Elimination of the guard ring in uniform avalanche photodiodes

A conventional
p^{+}-n
(or
n^{+}-p
) planar avalanche photodiode with a 10
^-4
cm
²
active area has ∼2.5 × 10
^-4
cm
²
total area because of its protecting guard ring and has a series resistance of ∼50 to 100 ohms. For narrow-band applications, multiplications greater than 10 are necessary to equal the available output power of a conventional nonavalanching
p-i-n
photodiode. In broad-band applications, significant multiplications are necessary to compete favorably with the
p-i-n
when the active area is less than 10
^-4
cm
²
or when the signal frequency is > 1 GHz. A
p-n^{+}
planar structure is discussed that eliminates the need for a guard ring because positive junction curvature occurs on the high-resistivity side. The
p-n^{+}
diodes can be designed to have resistances (R
s
∼2 ohms), capacitances (C < 1 pf), and RC cutoff frequencies (f
co
>100 GHz) equivalent to those of the
p-i-n
and to have uniform multiplication as well. Closer array spacings can be achieved than with the guard ring structure, as well as higher effective quantum efficiencies in the avalanche mode. Practical realization of the
p-n^{+}
structure has been achieved in silicon by a combination of epitaxial and doped-oxide processing. Seven-mil-diameter junctions with high breakdown voltage (110 V) and uniform avalanche properties have been constructed.