Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-06-29
2003-07-22
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S012000, C257S013000, C257S014000, C257S015000, C257S017000, C257S023000, C257S024000, C257S432000, C438S020000, C438S028000, C438S048000, C438S590000
Reexamination Certificate
active
06597011
ABSTRACT:
The present invention relates to an electro-optical or single carrier electronic device that utilises the simultaneous application of two non-parallel electric fields.
BACKGROUND OF THE INVENTION
Optoelectronic devices are well known. They use a means responsive to light to generate a photocurrent, a structure that has a semiconductor quantum well region and a means that responds to the photocurrent so as to electrically control the optical absorption of the semiconductor quantum well region. The optical absorption of a semiconductor quantum well region can vary in response to variations in applied electric field.
The absorption is excitonic in nature and arises from the quantization of discrete energy levels in the conduction band and valence band potential energy wells formed by sandwiching a narrow band gap (NBG) semiconductor between two wide band gap (WBG) semiconductors. If the thickness of the well layer is much smaller than the De Broglie wavelength of the electron, quantum size effects occur. If the minimum of the conduction band edge and valence band maximum of both potential wells occur in the NBG layer, quantized energy levels separated by the NBG energy manifest themselves in the optical absorption spectrum as discrete absorption features separated from the bulk absorption edge. These absorption features are due to the instantaneous creation of electron-hole pairs which experience enhanced coulomb attraction due to their spatial confinement in the potential well, thereby increasing the electron-hole binding energy. As a result, the electron-hole pair, or exciton, is stable against phonon collisions and is stable at room temperature. Modulation of the energy at which this exciton absorption is maximum can be accomplished using the Quantum Confined Stark Effect (QCSE) by a suitably orientated applied electric field, as described by Chemla, D. S. in U.S. Pat. No. 4,525,687.
Conventional optical modulation and photodetection devices employ schemes that use, in general, a reverse biased diode structure containing a not intentionally doped (NID) optically active region sandwiched between conductive layers of p-doped and n-doped semiconductor layers so as to form a p-i-n diode. The electric field being generated across the NID active region by application of a voltage source to the p-doped and n-doped contacts with polarity so as to reverse bias the p-i-n diode. This arrangement has the advantage of producing a low dark current.
When the device is illuminated with suitable wavelength light so as to coincide with the absorption properties of the NID active region, each photon absorbed in the active region instantaneously creates an electron-hole pair (to within ~50 femtoseconds). For optical radiation coupled to the active region with direction of propagation mostly perpendicular to the semiconductor layers, the p-doped and n-doped layers are composed of wider band gap material so as to render them transparent to the radiation.
Due to the applied electric field the photogenerated carriers (electrons and holes) are separated and swept to opposite sides of the device, electrons toward the n-doped contact and holes toward the p-doped contact, thereby generating a photocurrent in an external circuit. This photocurrent is superimposed on the dark current.
For function as a tunable photodetector the device described is an electrical two-port and optical one-port device. For function as an optical beam processing device with a modulated output optical beam constitutes an electrical two-port and optical two-port device.
The photocurrent generated using the optical non-linearity of the multiple quantum structure inside the active region can be used in an external circuit to provide voltage feedback to the device itself. This is commonly referred to as the Self Electro-optic Effect Device (SEED) as described by Miller, D. A. B. in U.S. Pat. No. 4,546,244. The SEED uses the electric field dependence of the exciton absorption (due to electric field modification of the quantum well potential energy) in the active layer by the use of QCSE, and the photocurrent generated can be used to provide positive or negative feedback to the device. This allows one to construct circuits which exhibit optical switching (positive feedback) and optical self linearization/modulation (negative feedback) characteristics.
Note that when the device is used with photocurrent feedback, the photocurrent cannot be amplified electrically without influencing the voltage across the device. A limitation in such a configuration is that the size of the photocurrent ( ) available for use in the external circuit is limited by exciton saturation (which saturates the absorption and broadens the optical non-linearity) and thereby places a maximum value to the input optical power (
i
). It can be shown that the characteristic response time ( ) of p-i-n SEED can be written as (see Miller, D. A. B., “Novel Analog Self-Electrooptic Effect Devices”, IEEE J. Quant. Electron., Vol. 29, No. 2, p. 678),
τ
r
=
k
(
G
i
,
where C is the capacitance, G=dA/dV is the modulator sensitivity defined as the derivative with respect to voltage, A is the voltage dependent absorption of the active region, and k is a constant determined by the incident photon energy. The above equation illustrates that smaller capacitance, larger input optical power and larger absorption sensitivity can be used to decrease the response time. Therefore, it is argued for a given C, G and k, the maximum incident power determines the maximum photocurrent that can be generated and thus speed of the device. Methods to amplify the photocurrent generated in the MQW active region can be via the use of transistor action. Miller in U.S. Pat. No. 4,546,244 teaches an integrated phototransistor/SEED device formed as a (p-type emitter
-type-base/p-type collector) followed by a p-i(MQW)-n. As an optically controlled modulator this device is an electrical two-port and optical three-port device where current amplification (controlled by an optical beam impinging upon the base region of the pnp transistor) is physically separate to the p-i(MQW)-n diode which must remain in reverse bias for absorption modulation.
Another method developed by Goossen et. al. IEEE J. Photonics Tech. Lett., Vol. 4, No. 4, p. 393, teach a device where a NID MQW is placed between the base-collector region of a heterojunction phototransistor (HPT) to form, for example, an (n-type emitter/p-type-base/NID MQW
-type collector) structure. Both these structures rely upon carrier flow along the growth direction and therefore suffer from low mobility due to the MQW barriers. To increase the carrier mobility toward that found in bulk material, the MQW confining barriers were reduced to form extremely shallow quantum wells. Exciton features are still present but speed of operation is at the expense of carrier confinement and thus exciton absorption strength. The present invention seeks to improve on known devices by physically separating the photocurrent transport from the perpendicular biased electric fields so as to produce an electrical four-port and optical two-port device, by the simultaneous application of non-parallel fields. As will be discussed later, the configuration of the present invention allows one to optimize the capacitance of the device without affecting the lateral response time. Two important consequences of this proposed configuration are:
(i) the thickness of the intrinsic region along the growth direction, (i.e. number of quantum wells and superlattice blocking layers), can be increased thereby reducing the capacitance seen by the QCSE modulating field; and
(ii) the lateral transit time of the photogenerated electrons and holes, which is determined by the source-drain separation and in-plane mobility, can be optimized for high speed operation.
In an optimum configuration one applies both perpendicular and parallel electric fields (that may be intrinsically or externally applied) to a material containing two-dimensional quantum wells (or superlattice) one dimensional q
Defence Science and Technology Organization
Lee Eddie
Ortiz Edgardo
Sand & Sebolt
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