Patent Application
20050040432
This invention relates to semiconductor light emission, and, more particularly to a method and device for producing controlled light emission, and which is also simultaneously capable of electrical signal amplification.
A part of the background hereof lies in the development of light emitters based on direct bandgap semiconductors such as III-V semiconductors. Such devices, including light emitting diodes and laser diodes, are in widespread commercial use.
Another part of the background hereof lies in the development of wide bandgap semiconductors to achieve high minority carrier injection efficiency in a device known as a heterojunction bipolar transistor (HBT), which was first proposed in 1948 (see e.g. U.S. Pat. No. 2,569,376; see also H. Kroemer, “Theory Of A Wide-Gap Emitter For Transistors” Proceedings Of The IRE, 45, 1535-1544 (1957)). These transistor devices are capable of operation at extremely high speeds. An InP HBT has recently been demonstrated to exhibit operation at a speed above 500 GHz.
It is among the objects of the present invention to provide devices and methods for producting controlled light emission, and to also provide devices capable of simultaneous control of optical and electrical outputs.
An aspect of the present invention involves a direct bandgap heterojunction transistor that exhibits light emission from the base layer. Modulation of the base current produces modulated light emission. [As used herein, “light” means optical radiation that can be within or outside the visible range.]
A further aspect of the invention involves three port operation of a light emitting HBT. Both spontaneous light emission and electrical signal output are modulated by a signal applied to the base of the HBT.
In accordance with one embodiment of the invention, a method is set forth for producing controllable light emission from a semiconductor device, including the following steps: providing a heterojunction bipolar transistor device that includes collector, base, and emitter regions; and applying electrical signals across terminals coupled with the collector, base, and emitter regions to cause light emission by radiative recombination in the base region. In a form of this embodiment, the step of applying electrical signals includes applying a collector-to-emitter voltage and modulating light output by applying a modulating base current.
In accordance with another embodiment of the invention, a device is set forth having an input port for receiving an electrical input signal, an electrical output port for outputting an electrical signal modulated by the input signal, and an optical output port for outputting an optical signal modulated by the input signal, the device comprising a heterojunction bipolar transistor device that includes collector, base, and emitter regions, the input port comprising an electrode coupled with the base region, the electrical output port comprising electrodes coupled with the collector and emitter regions, and the optical output port comprising an optical coupling with the base region.
In accordance with a further embodiment of the invention, a semiconductor laser is set forth, including: a heterojunction bipolar transistor structure comprising collector, base, and emitter of direct bandgap semiconductor materials; an optical resonant cavity enclosing at least a portion of the transistor structure; and means for coupling electrical signals with the collector, base, and emitter regions to cause laser emission from the device.
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.
This embodiment employs a fabrication process sequence which includes e-beam defined Ti/Pt/Au emitter contacts (145), a self-aligned emitter etch, a self-aligned Ti/Pt/Au base metal deposition, a base-collector etch, and collector metal deposition. A bisbenzocyclobutene (BCB) based etch-back process is employed for “backend” fabrication (i.e., to render the electrode and contact formation on the top of the transistor).
For conventional PN junction diode operation, the recombination process is based on both an electron injected from the n-side and a hole injected from the p-side, which in a bimolecular recombination process can be limited in speed. In the case of HBT light emission hereof, the base “hole” concentration is so high that when an electron is injected into the base, it recombines (bimolecular) rapidly. The base current merely re-supplies holes via relaxation to neutralize charge imbalance. For a heterojunction bipolar transistor (HBT), the base current can be classified into seven components, namely: (1) hole injection into the emitter region (iBp); (2) surface recombination current in the exposed extrinsic base region (iBsurf); (3) base ohmic contact recombination current (iBcont); (4) space charge recombination current (iBscr); (5) bulk base non-radiative recombination current due to the Hall-Shockley-Reed process (HSR) (iBHSR); (6) bulk base Auger recombination current (iBAug); and (7) bulk base radiative recombination current (iBrad).
For a relatively efficient HBT with ledge passivation on any exposed base region, the surface recombination current can be reduced significantly. Hence, the base current and recombination lifetime can be approximated as primarily bulk HSR recombination, the Auger process, and radiative recombination. The base current expressed in the following equation (1) is then related to excess minority carriers, Δn, in the neutral base region, the emitter area, AE, the charge, q, and the base recombination lifetime, τn as
iB=IBHSR+IBAUG+iBrad=qAE Δnτn (1)
The overall base recombination lifetime, τn, is related to the separate recombination components of Hall-Shockley-Read, τHSR, Auger, τAUG, and radiative recombination, τrad, as
τn=(1/τHSR+1/τAUG+1/τrad)−1 (2)
The light emission intensity Δl in the base is proportional to iBrad and is related to the minority carrier electron with the majority hole over the intrinsic carrier concentration, (np-ni2), in the neutral base region and the rate of radiative recombination process, B1 set forth in Equation (3) below, where the hole concentration can be approximated as equal to base dopant concentration, NB. The radiative base current espressed in equation (3) is then related to excess minority carriers, Δn, in the neutral base region, and the base recombination lifetime, τrad as
Brad=q AEB(np−ni2)=q AEB n p=q AEΔn(BNB)=qAEΔn/τrad (3)
For a high speed HBT, it is easy to predict that the base recombination lifetime can be less than half of the total response delay time. Hence, the optical recombination process in the base should be at least two times faster than the speed of the HBT. In other words, HBT speed, which can be extremely fast, is limiting.
In typical transistor operation, one of the three terminals of a transistor is common to both the input and output circuits. This leads to familiar configurations known as common emitter (CE), common base (CB), and common collector (CC). The common terminal (often ground reference) can be paired with one or the other of the two remaining terminals. Each pair is called a port, and two pairs for any configurations are called a two-port network. The two ports are usually identified as an input port and as an output port. In accordance with a feature hereof as illustrated in
The common emitter output characteristics of the test version of the
An output light modulation test was performed for this embodiment. A pattern generator (Tektronix Function Generator) produces an AC signal with peak-to-peak amplitude of 1 V. A bias tee combines this AC signal with a DC bias voltage of 1.1V from a DC supply. The InGaP/GaAs HBT turn-on voltage is VBE=1.5V. The HBT transistor's emission area (open space of the base region) is less than 1-μm×2-μm. The light from the small aperture (most of the HBT light is obscured in this test) is coupled into a multimode fiber probe with a core diameter of 25 μm. The light is fed into a Si APD detector with a 20-dB linear amplifier. A sampling oscilloscope displays both the input modulation signal and the output light signal. The optical emission wavelength is around 885 nm due to the compositionally graded InGaAs base (1.4% In).
The output peak-to-peak amplitude, Vpp, which is directly proportional to the light emission intensity, Δlout, as a function of base current, is shown in
It will be understood that other configurations and material systems can be used, including, as examples, GaAs and GaN based HBTs, or other direct bandgap material systems.
The principles hereof can also potentially have application to indirect bandgap materials (such as Ge and Si) in an HBT with a heavily doped base region, and with an optical port that is optically coupled with the base region. The light produced will generally be of less intensity than that produced by the direct bandgap HBT light emitters hereof. However, it may be useful to have this light generating and coupling capability in Ge-Si systems for various applications, including devices having one or more quantum wells and/or one or more quantum dot regions for enhancing recombination.
