Effect of emitter doping on device characteristics

The effect of emitter surface doping and emitter diffusion depth on the bandgap narrowing, temperature coefficient of collector current, temperature coefficient of current gain, and burst noise characteristics have been studied. The experimental investigation was done by fabricating two groups of n-p-n transistors. The first group of transistors was fabricated with three different emitter surface concentrations (
C_{0} = 5 \times 10^{21}
, 2 × 10
²⁰
, and 6 × 10
¹⁹
atoms/cm
³
) and fixed emitter depth (
X_{JE}= 1.5
µm), and the second group of transistors was fabricated with three different emitter depths (X
JE
= 0.8, 1.5, and 2.2 µm) and fixed emitter surface concentrations (
C_{0} = 2 \times 10^{20}
atoms/cm
³
). The heavy doping (5 × 10
²¹
atoms/cm
³
) was found to have introduced a narrowing of the bandgap energy and a corresponding degradation in gain. An annealing experiment was conducted by heating the devices to 800°C for 16 h in nitrogen, and this experiment had the effect of recovering the bandgap narrowing and improving the gain. The temperature coefficient of collector current
\alpha_{T}=(1/I_{c}) \cdot (\DeltaI_{c}/\DeltaT)
was determined for fixed V
BE
and the temperature coefficient of current gain
\beta_{T}=(1/\beta)(\Delta\beta/\DeltaT)
was calculated for fixed I
c
. For the first group of transistors, α
T
was found to have changed from 0.04 to 0.065 (1/°C) when C
0
was changed from 6 × 10
¹⁹
to 5 × 10
²¹
atoms/cm
³
. The corresponding change in β
T
was from 4 × 10
^-4
to 1 × 10
^-3
(1/°C), indicating that the current gain for this group of transistors was much more sensitive to doping than to the collector current. For the second group of transistors, however, α
T
and β
T
were found to have changed by the same orders of magnitude as X
JE
was changed from 0.85 to 2.2 µm: α
T
changed from 0.032 to 0.062 (1/°C) and β
T
changed from 3 × 10
^-3
to 6 × 10
^-3
(1/°C). Emitter doping also showed a relationship to burst noise characteristics. Ninety-five percent of devices with low C
0
(6 × 10
¹⁹
atoms/cm
³
) were found to be free of burst noise.