Impatt Diode

IMPATT DiodeName:- Tuhin Dutta

Stream:- ECE

Roll: 1011

Introduction When the pn junction diode is reverse-biased, then current does

not flow. However when the reverse voltage exceeds a certain value, the

junction breaks down and current flows with only slight increase of voltage. This breakdown is caused by avalanche multiplication of electrons and holes in the space charge region of the junction.

The pn junction in the avalanche breakdown condition exhibits negative resistance characteristics in the microwave frequency range.

Since the negative resistance is based upon avalanche multiplication and transit-time effect of carriers, the device has been called the IMPATT (Impact Avalanche Transit-Time) Diode.

Device StructureThe original suggestion for a microwave device employing

transit-time effect was made by W. T. Read and involved an n+-p-i-p+ structure such as that shown in figure. This device operates by injecting carriers into the drift region and is called an IMPATT diode.

The device consists essentially of two regions:

1) the n+p region, at which avalanche multiplication occurs, and

2) the i (essentially intrinsic) region, through which generated holes must drift in moving to p+ contact.

Principle of operation |N

D-N

A|

1020

1016

1012 x

E

x

realideal

Drift region

K A

~V(t)

p+ n+in

Avalanche region I(t)

IMPATT diode

V(t)=VDC+vAC(t)E(t)=EDC+ EAC(t)EDCEb

E(t) EAC(T/4)

x

Eb

n(x,t)T=0

E(t)

x

Eb

EAC(T/2)=0

n(x,t)T=T/4

E(t)

x

Eb

EAC(3T/4)n(x,t)

T=T/2

E(t)

x

Eb

EAC(T)=0

n(x,t)T=3T/4

T=T/4

IMPATT I-V Characteristics

0 T/2 T 3T/2 2T 5T/2Qinj

t0 T/2 T 3T/2 2T 5T/2

I

t0 T/2 T 3T/2 2T 5T/2

IDC

VDC

V

t

vAC

iAC

iAC~-vAC

rAC<0PAC=iACvAC/2<0Power is transferredto the field

Small Signal model

Drift region

K Ap+ n+n-n

Avalanche region

I(t)

xA W

CA=sS/xA

Current in avalanche region is delayed by /2 -inductance

2 /A

AS DC i

xL

v I E

rAC

1 2 /( ) i s DC

A A As

E v IL C

A

rAC>0 < A

rAC<0 > A

Resonance frequency

Typically f=vs/2W

Some IMPATT Circuits

Classification

Device structure is based on the doping profile. The three basic types of Impatt diodes are:-

1. Single drift region (SDR) - The SDR diode consists of a single avalanche zone and a single drift zone with p+nn+ structure.

2. Double drift region (DDR) – A DDR diode has a p+pnn+ structure that consist of two drift layers, one for electrons and other for holes on either side of the central avalanche zone.

3. Double avalanche region (DAR) – The DAR diode has a p+nipn+ structure that consist of one drift zone sandwiched between two avalanche zones. The electrons and holes from the two junctions travel across the central i-region in opposite directions and deliver power.

Applications These diodes make excellent microwave

generators for many applications like:-

1. Parametric amplifier,

2. Parametric up converter,

3. Parametric down converter,

4. Negative resistance parametric amplifier.

SummarySummary

• IMPATT stands for Impact Avalanche And Transit Time

• Operates in reverse-breakdown (avalanche) region

• Applied voltage causes momentary breakdown once per cycle

• This starts a pulse of current moving through the device

• Frequency depends on device thickness

• IMPact Ionization Transit Time• IMPATT devices can be used for oscillator and

amplifier applications• They can be fabricated with Si, GaAs, and InP• Can be used up 400 GHz.• Noisy oscillator• In general, IMPATTs have 10 dB higher AM

noise than that of Gunn diodes • IMPATT diode is not suitable for use as a local

oscillator in a receiver.

Thank YouThank You……