00491243 Fisica de semiconductores

7/27/2019 00491243 Fisica de semiconductores

http://slidepdf.com/reader/full/00491243-fisica-de-semiconductores 1/6

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 43, NO. 5, MAY 1996 68 5

Modeling and Characterization of the Reverse

Recovery of a High-Power GaAs Schottky Diode

S a m e e r P. Pendharkar, Craig R. Winterhal te r , and Krishna Shenai , Senior Member, IEEE

Abstract-The reverse recovery characteristics of high-power

GaAs Schottky rectifiers are reported at various temperatures.

Mixed device and circuit simulations were used to study the

internal plasma dynamics during the reverse recovery process.

In this approach, semiconductor transport and heat generation

and diffusion equations were solved self-consistently using a two-

dimensional (2-D) finite element grid structure under boundary

conditions imposed by the measurement circuit. The simulation

results are shown to be in good agreement with the measured data

at temperatures n the range of 25"C to 125"C. These results are

compared with the reverse recovery characteristics of a commer-

cial silicon P-1-N power rectifier under identical conditions and

it is shown that carrier depletion is the dominant mechanism

causing the reverse recovery in a GaAs Schottky diode. Thereverse recovery power loss is negligible in a GaAs Schottky

rectifier and is shown to decrease as the case temperature is

increased, contrary to the silicon Pd-N rectifier behavior.

I. INTRODUCTION

N many power electronic applications such as the mo-I or drives and pulse-width modulated (PWM) converters,

switching frequency in excess of a few hundred kilohertz is

used to reduce the size of the passive components [l]. With

this increase in switching frequency, there is a need for faster

devices with reduced switching power loss in addition to low

conduction power loss. A power rectifier can severely impairthe performance of fast-switching converter topologies and

is often one of the most crucial components. It is therefore

desirable to have a rectifier with low forward conducting

voltage drop [2], minimum reverse leakage current, minimum

reverse recovery, and a soft reverse recovery which limits the

EM1 and decreases the switching power losses and the stress.

One such rectifier is the GaAs Schottky diode.

The GaAs Schottky diode has several advantages over

the silicon P-i-N diode. These include improved switching

speed and reduced reverse recovery power loss. The improved

switching speed stems from the Schottky diode being a ma-

jority carrier device whereas the P-i-N structure is a minority

carrier device and is prone to minority carrier charge storage

effects [3], [4]. This characteristics of the P-i-N structure is

particularly deleterious when used in applications requiringManuscript received November 2, 1995; revised December 20, 1995. The

review of this paper wa s arranged by Editor N. Moll.S. P. Pendharkar was with the Electronics and Computer Engineering

Department, University of Wisconsin-Madison, WI 53706-169 1 USA. He isno w with Texas Instruments, Dallas,TX 75243 USA

C. R . Winterhalter is with the Electrical and Computer Engineering Depart-ment, University of Wisconsin-Madison, W1 53706-1691 USA.

K. Shenai is with the Department of Electrical Engineering and ComputerScience, University of Illinois at Chicago, Chicago, IL 60607-7053 USA.

Publisher Item Identifier S 0018-9383(96)03365-5.

fast switching speed. This mechanism is not present in the

Schottky structure, and hence, makes the Schottky structureclearly superior in high-speed power switching applications.

Although it is expected that the Schottky rectifier will

show better reverse recovery characteristics compared to P i - N

diodes, it is difficult to fabricate high-power Schottky rectifiers

with reverse breakdown voltage VBD > 100 V on silicon

substrates because of increased surface leakage currents. Com-

pound semiconductor materials are known to result in surface

Fermi-level pinning characteristics because of high density

of surface states [5]. In addition, these materials provide

improved electron transport due to higher carrier mobFor example, GaAs devices with identical values of VBD ar e

shown to result in nearly an order of magnitude improvement

in electrical conductivity compared to silicon devices [6].

Improvement in material growth and device technologies has

resulted in the fabrication of high-power G aAs Schottky diodes

which can be used to reduce the overall power losses in a

given converter. Although much w ork has been done to study

the reverse recovery characteristics of high-power Pi - N diodes

[7]-[ 101, limited results are available in the p ublished literature

on high-power Schottky diodes. This study is motivated by the

need to understand the reverse recovery of high-power GaAsSchottky rectifiers under different operating conditions.

This paper is organized in the following m anner. The paper

begins with a brief discussion of the parameter extraction

technique used to extract physically-based parameters for a

200-V GaAs Schottky diode using the measured data and

results obtained from a 2-D device simulator [l l] . Next,

the reverse recovery performance of the diode is studied at

elevated temperatures and the physics of switching dynamics

is analyzed. An advanced mixed device and circuit simulator

[12] is used to study the internal plasma dynamics of thediode under boundary conditions imposed by the measurement

circuit. For comparison, results obtained from a 600-V siliconP-1-N diode are also presented under identical static and

dynamic operating conditions. For both diodes, the simulation

results are show n to be in good agreement with the measured

data. Whereas the reverse recovery characteristic of a siliconP-i-N rectifier is degraded at elevated temperatures, that of the

GaAs Schottky rectifier is found to improve with temperature.

11. DIODEPARAMETERXTRACTION

To perform detailed 2-D device simulations, several impor-tant material and device design parameters are needed. The

parameters needed for 2-D simulation of a Schottky diode

are: the drift region doping density ( N D ) nd the drift region

0018-9383/96$05.00 0 996 IEEE



68 6 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 43, NO. 5 , MAY 1996

thickness ( d ) , he diode area A, the asymptotic value of barrier

height of the metal-semiconductor interface at zero electric

field $BO, the effective series resistance R, and a , a constant

used to model the barrier lowering effect caused by the dipole

charge present at the metal-semiconductor interface.

The device structures used in this study are shown in

Fig. 1. The Schottky rectifier was fabricated on epitaxialn-/n+ GaA s substrates grown by LPE. The field termination

was provided using field plates. In addition, diodes with pf

guard rings around the Schottky electrode were also fabricated.

The Schottky contact was formed by dry etching a silicon

oxynitride dielectric layer and sputter depositing and annealing

a titanium alloy. The structure and doping parameters of the

commercial P-i-N diode were obtained from the vendor. The

area A of the device was extracted from optical measurementsof completely processed wafers and was corrected for all

process-induced variations such as undercutting during wet

etching. This value of A agreed closely (within 2%) with

the value obtained from mask dimensions. The area can also

be calculated by finding the equivalent width of the device

by comparing measured forward I-V and simulated (2-D

simulation) I-V characteristics. The drift region doping density

and drift region thickness are obtained from the measured

avalanche breakdown voltage characteristics. Values of 11.5

,um and 4.5 x 1015 cm-3 were extracted for d and No ,

respectively. The value of No was found to be in goodagreement (within 10%) with that extracted from the measured

reverse biased C-'-V curve and SIMS measurements.

The zero electric field barrier height, $BO was extracted us -

ing the 1/C2 versus reverse voltage curve [13]. The measured

1/C2 curve was extrapolated to obtain the X-axis intercept

voltage V,. Using the relation

the value of junction built-in potential Vb, as extracted. Thecorrection factor ( k T / q ) accounts for the majority camerdistribution tail. The correction is simply the dipole moment

of the error distribution [13], [14], the true carrier distribution

minus the ab rupt distribution. Th e value of $BO was calculated

from the relation

BO vbi + vn (2)

where V, is the Fermi-level separation from the conduction

band energy in the bulk. This quantity is simply a function

of the doping concentration and is given by the followingequation

v, = -lll(%)T

4( 3 )

where N C is the conduction band density of states and is

given by [21

(4)

where To is 300 K and T is the,absolu te case temperature ofthe device under test. The values fo r NC varied from 4.6 x 1017

cm-3 at 25°C to 6.5 x 1017 cmP3 a t 100°C.

AluminumNODE

0 / Dielectric

i iI j58.0pm1 n - S i 1 j

N= 2.85 X o ' ~m -3

n+- Si

N= 1.04X10'8cm-3

Tiianvm

Alloy

ACATHODE

ANODE0 Ynum

nt GaAs

N= 2 o ~ 1 d ~ ~ m - ~

CATHODE

(b)

Fig 1

Schottky diodeCross sections of (a) 600-V silicon P-I-N diode, and (b) 200-V GaAs

The value of the effective series resistance R was obtained

from the results plotted in Fig. 4(a) in the voltage range of 0.9V-1.2 V [15]. The value of the static dipole barrier loweringconstant a was found using the following relation

1 - AA*T~e -&bn ln kT ) ( e q V / n k T - l )(5)

where A* is the effective Richardson constant and &, is the

effective electric field dependent barrier height of the metal-

semiconductor junction. A plot of ( 5 ) on a semi-log scale

was used to extract the diode ideality factor n. It should be

noted that the value of n is not required by the simulator. It is

included here just to take into account the tunneling currents[16]. As the tunneling current effects are not very significant

for a doping of 4.5 x 1015 ~ m - ~ , n 1 is expected. The



PENDHARKAR et al.: REVERSE RECOVERY OF A HIGH-POWER GaAs SCHOTTKY DIODE

1200

1000

I- 800zWUU

3 6000

687

-2D-SIMULATIONS

. o MEASUREMENT- 11

-C

~

value of n extracted from measurements at room temperature

was 1.012 which confirms the fact stated above. The value

of n was extracted at each measurement temperature. It was

found that the value of n increased slightly with temperature

consistent w ith literature [14]. The total barrier lowering A$,is given by

46% 46 0 - A 4 (6)

A$ = Aq5' + A 4" (7)

where A$' is the image force induced barrier lowering and

A$'' is the static dipole lowering given by the following

relationships

where

-A$' = /%

an d

A$" = QE,

where E, is given by

(9)

Using the above equations, the value of Q was extracted. The

value of a is dependent on temperature, and hence, Q was

extracted at each temperature from the measured I-V curves.

111. COMPARISON OF MEASUREDND

SIMULATEDTATIC ERFORMANCE

The parameters previously obtained were used to develop

a 2-D simulation grid and the static simulations were per-

formed using PISCES2B [ l l ] . A plot of the measured and

simulated reverse breakdown curves are shown in Fig. 2. This

plot shows that the simulated and measured diode structureshave nearly identical breakdown voltages confirming the fact

that the extracted drift region parameters are correct. The

measured reverse leakage current is found to be greater than

the simulated values. This is primarily attributed to the fact that

surface leakage currents are pronounced in Schottky diodes.

The surface leakage currents are not included in the simulator

leading to a lower reverse leakage current in simulations. In

Fig. 3, a plot of 1/C2 versus reverse voltage is shown for

both the simulated and measured data at 25OC. Again, there

is a good agreement between the measured and simulated

results. Th e I-V curves are shown in Fig. 4(a) at various

temperatures ranging from 25OC to 100OC. The agreement

between the measured and simulated data is good at lower

temperatures. In Fig. 4(b), the junction built-in potential Vb,is plotted versus temperature. This plot shows that has a

negative temperature coe fficient which is expected. A decrease

in Vbi with increasing temperature suggests that the zero-bias

capacitance of the d evice is increasing as the case temperature

is increased. The discrepancy between the measured and

simulated values of Vil, especially at higher temperatures,

results from the difficulty in making accurate capacitance

measurements at elevated temperatures. This is caused by an

increase in leakage currents above room temperature.

wa

8%0

40 0

200

0

0 40 80 120 160 200

CATHODE VOLTAGE (V)

Fig. 2.200-V GaAs Schottky diode at 25OC.Measured and simulated avalanche breakdown characteristics of arE+19

5E+19

4E+19

3E+19

2E+19

1E+19

- - - MEASUREMENT

1E+18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 1 2 3 4 5 6 7

REVERSE BIAS (V)

Fig. 3.200-V GaAs Schottky diode at 25OC.

Measured and simulated 1/C2 versus reverse bias voltage for a

I v . COMPARISON OF MEASUREDND SIMULATED

REVERSERECOVERY HARACTERISTICSThe reverse recovery of the rectifier was measured using

the circuit shown in Fig. 5(a). The reverse recovery was seen

by applying two pulses at the gate of the IGBT. The first

pulse turns the IGBT on, allowing current to flow through the

inductor therefore charging the inductor to the desired current

level. During this pulse, the diode is reverse-biased and nocurrent passes through it. At the falling edge of the first pulse,

the IGBT is switched off and the current begins to free-wheel

through the diod e. After the current has reached a steady value,



688

0.0 0.2 0.4 0.6 0.8 1.0 1.2

ANODE VOLTAGE (V)

(a)

0.7

h= 0.65

-I

IzLLI

I 0.6

9

2

52 0.55

0.5

, I

-2D-SIMULATIONS

o MEASUREMENT

25 50 75 100 125

TEMPERATURE ( "C )

(b)

Fig. 4. Measured and simulated (a) I-V characteristics, and (b ) junctionbuilt-in potential for a 200-V GaAs Schottky diode at various case temper-atures.

the IGBT is once again switched on and the diode is reverse-biased and current flows only through the inductor. This is the

point at which the reverse recovery of the diode is observed.

The reverse recovery was also studied using an advanced

mixed device and circuit simulator [la ]. The simulation circuit

used is show n in Fig. 5(b). In this circuit, the IGBT is replaced

by a switch in series with a small inductance. The working

of the circuit is very simple and is explained briefly. The

resistance R is used to establish the initial current in the

inductor L . The values of R and the supply voltage are

chosen appropriately to establish the initial current in L equal

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL 43, NO 5 , MAY 1996

B rR

Fig. 5 .

and circuit simulations of the reverse recovery of diodes.

Circuit schematics used for (a ) measurement, and (b) mixed device

to the measured value. Next, switch SI s opened so that

current freewheels through the diode, the device under test

(DUT). Then the switch Sz is closed to initiate the reverse

recovery process. The value of Lsmall s adjusted to obtain

the required d i l d t during the diode turn-off process. The

discrepancy in the measured and simulated waveforms is

attributed to the fact that switch SZ in series with Lsmall

does not properly simulate the IGBT behavior in the circuit

shown in Fig. 5(a). The DU T used in the circuit in Fig. 5(b)

is the 2-D diode grid developed from static mtasurements.

The measured and simulated reverse recovery waveforms are

shown in Fig. 6(a) at 25°C and 100°C. The results clearly

show the negative temperature coefficient of the diode reverse

recovery. This is further demonstrated in Table I, which shows

the total measured reverse recovery time t,,, and the measuredreverse recovery charge Qrr, at a d i / d t of 140 A l p s an d

at temperatures ranging from 25°C to 100OC. The reverserecovery charge was estimated by integrating the area enclosed

by the reverse current waveform.The reason for the negative temperature coefficient of th e

diode reverse recovery is explained below. Reverse recovery

in a junction diode is basically attributed to two capacitances

viz., depletion capacitance and diffusion capacitance. It ha s

been shown that, at high current densities, Schottky diode

can show some minority carrier injection [17], with effective

minority carrier lifetimes in the range of 0.1 ns to 1 ns. The

diffusion capacitance in high-power Schottky diodes can be

significant because of large d ie size. The diffusion capacitance

is proportional to T xT2 e ( - - Y $ b n l k T ) x {e ( q V l n k T )- )where



PENDHARKAR et al.: REVERSE RECOVERY OF A HIGH-POWER GaAs SCHOTTKY DIODE 689

2D-SIMULATIONS

MEASUREMENT

4

- 2 L ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’0 50 10 0

TIME (nsec)

(a)

8

6

4

2

0

-2

-40 40 80 120 160

TIME (nsec)

(b)

Fig. 6 .

100°C for (a) 200 V GaAs Schottky diode and @) 600 V P-2-N diode.Measured and simulated reverse recovery characteristics at 25OC and

TABLE I

MEASURED00-V GAASSCHOTTKYIODEREVERSE ECOVERYARAMETERS

Temperature trr (ns) Qrr (nC)

2572 52 32.5

50°C 50 21.3

75°C 49 20.3

100°C 40 8.5

T is the effective minority carrier lifetime. In P-i-N diodes, r

increases significantly with temperature causing the diffusion

capacitance to increase. This results in the degradation of

reverse recovery characteristics at higher temperatures. In

case of Schottky diodes, r does not increase significantly

with temperature, and hence, the exponential term dominates

causing a decrease in diffusion capacitance with tempera-

ture. This causes an improvement in the reverse recovery

performance of the Schottky diode with temperature. The

depletion capacitance in P-i-N as well as Schottky diode

increases som ewhat with temperature because of the negativetemperature d ependence of the built-in potential. Fig. 6(b)

plots the reverse recovery characteristics of a 600-V silicon

P i - N diode for comparison. As can be seen from Fig. 6(b),

the reverse recovery characteristics of a P-i-N diode degrade

with temperature for reasons explained above. Moreover, the

P-i-N diode shows a significantly degraded reverse recovery

characteristics compared to a G aAs Schottky diode because of

large diffusion capacitance (caused by large minority carrier

lifetimes).

v. CONCLUSION AND DISCUSSIONS

The static and reverse recovery performances of a high-voltage GaAs S chottky diode were studied using an advanced

2-D >mixeddevice and circuit simulator and compared to the

measured data. It was shown that the GaAs Schottky diode

reverse recovery mechanism greatly differs from that of the

silicon P-i-N diode. In addition, the GaAs Schottky diode was

shown to have superior performance in terms of improved

switching time and reduced reverse recovery power loss. A

systematic approach is described to extract accurate device

parameters from simple measurements that can be used toconstruct a 2-D simulation grid. In all cases, the simulation

results are show n to be in good agreement with the measured

data.

The mixed device and circuit simulation results presented

in this paper clearly demonstrate the usefulness of 2-D de-vice simulations at the circuit level in understanding the

complex carrier dynamics in high-power switching devices.

This approach is expected to become important in developing

next generation of high-power devices optimized for a given

application.

ACKNOWLEDGMENT

The authors are indebted to A. Saleh of Motorola Power

Products Division, Phoenix, AZ, for providing the GaAs

Schottky diode samples used in this work.

REFERENCES

N. Mohan, T. Undeland, and W. P. Robbins, Power Electronics;Converters, Applications and Design.M. S. Adler, “Factors determining forward voltage drop in field ter-minated diode (FTD),” ZEEE Trans. Electron. Devices, vol. ED-25, pp.529-536, 1978.R. N. Hall, “Power rectifiers and thyristors,” in Proc. IRE, 1952, vol.40, pp. 1512-1518.A. Munoz-Yague and P. Leturq, “High-level behavior of power recti-fiers,” IEEE Trans. Electron. Dev ices, vol. ED-25, pp . 4249, 1978.W. E. Spicer, P. W. Chye, P. R. Skeath, C. Y. Su , and I. Lindau,“New and unified model for Schottky barrier and 111-V insulatorinterface states formation,” J. Vac. Sci. Technol., vof. 16, pp. 1422-1433,Sept./Oct. 1979.

New York Wiley, 1989.



690 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 43 , NO. 5 , MA Y 1996

[6] K Shenai, R S Scott, and B J Baliga, “Optimum semiconductors

fo r high-power electronics,” ZEEE Trans Electron Devices, vol 36, pp.181 1-1823, Sept 1989

171 H Benda and E Spenke, “Reverse recovery processes in silicon powerrectifiers,” in Proc IEEE, 1967, vol 55, pp 1331-1354

181 F Berz, “Step recovery of PIN diodes,” Solid-State Elec tron, vol. 22,pp 927-932, 1979

[9] -, “Ramp recovery in PIN diodes,” Solid-State Electron, vol 23.

pp 783-792, 1980[ I O ] J A G Slatter and J P Whelan, “PIN diode recovery storage time,”

Solid-State Elec tron, vol 23, pp 1235-1242, 1980[1 ] ATLAS Users Manual, Silvaco International, Santa Clara, CA[12J MIXED MODE Users Manual, Silvaco International, Santa Clara, CA1131 S M Sze, Physics o Semiconductor Devices, 2nd ed New York

Wiley, 1981[ 141 K Shenai and R w Dutton, “Current t~~nSPOrtechanisms in atom-

lcallY abrupt metal-~emiconductor nterfaces,” Trans ElectronDevccec., vol 35, no 4, pp 46 848 2, Apr 1988

wl p Van Mleghem, EEE rans Devices, vO 1 41, no 12, I9g4[I61 C Y Chang and S M Sze, “Carrier transport across metal-

semiconductor barrier<,” Solid State Electron, vol 13, pp 727-740,

19701171 D L Scharfetter, “Minority carrier injection and charge storage in

epitaxial Schottky barrier diodes,” Solid-State Electron , vol 8, pp299-301, 1965

Sameer P. Pendharkar was bo m in Pune, India,on September 6, 1972 He studied for five yearsat the Indian Institute of Technology, Bombay,and received the M Tech degree (integrated) inelectrical engineering in 1994 He received the M Sdegree in electrical engineering from University of

Wisconsin-Madison, WI, in 1995Currently, he is with Texas Instruments, Dal-

las, TX, where he is working on developingApplication-Specific Power IC (ASPIC) technolo-gies

Craig R. Winterhalter received the B S degree in electrical engineering fromthe University of Wisconsin, Madison, i n 1995 He is currently pursuing theM S degree in electrical engineering, also at the University of Wisconsin,Madison His current research is in the area of power semiconductor devices

Krishna Shenai (M”79-SM’89), for a photograph and biography, see p.149

of the January issue of this TRANSACTIONS

Documents

00491243 Fisica de semiconductores