Abstract
A method for electrical and optical bistable switching, including the following steps: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing a quantum size region in the base region, and providing base, collector and emitter terminals respectively coupled with the base, collector, and emitter regions; providing input electrical signals with respect to the base, collector, and emitter terminals to obtain an electrical output signal and light emission from the base region; providing an optical resonant cavity that encloses at least a portion of the base region and the light emission therefrom, an optical output signal being obtained from a portion of the light in the optical resonant cavity; and modifying the input electrical signals to switch back and forth between a first state wherein the photon density in the cavity is below a predetermined threshold and the optical output is incoherent, and a second state wherein the photon density in the cavity is above the predetermined threshold and the optical output is coherent, said switching from the first to the second state being implemented by modifying the input electrical signals to reduce optical absorption by collector intra-cavity photon-assisted tunneling, and the switching from the second to the first state being implemented by modifying the input electrical signals to increase photon absorption by collector intra-cavity photon-assisted tunneling.
Claims
1. A method for electrical and optical bistable switching, comprising the steps of: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing a quantum size region in said base region, and providing base, collector and emitter terminals respectively coupled with said base, collector, and emitter regions; providing input electrical signals with respect to said base, collector, and emitter terminals to obtain an electrical output signal and light emission from said base region; providing an optical resonant cavity that encloses at least a portion of said base region, including the junction thereof with the collector region, and the light emission therefrom, an optical output signal being obtained from a portion of the light in said optical resonant cavity; and modifying said input electrical signals to switch back and forth between a first state wherein the photon density in said cavity is below a predetermined threshold and said optical output is incoherent, and a second state wherein the photon density in said cavity is above said predetermined threshold and said optical output is coherent, said switching from said first to said second state being implemented by modifying said input electrical signals to reduce optical absorption by collector intra-cavity photon-assisted tunneling, and said switching from said second to said first state being implemented by modifying said input electrical signals to increase photon absorption by collector intra-cavity photon-assisted tunneling.
2. The method as defined by claim 1, wherein said step of switching from said first state said second state includes applying a stepdownward voltage to said device to reduce photon absorption by collector intra-cavity photon-assisted tunneling.
3. The method as defined by claim 1, wherein said step of switching from said second state said first state includes applying a stepupward voltage to said device to increase photon absorption by collector intra-cavity photon-assisted tunneling.
4. The method as defined by claim 2, wherein said step of applying a stepdownward voltage to said device comprises applying a collector-emitter voltage, V.sub.CE, below a threshold voltage, V.sub.TD, to said device while keeping the base current, i.sub.B, substantially constant.
5. The method as defined by claim 3, wherein said step of applying a stepupward voltage to said device comprises applying a collector-emitter voltage, V.sub.CE, above a threshold voltage, V.sub.TD, to said device while keeping the base current, i.sub.B, substantially constant.
6. A method for electro-optical bistable switching, comprising the steps of: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing a quantum size region in said base region, and providing base, collector and emitter terminals respectively coupled with said base, collector, and emitter regions; providing electrical signals with respect to said base, collector, and emitter terminals to obtain an electrical output signal and light emission from said base region; providing an optical resonant cavity that encloses at least a portion of said base region, including the junction thereof with the collector region, and the light emission therefrom, an optical signal being obtained from a portion of the light in said optical resonant cavity; and forward sweeping a signal applied to said terminals and then backward sweeping said signal to obtain both an electrical output hysteresis characteristic of said device and an optical output hysteresis characteristic of said device.
7. The method as defined by claim 6, wherein said hysteresis is controllable with the device collector voltage.
8. The method as defined by claim 6, wherein said hysteresis is controllable with the device base current.
9. The method as defined by claim 6, wherein said hysteresis is controllable with the device collector voltage and base current.
10. The method as defined by claim 6, wherein said method is applied in an electro-optical flip-flop application.
11. The method as defined by claim 6, wherein said method is applied in an electro-optical logic application.
12. The method as defined by claim 6, wherein said method is applied in an electro-optical memory application.
13. A method for operating a transistor device, comprising the steps of: providing a semiconductor device that includes a semiconductor base region of a first conductivity type between semiconductor collector and emitter regions of a second conductivity type, providing at least one quantum size region in said base region, and providing base, collector and emitter terminals respectively coupled with said base, collector, and emitter regions; providing input electrical signals with respect to said base, collector, and emitter terminals to obtain an electrical output signal and light emission from said base region; providing an optical resonant cavity that encloses at least a portion of said base region, including the junction thereof with said collector region, and the light emission therefrom; and operating said device to modulate the photon density in said cavity, by controlling photon generation by said at least one quantum size region and controlling photon absorption by intra-cavity photon-assisted tunneling, said step of operating said device to modulate the photon density in said cavity including switching from a first state of transistor operation to a second state of transistor operation by applying a stepdownward voltage to said device to reduce photon absorption by collector intra-cavity photon-assisted tunneling; and said step of operating said device to modulate the photon density in said cavity further including switching from said second state of transistor operation to said first state of transistor operation by applying a stepupward voltage to said device to increase photon absorption by collector intra-cavity photon-assisted tunneling.
14. The method as defined by claim 13, wherein said step of providing an optical resonant cavity comprises providing lateral reflectors to form an edge-emitting cavity configuration.
15. The method as defined by claim 13, wherein said step of providing an optical resonant cavity comprises providing vertical reflectors to form a vertical cavity configuration.
16-19. (canceled)
20. The method as defined by claim 13, wherein in said first state the photon density in said cavity is below a predetermined threshold and said light emission is incoherent, and in said second state the photon density in said cavity is above said predetermined threshold and said light emission is coherent.
21. The method as defined by claim 13, wherein said step of operating said device to modulate the photon density in said cavity comprises controlling the base current and collector voltage of said semiconductor device.
22. The method as defined by claim 15, wherein an optical output signal is obtained from a portion of the light in said optical resonant cavity.
23. The method as defined by claim 14, wherein an optical output signal is obtained from a portion of the light in said optical resonant cavity.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic energy diagram of a quantum-well (QW) edge-emitting transistor laser as used in embodiments hereof, operating by e-h recombination in the base-QW and photon absorption by intra-cavity photon-assisted tunneling (ICPAT) at the collector junction. The collector current I.sub.C=I.sub.t+I.sub.ICPAT+I.sub.rT where I.sub.t=.sub.t I.sub.B+.sub.2 (I.sub.ICPAT+I.sub.rT).
[0021] FIG. 2 includes FIG. 2(a) and FIG. 2(b). FIG. 2(a) is a scanning electron micrograph of top view of the QW transistor laser with cavity length L=200 m, and FIG. 2(b) is a focused ion beam (FIB) micrograph of a cross-section of the edge-emitting transistor laser revealing the material and device layers.
[0022] FIG. 3 is a graph of emission spectra of the edge-emitting transistor laser (EETL) of a cavity length L=200 m operating at 15 C., I.sub.B=50 mA, and V.sub.CE=1.5V.
[0023] FIG. 4 includes FIG. 4(i) and FIG. 4(ii), with graphs for the 200 m transistor, laser (i) Collector I.sub.CV.sub.CE and (ii) LV.sub.CE characteristics, operating at 20 C. exhibiting a sharp current step-change and different ICPAT voltage switching (dotted lines). Four distinct operating regions are evident: (1) spontaneous recombination below threshold, I.sub.TH=43 mA (solid line), (2) stimulated recombination and coherent light above threshold (dashed line), (3) collector current and light output switching from stimulated to spontaneous recombination and emission at switch-UP voltages V.sub.TU(I.sub.B) (dotted line), and (4) spontaneous recombination and emission above current threshold and above voltage threshold V.sub.TU (solid line).
[0024] FIG. 5 includes FIG. 5(a) and FIG. 5(b). In these Figures there is shown transistor laser electro-optical hysteresis of the collector I.sub.CV.sub.CE (FIG. 5(a)), and the optical LV.sub.CE (I.sub.B) output characteristics (@ 20 C.) with threshold I.sub.TH=43.4 mA (FIG. 5(b)). In FIG. 5(a) when V.sub.CE increases from 2 to 3 V, the forward collector I.sub.CV.sub.CE and optical LV.sub.CE characteristics for the base current I.sub.B=72 to 90 mA (step I.sub.B=3 mA) exhibit sharp current and optical switching at switch-UP voltage (V.sub.TU) for a given base current (solid line). In FIG. 5(b) when V.sub.CE decreases from 3 to 2 V, the backward I.sub.CV.sub.CE and LV.sub.CE characteristics exhibit sharp current and optical switching at switch-DOWN voltage V.sub.TD (dotted line).
[0025] FIG. 6 includes FIG. 6(a) and FIG. 6(b). In these Figures there is shown transistor laser electrical hysterisis of the collector I.sub.CV.sub.CE (FIG. 6(a)), and optical hysterisis of the light LV.sub.CE characteristics operating at 10 C. at a fixed base current I.sub.B=84 mA (FIG. 6(b)). EETL bistable states for a given V.sub.CE=2.58 V are realized between lower electrical energy state (0) with a stimulated and higher optical energy state (coherent) and higher electrical energy state (1) with a spontaneous and lower optical energy state (incoherent). It can be observed that there is a voltage threshold difference between switch-UP V.sub.TU=2.6 V and switch-DOWN V.sub.TD=2.56 V which yields electro-optical hysteresis and bistability.
[0026] FIG. 7 includes FIG. 7(i) and FIG. 7(ii). In these Figures there is shown transistor laser electro-optical hysterisis and bistability oprtating at 20 C. for a given I.sub.B=84 mA (I.sub.TH=43 mA). The current difference is I.sub.C=15 mA and the optical light difference is L=1 mW at V.sub.TU (switch-UP)=2.60 V and V.sub.TD (switch-DOWN)=2.56 V. A transistor laser electro-optical bistability is consequently realized and demonstrated at V.sub.CE=2.58 V between the coherent state (0) and the incoherent state (1).
[0027] FIG. 8 includes FIG. 8(i) and FIG. 8(ii). In these Figures there is shown transistor laser electro-optical hysterisis and bistability oprtating at 10 C. for a given I.sub.B=84 mA (I.sub.TH=33 mA). The current difference is I.sub.C=25 mA and the optical light difference is L=2 mW at V.sub.TU (switch-UP)=3.46 V and V.sub.TD (switch-DOWN)=3.40 V. A transistor laser electro-optical bistability is realized and demonstrated at V.sub.CE=3.43 V between the coherent state (0) and the incoherent state (1).
[0028] FIG. 9 is a state diagram showing the step-up and step-down technique and states which exploit the electro-optical bistability characteristics of a transistor laser.
DETAILED DESCRIPTION
[0029] The schematic energy band diagram of a heterojunction transistor laser (n-p-n) with a quantum-well (QW) in the base, photon-assisted tunneling at the collector junction, and a reflecting optical cavity are shown in FIG. 1 operating with emitter current injection, base recombination and transport, and tunneling collector current. The base recombination hole current (I.sub.Br) is supplied by the external base current (I.sub.B), the intracavity photon-assisted tunneling hole current (I.sub.ICPAT, h), and the band-to-band tunneling hole current (I.sub.rT). The collector electron current (I.sub.C) is contributed from the base electron current reaching the collector junction (It), the intracavity photon-assisted tunneling electron current (I.sub.ICPAT, e), and the band-to-band tunneling electron current (I.sub.rT). The photon generation is due to e-h recombination at base quantum-well, and the photon absorption is due to intra-cavity photon-assisted e-h tunneling at the collector junction. The corresponding hole current contributes to the base for electron relaxation transport and excess (injected) carrier spontaneous and stimulated recombination, thus providing at the collector tunneling-modulation of the laser and tunneling-amplification of the transistor (see also M. Feng, J. Qiu, C. Y. Wang, and N. Holonyak, Jr., J. Appl. Phys. 120, 20451 (2016)). The cleaved mirrors provide the optical cavity with a photon trap and leads to the coherent light and laser output when the photon density is above the coherent threshold.
[0030] A quantum-well transistor laser (QWTL) that has been designed and fabricated for improved performance as used in accordance with a form of the invention is illustrated in FIG. 2(a) which shows a scanning electron micrograph of the top view of the coplanar common-emitter TL device and in FIG. 2(b) which shows a focused ion beam (FIB) micrograph of a cross-section of the edge-emitting TL. The QWTL in FIG. 2(a) has an emitter cavity width of 2 m and a cleaved cavity length 200 m and the emitter, base, and collector contacts denoted as E, B, and C, respectively. In FIG. 2(b), from the GaAs semi-insulating substrate (g) upward, the epitaxial structure of the transistor laser includes a 5000 heavily doped n-type GaAs buffer layer (f) and also 5000 of n-type Al.sub.0.95Ga.sub.0.05As serves as the lower cladding. The collector (e) includes a layer of Al.sub.0.4Ga.sub.0.6As that serves as a sub-collector and a 1000 lightly doped n-type GaAs collector layer. The base (d) comprises a conduction energy barrier (81 meV) formed by a 100 layer of heavily C-doped p-type Al.sub.0.1Ga.sub.0.9As, and a 910 heavily doped p-type GaAs base layer. Within the base region (d), there is incorporated an undoped 150 In.sub.xGa.sub.1-xAs QW designed for emission at 980 nm. On top of the base is a 400 lightly doped n-type In.sub.0.49Ga.sub.0.51P emitter layer (c). A 5000 n-type GaAs/Al.sub.0.92Ga.sub.0.08As top cladding layer (b) is grown on top of the emitter. A 1000 heavily doped n-type GaAs emitter contact layer (a) caps the stack of layers. [Alternatively, the device can be configured as a vertical cavity device with upper and lower reflectors, for example distributed Bragg reflectors (DBRs).]
[0031] FIG. 3 shows the transistor laser spectrum m, with peak wavelength at 977 nm operating at I.sub.B=50 mA and V.sub.CE=1.5V at 15 C. The two-headed arrow shows the peak relative amplitude of 22 dBm.
[0032] A 200 m cavity EETL operating at 20 C. shows the measured outputs of (i) the collector I.sub.CV.sub.CE and (ii) the optical LV.sub.CE family of characteristics in FIG. 4. As seen in the Figure, the collector I.sub.CV.sub.CE characteristics with a step-upward collector voltage exhibit sharp current changing at switch-UP voltage (V.sub.TU) as V.sub.CE increases from 0 to 4 V and I.sub.B increases from 0 to 90 mA with steps of I.sub.B=3 mA. Four different operating regions are identified: (1) the spontaneous region below base current threshold I.sub.TH=43 mA (solid line), (2) laser stimulated region above base current threshold (dashed line), (3) the I.sub.ICPAT switch-UP region (dotted line), and (4) the spontaneous region above base current threshold (I.sub.TH) and above collector voltage threshold (V.sub.TH) (solid line). This device shows the unique signature of TL operation: collector current gain compression at the laser threshold current I.sub.TH=43 mA. This unique characteristic is attributed to the change in base recombination lifetime as the device shifts operation from slow spontaneous (solid line) in region (1) to faster stimulated recombination (dashed line) in region (2).
[0033] The dotted line region (3) in the I.sub.CV.sub.CE family of characteristics represents electrical switching in the operation due to the base-QW shifting from stimulated to spontaneous recombination. The transistor operates in spontaneous recombination after collector I.sub.ICPAT switching (solid line) region (4). The dotted line region (3) in the LV.sub.CE family of characteristics represents optical switching owing to the cavity operation shifting from coherent to incoherent via intra-cavity photon assisted-tunneling. The transistor laser operates in incoherent recombination after collector I.sub.ICPAT switching, thus, yielding only incoherent light output at low intensity (solid line region).
[0034] For investigating the switching behavior by amplifying region in FIG. 4, the step-upward (forward) collector I.sub.CV.sub.CE family of characteristics at 20 C. (solid line) in FIG. 5(a) exhibits sharp current change at switch-UP voltage (V.sub.TU) as V.sub.CE increases from 2 to 3 V and I.sub.B increases from 72 to 90 mA with steps of I.sub.B=3 mA. For a given I.sub.B=84 mA (I.sub.TH=43 mA), the current difference is I.sub.C=15 mA at V.sub.TU=2.60 V. When V.sub.CE decreases from 3 to 2 V for step-downward (backward) operation, current switching at different switch-DOWN voltage (V.sub.TD) characteristics (dotted line) is exhibited in FIG. 5(a). For a given I.sub.B=84 mA (I.sub.TH=43 mA), the current difference is I.sub.C=15 mA at V.sub.TD=2.56 V.
[0035] When the device is operated at 10 C. and the threshold is reduced to I.sub.TH=33 mA, the forward collector I.sub.CV.sub.CE family of characteristics (solid line) in FIG. 6(a) exhibit sharper current change at V.sub.TU from 2.5 to 4 V and I.sub.B increases from 69 to 90 mA with steps of I.sub.B=3 mA. For a given I.sub.B=84 mA (I.sub.TH=33 mA), the current difference improves to I.sub.C=25 mA at V.sub.TU=3.46 V. When V.sub.CE decreases from 4 to 2.5 V for backward operation, different corner current switching at switch-DOWN voltage (V.sub.TD) characteristics (dotted line) are exhibited in FIG. 6(b). For I.sub.B=84 mA, the current difference increases to I.sub.C=25 mA at V.sub.TD=3.4 V. Due to the difference in switch-UP and switch-DOWN voltage, the electrical hystereses in the collector I.sub.CV.sub.CE family of characteristics are demonstrated at 20 C. and 10 C., respectively,
[0036] The physical mechanism of switch-UP can be explained by the base-QW shifting operation from stimulated to spontaneous recombination when the optical absorption rate by ICPAT increases with V.sub.CE and exceeds the stimulated photon generation rate at the base-QW for a given base current, the cavity photon density then drops below coherent threshold resulting in switching at switch-UP voltage V.sub.TU. After switching, the transistor is operating under spontaneous but above laser current threshold I.sub.TH. The mechanism of switch-DOWN can be explained by the base-QW shifting operation from spontaneous to stimulated (lasing) recombination when the optical absorption rate decreases with V.sub.CE and is lower than the spontaneous photon generation rate, the cavity photon density (incoherent) increases above the coherent threshold resulting in switching at switch-DOWN voltage V.sub.TD.
[0037] It can be observed that the output collector current (I.sub.C) and the input collector voltage (V.sub.CE) relation forms a hysteresis loop for a given base current above the laser current threshold. There is a threshold difference in switch-UP and switch-DOWN voltages and results in the hysteresis loop as shown in FIGS. 5(a) and 6(a). Also, the hysteresis loop area increases and switching slope reduces with temperature decreases. Thus, the electrical hysteresis family of characteristics is programmable with base current (I.sub.B), collector voltage (V.sub.CE), and the junction temperature.
[0038] FIG. 5(b) displays the TL step-upward (forward) optical LV.sub.CE (I.sub.B) family of characteristics operating at 20 C. When V.sub.CE increases from 2 to 3 V and I.sub.B increases from 72 to 90 mA with steps I.sub.B=3 mA, the forward LV.sub.CE characteristics exhibit sharp optical change at V.sub.TU in FIG. 5(b) (solid line) (e.g., L=1 mW for I.sub.B=84 mA and V.sub.TU=2.6 V). When V.sub.CE decreases from 3 to 2 V, the backward LV.sub.CE characteristics exhibit optical change at V.sub.TD in FIG. 5 (b) (e.g., L=1 mW for I.sub.B=84 mA and V.sub.TD=2.56 V (dotted line).
[0039] FIG. 6(b) displays the TL forward and backward optical LV.sub.CE (I.sub.B) family of characteristics operating @ 10 C. When V.sub.CE increases from 2.5 to 4 V and I.sub.B increases from 69 to 90 mA with steps I.sub.B=3 mA, the forward optical LV.sub.CE characteristics exhibit sharp optical change at V.sub.TU in FIG. 6(b) (solid line) (e.g., L=1.5 mW for I.sub.B=84 mA and V.sub.TU=3.45 V). Forward optical switching @ 10 C. in FIG. 6(b) (corresponding to forward electrical switching as shown in FIG. 6(a)) is due to the optical cavity shifting operation from coherent to incoherent. When the optical absorption rate by ICPAT increases with V.sub.CE and is greater than the optical generation rate at the base-QW, the cavity photon density drops below coherent threshold resulting in optical switching at V.sub.TU=3.45V.
[0040] When V.sub.CE decreases from 4 to 2.5 V, the backward LV.sub.CE characteristics operating @ 10 C. exhibit optical step-change at V.sub.TD in FIG. 6(b) (dotted line) (e.g., L=1.5 mW for I.sub.B=84 mA and V.sub.TD=3.4 V). Optical backward switching is owing to the optical cavity shifting operation from incoherent to coherent. When the optical absorption rate decreases with V.sub.CE and is below the photon generation rate, the cavity photon density increases above the threshold resulting in coherent light output.
[0041] FIG. 7(i) shows the electrical hysteresis of the collector current I.sub.CV.sub.CE and FIG. 7(ii) shows the optical hysteresis of the optical LV.sub.CE at 20 C. for a fixed I.sub.B=84 mA. When V.sub.CE increases from 2.5 to 2.58 V, the I.sub.C and L move forward from point (C) to (A). The base operation is in stimulated recombination, and the light ouput is coherent. The cavity photon density reduces by increasing of ICPAT optical absorption rate with a nearly constant QW optical generation rate limited by a fixed I.sub.B=84 mA. When V.sub.CE increases from 2.58 to 2.67 V, the I.sub.C and L (solid line) move forward from (A) to (D). The cavity operation shifts from coherent to incoherent when the cavity photon density reduces below the threshold at V.sub.TU=2.6 V by further increase of ICPAT photon absorption rate and the base operation shifits from stimulated to spontaneous recombination. The current difference is I.sub.C=15 mA and the optical light difference is L=1 mW at V.sub.TU=2.6 V.
[0042] When V.sub.CE decreases from 2.67 to 2.58 V in FIG. 7(ii), the I.sub.C and L (dotted line) move backward from point (D) to (B). The base-QW operates in spontaneous recombination and the light is incoherent. When V.sub.CE decreases from 2.58 to 2.5 V, the I.sub.C and L (dotted line) moves backward from point (B) to (C). The base shifts operation from spontaneous to stimulated recombination at V.sub.TD=2.56 V and the light output shifting from incoherent to coherent (laser) with further reduction of photon absorption rate by ICPAT. For a given I.sub.B=84 mA (I.sub.TH=43 mA), the current difference is I.sub.C=15 mA and the optical light difference is L=1 mW at V.sub.TD=2.56 V. A transistor laser electro-optical bistability is consequently realized and demonstrated at V.sub.CE=2.58 V between the coherent state (A) and the incoherent state (B).
[0043] FIG. 8(i) demonstrates the electrical hysteresis of the collector current I.sub.CV.sub.CE and FIG. 8(ii) displays the optical hysteresis in LV.sub.CE at 10 C. for I.sub.B=84 mA with a I.sub.TH=33 mA. When V.sub.CE increases from 3.33 to 3.43 V, the I.sub.C and L (solid line) move forward from point (C) to (A). The light ouput is coherent with the base operating in stimulated recombination. When V.sub.CE further increases from 3.43 to 3.51 V, the I.sub.C and L (solid line) move forward from point (A) to (D). The cavity operation shifts from coherent to incoherent with the photon density reduced below the coherent threshold at V.sub.TU=3.46 V by further increase of ICPAT photon absorption rate. As a result, the base operation shifits from stimulated to spontaneous recombination. The current difference is I.sub.C=25 mA and the light difference is L=2 mW at V.sub.TU=3.46 V.
[0044] When V.sub.CE decreases from 3.51 to 3.43 V in FIG. 8 (ii), the I.sub.C and L (dotted line) move backward from point (D) to (B). The base operates in spontaneous recombination, the light is incoherent. When V.sub.CE further decreases from 3.43 to 3.33 V, the I.sub.C and L (dotted line) move backward from point (B) to (C). The light output shifts from incoherent to coherent (laser) with further reduction of photon absorption by ICPAT at V.sub.TD=3.4 V. As a result, the base shifts operation from spontaneous to stimulated recombination. The current difference is I.sub.C=25 mA and the optical light difference is L=2 mW at V.sub.TD=3.4 V. It can be observed that the switching current and light difference are improved considerably with reduced device temperature in FIGS. 7 and 8. Transistor laser electro-optical bistability is thus realized and demonstrated at V.sub.CE=3.43 V between the coherent state (A) and the incoherent state (B).
[0045] FIG. 9 illustrates schematically the paths of bistability operation of the transistor laser. A three-port transistor laser with cavity photon density above laser threshold is labeled as a coherent energy state (0), the transistor operating under stimulated recombination in the base-QW providing lower collector current output and coherent light (laser) output. A transistor laser with cavity photon density below laser threshold is labeled as an incoherent energy state (1), the transistor operating under spontaneous recombination in the base-QW providing higher collector current output and incoherent light (light-emitting transistor) output. The switching from state (0) to state (1) is achieved by the reduction of cavity photon density below coherent threshold via increasing photon absorption rate with intra-cavity photon-assisted tunneling by increasing collector voltage above the switch-UP threshold (V.sub.CEV.sub.TU) for a given base current (I.sub.B). The switching from incoherent state (1) to coherent state (0) is achieved by the increase of cavity photon density above laser threshold via reducing photon absorption rate by decreasing collector voltage below collector switch-DOWN threshold (V.sub.CEV.sub.TD) for a given base current (I.sub.B).
[0046] The time delay for the electrical and optical switch-UP is expected to be advantageously short due to fast intra-cavity photon-assisted tunneling reducing the cavity photon density below the incoherent voltage threshold since the quantum tunneling time is characterized as 6 to 8 fs by field emission microscopy (see S. K. Sekatskii and V. S. Letokhov, Phys. Rev. B 64, 233311 (2001)) and calculates to be 20 to 50 fs (see Z. S. Wang, L. C. Kwek, C. H. Lai, and C. H. Oh, Phys. Rev. A 69, 052108 (2004)). The time delay for the electrical and optical switch-DOWN is expected to be relatively longer owing to the slow photon generation rate via spontaneous e-h recombination (10-50 picosecond HBT-LET) building up the cavity photon density above the coherent voltage threshold. The electrical and optical hystereses demonstrated for the transistor laser are due to the time delay of different operational paths of e-h and photon recombination/generation in forward and backward switching.
[0047] Room temperature operation of both the electrical and optical bistability of a transistor laser have been demonstrated. An electro-optical hysteresis with sharp square corners and different voltage thresholds of the collector I.sub.CV.sub.CE and LV.sub.CE characteristics operating at 20 and 10 C. for the step-upward and step-downward operations are observed and are complementary. Because of the switching path differences between coherent and incoherent cavity photon densities reacting with collector voltage modulation via intra-cavity photon-assisted tunneling (ICPAT)) resulting in the collector voltage difference in switch-UP and switch-DOWN operations, the TL bistability is realizable, controllable, and usable. The operations of the electro-optical hysteresis and bistability in the compact form of the transistor laser can be employed, for example, for high speed optical logic gate and flip-flop applications.
