Subnanosecond 300 V diffused step recovery diode

M. J. Chudobiak, D. J. Walkey

Abstract

A novel diffused step recovery diode structure is presented, which can operate with reverse voltages of several hundred volts and which exhibits long lifetimes of several microseconds. Experimental results for fabricated devices are presented for 300 V operation into a 50 load, with 1.7 ns (typical) and 0.9 ns (best-case) transition times.

Introduction: Step recovery diodes (SRDs) have remained extremely useful [1,2] in wave-shaping applications [3] in the three decades since their introduction [4]. No other device rivals their combination of fast switching speed, ease of use, and wide availability. Unfortunately, SRDs are generally limited to breakdown voltages of less than 100 V. This low V_BR is partly due to the use of abrupt junctions in modern SRDs.

Figure 1 shows a typical SRD pulse-sharpening circuit. The SRD is biased with a DC current, I_F, which stores charge in the SRD. When the input V_IN rises, the stored charge in the SRD is withdrawn. Until all of the charge is withdrawn, the diode acts as a low impedance. After this time, the diode voltage "snaps" to the full reverse voltage, V_OP.

Fig. 1 SRD circuit used to sharpen the falling edge of a pulse.

Theory: Figure 2 shows the electric field evolution in a high-voltage abrupt-junction p-i-n SRD structure during the reverse transient. The fast "snap" portion of the transient begins when the two space-charge regions overlap, generating a significant electric field throughout the entire active layer. The diode voltage at this instant is the ramp voltage, shown in Figure 1. If the i-layer width is not kept very small, the ramp voltage will be a significant portion of V_OP. This limits the operating voltage of abrupt p-i-n SRDs, since a high V_BR requires a wide i-layer width.

Fig. 2 Evolution of the electric field during the reverse transient in an abrupt p-i-n diode. The electric field profiles are high and narrow.

Attempts to extend the operating range of SRDs have been made by connecting several identical devices in series [5], but this approach generally results in a low effective carrier lifetime and increased mechanical complexity.

This letter reports on the use of a diffused-gaussian doping structure, similar to those used in some power rectifiers, as a high-voltage SRD. Doping profiles of the form:

(1)

are considered, where N_S1 and N_S2 are surface dopings, N_B is the substrate doping, L is the substrate thickness, and is the dopant-diffusion characteristic length. If N_B is chosen such that:

(2)

where N_B > 0 is p-type doping, j_R is the reverse current density V_OP/R_LA, and v_S is the electron saturation velocity, the electric field evolution will be of the form shown in Figure 3. The compensating effect of the dopants lowers and widens the electric field profiles. Thus the two space-charge regions overlap at a lower V_RAMP. Also, the elimination of abrupt junctions allows higher breakdown voltages to be achieved.

Fig. 3 Evolution of the electric field during the reverse transient in the diffused structure presented here. The electric field profiles are low and wide.

(This diffused SRD structure should not be confused with lower-voltage ones designed to incorporate a built-in electric field. The injection levels in the device discussed here are much too high to have useful built-in fields.)

If (2) is not satisfied, only one space-charge region will develop (at the p+ n- junction for N_B < 0, and at the p- n+ junction for N_B > j_R/qv_S [6]). Since the snap transient begins when the space-charge region extends through the entire middle layer, V_RAMP equals V_OP for these cases, and no step recovery occurs.

The switching speed of this SRD structure can be estimated by assuming that the charge remaining in the active layer of the diode just before the snap transient occurs is the minimum consistent with quasi-neutrality. This is equivalent to assuming that V_RAMP 0. The active layer of the diode (x₁ < x < x₂) is assumed to be given by the maximum extent of the overlapped space-charge region just before breakdown. (It is also assumed that the diode is operated with the largest possible voltage, with V_OP = V_BR.) Then

(3)

where I_R/2 is the average current during the snap transient. Also,

(4)

If one notes that E_C, the peak electric field at breakdown (typically 200 kV/cm), is also related to the integrated doping, (3) and (4) can be combined to yield a convenient expression for t_R:

(5)

Fabrication: A device was designed with the profile given by N_S1 = N_S2 = 10¹⁷ cm^-3, N_B = 1.5 10¹⁴ cm^-3, L = 150, A = 1.4 mm², and = 30.6. This profile was fabricated by implanting 2.7 10¹⁴ cm^-2 doses of phosphorus and boron into opposite sides of a thin 4 inch, 85 -cm, float-zone p-type silicon wafer, and performing a 180-hour long drive-in diffusion at 1250 C. After metallization, individual devices were obtained by scribing and cleaving the wafer, yielding nearly-ideal one-dimensional structures.

Experiment: Figure 4 shows experimental results for the circuit of Figure 1, with a 50 load. The widest pulse is the input, and the narrower pulses show the outputs for different I_F. For each output, the fall time is about 1.7 ns, agreeing well with the 2.0 ns predicted by (5). This combination of high V_OP and low t_R is considerably better than what can be achieved with commercial single-device SRDs. Also, for the widest output pulse, the forward bias current was only 12 mA. This corresponds to a carrier lifetime of 4500 ns, which is about an order of magnitude better than commercial SRDs. The wider active-layer width possible in a diffused structure is largely responsible for this high effective lifetime. Similar results been observed when these diodes were used in standard rise-time sharpening circuits, although at the lowest biases (2 mA), a very fast sharpened rise time of 0.9 ns was observed. This is shown in Figure 5.

Fig. 4Output of Fig. 1 for I_F = 2,4,6,8,10, and 12 mA. The widest pulse is the input waveform. The fall time of each output is approximately 1.7 ns. (Scale: 50 mV/div 70 dB = 158 V/div, and 5 ns/div)

Fig. 5 Output of a rise-time sharpening circuit for I_F = 0,2,4,6 and 8 mA. The input and output for the I_F = 0 mA case are the same. Note the subnanosecond risetime for the I_F = 2 mA output. (Scale: 50 mV/div 70 dB = 158 V/div, and 2 ns/div)

Conclusions: A new step-recovery diode structure has been proposed. By using a diffused structure, higher breakdown voltages and lower ramp voltage can be obtained. Experimental results show that a single device can typically switch 300 V into 50 in 1.7 ns. The best-case results had a switching time of 0.9 ns. Very long carrier lifetimes are observed, which are desirable for pulse-sharpening.

M. J. Chudobiak (Avtech Electrosystems Ltd., PO Box 5120, Stn. LCD, Ottawa, Ontario, K2C 3H4, Canada)

D. J. Walkey (Dept. of Electronics, Carleton University, Ottawa, Ontario, K1S 5B6, Canada )

References

[1] M. J. LESHA and F. J. PAOLONI, "Generation of balanced subnanosecond pulses using step-recovery diodes", Electron. Lett., 1995, 31, (7), pp.510-511

[2] L. HOWARD and K. DANESHVAR, "Nanosecond-pulse generator for laser diodes", Rev. Sci. Instr., 1989, 60, (10), pp. 3343-3345

[3] ROBERT D. HALL and STEWART M. KRAKAUER, "The Step Recovery Diode for Microwave Harmonic Generation and Nanosecond Pulse Generation", Electronic Components, Nov. 1965. pp. 1046-1051.

[4] J. L. Moll, S. Krakauer, and R. Shen, "P-n junction charge storage diodes", Proc. IRE, 1962, 50, (1), pp. 43-53

[5] DAVID BROWN and DON MARTIN, "Subnanosecond high-voltage pulse generator", Rev. Sci. Instr., 1987, 58, (8), pp. 1523-1529.

[6] HANSJOCHEN BENDA and EBERHARD SPENKE, "Reverse Recovery Processes in Silicon Power Rectifiers", Proc. IEEE, 1967, 55, (8), pp. 1331-1354