Oct 20, 2006 - Document layout change ... How can you actually measure the electrical characteristics of a diode? rf and ... time) measured from the falling slope of the pulse to the time where it reaches 3 mA reverse bias given an estimate.
Carrier Lifetime and Forward Resistance in RF PIN-Diodes Revision History: 2006-10-20, Rev. 2.0 Previous Version: 2000-07-28 Page
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Application Note
3
Rev. 2.0, 2006-10-20
Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes
1
Carrier Lifetime and Forward Resistance in RF PIN-Diodes
This abstract summarizes the fundamentals of RF PIN Diode physics. General design considerations of PIN Diodes are discussed and a measuring method for the carrier lifetime is introduced. PIN diodes are used as switches or controlled resistors for signals at radio and microwave frequencies. Figure 1 shows a theoretical sketch of a PIN diode.
AN034_PIN_dioden_construction.vsd
Figure 1
PIN Diode construction
A lightly doped I-region (intrinsic) separates the heavily doped P+ and N+ regions. This forms a capacitor at zero bias: (1) εA C 0 = -----W
Where:
C0: capacitance at zero bias ε: dielectric constant of I-zone material The intrinsic region has a high resistance which can be calculated at zero bias by the following equation: (2) σW r 0 = --------A
Where:
r0: resistance at zero bias σ: specific resistance of I-region (dependant on the doping concentration)
Application Note
4
Rev. 2.0, 2006-10-20
Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes In forward bias the injection of carriers from the P+ and N+ region reduces the specific resistance to a level below that obtained from doping alone. The carrier concentration is determined by both injection and recombination processes; recombination is quantified by the term ‘carrier lifetime‘. The resulting resistance for radio frequencies can be approximated by the following equation: (3) 2
W r f = ---------------------------------( µ n + µ p ) ⋅ τI f
Where:
rf: forward resistance for radio frequencies If: forward current µn: electron mobility µp: hole mobility τ: carrier lifetime
RF Resistance vs. Forward Current 10000
rf [Ohm]
1000 BA595
100
BAR64 BAR63
10
BA592
1
0.1 0.01
0.1
1
10
If [m A] AN034_typical_rf_values.vsd
Figure 2
Typical values
To make things more complicated τ is not constant- it decreases with higher If - because recombination is facilitated when more carriers are available. Additionally τ is reduced by ‘traps‘(e.g. fold atoms) and border effects around the active I-zone. Refer to Figure 2 for a typical values of rf resistances.
Application Note
5
Rev. 2.0, 2006-10-20
Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes
Table 1
Overview of diode parameters
A
W
τ
rf
C0
BA595
Medium
High
1600 ns
High
230 fF
BAR64
High
Medium
1400 ns
Medium
300 fF
BAR63
Low
Low
80 ns
Low
300 fF
BA592
Medium
Very low
120 ns
Very low
1200 fF
As a rule of thumb PIN diodes should be operated at frequencies considerably higher than the reciprocal of τ: (4) 10 f ≥ -----τ
This avoids non-linear intermodulation and harmonics effects by preventing modulation of the charge carrier concentration. How can you actually measure the electrical characteristics of a diode? rf and Ct under bias condition can be obtained by using an impedance analyzer e.g. HP4291. Another test setup gives us the carrier lifetime (Figure 3).
AN034_Test_setup_carrier_lifetim e.vsd
Figure 3
Test setup for measuring the carrier lifetime
Application Note
6
Rev. 2.0, 2006-10-20
Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes The pulse width of the negative pulse from the generator must exceed the expected carrier lifetime. The DC conditions and pulse amplitude should be adjusted to obtain the oscilloscope trace shown in Figure 4: in this example If = 10 mA (0.5 V across 50 Ω) in forward bias, Ir = 6 mA in reverse bias. The time τrr (reverse recovery time) measured from the falling slope of the pulse to the time where it reaches 3 mA reverse bias given an estimate for τ, which is due to the following relationship between τrr and τ: (5) I τ rr = τIn ⋅ 1 + ---f I r
For chosen If and Ir the natural logarithm in the equation reduces to 1.
τ AN034_m easurem ent_carrier_lifetim e.vsd
Figure 4
Oscilloscope trace showing the measurement of carrier lifetime
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