micrograph picture of the cross-section
of a transistor laser (800-mm width) and
room-temperature CW operations (center
part bullseye). The middle portion of the
figure shows the transistor current gain
(b = DIC/DIB, bstim < bspon) vs. the base
current and reveals the shift (the “step”)
in base operation (IB = 45 m A) from
spontaneous to stimulated recombination at the threshold of laser operation.
The laser spectrum is inserted to show,
at IB = 60 m A, the laser operation at
l = 1,010 nm. The bottom portion of the
right-side figure demonstrates a 19-GHz
TL bandwidth and the absence of carrier-photon resonance owing to the fast base
carrier lifetime, tB,spon 29 ps.
It is not an exaggeration to suggest that
the transistor laser,
already so revealing
and offering still more,
may well be the most
important thing that
has happened to
the transistor.
Tunnel junction transistor
laser and signal mixing
The transistor laser acts also as an
internal optical-signal detector (internal
photon-assisted Franz-Keldysh [FK]
effect); the term Ifk T in the figure below
is sensitive to the collector voltage and
a mechanism for aiding carrier trans-
fer across the TL base (IE —> IC). The
left part of the figure shows the sche-
matic band diagram of a tunnel junc-
tion transistor laser (TJ–TL) with all
the key physical processes labeled. IE
is the emitter current (minority carrier
injection into the base) with the junction
in forward bias; IB is the current resup-
ply of holes by the usual base ohmic
contact; If k T is the internal re-supply of
holes by the FK photon-assisted tunnel-
ing; Ir T represents the resupply of holes
via ordinary direct (narrow-junction)
tunneling of electrons; and It is the usual
base minority carrier current transport of
injected electrons that do not recombine
and are collected. The collector current
is obviously IC = It + Ir T + Ifk T, with the
base hole recombination components
adding up as IBr = IB + Ifk T + Ir T.
Tunnel junction transistor laser
hnQW
In0.15Ga0.85As QW
EFN(e)
EC
Lz IE
n-In0.49Ga0.51P
E
p-GaAs
IfkT
It
0 0.4 LI-V, DIB = 4 mA ( 20° C) TJ transistor laser LI-V of comparison TL ITH,a 1. 2 1.0 . 8 0.6 0.4 0.2 0 80 ( 1) ( 2)
Optical output per Facet, L [m W]
(Left) Schematic band diagram of a quantum-well (QW) TJ-TL shown with a generic
resonator cavity. Note the collector current Ic = It + Ifk T + Ir T consists of the direct
transport term It, photon-assisted Franz-Keldysh current Ifk T, and regular tunneling Ir T.
(Right) The dependence of optical output of the TJ-TL on VCE indicates the enhance-
ment (VCE < 0.8 V) and quenching (VCE ≥ 0.8 V) of the laser output by FK “self” photon-
assisted tunneling (photon absorption). The L-VCE(IB) of the comparison TL (right) shows
similar behavior occurring gradually except requiring higher bias voltage VCE ≥ 1. 6 V.
IB = 80 mA
ITH,b
VCE [V]
2.0
of the TJ-TL, the tunneling process
occurs predominantly via Franz-Keldysh (photon-assisted) absorption (IC ≈
It + IfkT). Direct tunneling (not photon-assisted) can be observed at higher VCE
biases (IC = It + IfkT + IrT). The collector
I-V characteristics of the TJ-TL agree
well in form with the optical output,
with the L-VCE(IB) characteristics
shown in the right-side portion of the
figure. In the operation of the TJ-TL
under weak collector junction field,
collector tunneling (photon-assisted,
If k T > 0) enables the efficient supply
of holes to the quantum-well active
region, and thus improves the laser
optical output by two times that of the
comparison TL. We note that the holes
supplied by collector tunneling need
only relax a distance of roughly 30 nm
(from collector to the base quantum-well), as opposed to the lateral distance
of 5 mm traversed by holes supplied by
an ordinary base ohmic contact (IB).
The photon absorption resulting from
the weak collector junction field is not
sufficient to overcome the photon gain
established by emitter and base carrier
injection (IE, IB > 0). However, under
stronger reverse-biased collector junction field (region 2 of the central portion of the figure), the optical output is
reduced and at large bias quenched by
Franz-Keldysh absorption.
The collector tunnel junction thus
enables the laser output to be controlled
effectively by the use of a third terminal control voltage. This is unique
to the three-terminal TL and offers
signal impedance-matching advantages.
Despite relying on only the internal bulk
FK effect, the proximity of the collector
tunnel junction to the photon generation
source (QW) and the strong coupling of
the tunneling process to the cavity optical field make possible an effective direct
voltage modulation mechanism. This
enables the TJ-TL to be directly modulated via a current (dIE, dIB) as well as by
voltage control (dVCE, dVBC).
This is one of the benefits of three-terminal TL operation, multiple input
capabilities and better impedance matching. Moreover, the TL collector can be
48 | OPN Optics & Photonics News
www.osa-opn.org
