Optical bistable devices are fundamental to digital photonics as building blocks of switches, logic gates, and memories in future computer systems. Here, we demonstrate both optical and electrical bistability and capability for switching in a single transistor operated at room temperature. The electro-optical hysteresis is explained by the interaction of electron-hole (e-h) generation and recombination dynamics with the cavity photon modulation in different switching paths. The switch-UP and switch-DOWN threshold voltages are determined by the rate difference of photon generation at the base quantum-well and the photon absorption via intra-cavity photon-assisted tunneling controlled by the collector voltage. Thus, the transistor laser electro-optical bistable switching is programmable with base current and collector voltage, and the basis for high speed optical logic processors.
Bistability occurs in electrical or optical systems in which there is a region the output signal has two stable energy states for a given input. Switching between these states can be achieved by a change of input level. The input-output relationship forms a hysteresis loop, thus, giving the bistability. Electrical bistable devices are fundamental to digital electronics as building blocks of switches, logic gates, and memories in current computer systems. For example, any arrangement of transistors (CMOS or BJT) achieving two distinct stable states can be used as a storage element of a static random-access memory (SRAM) cell.
Today, digital electronic computers are bandwidth limited by the signal delay of RC time constants and carrier transit times of electronic logic. To overcome these problems, an optical digital computer has been considered. Optics is capable of communicating high bandwidth channels in parallel without suffering interference. Similarly, optical bistable devices are fundamental to digital photonics as building blocks of optical switches, optical logic gates, and optical memories. Two features are required to realize an optical bistable device: nonlinearity and feedback.1–5 An optical bistable system can be realized by the use of a nonlinear optical element whose output beam is used in a feedback system to control the transmission of light through the element. However, the full application of optics is yet to be applied to digital computers because of the lack of suitable optical logic processors with a scalable size and speed.
In 1991, semiconductor bistable photonic devices were reported based on the monolithic integration of a vertical cavity surface emitting laser (VCSEL) and a latching PNPN photothyristor6 as well as two PNPN photothyristors.7 However, the major issue with a laser-photothyristor pair is that the PNPN-thyristor stored charge and has an extremely slow switching speed, typically in the MHz range. This fundamental limitation is due to the saturated nature of the PNPN switching operation. Once turned on, the PNPN device accumulates large quantities of charge in its base, and takes a long time to turn off. This sets a fundamental limit to the speed of the laser-photothyristor to MHz switching. Others based on external optical components such as semiconductor optical amplifiers (SOAs), electro-absorption modulators (EAMs), and Mach-Zehnder modulators (MZMs) are crippled by low coupling efficiencies and low extinction ratios. Furthermore, these components are usually built with large lateral dimensions for ease of optical coupling, and long lengths to increase the extinction ratios. Such difficulties and large device dimensions (∼mm) are difficult for achieving high density integrated designs as required for logic applications.
In 2004, with quantum-wells (QWs) incorporated near the collector in the of a III-V heterojunction bipolar transistor (HBT), the radiative spontaneous recombination lifetime (τsp) of the device is reduced to a few pico-second.8,9 As a result, QW-HBTs with short base-collector metal contacts as in a two-terminal LED have demonstrated a record light emitting diode (LED) modulation bandwidth of f–3dB ∼ 7 GHz and confirm a fast τsp ∼ 23 ps operated at room temperature.10 Furthermore, the incorporation of an optical cavity for higher Q enclosing the QW-HBT provides with higher photon density field-enhancement stimulated recombination, coherent light output, and invention of the transistor laser (TL).11–13 The frequency response and resonance behavior of the semiconductor laser can be derived from the well-known Statz-deMars' coupled carrier-photon interaction rate equations.14 The modulation bandwidth is related to e-h radiation recombination lifetimes, photon lifetimes, and cavity photon density. The transistor laser can thus improve modulation bandwidth and bit-error-rate owing to fast radiative recombination lifetimes determined by the thin base and ability of the transistor to inject and collect stored charge within picoseconds (forcing the base QW recombination to compete with E-C transport).15–17
Optical absorption of a direct-gap semiconductor can be enhanced in the presence of a static electrical field and has been explained as photon-assisted tunneling (PAT) in semiconductor surfaces18–20 and used in a semiconductor PN junction diode.21 However, previous studies have not included the effect of electro-optical cavity coupling and quality factor, Q. In the transistor laser, the coherent photons generated at the base quantum-well interact with the collector field and “assist” optical cavity electron tunneling from the base valence band to the adjacent conduction band of the collector junction. In Feng and Holonyak's idea, the optical absorption can be further enhanced by the cavity coherent photon intensity of the transistor laser.22 The transistor laser intra-cavity photon-assisted tunneling (ICPAT) modulation via collector voltage (tunneling-collector voltage) is a unique property and the basis of ultra-high speed direct laser voltage modulation and switching.23–27
Due to the “planar” geometry of the transistor structure, the active devices or passive components can be conveniently replicated into electrical logic building blocks (integrated circuits, ICs) for computing and for all other traditional (electronics) information processing functions. For the uniqueness of the transistor laser and its “third” port, an optical dimension is convenient for ICs and computers. All the required components can be fabricated on a single epitaxial structure for transistors, lasers, detectors, and IC replication, thus facilitating the electronic-photonics integrated circuits (EOICs) on a very large scale. The transistor laser fundamentally enables the development of high-speed digital computation in the optical domain. The TL possesses the unique 3-port electrical and optical characteristics for direct current or voltage modulation and allows the design of ultra-high speed integrated optical switches.28
Previously, we realized the electro-optical bistability via intracavity photon-assisted tunneling, and demonstrated also in a ring-cavity transistor laser (RTL), operated at the low temperature of −50 °C.29,30 The optical output was measured by placing the fiber coupling to the side wall of the ring-cavity transistor laser; thus, the ratio of coherent to incoherent light proportionately reduced. In the present work, we demonstrate both the collector current IC-VCE and the optical L-VCE family of hystereses operating at room temperature in an edge-emitting transistor laser (EETL). The transistor laser electrical and optical bistabilities are controllable by base current (IB) and collector voltage (VCE). The current switching is due to the transistor base operation shifting between stimulated and spontaneous e-h recombination processes at the base-QW. The optical switching of coherent and incoherent energy states is due to transistor laser cavity photon density modulation via intra-cavity photon-assisted tunneling controlled by the collector voltage. Different switching paths between optical (coherent/incoherent) and electrical (stimulated/spontaneous) energy states result in different thresholds of input collector voltage. Thus, the operation principles as physical processes and operating mechanisms in transistor laser electro-optical bistabilities are considerably different than the optical hystereses in cavities containing the nonlinear absorptive (dissipative) and dispersive gain media that have been proposed and observed earlier.1–7
II. ENERGY BAND DIAGRAM OF A TRANSISTOR LASER
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 is shown in Fig. 1 operating with emitter current injection, base recombination, and transport, and tunneling collector current. The base recombination hole current (IBr) is supplied by the external base current (IB), the intra-cavity photon-assisted tunneling hole current (IICPAT, h), and the band-to-band tunneling hole current (IrT). The collector electron current (IC) is contributed from the base electron current reaching the collector junction (It), the intra-cavity photon-assisted tunneling electron current (IICPAT, e), and the band-to-band tunneling electron current (IrT). The photon generation is due to e-h recombination at the 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.27 The cleaved mirrors provide the optical cavity with a photon “trap” and lead to the coherent light and laser output when the photon density is above the coherent threshold. Different than the transistor invented by Bardeen and Brattain (1947) with the base operation in only spontaneous recombination, the transistor laser of Feng and Holonyak (2004) possesses both base spontaneous and a stimulated recombination.
III. EPITAXIAL LAYER STRUCTURE AND FABRICATION OF A TRANSISTOR LASER
Here, we describe a quantum-well transistor laser (QWTL) that has been designed and fabricated for improved performance as shown in Fig. 2(a) with a scanning electron micrograph of the top view of the coplanar common-emitter TL device and Fig. 2(b) with a focused ion beam (FIB) micrograph of a cross-section of the EETL. The QWTL in Fig. 2(a) has an emitter cavity width of ∼2 μm and a cleaved cavity length ∼200 μm and the emitter, base, and collector contacts are 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 of the present work consists of a 5000 Å heavily doped n-type GaAs buffer layer and 5000 Å of n-type Al0.95Ga0.05 As serves as the lower cladding (f). The collector (e) is composed of Al0.4Ga0.6 As that serves as the sub-collector and a 1000 Å lightly doped n-type GaAs collector layer. The base (b) is composed of a conduction energy barrier (81 meV) formed by a 100 Å layer of heavily C-doped p-type Al0.1Ga0.9 As, and a 910 Å heavily doped p-type GaAs base layer. Within the base layer, there is incorporated an undoped 150 Å InxGa1-xAs QW designed for emission at λ ≈ 980 nm. On top of the base is a 400 Å lightly doped n-type In0.49Ga0.51P emitter layer (c). A 5000 Å n-type GaAs/Al0.92Ga0.08As top cladding layer (b) is grown on top of the emitter. A 1000 Å heavily doped n-type GaAs emitter contact layer (a) caps the stack of layers. Figure 3 shows the transistor laser spectrum with peak wavelength at 977 nm operating at IB = 50 mA and VCE = 1.5 V at 15 °C. The large 22 dB SMSR is related to the cavity structure and high differential gain of TL, we apply an oxide confined process to improve the cavity width to be < 2 μm for a cleaved mirror cavity length of 200 μm.
IV. COLLECTOR IC-VCE AND OPTICAL L-VCE FAMILY OF CHARACTERISTICS
A 2 μm × 200 μm cavity EETL operating at 20 °C shows the measured outputs of (a) the collector IC–VCE and (b) the optical L–VCE family of characteristics in Fig. 4. The collector IC-VCE characteristics with a step-upward collector voltage exhibit sharp current changing at switch-UP voltage (VTU) as VCE increases from 0 to 4 V and IB increases from 0 to 90 mA with steps of ΔIB = 3 mA. Four different operating regions are identified: (1) the spontaneous region below base current threshold ITH = 43 mA (black, JTH = 10.75 kA/cm2), (2) laser stimulated region above base current threshold (red), (3) the IICPAT switch-UP region (blue), and (4) the spontaneous region above the base current threshold (ITH) and above the collector voltage threshold (VTH) (black). This device shows the unique signature of TL operation: collector current gain compression at the laser threshold current ITH. This unique characteristic is attributed to the change in base recombination lifetime as the device shifts operation from “slow” spontaneous (black) in region 1 to “faster” stimulated recombination (red) in region 2.
The blue region 3 in the IC–VCE 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 IICPAT switching (black region 4). The blue region in the L–VCE 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 IICPAT switching, thus, yielding only incoherent light output at low intensity (black region).
V. ELECTRICAL HYSTERESIS IN COLLECTOR IC-VCE CHARACTERISTICS @ 20 and 10 °C
Detailed electrical switching characteristics of the TL at 20 °C are shown in Fig. 5(a). The step-upward (forward) collector IC-VCE family of characteristics (red) in Fig. 5(a) exhibits a sharp current change at switch-UP voltage (VTU) as VCE increases from 2 to 3 V and IB increases from 72 to 90 mA with steps of ΔIB = 3 mA. For a given IB = 84 mA (ITH = 43 mA), the current difference is ΔIC = 15 mA at VTU = 2.60 V. When VCE decreases from 3 to 2 V for step-downward (backward) operation, current switching at different switch-DOWN voltage (VTD) is shown in Fig. 5(a) with black. For given IB = 84 mA (ITH = 43 mA (JTH = 10.75 kA/cm2)), the current difference is ΔIC = 15 mA at VTD = 2.56 V.
When the device is operated at 10 °C and the threshold is reduced to ITH = 33 mA (JTH = 8.25 kA/cm2), the forward collector IC-VCE family of characteristics (red) in Fig. 6(a) exhibits a sharper current change at VTU from 2.5 to 4 V and IB increases from 69 to 90 mA with steps of ΔIB = 3 mA. For given IB = 84 mA (ITH = 33 mA), the current difference improves to ΔIC = 25 mA at VTU = 3.46 V. When VCE decreases from 4 to 2.5 V for backward operation, different corner current switching at switch-DOWN voltage (VTD) characteristics (black) is shown in Fig. 6(a). For IB = 84 mA, the current difference increases to ΔIC = 25 mA at VTD = 3.4 V. Due to the difference in switch-UP and switch-DOWN voltage, the electrical hystereses in the collector IC-VCE family of characteristics are demonstrated at 20 °C and 10 °C, respectively.
In 2016, we study the effect of intra-cavity coherent photon intensity on photon-assisted tunneling in the transistor laser and realized photon field-enhancement on optical absorption which has an inherent advantage to achieve a large extinction ratio.22 Thus, this process is named Feng-Holonyak intra-cavity photon-assisted tunneling (FH-ICPAT). The physical mechanism of switch-UP is explained by the base-QW shifting operation from stimulated to spontaneous recombination when the optical absorption rate by FH-ICPAT increases with VCE and exceeds the stimulated photon generation rate at the base-QW for a given base current, the cavity photon density then drops below the coherent threshold resulting in switching at switch-UP voltage VTU. After switching, the transistor is operating under spontaneous but above the laser IB current threshold, ITH. The mechanism of switch-DOWN is explained by the base-QW shifting operation from spontaneous to stimulated (lasing) recombination when the optical absorption rate decreases with VCE and is lower than the spontaneous photon generation rate, the cavity photon density increases from the incoherent state above to the coherent threshold resulting in switching at switch-DOWN voltage VTD.
Note that the output collector current (IC) and the input collector voltage (VCE) relationship 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 the switching slope reduces as temperature decreases. Thus, the electrical hysteresis family of characteristics is programmable with base current (IB), collector voltage (VCE), and the junction temperature.
VI. OPTICAL HYSTERESIS IN OPTICAL L-VCE CHARACTERISTICS @ 20 °C AND 10 °C
Figure 5(b) displays the TL step-upward (forward) optical L-VCE (IB) family of characteristics at 20 °C. When VCE increases from 2 to 3 V and IB increases from 72 to 90 mA with steps ΔIB = 3 mA, the forward L-VCE characteristics exhibit a sharp optical change at VTU in Fig. 5(b) (red). A difference in optical output power ΔL = 1 mW is observed for IB = 84 mA at VTU = 2.6 V. When VCE decreases from 3 to 2 V, the backward L-VCE characteristics exhibit an optical change at VTD in Fig. 5(b) (black). Similarly, ΔL = 1 mW is observed for IB = 84 mA at VTD = 2.56 V.
Figure 6(b) displays the TL forward (red) and backward (black) optical L-VCE (IB) family of characteristics at 10 °C. When VCE increases from 2.5 to 4 V and IB increases from 69 to 90 mA with steps ΔIB = 3 mA, the forward optical L-VCE characteristics exhibit a sharp optical change at VTU in Fig. 6(b) (red). A larger difference in optical output power of ΔL = 1.5 mW is observed for IB = 84 mA at VTU = 3.45 V. Forward optical switching at 10 °C in Fig. 6(b), inversely corresponding to forward electrical switching as shown in Fig. 6(a), is due to the optical cavity shifting operation from the coherent to incoherent state. When the optical absorption rate by FH-ICPAT increases with VCE and is greater than the optical generation rate at the base-QW, the cavity photon density drops below the coherent threshold resulting in optical switching at VTU = 3.45 V for IB = 84 mA.
When VCE decreases from 4 to 2.5 V, the backward L-VCE characteristics at 10 °C exhibit an optical step-change at VTD in Fig. 6(b) (black). Similarly, ΔL = 1.5 mW is overserved for IB = 84 mA at VTD = 3.4 V. Optical backward switching is due to the optical cavity shifting operation from incoherent to coherent. When the optical absorption rate decreases with VCE and is below the photon generation rate, the cavity photon density increases above the threshold resulting in coherent light output.
VII. ELECTRO-OPTICAL HYSTERESIS AND BISTABILITY
Figure 7(a) demostrates the electrical hysteresis of the collector current IC-VCE and Fig. 7(b) displays the optical hysteresis of the optical L-VCE at 20 °C for fixed IB = 84 mA. When VCE increases from 2.5 to 2.58 V, the IC and L (red) move forward from point (C) to (A). The base operation is in stimulated recombination, the light ouput is coherent. The cavity photon density reduces by increasing FH-ICPAT optical absorption rate with a nealy constant QW optical generation rate limited by a fixed IB = 84 mA. When VCE increases from 2.58 to 2.67 V, the IC and L (red) move forward from (A) to (D). The cavity operation shifts from coherent to incoherent when the cavity photon density reduces below the threshold at VTU = 2.6 V by further increase of FH-ICPAT photon absorption rate and the base operation shifits from stimulated to spontaneous recombination. The current difference is ΔIC = 15 mA and the optical light difference is ΔL = 1 mW at VTU = 2.6 V.
When VCE decreases from 2.67 to 2.58 V in Fig. 7(b), the IC and L (black) move backward from point (D) to (B). The base-QW operates in spontaneous recombination and the light is incoherent. When VCE decreases from 2.58 to 2.5 V, the IC and L (black) moves backward from point (B) to (C). The base shifts operation from spontaneous to stimulated recombination at VTD = 2.56 V and the light output shifting from incoherent to coherent (laser) with further reduction of the photon absorption rate by FH-ICPAT. For given IB = 84 mA (ITH = 43 mA, JTH = 10.75 kA/cm2), the current difference is ΔIC = 15 mA and the optical light difference is ΔL = 1 mW at VTD = 2.56 V. A transistor laser electro-optical bistability is consequently realized and demonstrated at VCE = 2.58 V between the coherent state (A) and the incoherent state (B).
Figure 8(a) demostrates the electrical hysteresis of the collector current IC-VCE and Fig. 8(b) displays the optical hysteresis in L-VCE at 10 °C for IB = 84 mA with ITH = 33 mA (JTH = 8.25 kA/cm2). When VCE increases from 3.33 to 3.43 V, the IC and L (red) move forward from point (C) to (A). The light ouput is coherent with the base operating in stimulated recombination. When VCE further increases from 3.43 to 3.51 V, the IC and L (red) 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 VTU = 3.46 V by further increase of the FH-ICPAT photon absorption rate. As a result, the base operation shifits from stimulated to spontaneous recombination. The current difference is ΔIC = 25 mA and the light difference is ΔL = 2 mW at VTU = 3.46 V.
When VCE decreases from 3.51 to 3.43 V in Fig. 8(b), the IC and L (black) move backward from point (D) to (B). The base operates in spontaneous recombination, the light is incoherent. When VCE further decreases from 3.43 to 3.33 V, the IC and L (black) move backward from point (B) to (C). The light output shifts from incoherent to coherent (laser) with further reduction of photon absorption by FH-ICPAT at VTD = 3.4 V. As a result, the base shifts operation from spontaneous to stimulated recombination. The current difference is ΔIC = 25 mA and the optical light difference is ΔL = 2 mW at VTD = 3.4 V. Noticeably, the switching current and light difference are improved considerably with reduction of device temperature from 20 °C to 10 °C. A transistor laser electro-optical bistability is realized and demonstrated at VCE = 3.43 V at 10 °C between the coherent state (A) and the incoherent state (B).
VIII. PRINCIPLE OF THE TRANSISTOR LASER BISTABILITY
Figure 9 illustrates schematically the paths of bistability operation of the transistor laser. A (three-port) transistor laser with cavity photon density above the laser threshold can be 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 the laser threshold can be assigned as an incoherent energy state (1), the transistor operating under spontaneous recombination in the base-QW providing higher collector current output and incoherent light (LET) output. The switching from state (0) to state (1) is achieved by the reduction of cavity photon density below the coherent threshold via increasing the photon absorption rate with intra-cavity photon assisted tunneling by increasing collector voltage above the switch-UP threshold (VCE ≥ VTU) for a given base current (IB). The switching from the incoherent state (1) to coherent state (0) is achieved by the increase of cavity photon density above the laser threshold via reducing the photon absorption rate by decreasing collector voltage below the collector switch-DOWN threshold (VCE ≤ VTD) for a given base current (IB).
The time delay for the electrical and optical switch-UP is expected to be shorter due to fast intra-cavity photon assisted tunneling reducing the cavity photon density below the incoherent voltage threshold. The quantum tunneling time is characterized as ∼6 to 8 fs by field emission microscopy31 and calculated to be ∼20 to 50 fs.32 The time delay for the electrical and optical switch-DOWN is expected to be longer owing to the slow photon generation rate via spontaneous e-h recombination, 10 to 50 ps for HBT-LET,8–10 building up the cavity photon density above the coherent voltage threshold. The electrical and optical hystereses observed for the TL are due to the time delay of different operational paths of e-h and photon recombination/generation in forward and backward switching.
Since base current (IB) provides the cavity photon generation rate at based QW and the collector voltage (VCE) controls the photon absorption rate, we can program various base current and collector voltage levels to achieve different hysteresis properties. Furthermore, the cavity Q determines the laser threshold and the ratio of IB/ITH which affected the laser junction temperature under operation. Thus, both electrical and optical hysteresis properties can be improved via the design of a high Q microcavity to a lower threshold and junction temperature, and the base QW design and the collector bandgap engineering for a more efficient photon generation and absorption rate.
Room temperature operation of both the electrical and optical bistability of a transistor laser is demonstrated. An electro-optical hysteresis with sharp square corners and different voltage thresholds of the collector IC–VCE and L–VCE 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 Feng-Holonyak intra-cavity photon-assisted tunneling (FH-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 TL can be employed for high speed optical logic gate and flip-flop applications. The transistor laser and the data we have assembled since 2004 and now have taken to higher integrated functionality (optical and electrical hystereses and bistabilities) at room temperature are presented here.
This work was supported in part by Dr. Kenneth C. Goretta, Air Force of Scientific Research (AFOSR) under Grant FA9550-15-1–0122. Mr. Curtis Wang would like to thank the National Defense Science and Engineering Graduate (NDSEG) Fellowship for the support.