Reduction of Turn-on, Knee, and Offset Voltages of InAlGaP/GaAs HBTs Using $\delta$-Doping in the InAlGaP Emitter

In
_0.5
(Al
_0.3
Ga
_0.7
)
_0.5
P/GaAs heterojunction bipolar transistors (HBTs) are demonstrated as an alternative to AlGaAs/GaAs and InGaP/GaAs HBTs for microwave applications. The effects of δ -doping in the InAlGaP emitter on both electron transport and transistor performance are systemically explored. For comparison, a reference HBT with an identical epilayer design to that of the δ-doped HBT, except for the absence of a δ-doping layer, is processed at the same time. The two design options are established to yield comparable dc current gains β . InAlGaP can be selectively etched over GaAs, making the device etching routine. Notably, δ-doping simultaneously reduces the knee voltage
V
_knee
, the offset voltage
V
_offset
, and the turn-on voltage
V
_{BE, on}
of the HBTs of interest. This paper also sheds light on the behavior of the offset voltage for these devices. An unexpected attribute of the investigated devices is the effect of the base current on the offset voltage. The offset voltage is analyzed using a modified large-signal model. A further important inference of this paper is that the β of the transistor is flat across a large range of currents at room temperature. The use of InAlGaP as the emitter layer substantially decreases the variation of β with substrate temperature in the HBTs. The stability of the dc current gain with varying temperature is attributable to the inherent characteristics of this material system. The results make InAlGaP/GaAs δ-HBTs viable contenders for millimeter-wave application.