582
21 Diodes
(a)
(b)
(c)
Fig. 21.68 a Donor-doping profile according to (21.175) in p + n or Schottky diodes for m = 0 (abrupt junction), m =1
(linearly graded junction) and two values with m < 0 (hyperabrupt junctions). b Bias dependence of capacitance for
diodes with abrupt junction (‘A’, γ = 0.5), hyperabrupt junction (‘H’, γ > 0.5) and ‘epilinear’ junction (‘L’, γ = 2). c
Frequency-voltage tuning characteristics (scaled to 1.0 for V = 0) for the three diode types. Parts (b) and (c) adapted
from [1794], reprinted with permission
21.5.6 Fast-Recovery Diodes
Fast-recovery diodes are designed for high switching speeds. The switching speed from the forward to
the reverse regime is given by the time t 0 = t 1 + t 2 with t 1 being the time to reduce the minority carrier
density to the equilibrium value (e.g. p n → p n 0 ) and t 2 being the time in which the current decreased
exponentially (Fig. 21.69). The time t 1 can be drastically reduced by incorporation of deep levels that
act as recombination centers. A prominent example is Si:Au. However, this concept is limited since
the reverse generation current, e.g. (21.154), depends on the trap density. For direct semiconductors,
recombination times are short, e.g. 0.1 ns or less for GaAs. In silicon, they can be extremely long (up to
ms) or at least 1–5 ns. Schottky diodes are suitable for high-speed applications since they are majority
carrier devices and minority charge carrier storage can be neglected.
21 Diodes
(a)
(b)
(c)
Fig. 21.68 a Donor-doping profile according to (21.175) in p + n or Schottky diodes for m = 0 (abrupt junction), m =1
(linearly graded junction) and two values with m < 0 (hyperabrupt junctions). b Bias dependence of capacitance for
diodes with abrupt junction (‘A’, γ = 0.5), hyperabrupt junction (‘H’, γ > 0.5) and ‘epilinear’ junction (‘L’, γ = 2). c
Frequency-voltage tuning characteristics (scaled to 1.0 for V = 0) for the three diode types. Parts (b) and (c) adapted
from [1794], reprinted with permission
21.5.6 Fast-Recovery Diodes
Fast-recovery diodes are designed for high switching speeds. The switching speed from the forward to
the reverse regime is given by the time t 0 = t 1 + t 2 with t 1 being the time to reduce the minority carrier
density to the equilibrium value (e.g. p n → p n 0 ) and t 2 being the time in which the current decreased
exponentially (Fig. 21.69). The time t 1 can be drastically reduced by incorporation of deep levels that
act as recombination centers. A prominent example is Si:Au. However, this concept is limited since
the reverse generation current, e.g. (21.154), depends on the trap density. For direct semiconductors,
recombination times are short, e.g. 0.1 ns or less for GaAs. In silicon, they can be extremely long (up to
ms) or at least 1–5 ns. Schottky diodes are suitable for high-speed applications since they are majority
carrier devices and minority charge carrier storage can be neglected.