Power diode V-I and reverse recovery

A power diode is a two-terminal P-N junction (with a lightly doped $n^-$ drift layer, the P-i-N structure) that conducts when its anode is positive with respect to the cathode and blocks when reverse biased. It is designed for large current and high reverse voltage.

Structure

   Anode
     |
   [ p+ ]   heavily doped
   [ n- ]   drift region (sets
   [    ]   reverse voltage rating)
   [ n+ ]   substrate
     |
   Cathode

V-I characteristic

              iD
               ^         /  forward
               |        /   conduction
               |       /
               |      /
     -VRRM     |    _/ slope 1/r
  ----+--------+---'--------------> vD
      |   reverse    Vcut-in
      |   leakage    (0.7-1 V)
      |   (very small)
      |
      v reverse breakdown (avalanche)

• Forward bias: after the cut-in voltage (about 0.7 to 1 V), the current rises rapidly. The forward drop at rated current is about 1 to 2 V because of the drift-region resistance (on-state resistance $r$).
• Reverse bias: only a small leakage current (µA to mA) flows until the reverse breakdown voltage $V_{BR}$. The diode is rated for a repetitive peak reverse voltage $V_{RRM}$ below this. Beyond breakdown, avalanche current flows and the diode may be damaged.

Reverse recovery characteristic

Reverse recovery: when a conducting diode is switched to reverse bias, its current does not stop at zero. The minority carriers stored in the junction must first be removed, so the current falls through zero, flows in the reverse direction for a short time and then decays to zero.

  iD
   ^ IF
   |-------.
   |        \  slope = di/dt
   |         \
 --+----------\--------------------> t
   |           \      .--------
   |            \   .'
   |   - IRR ....\.'   tb
   |          ta  |<->|
   |         |<-->|
   |         |<---trr--->|

• $t_a$: time from current zero to the peak reverse current $I_{RR}$, while charge stored in the depletion region is removed.
• $t_b$: time for the reverse current to decay from $I_{RR}$ to about $0.25 I_{RR}$, while charge in the bulk is removed.
• Reverse recovery time $t_{rr} = t_a + t_b$; softness factor $S = t_b / t_a$.
• Peak reverse current: $I_{RR} = t_a \dfrac{di}{dt}$.
• Reverse recovery (stored) charge $Q_{RR}$ is the area under the reverse current, approximately a triangle:

$$Q_{RR} = \tfrac{1}{2} I_{RR}\, t_{rr}$$

Depending on $S$, diodes are soft recovery ($S \approx 1$, less oscillation) or fast/abrupt recovery (small $S$). Reverse recovery causes extra switching loss and voltage spikes, so fast-recovery diodes are used in choppers and inverters.