Optical bandwidth enhancement of light emitting and lasing transistor devices and circuits

A method for producing wide bandwidth laser emission responsive to high frequency electrical input signals, including the following steps: providing a heterojunction bipolar transistor device having collector, base, and emitter regions; providing at least one quantum size region in the base region, and enclosing at least a portion of the base region in an optical resonant cavity; coupling electrical signals, including the high frequency electrical input signals, with respect to the collector, base and emitter region, to cause laser emission from the transistor device; and reducing the operating beta of the transistor laser device to enhance the optical bandwidth of the laser emission in response to the high frequency electrical signals.

FIELD OF THE INVENTION

This invention relates to light emitting and lasing transistor devices used for optical communications and other purposes and, more particularly, to enhancement of the optical bandwidth of such devices.

BACKGROUND OF THE INVENTION

The transistor laser has already been demonstrated to exhibit a useful relatively wide optical bandwidth. However, for applications including, but not limited to, optical communications, greater optical bandwidth is very advantageous and desirable.

It is among the objects hereof to enhance the optical bandwidth of light emitting and lasing transistor devices and circuits.

SUMMARY OF THE INVENTION

In a transistor laser (TL), the usual electrical collector (IC) is complemented with a quantum well “optical collector” (hν, output port #2) inserted in the base-region of a heterojunction bipolar transistor. Besides its usual role in providing a high impedance output with current gain, β(=IC/IB), the electrical collector, with its proximity to the QW (≦300 Å), acts as a sensitive “read-out” (a “probe”) of the base recombination and transport processes (see e.g. H. W. Then, M. Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505 (2007); and H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007)). In accordance with an aspect hereof, the collector circuit is utilized to enhance the optical characteristics of light emitting and lasing transistor device via external control.

As will be demonstrated, applying an electrical AC auxiliary base signal to a transistor laser allows achievement of a faster stimulated recombination rate (higher peak photon operation and output), shortens the base carrier lifetime (reducing β and increasing laser differential gain), and hence, manipulates the TL into higher speed performance. This technique, applicable only in a three-terminal device configuration, is much more convenient and practical than, for example, optical methods that manipulate the laser cavity Q.

In accordance with an embodiment of a first aspect of the invention, a method is set forth for producing wide bandwidth laser emission responsive to high frequency electrical input signals, including the following steps: providing a heterojunction bipolar transistor device comprising collector, base, and emitter regions; providing at least one quantum size region in said base region, and enclosing at least a portion of said base region in an optical resonant cavity; coupling electrical signals, including said high frequency electrical input signals, with respect to said collector, base and emitter region, to cause laser emission from said transistor device; and reducing the operating beta of the transistor laser device to enhance the optical bandwidth of said laser emission in response to said high frequency electrical signals. In a preferred embodiment of this aspect of the invention, the step of reducing the operating beta of the transistor laser device comprises applying an auxiliary electrical signal to the transistor laser device. In this embodiment, the high frequency electrical input signals are in the range about 0.1-20 GHz, and said auxiliary signal has a frequency in the range 1 KHz-10 MHz. In this embodiment, the step of applying an auxiliary electrical signal to said transistor laser device comprises applying said auxiliary signal to the base region of said transistor laser device. In a form of this embodiment, the step of providing a transistor device comprises providing said transistor operating in a common emitter configuration, and wherein said input electrical signal and said auxiliary electrical signal are applied to an input electrical input port defined across the base to emitter terminals of said transistor device.

Another aspect of the invention is adapted for use in operation of a transistor laser device having an electrical input port for receiving an electrical input signal, an electrical output port, and an optical output port for outputting an optical signal modulated by said input signal, said device comprising a heterojunction bipolar transistor device that includes collector, base, and emitter regions, a quantum size region in said base region, and an optical resonant cavity enclosing at least a portion of said base region, said input port including an electrode coupled with said base region, said electrical output port including electrodes coupled with said collector and emitter regions, and said optical output port comprising an optical coupling with said base region, electrical signals, including said input electrical signal, being coupled with respect to said collector, base, and emitter regions to cause laser emission from the optical output port of said transistor laser. In this setting, a method is set forth for enhancing the response, by said laser emission at the optical output port, to high frequency electrical input signals, comprising increasing the transport of carriers to said quantum size region. In a preferred embodiment of this aspect of the invention, the step of increasing the transport of carriers to said quantum size region comprises applying an auxiliary electrical signal to said electrical input port of said transistor laser. In an embodiment hereof, the electrical input signal has a frequency in the range about 0.1-20 GHz, and said applying of an auxiliary electrical signal to said electrical input port comprises applying an auxiliary signal at a frequency in the range 1 KHz-10 MHz.

Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION

Devices used as a starting point for demonstrating examples of embodiments of the invention are three-port n-p-n HBT (n-InGaP/p-GaAs+InGaAs QW/n-AlGaAs) fabricated as described earlier in M. Feng, N. Holonyak, Jr., A. James, K. Cimino, G. Walter, and R. Chan, Appl. Phys. Lett. 89, 113504 (2006), supra. In particular, the TL crystal is a stack of AlGaAs, GaAs, InGaAs, and InGaP thin layers grown on a GaAs substrate by metalorganic chemical vapor deposition. Upward from the collector the p-type base region is a series of layers graded in doping, as described. The transistor contacts are realized by top-down metallization on ledges, steps, or apertures processed by photolithography and etching down to the relevant epitaxial layers of the TL crystal. The p-type 980 Å base active region for this example contains an InGaAs QW of size (width) LZ≈120 Å, giving a recombination radiation wavelength λ˜103nm. The cleave-to-cleave emitter-base (EB) cavity length for this example is 200 μm, and the distance from emitter to the electrical collector is WEC=880 Å, and emitter-to-QW collector WEQW=590 Å.

Although both the electrical and optical collector perform the similar function of carrier “collection”, their response times to the injection current (emitter current, IE) differ because of the difference in the transport time for carriers to reach each collector and the different junction parasitics (emitter-collector device size asymmetry). In the “emitter-up” form of HBT construction, the collector cross-section (area) is usually significantly larger than the emitter cross-section. The asymmetry results in a sizeable base-collector (BC) junction capacitance, Cjc, incurring a large charging delay time. The transistor electrical delay time, τEC=τt,1+Cje/gm+(RE+RC+1/gm)Cjc+τC, where τt,1and τCare the base and collector transit times respectively, Cjeand Cjcare, respectively, the EB and BC junction capacitances, gm=dIC/dVBEis the transistor transconductance, and REand RCare, respectively, the EB and BC resistances. Treating the base QW as an “optical collector” and removing the terms associated with the BC junction, one obtains for the TL optical delay time, τEQW=τt,2+Cje/gm, where τt,2is the emitter-to-QW transit time over a distance WEQW. For the TL of this example, τEQWand τECare dominated by junction capacitance charging delays. The times τEC=240 ps and τECW=4 ps are determined from measured microwave S-parameters and the transit time values τt,1=WEC2/2D and τt,2=WEQW2/2D (D being the diffusion constant in the following Table, which shows values of some key device parameters).

It is clear that the “optical” collector can “respond” faster than the electrical collector. Therefore, the overall response time of the system can be enhanced by increasing the rate of recombination (stimulated, for this laser example) at the QW. This should then be manifest as a “compression” in the collector I-V characteristics or reduction in the β(=ICO/IBO) (see e.g. R. Chan, M. Feng, N. Holonyak, Jr., A. James, and G. Walter, Appl. Phys. Lett. 88, 143508 (2006)). Experimentally, there are a number of ways to achieve this, one of which is to increase the Q of the cavity (see G. Walter, A. James, N. Holonyak, Jr., M. Feng, and R. Chan, Appl. Phys. Lett. 88, 232105 (2006)). In accordance with an embodiment of the invention, an electrical method is utilized. A low frequency AC auxiliary base signal is applied in order to peak the photon output and reduce the electrical β of the TL. The AC signal is applied in addition to the usual base bias current, IBO, and collector-emitter voltage, VCEO, as shown inFIG. 1.FIG. 1shows the transistor laser, TL, in common emitter configuration for this example, with a 50 ohm load, and with applied DC bias voltage VCEOand bias current IBO. The small signal AC input, and the auxiliary relatively low frequency AC input, are combined by adder110and input to the base of the TL. The resulting collector I-V characteristics and fiber-coupled peak optical output characteristics with and without the AC auxiliary signal are shown inFIGS. 2 and 3, respectively (see also, R. Chan, M. Feng, N. Holonyak, Jr., A. James, and G. Walter, Appl. Phys. Lett. 88, 143508 (2006), supra). The typical device optical output per facet is 1.2 mW at IBO=40 mA. With the AC auxiliary base signal, IBrises and falls and the photon output of the TL peaks following the peak of IB(point A inFIG. 2b).

Here, an additional effect, unique only to transistor operation is in play. In this example, the AC output collector signals, icand vce, are constrained to vary along a 50-ohm termination load-line, whose maximum and minimum amplitudes are limited by the transistor saturating at point A and cutting off at point B (FIG. 2b). ICis thus ‘clipped’ as is shown by the pulse-like VCEwaveform despite an input IBthat is purely sinusoidal (FIG. 2c). The average collector operating current, ICOis therefore reduced (from O1to O2inFIG. 2b), while the same supply of recombination current (IBO) is maintained. Consequently, with the applied AC auxiliary base signal, β decreases from βDC=1.3 to βAC=0.5. The frequency of the AC auxiliary signal is thus chosen to maximize the amplitude of IB(for peak photon output) and VCE(for reduction of ICO).

The reduction of β from βDCto βACresults in an increase in the proportion of injected carriers (IEO) that are channeled to the “faster” QW collector (IBO) and enhances the laser differential gain, which is defined as the measure of the coherent photons generated per unit length per injected carrier (see H. W. Then, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 183505 (2007); H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007)). These factors are advantageous towards improving the modulation bandwidth of the TL. From the charge control model (see M. Feng, N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys. Lett. 91, 053501 (2007), and neglecting the bulk recombination term, ICO=Q1/τt,1and IBO≈Q2/τt,2, where Q1and Q2are respectively the ‘tilted’ carrier populations responsible for the transport of carriers to the reverse-biased BC junction and to the QW. A measure of the proportion of the carriers transported to the QW, κ. is then be defined as,

κ=Q2Q2+Q1≈1β⁡(WECWEQW)2+1.(1)
As β reduces from βDC=1.3 to βAC=0.5, κ increases from κDC=0.26 to κAC=0.47.

To illustrate bandwidth improvement for this example, one can apply the continuity condition to the Q1and Q2populations and incorporate the effects of transit and junction charging delays, and obtain d(Q1+Q2)/dt=IE−Q1/τEC−Q2/τEQW−(Q1+Q2)/τbulk, where τbulkis the bulk recombination lifetime in the GaAs region (outside of QW). By eliminating Q1in favor of Q2, we obtain dQ2/dt=κIE−Q2/τ, and

τ=1(1τEQW+1τbulk)⁢κ+(1τEC+1τbulk)⁢(1-κ),(2)
where τ defines the effective base carrier lifetime, i.e., the average time an injected carrier (electron) “survives” in the base region before it recombines with a hole in the QW or is swept out by the reverse-biased field of the BC junction. An increased κ (from κDCto κAC) means that more carriers are “channeled” to the faster QW collector (τEQW<τEC), and τ speeds up by 1.6 times from 13 ps to 7.6 ps for the transfer laser of this example. By considering the small-signal variations in Q2(=Q2O+q2ejω) and IE(=IEO+iEejω), one obtains for the response function a 3-dB bandwidth f3dB=1/(2 π τ). The analysis is further developed to include photon-carrier interaction (see H. Statz and G. deMars, Quantum Electronics (Columbia University Press, New York, N.Y., 1960), p. 650), and gives as the laser modulation response function

p⁡(ω)iE⁡(ω)=H⁡(ω)⁢(1j⁢ω2⁢⁢π⁢⁢f3⁢⁢d⁢⁢B+1),(3)
where H(ω) is the intrinsic laser response described in M. Feng, N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys. Lett. 91, 053501 (2007), supra. The bandwidth of the intrinsic laser is determined by the square-root of the laser differential gain, ∂g/∂N and photon density, Po. The intrinsic bandwidth is then reduced by the effects of junction parasitics via the pole at f3dBin the response function of Eq. 3. In the TL, the laser differential gain is conveniently extracted from β (see H. W. Then, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007), supra). From the definition of laser differential gain, it is seen that the enhancement of κ, i.e., the 1.8 times improvement in the transport of injected carriers to the QW collector, will result in a similar increase in laser differential gain because of an increased proportion of injected carriers participating in the generation of photons. Using the parameters in the above Table, one can calculate the optical response of the TL giving bandwidths, f3dB=21 GHz employing an AC auxiliary signal and 12 GHz under DC bias. This agrees well with the measured optical frequency response of the TL shown inFIG. 4.

The improvement in speed performance is not connected with issues of threshold per se. Under the application of the auxiliary AC signal, the measured peak threshold base current agrees with ITH, the threshold current under DC or CW operation. At a DC operating bias of IBO=40 mA (VCEO=1.5 V), the TL of this example operates with a peak (fiber-coupled) photon intensity, Ppeak,AC=110 μW, 1.6 times higher than under DC bias alone (Ppeak,DC=70 μW). This is consistent with the finding that laser threshold remains unchanged.