Direct measurement of the potential spike energy in AlGaAs/GaAs single-heterojunction bipolar transistors

A balanced two-step current transport theory, i.e., thermionic emission followed by Shockley diffusion, is applied to study the emitter-base (EB) potential spike energy in the AlGaAs/GaAs single-heterojunction bipolar transistor. It is found, surprisingly, that when the transistor is operated in the active region the
I-V
characteristics of the collector current (I
C
) versus base-emitter applied voltage (V
BE
) exhibits an ideality factor of 1.237. This non-1kT transfer characteristics is due to the bias-dependent potential spike energy at the emitter-base heterojunction. The reverse I-V characteristics of emitter current (I
E
) versus base-collector bias (V
BC
), however, shows the traditional 1kT behavior. The difference between I
C
and I
E
at the same applied voltage (
V_{BE} = V_{BC}
) determines the potential spike energy (ΔE). It turns out that Δ E/q = 0.19
(V_{BE} - 0.48)
where
q
is the unit charge. This indicates that the potential spike appears only when the applied voltage
V_{BE} > 0.48
V.