Fiber optic communications typically employ a modulated light source, such as a laser, a photodiode light detector, and an optical fiber interconnecting the laser and the photodiode. The laser typically is modulated to emit light pulses, which are to be received by remote photodiodes and converted into electrical signal outputs by the photodiodes.
Conventional photodiodes typically are arranged as PIN type photodiodes. FIG. 1 shows a common PIN photodiode 10 of the prior art. As shown in this figure, the photodiode 10 is constructed as a chip having three semiconductor regions: a p region 12, an n region 14, and an intermediate i (intrinsic) region 16. The p and n regions 12 and 14 normally are doped to high carrier concentrations while the i region 16 typically is unintentionally doped to have a small, residual p or n type carrier concentration. A p contact 18, or anode, is connected to the p region 12 and an n contact 20, or cathode, is connected to the n region 14. Normally, the p region 12 is coated with a dielectric coating 22 which prevents surface current leakage around the sides of the device and also serves as anti-reflective coating if the device is illuminated from the top side. The n region 14 is coated with an antireflective coating 24 which prevents reflection of the incident light away from the device. In some arrangements, a thin buffer coating (not shown) on the order of 0.1 .mu.m to 0.2 .mu.m is placed between the n region 14 and the i region 16 to prevent diffusion therebetween and avoid inadvertent doping of the i region.
PIN photodiodes such as that shown in FIG. 1 are negatively biased such that the entire i region 16 is depleted and substantially no current flows through the device under dark conditions. When incident light, for example light exiting an optical fiber, passes through either the transparent p region 12 or the transparent n region 14, it is absorbed by the i region 16 and the photons of light, hv, are converted into electron-hole pairs which create a net current flow through the photodiode 10.
A high performance photodiode must meet the speed requirements of high-bit-rate systems and must be efficient in converting optical signals at the operating frequencies to electrical signal current. Presently, high-bit-rate systems typically operate in the 10 Ghz to 20 Ghz frequency range. To meet the speed requirements of such systems, the photodiode used must have an adequately wide bandwidth. To operate at this wide bandwidth, the photodiode must be configured so as to minimize device capacitance. In particular, the capacitance of the pn junction, C.sub.j, must be minimized. The junction capacitance can be calculated by the mathematical expression C.sub.j =.epsilon.A/W, where .epsilon. is the dielectric permittivity, A is the junction area, and W is the depletion region thickness. In keeping with this expression, previous attempts at minimizing device capacitance have focused on minimizing the junction area, A, and reducing the unintentional doping of the i region. Although an effective means of reducing capacitance, junction area minimization creates difficulties with optical fiber/photodiode alignment. Misalignment of the optical fiber with the photodiode can result in part or all of the optical signals not being absorbed in the i region of the photodiode, reducing the optical to electrical conversion efficiency and generating tails on the pulses resulting in a bandwidth reduction.
From the above expression, it is apparent that increasing the depletion region thickness, W, is another means of reducing capacitance. This normally is accomplished by increasing the thickness of the i region which, as mentioned above typically is completely depleted by the negative bias applied to the photodiode. Although increasing i region thickness does effectively reduce capacitance of the photodiode, attempts to do so create manufacturing difficulties. In particular, it is difficult to grow relatively thick i regions composed of, for example, indium gallium arsenide (InGaAs) with high yield results.
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