gain is base recombination limited, base current 1s = Qs/‘r,,, where Qs is the electron charge in the base and l/~, is the electron recomibination rate. Because base thickness is typically much less than the minority carrier diffusion length, but more than the electron mean free path, Q, a xg, the base current scales as 1s cc xg. The collector current Ic is limited by electron diffusion across the base with an effective electron velocity V,, a D/xR, where D is the diffusion constant. Therefore, common emitter current gain fi = Ic/1s a l/x$. In an abrupt heterojunction bipolar transistor collector, current is limited by injection at the emitter-base conduction band spike hE, when Utherm exp( AEJk,T) 4 D/x& In this expression, vtherm 6 1 x lo7 cm s ’ is the x-directed average thermal velocity in the emitter and k,T = 0.025 eV is the thermal energy at room temperature. Thus, for example, the collector current in an abrupt junction HBT with AE, > 0.2 eV, xg> 100 A and D = 25 cm’ s ’ does not directly depend on base thickness xs even if base transport is diffusive.’ In such a transistor, the base thickness dependence of /3 arises solely from the xg dependence of base current, 1,. In our experiments, we observe a changeover from a l/xg dependence for xgs 1000 A to a l/xi dependence for xsz 1000 A. Using high frequency and collector breakdown measurements, we establish that this behavior is related to nondiffusive electron transport in the base. Single crystal, Ab.4,1no.szAs/Ina53G~.~7As layer structures were grown on semi-insulating (100) InP substrates by solid source molecular beam epitaxy.2 Base thicknesses are in the range 200 A<xB<4000 A, base doping level is p = 1.5X 1019 cm 3, and the collector space charge region is xc== 3000 A thick for all samples except the device with xs = 1500 A for which xc = 5000 A. By proper control of growth rate and substrate temperature, we are able to ensure the appropriate superposition of the metallurgical junction with the emitter-base junction. Following crystal growth, HBTs similar to those described in Ref. 3 were fabricated. Since emitter size effect is negligible in these devices,3 small area devices with emitter stripe widths of 2.5 pm were chosen to eliminate emitter-current crowding. It is also important to establish that current gain is not limited by nonideal 2ksT current components, particularly in thin base transistors where the neutral base recombination is is small. Figure 1 (a) shows the Gummel plot for a HBT with xs = 200 A. Note, the base current ideality factor is essentially identical to that for the collector current indicating negligible 2ksT effects. The abruptness of the emitter-base heterojunction is confirmed by carefully measuring the temperature dependence of the collector current Ic = Is[exp(eVBE/nkBT) l] for Vcs = 0 V. Typical results of measured IsI ycB=O p are shown in Fig. 1 (b). From these data, we are able to determine an effective barrier energy for electrons of c$1.23 eV. This corresponds to (Egb + AEJ, where Egb = 0.76 eV is the bandgap of the In0.53Gae.47As base and AE, = 0.47 eV, the energy of the conduction band offset, is the excess kinetic energy with which electrons are injuected from the emitter into the base. By way of contrast, in a transistor with graded emitter-base junction, we would obtain I$ EB6 and only low energy electrons in thermal equilibrium with the lattice could be introduced into the base. An increase in the excess initial kinetic energy of electrons injected into the base of an abrupt HBT extends the region over which nonequilibrium electron transport is important and potentially increases device speed.4